Add SessionDiagnostic derive macro.

Co-authored-by: Oliver Scherer <github35764891676564198441@oli-obk.de>

Cargo.lock

@@ -4086,6 +4086,7 @@ dependencies = [
  "rustc_hir_pretty",
  "rustc_index",
  "rustc_infer",
+ "rustc_macros",
  "rustc_middle",
  "rustc_session",
  "rustc_span",

compiler/rustc_macros/src/lib.rs

@@ -1,3 +1,4 @@
+#![feature(proc_macro_diagnostic)]
 #![allow(rustc::default_hash_types)]
 #![recursion_limit = "128"]
 
@@ -9,6 +10,7 @@ mod hash_stable;
 mod lift;
 mod query;
 mod serialize;
+mod session_diagnostic;
 mod symbols;
 mod type_foldable;
 
@@ -36,3 +38,14 @@ decl_derive!([MetadataDecodable] => serialize::meta_decodable_derive);
 decl_derive!([MetadataEncodable] => serialize::meta_encodable_derive);
 decl_derive!([TypeFoldable, attributes(type_foldable)] => type_foldable::type_foldable_derive);
 decl_derive!([Lift, attributes(lift)] => lift::lift_derive);
+decl_derive!(
+    [SessionDiagnostic, attributes(
+        message,
+        lint,
+        error,
+        label,
+        suggestion,
+        suggestion_short,
+        suggestion_hidden,
+        suggestion_verbose)] => session_diagnostic::session_diagnostic_derive
+);

