macros: add diagnostic derive for lints

`SessionDiagnostic` isn't suitable for use on lints as whether or not it
creates an error or a warning is decided at compile-time by the macro,
whereas lints decide this at runtime based on the location of the lint
being reported (as it will depend on the user's `allow`/`deny`
attributes, etc). Re-using most of the machinery for
`SessionDiagnostic`, this macro introduces a `LintDiagnostic` derive
which implements a `DecorateLint` trait, taking a
`LintDiagnosticBuilder` and adding to the lint according to the
diagnostic struct.

Cargo.lock

@@ -4009,6 +4009,7 @@ dependencies = [
  "rustc_hir",
  "rustc_index",
  "rustc_infer",
+ "rustc_macros",
  "rustc_middle",
  "rustc_parse_format",
  "rustc_session",

compiler/rustc_lint/Cargo.toml

@@ -22,3 +22,4 @@ rustc_trait_selection = { path = "../rustc_trait_selection" }
 rustc_parse_format = { path = "../rustc_parse_format" }
 rustc_infer = { path = "../rustc_infer" }
 rustc_type_ir = { path = "../rustc_type_ir" }
+rustc_macros = { path = "../rustc_macros" }

compiler/rustc_lint/src/types.rs

@@ -5,6 +5,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{fluent, Applicability, DiagnosticMessage};
 use rustc_hir as hir;
 use rustc_hir::{is_range_literal, Expr, ExprKind, Node};
+use rustc_macros::LintDiagnostic;
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, SizeSkeleton};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, AdtKind, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
@@ -1553,13 +1554,20 @@ impl InvalidAtomicOrdering {
         let Some(fail_ordering) = Self::match_ordering(cx, fail_order_arg) else { return };
 
         if matches!(fail_ordering, sym::Release | sym::AcqRel) {
-            cx.struct_span_lint(INVALID_ATOMIC_ORDERING, fail_order_arg.span, |diag| {
+            #[derive(LintDiagnostic)]
-                diag.build(fluent::lint::atomic_ordering_invalid)
+            #[lint(lint::atomic_ordering_invalid)]
-                    .set_arg("method", method)
+            #[help]
-                    .span_label(fail_order_arg.span, fluent::lint::label)
+            struct InvalidAtomicOrderingDiag {
-                    .help(fluent::lint::help)
+                method: Symbol,
-                    .emit();
+                #[label]
-            });
+                fail_order_arg_span: Span,
+            }
+
+            cx.emit_spanned_lint(
+                INVALID_ATOMIC_ORDERING,
+                fail_order_arg.span,
+                InvalidAtomicOrderingDiag { method, fail_order_arg_span: fail_order_arg.span },
+            );
         }
 
         let Some(success_ordering) = Self::match_ordering(cx, success_order_arg) else { return };

compiler/rustc_macros/src/diagnostics/diagnostic.rs

@@ -1,33 +1,24 @@
 #![deny(unused_must_use)]
 
-use crate::diagnostics::error::{
+use crate::diagnostics::diagnostic_builder::{DiagnosticDeriveBuilder, DiagnosticDeriveKind};
-    invalid_nested_attr, span_err, throw_invalid_attr, throw_invalid_nested_attr, throw_span_err,
+use crate::diagnostics::error::{span_err, DiagnosticDeriveError};
-    SessionDiagnosticDeriveError,
+use crate::diagnostics::utils::{build_field_mapping, SetOnce};
-};
+use proc_macro2::TokenStream;
-use crate::diagnostics::utils::{
+use quote::quote;
-    build_field_mapping, report_error_if_not_applied_to_span, report_type_error, type_is_unit,
+use syn::spanned::Spanned;
-    type_matches_path, Applicability, FieldInfo, FieldInnerTy, HasFieldMap, SetOnce,
+use synstructure::Structure;
-};
-use proc_macro2::{Ident, TokenStream};
-use quote::{format_ident, quote};
-use std::collections::HashMap;
-use std::str::FromStr;
-use syn::{
-    parse_quote, spanned::Spanned, Attribute, Meta, MetaList, MetaNameValue, NestedMeta, Path, Type,
-};
-use synstructure::{BindingInfo, Structure};
 
 /// The central struct for constructing the `into_diagnostic` method from an annotated struct.
 pub(crate) struct SessionDiagnosticDerive<'a> {
     structure: Structure<'a>,
     sess: syn::Ident,
-    builder: SessionDiagnosticDeriveBuilder,
+    builder: DiagnosticDeriveBuilder,
 }
 
 impl<'a> SessionDiagnosticDerive<'a> {
     pub(crate) fn new(diag: syn::Ident, sess: syn::Ident, structure: Structure<'a>) -> Self {
         Self {
-            builder: SessionDiagnosticDeriveBuilder {
+            builder: DiagnosticDeriveBuilder {
                 diag,
                 fields: build_field_mapping(&structure),
                 kind: None,
@@ -43,69 +34,21 @@ impl<'a> SessionDiagnosticDerive<'a> {
         let SessionDiagnosticDerive { mut structure, sess, mut builder } = self;
 
         let ast = structure.ast();
-        let attrs = &ast.attrs;
-
         let (implementation, param_ty) = {
             if let syn::Data::Struct(..) = ast.data {
-                let preamble = {
+                let preamble = builder.preamble(&structure);
-                    let preamble = attrs.iter().map(|attr| {
+                let (attrs, args) = builder.body(&mut structure);
-                        builder
-                            .generate_structure_code(attr)
-                            .unwrap_or_else(|v| v.to_compile_error())
-                    });
-
-                    quote! {
-                        #(#preamble)*;
-                    }
-                };
-
-                // Keep track of which fields are subdiagnostics or have no attributes.
-                let mut subdiagnostics_or_empty = std::collections::HashSet::new();
-
-                // Generates calls to `span_label` and similar functions based on the attributes
-                // on fields. Code for suggestions uses formatting machinery and the value of
-                // other fields - because any given field can be referenced multiple times, it
-                // should be accessed through a borrow. When passing fields to `add_subdiagnostic`
-                // or `set_arg` (which happens below) for Fluent, we want to move the data, so that
-                // has to happen in a separate pass over the fields.
-                let attrs = structure
-                    .clone()
-                    .filter(|field_binding| {
-                        let attrs = &field_binding.ast().attrs;
-
-                        (!attrs.is_empty()
-                            && attrs.iter().all(|attr| {
-                                "subdiagnostic"
-                                    != attr.path.segments.last().unwrap().ident.to_string()
-                            }))
-                            || {
-                                subdiagnostics_or_empty.insert(field_binding.binding.clone());
-                                false
-                            }
-                    })
-                    .each(|field_binding| builder.generate_field_attrs_code(field_binding));
-
-                structure.bind_with(|_| synstructure::BindStyle::Move);
-                // When a field has attributes like `#[label]` or `#[note]` then it doesn't
-                // need to be passed as an argument to the diagnostic. But when a field has no
-                // attributes or a `#[subdiagnostic]` attribute then it must be passed as an
-                // argument to the diagnostic so that it can be referred to by Fluent messages.
-                let args = structure
-                    .filter(|field_binding| {
-                        subdiagnostics_or_empty.contains(&field_binding.binding)
-                    })
-                    .each(|field_binding| builder.generate_field_attrs_code(field_binding));
 
                 let span = ast.span().unwrap();
                 let diag = &builder.diag;
-                let init = match (builder.kind, builder.slug) {
+                let init = match (builder.kind.value(), builder.slug.value()) {
                     (None, _) => {
                         span_err(span, "diagnostic kind not specified")
                             .help("use the `#[error(...)]` attribute to create an error")
                             .emit();
-                        return SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
                     }
-                    (Some((kind, _)), None) => {
+                    (Some(kind), None) => {
                         span_err(span, "diagnostic slug not specified")
                             .help(&format!(
                                 "specify the slug as the first argument to the attribute, such as \
@@ -113,14 +56,20 @@ impl<'a> SessionDiagnosticDerive<'a> {
                                 kind.descr()
                             ))
                             .emit();
-                        return SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
                     }
-                    (Some((SessionDiagnosticKind::Error, _)), Some((slug, _))) => {
+                    (Some(DiagnosticDeriveKind::Lint), _) => {
+                        span_err(span, "only `#[error(..)]` and `#[warn(..)]` are supported")
+                            .help("use the `#[error(...)]` attribute to create a error")
+                            .emit();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some(DiagnosticDeriveKind::Error), Some(slug)) => {
                         quote! {
                             let mut #diag = #sess.struct_err(rustc_errors::fluent::#slug);
                         }
                     }
-                    (Some((SessionDiagnosticKind::Warn, _)), Some((slug, _))) => {
+                    (Some(DiagnosticDeriveKind::Warn), Some(slug)) => {
                         quote! {
                             let mut #diag = #sess.struct_warn(rustc_errors::fluent::#slug);
                         }
@@ -139,10 +88,12 @@ impl<'a> SessionDiagnosticDerive<'a> {
                     #diag
                 };
                 let param_ty = match builder.kind {
-                    Some((SessionDiagnosticKind::Error, _)) => {
+                    Some((DiagnosticDeriveKind::Error, _)) => {
                         quote! { rustc_errors::ErrorGuaranteed }
                     }
-                    Some((SessionDiagnosticKind::Warn, _)) => quote! { () },
+                    Some((DiagnosticDeriveKind::Lint | DiagnosticDeriveKind::Warn, _)) => {
+                        quote! { () }
+                    }
                     _ => unreachable!(),
                 };
 
@@ -154,7 +105,7 @@ impl<'a> SessionDiagnosticDerive<'a> {
                 )
                 .emit();
 
-                let implementation = SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
+                let implementation = DiagnosticDeriveError::ErrorHandled.to_compile_error();
                 let param_ty = quote! { rustc_errors::ErrorGuaranteed };
                 (implementation, param_ty)
             }
@@ -176,523 +127,99 @@ impl<'a> SessionDiagnosticDerive<'a> {
     }
 }
 
-/// What kind of session diagnostic is being derived - an error or a warning?
+/// The central struct for constructing the `decorate_lint` method from an annotated struct.
-#[derive(Copy, Clone)]
+pub(crate) struct LintDiagnosticDerive<'a> {
-enum SessionDiagnosticKind {
+    structure: Structure<'a>,
-    /// `#[error(..)]`
+    builder: DiagnosticDeriveBuilder,
-    Error,
-    /// `#[warn(..)]`
-    Warn,
 }
 
-impl SessionDiagnosticKind {
+impl<'a> LintDiagnosticDerive<'a> {
-    /// Returns human-readable string corresponding to the kind.
+    pub(crate) fn new(diag: syn::Ident, structure: Structure<'a>) -> Self {
-    fn descr(&self) -> &'static str {
+        Self {
-        match self {
+            builder: DiagnosticDeriveBuilder {
-            SessionDiagnosticKind::Error => "error",
+                diag,
-            SessionDiagnosticKind::Warn => "warning",
+                fields: build_field_mapping(&structure),
-        }
+                kind: None,
-    }
+                code: None,
-}
+                slug: None,
-
-/// Tracks persistent information required for building up the individual calls to diagnostic
-/// methods for the final generated method. This is a separate struct to `SessionDiagnosticDerive`
-/// only to be able to destructure and split `self.builder` and the `self.structure` up to avoid a
-/// double mut borrow later on.
-struct SessionDiagnosticDeriveBuilder {
-    /// The identifier to use for the generated `DiagnosticBuilder` instance.
-    diag: syn::Ident,
-
-    /// Store a map of field name to its corresponding field. This is built on construction of the
-    /// derive builder.
-    fields: HashMap<String, TokenStream>,
-
-    /// Kind of diagnostic requested via the struct attribute.
-    kind: Option<(SessionDiagnosticKind, proc_macro::Span)>,
-    /// Slug is a mandatory part of the struct attribute as corresponds to the Fluent message that
-    /// has the actual diagnostic message.
-    slug: Option<(Path, proc_macro::Span)>,
-    /// Error codes are a optional part of the struct attribute - this is only set to detect
-    /// multiple specifications.
-    code: Option<(String, proc_macro::Span)>,
-}
-
-impl HasFieldMap for SessionDiagnosticDeriveBuilder {
-    fn get_field_binding(&self, field: &String) -> Option<&TokenStream> {
-        self.fields.get(field)
-    }
-}
-
-impl SessionDiagnosticDeriveBuilder {
-    /// Establishes state in the `SessionDiagnosticDeriveBuilder` resulting from the struct
-    /// attributes like `#[error(..)`, such as the diagnostic kind and slug. Generates
-    /// diagnostic builder calls for setting error code and creating note/help messages.
-    fn generate_structure_code(
-        &mut self,
-        attr: &Attribute,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
-        let diag = &self.diag;
-        let span = attr.span().unwrap();
-
-        let name = attr.path.segments.last().unwrap().ident.to_string();
-        let name = name.as_str();
-        let meta = attr.parse_meta()?;
-
-        let is_help_or_note = matches!(name, "help" | "note");
-
-        let nested = match meta {
-            // Most attributes are lists, like `#[error(..)]`/`#[warning(..)]` for most cases or
-            // `#[help(..)]`/`#[note(..)]` when the user is specifying a alternative slug.
-            Meta::List(MetaList { ref nested, .. }) => nested,
-            // Subdiagnostics without spans can be applied to the type too, and these are just
-            // paths: `#[help]` and `#[note]`
-            Meta::Path(_) if is_help_or_note => {
-                let fn_name = proc_macro2::Ident::new(name, attr.span());
-                return Ok(quote! { #diag.#fn_name(rustc_errors::fluent::_subdiag::#fn_name); });
-            }
-            _ => throw_invalid_attr!(attr, &meta),
-        };
-
-        // Check the kind before doing any further processing so that there aren't misleading
-        // "no kind specified" errors if there are failures later.
-        match name {
-            "error" => self.kind.set_once((SessionDiagnosticKind::Error, span)),
-            "warning" => self.kind.set_once((SessionDiagnosticKind::Warn, span)),
-            "help" | "note" => (),
-            _ => throw_invalid_attr!(attr, &meta, |diag| {
-                diag.help("only `error`, `warning`, `help` and `note` are valid attributes")
-            }),
-        }
-
-        // First nested element should always be the path, e.g. `#[error(typeck::invalid)]` or
-        // `#[help(typeck::another_help)]`.
-        let mut nested_iter = nested.into_iter();
-        if let Some(nested_attr) = nested_iter.next() {
-            // Report an error if there are any other list items after the path.
-            if is_help_or_note && nested_iter.next().is_some() {
-                throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
-                    diag.help("`help` and `note` struct attributes can only have one argument")
-                });
-            }
-
-            match nested_attr {
-                NestedMeta::Meta(Meta::Path(path)) if is_help_or_note => {
-                    let fn_name = proc_macro2::Ident::new(name, attr.span());
-                    return Ok(quote! { #diag.#fn_name(rustc_errors::fluent::#path); });
-                }
-                NestedMeta::Meta(Meta::Path(path)) => {
-                    self.slug.set_once((path.clone(), span));
-                }
-                NestedMeta::Meta(meta @ Meta::NameValue(_))
-                    if !is_help_or_note
-                        && meta.path().segments.last().unwrap().ident.to_string() == "code" =>
-                {
-                    // don't error for valid follow-up attributes
-                }
-                nested_attr => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
-                    diag.help("first argument of the attribute should be the diagnostic slug")
-                }),
-            };
-        }
-
-        // Remaining attributes are optional, only `code = ".."` at the moment.
-        let mut tokens = Vec::new();
-        for nested_attr in nested_iter {
-            let meta = match nested_attr {
-                syn::NestedMeta::Meta(meta) => meta,
-                _ => throw_invalid_nested_attr!(attr, &nested_attr),
-            };
-
-            let path = meta.path();
-            let nested_name = path.segments.last().unwrap().ident.to_string();
-            // Struct attributes are only allowed to be applied once, and the diagnostic
-            // changes will be set in the initialisation code.
-            if let Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) = &meta {
-                let span = s.span().unwrap();
-                match nested_name.as_str() {
-                    "code" => {
-                        self.code.set_once((s.value(), span));
-                        let code = &self.code.as_ref().map(|(v, _)| v);
-                        tokens.push(quote! {
-                            #diag.code(rustc_errors::DiagnosticId::Error(#code.to_string()));
-                        });
-                    }
-                    _ => invalid_nested_attr(attr, &nested_attr)
-                        .help("only `code` is a valid nested attributes following the slug")
-                        .emit(),
-                }
-            } else {
-                invalid_nested_attr(attr, &nested_attr).emit()
-            }
-        }
-
-        Ok(tokens.drain(..).collect())
-    }
-
-    fn generate_field_attrs_code(&mut self, binding_info: &BindingInfo<'_>) -> TokenStream {
-        let field = binding_info.ast();
-        let field_binding = &binding_info.binding;
-
-        let inner_ty = FieldInnerTy::from_type(&field.ty);
-
-        // When generating `set_arg` or `add_subdiagnostic` calls, move data rather than
-        // borrow it to avoid requiring clones - this must therefore be the last use of
-        // each field (for example, any formatting machinery that might refer to a field
-        // should be generated already).
-        if field.attrs.is_empty() {
-            let diag = &self.diag;
-            let ident = field.ident.as_ref().unwrap();
-            quote! {
-                #diag.set_arg(
-                    stringify!(#ident),
-                    #field_binding
-                );
-            }
-        } else {
-            field
-                .attrs
-                .iter()
-                .map(move |attr| {
-                    let name = attr.path.segments.last().unwrap().ident.to_string();
-                    let (binding, needs_destructure) = match (name.as_str(), &inner_ty) {
-                        // `primary_span` can accept a `Vec<Span>` so don't destructure that.
-                        ("primary_span", FieldInnerTy::Vec(_)) => {
-                            (quote! { #field_binding.clone() }, false)
-                        }
-                        // `subdiagnostics` are not derefed because they are bound by value.
-                        ("subdiagnostic", _) => (quote! { #field_binding }, true),
-                        _ => (quote! { *#field_binding }, true),
-                    };
-
-                    let generated_code = self
-                        .generate_inner_field_code(
-                            attr,
-                            FieldInfo {
-                                binding: binding_info,
-                                ty: inner_ty.inner_type().unwrap_or(&field.ty),
-                                span: &field.span(),
             },
-                            binding,
+            structure,
-                        )
+        }
-                        .unwrap_or_else(|v| v.to_compile_error());
+    }
 
-                    if needs_destructure {
+    pub(crate) fn into_tokens(self) -> TokenStream {
-                        inner_ty.with(field_binding, generated_code)
+        let LintDiagnosticDerive { mut structure, mut builder } = self;
+
+        let ast = structure.ast();
+        let implementation = {
+            if let syn::Data::Struct(..) = ast.data {
+                let preamble = builder.preamble(&structure);
+                let (attrs, args) = builder.body(&mut structure);
+
+                let diag = &builder.diag;
+                let span = ast.span().unwrap();
+                let init = match (builder.kind.value(), builder.slug.value()) {
+                    (None, _) => {
+                        span_err(span, "diagnostic kind not specified")
+                            .help("use the `#[error(...)]` attribute to create an error")
+                            .emit();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some(kind), None) => {
+                        span_err(span, "diagnostic slug not specified")
+                            .help(&format!(
+                                "specify the slug as the first argument to the attribute, such as \
+                                 `#[{}(typeck::example_error)]`",
+                                kind.descr()
+                            ))
+                            .emit();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some(DiagnosticDeriveKind::Error | DiagnosticDeriveKind::Warn), _) => {
+                        span_err(span, "only `#[lint(..)]` is supported")
+                            .help("use the `#[lint(...)]` attribute to create a lint")
+                            .emit();
+                        return DiagnosticDeriveError::ErrorHandled.to_compile_error();
+                    }
+                    (Some(DiagnosticDeriveKind::Lint), Some(slug)) => {
+                        quote! {
+                            let mut #diag = #diag.build(rustc_errors::fluent::#slug);
+                        }
+                    }
+                };
+
+                let implementation = quote! {
+                    #init
+                    #preamble
+                    match self {
+                        #attrs
+                    }
+                    match self {
+                        #args
+                    }
+                    #diag.emit();
+                };
+
+                implementation
             } else {
-                        generated_code
-                    }
-                })
-                .collect()
-        }
-    }
-
-    fn generate_inner_field_code(
-        &mut self,
-        attr: &Attribute,
-        info: FieldInfo<'_>,
-        binding: TokenStream,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
-        let meta = attr.parse_meta()?;
-        match meta {
-            Meta::Path(_) => self.generate_inner_field_code_path(attr, info, binding),
-            Meta::List(MetaList { .. }) => self.generate_inner_field_code_list(attr, info, binding),
-            _ => throw_invalid_attr!(attr, &meta),
-        }
-    }
-
-    fn generate_inner_field_code_path(
-        &mut self,
-        attr: &Attribute,
-        info: FieldInfo<'_>,
-        binding: TokenStream,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
-        assert!(matches!(attr.parse_meta()?, Meta::Path(_)));
-        let diag = &self.diag;
-
-        let meta = attr.parse_meta()?;
-
-        let ident = &attr.path.segments.last().unwrap().ident;
-        let name = ident.to_string();
-        let name = name.as_str();
-        match name {
-            "skip_arg" => {
-                // Don't need to do anything - by virtue of the attribute existing, the
-                // `set_arg` call will not be generated.
-                Ok(quote! {})
-            }
-            "primary_span" => {
-                report_error_if_not_applied_to_span(attr, &info)?;
-                Ok(quote! {
-                    #diag.set_span(#binding);
-                })
-            }
-            "label" => {
-                report_error_if_not_applied_to_span(attr, &info)?;
-                Ok(self.add_spanned_subdiagnostic(binding, ident, parse_quote! { _subdiag::label }))
-            }
-            "note" | "help" => {
-                let path = match name {
-                    "note" => parse_quote! { _subdiag::note },
-                    "help" => parse_quote! { _subdiag::help },
-                    _ => unreachable!(),
-                };
-                if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
-                    Ok(self.add_spanned_subdiagnostic(binding, ident, path))
-                } else if type_is_unit(&info.ty) {
-                    Ok(self.add_subdiagnostic(ident, path))
-                } else {
-                    report_type_error(attr, "`Span` or `()`")?;
-                }
-            }
-            "subdiagnostic" => Ok(quote! { #diag.subdiagnostic(#binding); }),
-            _ => throw_invalid_attr!(attr, &meta, |diag| {
-                diag.help(
-                    "only `skip_arg`, `primary_span`, `label`, `note`, `help` and `subdiagnostic` \
-                     are valid field attributes",
-                )
-            }),
-        }
-    }
-
-    fn generate_inner_field_code_list(
-        &mut self,
-        attr: &Attribute,
-        info: FieldInfo<'_>,
-        binding: TokenStream,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
-        let meta = attr.parse_meta()?;
-        let Meta::List(MetaList { ref path, ref nested, .. }) = meta  else { unreachable!() };
-
-        let ident = &attr.path.segments.last().unwrap().ident;
-        let name = path.segments.last().unwrap().ident.to_string();
-        let name = name.as_ref();
-        match name {
-            "suggestion" | "suggestion_short" | "suggestion_hidden" | "suggestion_verbose" => {
-                return self.generate_inner_field_code_suggestion(attr, info);
-            }
-            "label" | "help" | "note" => (),
-            _ => throw_invalid_attr!(attr, &meta, |diag| {
-                diag.help(
-                    "only `label`, `note`, `help` or `suggestion{,_short,_hidden,_verbose}` are \
-                     valid field attributes",
-                )
-            }),
-        }
-
-        // For `#[label(..)]`, `#[note(..)]` and `#[help(..)]`, the first nested element must be a
-        // path, e.g. `#[label(typeck::label)]`.
-        let mut nested_iter = nested.into_iter();
-        let msg = match nested_iter.next() {
-            Some(NestedMeta::Meta(Meta::Path(path))) => path.clone(),
-            Some(nested_attr) => throw_invalid_nested_attr!(attr, &nested_attr),
-            None => throw_invalid_attr!(attr, &meta),
-        };
-
-        // None of these attributes should have anything following the slug.
-        if nested_iter.next().is_some() {
-            throw_invalid_attr!(attr, &meta);
-        }
-
-        match name {
-            "label" => {
-                report_error_if_not_applied_to_span(attr, &info)?;
-                Ok(self.add_spanned_subdiagnostic(binding, ident, msg))
-            }
-            "note" | "help" if type_matches_path(&info.ty, &["rustc_span", "Span"]) => {
-                Ok(self.add_spanned_subdiagnostic(binding, ident, msg))
-            }
-            "note" | "help" if type_is_unit(&info.ty) => Ok(self.add_subdiagnostic(ident, msg)),
-            "note" | "help" => {
-                report_type_error(attr, "`Span` or `()`")?;
-            }
-            _ => unreachable!(),
-        }
-    }
-
-    fn generate_inner_field_code_suggestion(
-        &mut self,
-        attr: &Attribute,
-        info: FieldInfo<'_>,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
-        let diag = &self.diag;
-
-        let mut meta = attr.parse_meta()?;
-        let Meta::List(MetaList { ref path, ref mut nested, .. }) = meta  else { unreachable!() };
-
-        let (span_field, mut applicability) = self.span_and_applicability_of_ty(info)?;
-
-        let mut msg = None;
-        let mut code = None;
-
-        let mut nested_iter = nested.into_iter().peekable();
-        if let Some(nested_attr) = nested_iter.peek() {
-            if let NestedMeta::Meta(Meta::Path(path)) = nested_attr {
-                msg = Some(path.clone());
-            }
-        };
-        // Move the iterator forward if a path was found (don't otherwise so that
-        // code/applicability can be found or an error emitted).
-        if msg.is_some() {
-            let _ = nested_iter.next();
-        }
-
-        for nested_attr in nested_iter {
-            let meta = match nested_attr {
-                syn::NestedMeta::Meta(ref meta) => meta,
-                syn::NestedMeta::Lit(_) => throw_invalid_nested_attr!(attr, &nested_attr),
-            };
-
-            let nested_name = meta.path().segments.last().unwrap().ident.to_string();
-            let nested_name = nested_name.as_str();
-            match meta {
-                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
-                    let span = meta.span().unwrap();
-                    match nested_name {
-                        "code" => {
-                            let formatted_str = self.build_format(&s.value(), s.span());
-                            code = Some(formatted_str);
-                        }
-                        "applicability" => {
-                            applicability = match applicability {
-                                Some(v) => {
                 span_err(
-                                        span,
+                    ast.span().unwrap(),
-                                        "applicability cannot be set in both the field and \
+                    "`#[derive(LintDiagnostic)]` can only be used on structs",
-                                         attribute",
                 )
                 .emit();
-                                    Some(v)
-                                }
-                                None => match Applicability::from_str(&s.value()) {
-                                    Ok(v) => Some(quote! { #v }),
-                                    Err(()) => {
-                                        span_err(span, "invalid applicability").emit();
-                                        None
-                                    }
-                                },
-                            }
-                        }
-                        _ => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
-                            diag.help(
-                                "only `message`, `code` and `applicability` are valid field \
-                                 attributes",
-                            )
-                        }),
-                    }
-                }
-                _ => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
-                    if matches!(meta, Meta::Path(_)) {
-                        diag.help("a diagnostic slug must be the first argument to the attribute")
-                    } else {
-                        diag
-                    }
-                }),
-            }
-        }
 
-        let applicability =
+                DiagnosticDeriveError::ErrorHandled.to_compile_error()
-            applicability.unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
+            }
+        };
 
-        let name = path.segments.last().unwrap().ident.to_string();
+        let diag = &builder.diag;
-        let method = format_ident!("span_{}", name);
+        structure.gen_impl(quote! {
-
+            gen impl<'__a> rustc_errors::DecorateLint<'__a, ()> for @Self {
-        let msg = msg.unwrap_or_else(|| parse_quote! { _subdiag::suggestion });
+                fn decorate_lint(self, #diag: rustc_errors::LintDiagnosticBuilder<'__a, ()>) {
-        let msg = quote! { rustc_errors::fluent::#msg };
+                    use rustc_errors::IntoDiagnosticArg;
-        let code = code.unwrap_or_else(|| quote! { String::new() });
+                    #implementation
-
-        Ok(quote! { #diag.#method(#span_field, #msg, #code, #applicability); })
-    }
-
-    /// Adds a spanned subdiagnostic by generating a `diag.span_$kind` call with the current slug
-    /// and `fluent_attr_identifier`.
-    fn add_spanned_subdiagnostic(
-        &self,
-        field_binding: TokenStream,
-        kind: &Ident,
-        fluent_attr_identifier: Path,
-    ) -> TokenStream {
-        let diag = &self.diag;
-        let fn_name = format_ident!("span_{}", kind);
-        quote! {
-            #diag.#fn_name(
-                #field_binding,
-                rustc_errors::fluent::#fluent_attr_identifier
-            );
                 }
             }
-
+        })
-    /// Adds a subdiagnostic by generating a `diag.span_$kind` call with the current slug
-    /// and `fluent_attr_identifier`.
-    fn add_subdiagnostic(&self, kind: &Ident, fluent_attr_identifier: Path) -> TokenStream {
-        let diag = &self.diag;
-        quote! {
-            #diag.#kind(rustc_errors::fluent::#fluent_attr_identifier);
-        }
-    }
-
-    fn span_and_applicability_of_ty(
-        &self,
-        info: FieldInfo<'_>,
-    ) -> Result<(TokenStream, Option<TokenStream>), SessionDiagnosticDeriveError> {
-        match &info.ty {
-            // If `ty` is `Span` w/out applicability, then use `Applicability::Unspecified`.
-            ty @ Type::Path(..) if type_matches_path(ty, &["rustc_span", "Span"]) => {
-                let binding = &info.binding.binding;
-                Ok((quote!(*#binding), None))
-            }
-            // If `ty` is `(Span, Applicability)` then return tokens accessing those.
-            Type::Tuple(tup) => {
-                let mut span_idx = None;
-                let mut applicability_idx = None;
-
-                for (idx, elem) in tup.elems.iter().enumerate() {
-                    if type_matches_path(elem, &["rustc_span", "Span"]) {
-                        if span_idx.is_none() {
-                            span_idx = Some(syn::Index::from(idx));
-                        } else {
-                            throw_span_err!(
-                                info.span.unwrap(),
-                                "type of field annotated with `#[suggestion(...)]` contains more \
-                                 than one `Span`"
-                            );
-                        }
-                    } else if type_matches_path(elem, &["rustc_errors", "Applicability"]) {
-                        if applicability_idx.is_none() {
-                            applicability_idx = Some(syn::Index::from(idx));
-                        } else {
-                            throw_span_err!(
-                                info.span.unwrap(),
-                                "type of field annotated with `#[suggestion(...)]` contains more \
-                                 than one Applicability"
-                            );
-                        }
-                    }
-                }
-
-                if let Some(span_idx) = span_idx {
-                    let binding = &info.binding.binding;
-                    let span = quote!(#binding.#span_idx);
-                    let applicability = applicability_idx
-                        .map(|applicability_idx| quote!(#binding.#applicability_idx))
-                        .unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
-
-                    return Ok((span, Some(applicability)));
-                }
-
-                throw_span_err!(info.span.unwrap(), "wrong types for suggestion", |diag| {
-                    diag.help(
-                        "`#[suggestion(...)]` on a tuple field must be applied to fields of type \
-                         `(Span, Applicability)`",
-                    )
-                });
-            }
-            // If `ty` isn't a `Span` or `(Span, Applicability)` then emit an error.
-            _ => throw_span_err!(info.span.unwrap(), "wrong field type for suggestion", |diag| {
-                diag.help(
-                    "`#[suggestion(...)]` should be applied to fields of type `Span` or \
-                     `(Span, Applicability)`",
-                )
-            }),
-        }
     }
 }

compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs

@@ -0,0 +1,590 @@
+#![deny(unused_must_use)]
+
+use crate::diagnostics::error::{
+    invalid_nested_attr, span_err, throw_invalid_attr, throw_invalid_nested_attr, throw_span_err,
+    DiagnosticDeriveError,
+};
+use crate::diagnostics::utils::{
+    report_error_if_not_applied_to_span, report_type_error, type_is_unit, type_matches_path,
+    Applicability, FieldInfo, FieldInnerTy, HasFieldMap, SetOnce,
+};
+use proc_macro2::{Ident, TokenStream};
+use quote::{format_ident, quote};
+use std::collections::HashMap;
+use std::str::FromStr;
+use syn::{
+    parse_quote, spanned::Spanned, Attribute, Meta, MetaList, MetaNameValue, NestedMeta, Path, Type,
+};
+use synstructure::{BindingInfo, Structure};
+
+/// What kind of diagnostic is being derived - an error, a warning or a lint?
+#[derive(Copy, Clone)]
+pub(crate) enum DiagnosticDeriveKind {
+    /// `#[error(..)]`
+    Error,
+    /// `#[warn(..)]`
+    Warn,
+    /// `#[lint(..)]`
+    Lint,
+}
+
+impl DiagnosticDeriveKind {
+    /// Returns human-readable string corresponding to the kind.
+    pub fn descr(&self) -> &'static str {
+        match self {
+            DiagnosticDeriveKind::Error => "error",
+            DiagnosticDeriveKind::Warn => "warning",
+            DiagnosticDeriveKind::Lint => "lint",
+        }
+    }
+}
+
+/// Tracks persistent information required for building up individual calls to diagnostic methods
+/// for generated diagnostic derives - both `SessionDiagnostic` for errors/warnings and
+/// `LintDiagnostic` for lints.
+pub(crate) struct DiagnosticDeriveBuilder {
+    /// The identifier to use for the generated `DiagnosticBuilder` instance.
+    pub diag: syn::Ident,
+
+    /// Store a map of field name to its corresponding field. This is built on construction of the
+    /// derive builder.
+    pub fields: HashMap<String, TokenStream>,
+
+    /// Kind of diagnostic requested via the struct attribute.
+    pub kind: Option<(DiagnosticDeriveKind, proc_macro::Span)>,
+    /// Slug is a mandatory part of the struct attribute as corresponds to the Fluent message that
+    /// has the actual diagnostic message.
+    pub slug: Option<(Path, proc_macro::Span)>,
+    /// Error codes are a optional part of the struct attribute - this is only set to detect
+    /// multiple specifications.
+    pub code: Option<(String, proc_macro::Span)>,
+}
+
+impl HasFieldMap for DiagnosticDeriveBuilder {
+    fn get_field_binding(&self, field: &String) -> Option<&TokenStream> {
+        self.fields.get(field)
+    }
+}
+
+impl DiagnosticDeriveBuilder {
+    pub fn preamble<'s>(&mut self, structure: &Structure<'s>) -> TokenStream {
+        let ast = structure.ast();
+        let attrs = &ast.attrs;
+        let preamble = attrs.iter().map(|attr| {
+            self.generate_structure_code_for_attr(attr).unwrap_or_else(|v| v.to_compile_error())
+        });
+
+        quote! {
+            #(#preamble)*;
+        }
+    }
+
+    pub fn body<'s>(&mut self, structure: &mut Structure<'s>) -> (TokenStream, TokenStream) {
+        // Keep track of which fields are subdiagnostics or have no attributes.
+        let mut subdiagnostics_or_empty = std::collections::HashSet::new();
+
+        // Generates calls to `span_label` and similar functions based on the attributes
+        // on fields. Code for suggestions uses formatting machinery and the value of
+        // other fields - because any given field can be referenced multiple times, it
+        // should be accessed through a borrow. When passing fields to `add_subdiagnostic`
+        // or `set_arg` (which happens below) for Fluent, we want to move the data, so that
+        // has to happen in a separate pass over the fields.
+        let attrs = structure
+            .clone()
+            .filter(|field_binding| {
+                let attrs = &field_binding.ast().attrs;
+
+                (!attrs.is_empty()
+                    && attrs.iter().all(|attr| {
+                        "subdiagnostic" != attr.path.segments.last().unwrap().ident.to_string()
+                    }))
+                    || {
+                        subdiagnostics_or_empty.insert(field_binding.binding.clone());
+                        false
+                    }
+            })
+            .each(|field_binding| self.generate_field_attrs_code(field_binding));
+
+        structure.bind_with(|_| synstructure::BindStyle::Move);
+        // When a field has attributes like `#[label]` or `#[note]` then it doesn't
+        // need to be passed as an argument to the diagnostic. But when a field has no
+        // attributes or a `#[subdiagnostic]` attribute then it must be passed as an
+        // argument to the diagnostic so that it can be referred to by Fluent messages.
+        let args = structure
+            .filter(|field_binding| subdiagnostics_or_empty.contains(&field_binding.binding))
+            .each(|field_binding| self.generate_field_attrs_code(field_binding));
+
+        (attrs, args)
+    }
+
+    /// Establishes state in the `DiagnosticDeriveBuilder` resulting from the struct
+    /// attributes like `#[error(..)`, such as the diagnostic kind and slug. Generates
+    /// diagnostic builder calls for setting error code and creating note/help messages.
+    fn generate_structure_code_for_attr(
+        &mut self,
+        attr: &Attribute,
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
+        let diag = &self.diag;
+        let span = attr.span().unwrap();
+
+        let name = attr.path.segments.last().unwrap().ident.to_string();
+        let name = name.as_str();
+        let meta = attr.parse_meta()?;
+
+        let is_help_or_note = matches!(name, "help" | "note");
+
+        let nested = match meta {
+            // Most attributes are lists, like `#[error(..)]`/`#[warning(..)]` for most cases or
+            // `#[help(..)]`/`#[note(..)]` when the user is specifying a alternative slug.
+            Meta::List(MetaList { ref nested, .. }) => nested,
+            // Subdiagnostics without spans can be applied to the type too, and these are just
+            // paths: `#[help]` and `#[note]`
+            Meta::Path(_) if is_help_or_note => {
+                let fn_name = proc_macro2::Ident::new(name, attr.span());
+                return Ok(quote! { #diag.#fn_name(rustc_errors::fluent::_subdiag::#fn_name); });
+            }
+            _ => throw_invalid_attr!(attr, &meta),
+        };
+
+        // Check the kind before doing any further processing so that there aren't misleading
+        // "no kind specified" errors if there are failures later.
+        match name {
+            "error" => self.kind.set_once((DiagnosticDeriveKind::Error, span)),
+            "warning" => self.kind.set_once((DiagnosticDeriveKind::Warn, span)),
+            "lint" => self.kind.set_once((DiagnosticDeriveKind::Lint, span)),
+            "help" | "note" => (),
+            _ => throw_invalid_attr!(attr, &meta, |diag| {
+                diag.help("only `error`, `warning`, `help` and `note` are valid attributes")
+            }),
+        }
+
+        // First nested element should always be the path, e.g. `#[error(typeck::invalid)]` or
+        // `#[help(typeck::another_help)]`.
+        let mut nested_iter = nested.into_iter();
+        if let Some(nested_attr) = nested_iter.next() {
+            // Report an error if there are any other list items after the path.
+            if is_help_or_note && nested_iter.next().is_some() {
+                throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
+                    diag.help("`help` and `note` struct attributes can only have one argument")
+                });
+            }
+
+            match nested_attr {
+                NestedMeta::Meta(Meta::Path(path)) if is_help_or_note => {
+                    let fn_name = proc_macro2::Ident::new(name, attr.span());
+                    return Ok(quote! { #diag.#fn_name(rustc_errors::fluent::#path); });
+                }
+                NestedMeta::Meta(Meta::Path(path)) => {
+                    self.slug.set_once((path.clone(), span));
+                }
+                NestedMeta::Meta(meta @ Meta::NameValue(_))
+                    if !is_help_or_note
+                        && meta.path().segments.last().unwrap().ident.to_string() == "code" =>
+                {
+                    // don't error for valid follow-up attributes
+                }
+                nested_attr => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
+                    diag.help("first argument of the attribute should be the diagnostic slug")
+                }),
+            };
+        }
+
+        // Remaining attributes are optional, only `code = ".."` at the moment.
+        let mut tokens = Vec::new();
+        for nested_attr in nested_iter {
+            let meta = match nested_attr {
+                syn::NestedMeta::Meta(meta) => meta,
+                _ => throw_invalid_nested_attr!(attr, &nested_attr),
+            };
+
+            let path = meta.path();
+            let nested_name = path.segments.last().unwrap().ident.to_string();
+            // Struct attributes are only allowed to be applied once, and the diagnostic
+            // changes will be set in the initialisation code.
+            if let Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) = &meta {
+                let span = s.span().unwrap();
+                match nested_name.as_str() {
+                    "code" => {
+                        self.code.set_once((s.value(), span));
+                        let code = &self.code.as_ref().map(|(v, _)| v);
+                        tokens.push(quote! {
+                            #diag.code(rustc_errors::DiagnosticId::Error(#code.to_string()));
+                        });
+                    }
+                    _ => invalid_nested_attr(attr, &nested_attr)
+                        .help("only `code` is a valid nested attributes following the slug")
+                        .emit(),
+                }
+            } else {
+                invalid_nested_attr(attr, &nested_attr).emit()
+            }
+        }
+
+        Ok(tokens.drain(..).collect())
+    }
+
+    fn generate_field_attrs_code(&mut self, binding_info: &BindingInfo<'_>) -> TokenStream {
+        let field = binding_info.ast();
+        let field_binding = &binding_info.binding;
+
+        let inner_ty = FieldInnerTy::from_type(&field.ty);
+
+        // When generating `set_arg` or `add_subdiagnostic` calls, move data rather than
+        // borrow it to avoid requiring clones - this must therefore be the last use of
+        // each field (for example, any formatting machinery that might refer to a field
+        // should be generated already).
+        if field.attrs.is_empty() {
+            let diag = &self.diag;
+            let ident = field.ident.as_ref().unwrap();
+            quote! {
+                #diag.set_arg(
+                    stringify!(#ident),
+                    #field_binding
+                );
+            }
+        } else {
+            field
+                .attrs
+                .iter()
+                .map(move |attr| {
+                    let name = attr.path.segments.last().unwrap().ident.to_string();
+                    let (binding, needs_destructure) = match (name.as_str(), &inner_ty) {
+                        // `primary_span` can accept a `Vec<Span>` so don't destructure that.
+                        ("primary_span", FieldInnerTy::Vec(_)) => {
+                            (quote! { #field_binding.clone() }, false)
+                        }
+                        // `subdiagnostics` are not derefed because they are bound by value.
+                        ("subdiagnostic", _) => (quote! { #field_binding }, true),
+                        _ => (quote! { *#field_binding }, true),
+                    };
+
+                    let generated_code = self
+                        .generate_inner_field_code(
+                            attr,
+                            FieldInfo {
+                                binding: binding_info,
+                                ty: inner_ty.inner_type().unwrap_or(&field.ty),
+                                span: &field.span(),
+                            },
+                            binding,
+                        )
+                        .unwrap_or_else(|v| v.to_compile_error());
+
+                    if needs_destructure {
+                        inner_ty.with(field_binding, generated_code)
+                    } else {
+                        generated_code
+                    }
+                })
+                .collect()
+        }
+    }
+
+    fn generate_inner_field_code(
+        &mut self,
+        attr: &Attribute,
+        info: FieldInfo<'_>,
+        binding: TokenStream,
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
+        let meta = attr.parse_meta()?;
+        match meta {
+            Meta::Path(_) => self.generate_inner_field_code_path(attr, info, binding),
+            Meta::List(MetaList { .. }) => self.generate_inner_field_code_list(attr, info, binding),
+            _ => throw_invalid_attr!(attr, &meta),
+        }
+    }
+
+    fn generate_inner_field_code_path(
+        &mut self,
+        attr: &Attribute,
+        info: FieldInfo<'_>,
+        binding: TokenStream,
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
+        assert!(matches!(attr.parse_meta()?, Meta::Path(_)));
+        let diag = &self.diag;
+
+        let meta = attr.parse_meta()?;
+
+        let ident = &attr.path.segments.last().unwrap().ident;
+        let name = ident.to_string();
+        let name = name.as_str();
+        match name {
+            "skip_arg" => {
+                // Don't need to do anything - by virtue of the attribute existing, the
+                // `set_arg` call will not be generated.
+                Ok(quote! {})
+            }
+            "primary_span" => {
+                report_error_if_not_applied_to_span(attr, &info)?;
+                Ok(quote! {
+                    #diag.set_span(#binding);
+                })
+            }
+            "label" => {
+                report_error_if_not_applied_to_span(attr, &info)?;
+                Ok(self.add_spanned_subdiagnostic(binding, ident, parse_quote! { _subdiag::label }))
+            }
+            "note" | "help" => {
+                let path = match name {
+                    "note" => parse_quote! { _subdiag::note },
+                    "help" => parse_quote! { _subdiag::help },
+                    _ => unreachable!(),
+                };
+                if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
+                    Ok(self.add_spanned_subdiagnostic(binding, ident, path))
+                } else if type_is_unit(&info.ty) {
+                    Ok(self.add_subdiagnostic(ident, path))
+                } else {
+                    report_type_error(attr, "`Span` or `()`")?
+                }
+            }
+            "subdiagnostic" => Ok(quote! { #diag.subdiagnostic(#binding); }),
+            _ => throw_invalid_attr!(attr, &meta, |diag| {
+                diag.help(
+                    "only `skip_arg`, `primary_span`, `label`, `note`, `help` and `subdiagnostic` \
+                     are valid field attributes",
+                )
+            }),
+        }
+    }
+
+    fn generate_inner_field_code_list(
+        &mut self,
+        attr: &Attribute,
+        info: FieldInfo<'_>,
+        binding: TokenStream,
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
+        let meta = attr.parse_meta()?;
+        let Meta::List(MetaList { ref path, ref nested, .. }) = meta  else { unreachable!() };
+
+        let ident = &attr.path.segments.last().unwrap().ident;
+        let name = path.segments.last().unwrap().ident.to_string();
+        let name = name.as_ref();
+        match name {
+            "suggestion" | "suggestion_short" | "suggestion_hidden" | "suggestion_verbose" => {
+                return self.generate_inner_field_code_suggestion(attr, info);
+            }
+            "label" | "help" | "note" => (),
+            _ => throw_invalid_attr!(attr, &meta, |diag| {
+                diag.help(
+                    "only `label`, `note`, `help` or `suggestion{,_short,_hidden,_verbose}` are \
+                     valid field attributes",
+                )
+            }),
+        }
+
+        // For `#[label(..)]`, `#[note(..)]` and `#[help(..)]`, the first nested element must be a
+        // path, e.g. `#[label(typeck::label)]`.
+        let mut nested_iter = nested.into_iter();
+        let msg = match nested_iter.next() {
+            Some(NestedMeta::Meta(Meta::Path(path))) => path.clone(),
+            Some(nested_attr) => throw_invalid_nested_attr!(attr, &nested_attr),
+            None => throw_invalid_attr!(attr, &meta),
+        };
+
+        // None of these attributes should have anything following the slug.
+        if nested_iter.next().is_some() {
+            throw_invalid_attr!(attr, &meta);
+        }
+
+        match name {
+            "label" => {
+                report_error_if_not_applied_to_span(attr, &info)?;
+                Ok(self.add_spanned_subdiagnostic(binding, ident, msg))
+            }
+            "note" | "help" if type_matches_path(&info.ty, &["rustc_span", "Span"]) => {
+                Ok(self.add_spanned_subdiagnostic(binding, ident, msg))
+            }
+            "note" | "help" if type_is_unit(&info.ty) => Ok(self.add_subdiagnostic(ident, msg)),
+            "note" | "help" => report_type_error(attr, "`Span` or `()`")?,
+            _ => unreachable!(),
+        }
+    }
+
+    fn generate_inner_field_code_suggestion(
+        &mut self,
+        attr: &Attribute,
+        info: FieldInfo<'_>,
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
+        let diag = &self.diag;
+
+        let mut meta = attr.parse_meta()?;
+        let Meta::List(MetaList { ref path, ref mut nested, .. }) = meta  else { unreachable!() };
+
+        let (span_field, mut applicability) = self.span_and_applicability_of_ty(info)?;
+
+        let mut msg = None;
+        let mut code = None;
+
+        let mut nested_iter = nested.into_iter().peekable();
+        if let Some(nested_attr) = nested_iter.peek() {
+            if let NestedMeta::Meta(Meta::Path(path)) = nested_attr {
+                msg = Some(path.clone());
+            }
+        };
+        // Move the iterator forward if a path was found (don't otherwise so that
+        // code/applicability can be found or an error emitted).
+        if msg.is_some() {
+            let _ = nested_iter.next();
+        }
+
+        for nested_attr in nested_iter {
+            let meta = match nested_attr {
+                syn::NestedMeta::Meta(ref meta) => meta,
+                syn::NestedMeta::Lit(_) => throw_invalid_nested_attr!(attr, &nested_attr),
+            };
+
+            let nested_name = meta.path().segments.last().unwrap().ident.to_string();
+            let nested_name = nested_name.as_str();
+            match meta {
+                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                    let span = meta.span().unwrap();
+                    match nested_name {
+                        "code" => {
+                            let formatted_str = self.build_format(&s.value(), s.span());
+                            code = Some(formatted_str);
+                        }
+                        "applicability" => {
+                            applicability = match applicability {
+                                Some(v) => {
+                                    span_err(
+                                        span,
+                                        "applicability cannot be set in both the field and \
+                                         attribute",
+                                    )
+                                    .emit();
+                                    Some(v)
+                                }
+                                None => match Applicability::from_str(&s.value()) {
+                                    Ok(v) => Some(quote! { #v }),
+                                    Err(()) => {
+                                        span_err(span, "invalid applicability").emit();
+                                        None
+                                    }
+                                },
+                            }
+                        }
+                        _ => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
+                            diag.help(
+                                "only `message`, `code` and `applicability` are valid field \
+                                 attributes",
+                            )
+                        }),
+                    }
+                }
+                _ => throw_invalid_nested_attr!(attr, &nested_attr, |diag| {
+                    if matches!(meta, Meta::Path(_)) {
+                        diag.help("a diagnostic slug must be the first argument to the attribute")
+                    } else {
+                        diag
+                    }
+                }),
+            }
+        }
+
+        let applicability =
+            applicability.unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
+
+        let name = path.segments.last().unwrap().ident.to_string();
+        let method = format_ident!("span_{}", name);
+
+        let msg = msg.unwrap_or_else(|| parse_quote! { _subdiag::suggestion });
+        let msg = quote! { rustc_errors::fluent::#msg };
+        let code = code.unwrap_or_else(|| quote! { String::new() });
+
+        Ok(quote! { #diag.#method(#span_field, #msg, #code, #applicability); })
+    }
+
+    /// Adds a spanned subdiagnostic by generating a `diag.span_$kind` call with the current slug
+    /// and `fluent_attr_identifier`.
+    fn add_spanned_subdiagnostic(
+        &self,
+        field_binding: TokenStream,
+        kind: &Ident,
+        fluent_attr_identifier: Path,
+    ) -> TokenStream {
+        let diag = &self.diag;
+        let fn_name = format_ident!("span_{}", kind);
+        quote! {
+            #diag.#fn_name(
+                #field_binding,
+                rustc_errors::fluent::#fluent_attr_identifier
+            );
+        }
+    }
+
+    /// Adds a subdiagnostic by generating a `diag.span_$kind` call with the current slug
+    /// and `fluent_attr_identifier`.
+    fn add_subdiagnostic(&self, kind: &Ident, fluent_attr_identifier: Path) -> TokenStream {
+        let diag = &self.diag;
+        quote! {
+            #diag.#kind(rustc_errors::fluent::#fluent_attr_identifier);
+        }
+    }
+
+    fn span_and_applicability_of_ty(
+        &self,
+        info: FieldInfo<'_>,
+    ) -> Result<(TokenStream, Option<TokenStream>), DiagnosticDeriveError> {
+        match &info.ty {
+            // If `ty` is `Span` w/out applicability, then use `Applicability::Unspecified`.
+            ty @ Type::Path(..) if type_matches_path(ty, &["rustc_span", "Span"]) => {
+                let binding = &info.binding.binding;
+                Ok((quote!(*#binding), None))
+            }
+            // If `ty` is `(Span, Applicability)` then return tokens accessing those.
+            Type::Tuple(tup) => {
+                let mut span_idx = None;
+                let mut applicability_idx = None;
+
+                for (idx, elem) in tup.elems.iter().enumerate() {
+                    if type_matches_path(elem, &["rustc_span", "Span"]) {
+                        if span_idx.is_none() {
+                            span_idx = Some(syn::Index::from(idx));
+                        } else {
+                            throw_span_err!(
+                                info.span.unwrap(),
+                                "type of field annotated with `#[suggestion(...)]` contains more \
+                                 than one `Span`"
+                            );
+                        }
+                    } else if type_matches_path(elem, &["rustc_errors", "Applicability"]) {
+                        if applicability_idx.is_none() {
+                            applicability_idx = Some(syn::Index::from(idx));
+                        } else {
+                            throw_span_err!(
+                                info.span.unwrap(),
+                                "type of field annotated with `#[suggestion(...)]` contains more \
+                                 than one Applicability"
+                            );
+                        }
+                    }
+                }
+
+                if let Some(span_idx) = span_idx {
+                    let binding = &info.binding.binding;
+                    let span = quote!(#binding.#span_idx);
+                    let applicability = applicability_idx
+                        .map(|applicability_idx| quote!(#binding.#applicability_idx))
+                        .unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
+
+                    return Ok((span, Some(applicability)));
+                }
+
+                throw_span_err!(info.span.unwrap(), "wrong types for suggestion", |diag| {
+                    diag.help(
+                        "`#[suggestion(...)]` on a tuple field must be applied to fields of type \
+                         `(Span, Applicability)`",
+                    )
+                });
+            }
+            // If `ty` isn't a `Span` or `(Span, Applicability)` then emit an error.
+            _ => throw_span_err!(info.span.unwrap(), "wrong field type for suggestion", |diag| {
+                diag.help(
+                    "`#[suggestion(...)]` should be applied to fields of type `Span` or \
+                     `(Span, Applicability)`",
+                )
+            }),
+        }
+    }
+}

compiler/rustc_macros/src/diagnostics/error.rs

@@ -4,16 +4,16 @@ use quote::quote;
 use syn::{spanned::Spanned, Attribute, Error as SynError, Meta, NestedMeta};
 
 #[derive(Debug)]
-pub(crate) enum SessionDiagnosticDeriveError {
+pub(crate) enum DiagnosticDeriveError {
     SynError(SynError),
     ErrorHandled,
 }
 
-impl SessionDiagnosticDeriveError {
+impl DiagnosticDeriveError {
     pub(crate) fn to_compile_error(self) -> TokenStream {
         match self {
-            SessionDiagnosticDeriveError::SynError(e) => e.to_compile_error(),
+            DiagnosticDeriveError::SynError(e) => e.to_compile_error(),
-            SessionDiagnosticDeriveError::ErrorHandled => {
+            DiagnosticDeriveError::ErrorHandled => {
                 // Return ! to avoid having to create a blank DiagnosticBuilder to return when an
                 // error has already been emitted to the compiler.
                 quote! {
@@ -24,9 +24,9 @@ impl SessionDiagnosticDeriveError {
     }
 }
 
-impl From<SynError> for SessionDiagnosticDeriveError {
+impl From<SynError> for DiagnosticDeriveError {
     fn from(e: SynError) -> Self {
-        SessionDiagnosticDeriveError::SynError(e)
+        DiagnosticDeriveError::SynError(e)
     }
 }
 
@@ -34,9 +34,9 @@ impl From<SynError> for SessionDiagnosticDeriveError {
 pub(crate) fn _throw_err(
     diag: Diagnostic,
     f: impl FnOnce(Diagnostic) -> Diagnostic,
-) -> SessionDiagnosticDeriveError {
+) -> DiagnosticDeriveError {
     f(diag).emit();
-    SessionDiagnosticDeriveError::ErrorHandled
+    DiagnosticDeriveError::ErrorHandled
 }
 
 /// Helper function for printing `syn::Path` - doesn't handle arguments in paths and these are
@@ -60,7 +60,7 @@ pub(crate) fn span_err(span: impl MultiSpan, msg: &str) -> Diagnostic {
 /// Emit a diagnostic on span `$span` with msg `$msg` (optionally performing additional decoration
 /// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
 ///
-/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
 macro_rules! throw_span_err {
     ($span:expr, $msg:expr) => {{ throw_span_err!($span, $msg, |diag| diag) }};
     ($span:expr, $msg:expr, $f:expr) => {{
@@ -87,7 +87,7 @@ pub(crate) fn invalid_attr(attr: &Attribute, meta: &Meta) -> Diagnostic {
 /// Emit a error diagnostic for an invalid attribute (optionally performing additional decoration
 /// using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
 ///
-/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
 macro_rules! throw_invalid_attr {
     ($attr:expr, $meta:expr) => {{ throw_invalid_attr!($attr, $meta, |diag| diag) }};
     ($attr:expr, $meta:expr, $f:expr) => {{
@@ -129,7 +129,7 @@ pub(crate) fn invalid_nested_attr(attr: &Attribute, nested: &NestedMeta) -> Diag
 /// Emit a error diagnostic for an invalid nested attribute (optionally performing additional
 /// decoration using the `FnOnce` passed in `diag`) and return `Err(ErrorHandled)`.
 ///
-/// For methods that return a `Result<_, SessionDiagnosticDeriveError>`:
+/// For methods that return a `Result<_, DiagnosticDeriveError>`:
 macro_rules! throw_invalid_nested_attr {
     ($attr:expr, $nested_attr:expr) => {{ throw_invalid_nested_attr!($attr, $nested_attr, |diag| diag) }};
     ($attr:expr, $nested_attr:expr, $f:expr) => {{

compiler/rustc_macros/src/diagnostics/mod.rs

@@ -1,10 +1,11 @@
 mod diagnostic;
+mod diagnostic_builder;
 mod error;
 mod fluent;
 mod subdiagnostic;
 mod utils;
 
-use diagnostic::SessionDiagnosticDerive;
+use diagnostic::{LintDiagnosticDerive, SessionDiagnosticDerive};
 pub(crate) use fluent::fluent_messages;
 use proc_macro2::TokenStream;
 use quote::format_ident;
@@ -58,11 +59,53 @@ use synstructure::Structure;
 /// See rustc dev guide for more examples on using the `#[derive(SessionDiagnostic)]`:
 /// <https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-structs.html>
 pub fn session_diagnostic_derive(s: Structure<'_>) -> TokenStream {
-    // Names for the diagnostic we build and the session we build it from.
+    SessionDiagnosticDerive::new(format_ident!("diag"), format_ident!("sess"), s).into_tokens()
-    let diag = format_ident!("diag");
+}
-    let sess = format_ident!("sess");
 
-    SessionDiagnosticDerive::new(diag, sess, s).into_tokens()
+/// Implements `#[derive(LintDiagnostic)]`, which allows for lints to be specified as a struct,
+/// independent from the actual lint emitting code.
+///
+/// ```ignore (rust)
+/// #[derive(LintDiagnostic)]
+/// #[lint(lint::atomic_ordering_invalid_fail_success)]
+/// pub struct AtomicOrderingInvalidLint {
+///     method: Symbol,
+///     success_ordering: Symbol,
+///     fail_ordering: Symbol,
+///     #[label(lint::fail_label)]
+///     fail_order_arg_span: Span,
+///     #[label(lint::success_label)]
+///     #[suggestion(
+///         code = "std::sync::atomic::Ordering::{success_suggestion}",
+///         applicability = "maybe-incorrect"
+///     )]
+///     success_order_arg_span: Span,
+/// }
+/// ```
+///
+/// ```fluent
+/// lint-atomic-ordering-invalid-fail-success = `{$method}`'s success ordering must be at least as strong as its failure ordering
+///     .fail-label = `{$fail_ordering}` failure ordering
+///     .success-label = `{$success_ordering}` success ordering
+///     .suggestion = consider using `{$success_suggestion}` success ordering instead
+/// ```
+///
+/// Then, later, to emit the error:
+///
+/// ```ignore (rust)
+/// cx.struct_span_lint(INVALID_ATOMIC_ORDERING, fail_order_arg_span, AtomicOrderingInvalidLint {
+///     method,
+///     success_ordering,
+///     fail_ordering,
+///     fail_order_arg_span,
+///     success_order_arg_span,
+/// });
+/// ```
+///
+/// See rustc dev guide for more examples on using the `#[derive(LintDiagnostic)]`:
+/// <https://rustc-dev-guide.rust-lang.org/diagnostics/sessiondiagnostic.html>
+pub fn lint_diagnostic_derive(s: Structure<'_>) -> TokenStream {
+    LintDiagnosticDerive::new(format_ident!("diag"), s).into_tokens()
 }
 
 /// Implements `#[derive(SessionSubdiagnostic)]`, which allows for labels, notes, helps and

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -1,8 +1,7 @@
 #![deny(unused_must_use)]
 
 use crate::diagnostics::error::{
-    span_err, throw_invalid_attr, throw_invalid_nested_attr, throw_span_err,
+    span_err, throw_invalid_attr, throw_invalid_nested_attr, throw_span_err, DiagnosticDeriveError,
-    SessionDiagnosticDeriveError,
 };
 use crate::diagnostics::utils::{
     report_error_if_not_applied_to_applicability, report_error_if_not_applied_to_span,
@@ -214,7 +213,7 @@ impl<'a> HasFieldMap for SessionSubdiagnosticDeriveBuilder<'a> {
 }
 
 impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
-    fn identify_kind(&mut self) -> Result<(), SessionDiagnosticDeriveError> {
+    fn identify_kind(&mut self) -> Result<(), DiagnosticDeriveError> {
         for attr in self.variant.ast().attrs {
             let span = attr.span().unwrap();
 
@@ -351,7 +350,7 @@ impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
         &mut self,
         binding: &BindingInfo<'_>,
         is_suggestion: bool,
-    ) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+    ) -> Result<TokenStream, DiagnosticDeriveError> {
         let ast = binding.ast();
 
         let inner_ty = FieldInnerTy::from_type(&ast.ty);
@@ -411,7 +410,7 @@ impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
         Ok(inner_ty.with(binding, generated))
     }
 
-    fn into_tokens(&mut self) -> Result<TokenStream, SessionDiagnosticDeriveError> {
+    fn into_tokens(&mut self) -> Result<TokenStream, DiagnosticDeriveError> {
         self.identify_kind()?;
         let Some(kind) = self.kind.map(|(kind, _)| kind) else {
             throw_span_err!(

compiler/rustc_macros/src/diagnostics/utils.rs

@@ -1,4 +1,4 @@
-use crate::diagnostics::error::{span_err, throw_span_err, SessionDiagnosticDeriveError};
+use crate::diagnostics::error::{span_err, throw_span_err, DiagnosticDeriveError};
 use proc_macro::Span;
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote, ToTokens};
@@ -34,7 +34,7 @@ pub(crate) fn type_is_unit(ty: &Type) -> bool {
 pub(crate) fn report_type_error(
     attr: &Attribute,
     ty_name: &str,
-) -> Result<!, SessionDiagnosticDeriveError> {
+) -> Result<!, DiagnosticDeriveError> {
     let name = attr.path.segments.last().unwrap().ident.to_string();
     let meta = attr.parse_meta()?;
 
@@ -59,7 +59,7 @@ fn report_error_if_not_applied_to_ty(
     info: &FieldInfo<'_>,
     path: &[&str],
     ty_name: &str,
-) -> Result<(), SessionDiagnosticDeriveError> {
+) -> Result<(), DiagnosticDeriveError> {
     if !type_matches_path(&info.ty, path) {
         report_type_error(attr, ty_name)?;
     }
@@ -71,7 +71,7 @@ fn report_error_if_not_applied_to_ty(
 pub(crate) fn report_error_if_not_applied_to_applicability(
     attr: &Attribute,
     info: &FieldInfo<'_>,
-) -> Result<(), SessionDiagnosticDeriveError> {
+) -> Result<(), DiagnosticDeriveError> {
     report_error_if_not_applied_to_ty(
         attr,
         info,
@@ -84,7 +84,7 @@ pub(crate) fn report_error_if_not_applied_to_applicability(
 pub(crate) fn report_error_if_not_applied_to_span(
     attr: &Attribute,
     info: &FieldInfo<'_>,
-) -> Result<(), SessionDiagnosticDeriveError> {
+) -> Result<(), DiagnosticDeriveError> {
     report_error_if_not_applied_to_ty(attr, info, &["rustc_span", "Span"], "`Span`")
 }
 
@@ -166,10 +166,12 @@ pub(crate) struct FieldInfo<'a> {
 /// Small helper trait for abstracting over `Option` fields that contain a value and a `Span`
 /// for error reporting if they are set more than once.
 pub(crate) trait SetOnce<T> {
-    fn set_once(&mut self, value: T);
+    fn set_once(&mut self, _: (T, Span));
+
+    fn value(self) -> Option<T>;
 }
 
-impl<T> SetOnce<(T, Span)> for Option<(T, Span)> {
+impl<T> SetOnce<T> for Option<(T, Span)> {
     fn set_once(&mut self, (value, span): (T, Span)) {
         match self {
             None => {
@@ -182,6 +184,10 @@ impl<T> SetOnce<(T, Span)> for Option<(T, Span)> {
             }
         }
     }
+
+    fn value(self) -> Option<T> {
+        self.map(|(v, _)| v)
+    }
 }
 
 pub(crate) trait HasFieldMap {

compiler/rustc_macros/src/lib.rs

@@ -127,6 +127,7 @@ decl_derive!(
         // struct attributes
         warning,
         error,
+        lint,
         note,
         help,
         // field attributes
@@ -139,6 +140,24 @@ decl_derive!(
         suggestion_hidden,
         suggestion_verbose)] => diagnostics::session_diagnostic_derive
 );
+decl_derive!(
+    [LintDiagnostic, attributes(
+        // struct attributes
+        warning,
+        error,
+        lint,
+        note,
+        help,
+        // field attributes
+        skip_arg,
+        primary_span,
+        label,
+        subdiagnostic,
+        suggestion,
+        suggestion_short,
+        suggestion_hidden,
+        suggestion_verbose)] => diagnostics::lint_diagnostic_derive
+);
 decl_derive!(
     [SessionSubdiagnostic, attributes(
         // struct/variant attributes

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -17,7 +17,7 @@ use rustc_span::symbol::Ident;
 use rustc_span::Span;
 
 extern crate rustc_macros;
-use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_macros::{SessionDiagnostic, LintDiagnostic, SessionSubdiagnostic};
 
 extern crate rustc_middle;
 use rustc_middle::ty::Ty;
@@ -535,3 +535,20 @@ struct LabelWithTrailingList {
     //~^ ERROR `#[label(...)]` is not a valid attribute
     span: Span,
 }
+
+#[derive(SessionDiagnostic)]
+#[lint(typeck::ambiguous_lifetime_bound)]
+//~^ ERROR only `#[error(..)]` and `#[warn(..)]` are supported
+struct LintsBad {
+}
+
+#[derive(LintDiagnostic)]
+#[lint(typeck::ambiguous_lifetime_bound)]
+struct LintsGood {
+}
+
+#[derive(LintDiagnostic)]
+#[error(typeck::ambiguous_lifetime_bound)]
+//~^ ERROR only `#[lint(..)]` is supported
+struct ErrorsBad {
+}

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr

@@ -363,6 +363,28 @@ error: `#[label(...)]` is not a valid attribute
 LL |     #[label(typeck::label, foo("..."))]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: only `#[error(..)]` and `#[warn(..)]` are supported
+  --> $DIR/diagnostic-derive.rs:540:1
+   |
+LL | / #[lint(typeck::ambiguous_lifetime_bound)]
+LL | |
+LL | | struct LintsBad {
+LL | | }
+   | |_^
+   |
+   = help: use the `#[error(...)]` attribute to create a error
+
+error: only `#[lint(..)]` is supported
+  --> $DIR/diagnostic-derive.rs:551:1
+   |
+LL | / #[error(typeck::ambiguous_lifetime_bound)]
+LL | |
+LL | | struct ErrorsBad {
+LL | | }
+   | |_^
+   |
+   = help: use the `#[lint(...)]` attribute to create a lint
+
 error: cannot find attribute `nonsense` in this scope
   --> $DIR/diagnostic-derive.rs:53:3
    |
@@ -395,7 +417,7 @@ LL |         arg: impl IntoDiagnosticArg,
    |                   ^^^^^^^^^^^^^^^^^ required by this bound in `DiagnosticBuilder::<'a, G>::set_arg`
    = note: this error originates in the derive macro `SessionDiagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 46 previous errors
+error: aborting due to 48 previous errors
 
 Some errors have detailed explanations: E0277, E0425.
 For more information about an error, try `rustc --explain E0277`.