compiler/rustc_macros/src/session_diagnostic.rs

@@ -0,0 +1,665 @@
+#![deny(unused_must_use)]
+use quote::format_ident;
+use quote::quote;
+
+use proc_macro::Diagnostic;
+use syn::spanned::Spanned;
+
+use std::collections::{HashMap, HashSet};
+
+/// Implements #[derive(SessionDiagnostic)], which allows for errors to be specified as a struct, independent
+/// from the actual diagnostics emitting code.
+/// ```ignore (pseudo-rust)
+/// # extern crate rustc_errors;
+/// # use rustc_errors::Applicability;
+/// # extern crate rustc_span;
+/// # use rustc_span::{symbol::Ident, Span};
+/// # extern crate rust_middle;
+/// # use rustc_middle::ty::Ty;
+/// #[derive(SessionDiagnostic)]
+/// #[code = "E0505"]
+/// #[error = "cannot move out of {name} because it is borrowed"]
+/// pub struct MoveOutOfBorrowError<'tcx> {
+///     pub name: Ident,
+///     pub ty: Ty<'tcx>,
+///     #[label = "cannot move out of borrow"]
+///     pub span: Span,
+///     #[label = "`{ty}` first borrowed here"]
+///     pub other_span: Span,
+///     #[suggestion(message = "consider cloning here", code = "{name}.clone()")]
+///     pub opt_sugg: Option<(Span, Applicability)>
+/// }
+/// ```
+/// Then, later, to emit the error:
+///
+/// ```ignore (pseudo-rust)
+/// sess.emit_err(MoveOutOfBorrowError {
+///     expected,
+///     actual,
+///     span,
+///     other_span,
+///     opt_sugg: Some(suggestion, Applicability::MachineApplicable),
+/// });
+/// ```
+pub fn session_diagnostic_derive(s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
+    // Names for the diagnostic we build and the session we build it from.
+    let diag = format_ident!("diag");
+    let sess = format_ident!("sess");
+
+    SessionDiagnosticDerive::new(diag, sess, s).into_tokens()
+}
+
+// Checks whether the type name of `ty` matches `name`.
+//
+// Given some struct at a::b::c::Foo, this will return true for c::Foo, b::c::Foo, or
+// a::b::c::Foo. This reasonably allows qualified names to be used in the macro.
+fn type_matches_path(ty: &syn::Type, name: &[&str]) -> bool {
+    if let syn::Type::Path(ty) = ty {
+        ty.path
+            .segments
+            .iter()
+            .map(|s| s.ident.to_string())
+            .rev()
+            .zip(name.iter().rev())
+            .all(|(x, y)| &x.as_str() == y)
+    } else {
+        false
+    }
+}
+
+/// The central struct for constructing the as_error method from an annotated struct.
+struct SessionDiagnosticDerive<'a> {
+    structure: synstructure::Structure<'a>,
+    builder: SessionDiagnosticDeriveBuilder<'a>,
+}
+
+impl std::convert::From<syn::Error> for SessionDiagnosticDeriveError {
+    fn from(e: syn::Error) -> Self {
+        SessionDiagnosticDeriveError::SynError(e)
+    }
+}
+
+/// Equivalent to rustc:errors::diagnostic::DiagnosticId, except stores the quoted expression to
+/// initialise the code with.
+enum DiagnosticId {
+    Error(proc_macro2::TokenStream),
+    Lint(proc_macro2::TokenStream),
+}
+
+#[derive(Debug)]
+enum SessionDiagnosticDeriveError {
+    SynError(syn::Error),
+    ErrorHandled,
+}
+
+impl SessionDiagnosticDeriveError {
+    fn to_compile_error(self) -> proc_macro2::TokenStream {
+        match self {
+            SessionDiagnosticDeriveError::SynError(e) => e.to_compile_error(),
+            SessionDiagnosticDeriveError::ErrorHandled => {
+                // Return ! to avoid having to create a blank DiagnosticBuilder to return when an
+                // error has already been emitted to the compiler.
+                quote! {
+                    unreachable!()
+                }
+            }
+        }
+    }
+}
+
+fn span_err(span: impl proc_macro::MultiSpan, msg: &str) -> proc_macro::Diagnostic {
+    Diagnostic::spanned(span, proc_macro::Level::Error, msg)
+}
+
+/// For methods that return a Result<_, SessionDiagnosticDeriveError>: emit a diagnostic on
+/// span $span with msg $msg (and, optionally, perform additional decoration using the FnOnce
+/// passed in `diag`). Then, return Err(ErrorHandled).
+macro_rules! throw_span_err {
+    ($span:expr, $msg:expr) => {{ throw_span_err!($span, $msg, |diag| diag) }};
+    ($span:expr, $msg:expr, $f:expr) => {{
+        return Err(_throw_span_err($span, $msg, $f));
+    }};
+}
+
+/// When possible, prefer using throw_span_err! over using this function directly. This only exists
+/// as a function to constrain `f` to an impl FnOnce.
+fn _throw_span_err(
+    span: impl proc_macro::MultiSpan,
+    msg: &str,
+    f: impl FnOnce(proc_macro::Diagnostic) -> proc_macro::Diagnostic,
+) -> SessionDiagnosticDeriveError {
+    let diag = span_err(span, msg);
+    f(diag).emit();
+    SessionDiagnosticDeriveError::ErrorHandled
+}
+
+impl<'a> SessionDiagnosticDerive<'a> {
+    fn new(diag: syn::Ident, sess: syn::Ident, structure: synstructure::Structure<'a>) -> Self {
+        // Build the mapping of field names to fields. This allows attributes to peek values from
+        // other fields.
+        let mut fields_map = HashMap::new();
+
+        // Convenience bindings.
+        let ast = structure.ast();
+
+        if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &ast.data {
+            for field in fields.iter() {
+                if let Some(ident) = &field.ident {
+                    fields_map.insert(ident.to_string(), field);
+                }
+            }
+        }
+
+        Self {
+            builder: SessionDiagnosticDeriveBuilder { diag, sess, fields: fields_map, kind: None },
+            structure,
+        }
+    }
+    fn into_tokens(self) -> proc_macro2::TokenStream {
+        let SessionDiagnosticDerive { structure, mut builder } = self;
+
+        let ast = structure.ast();
+        let attrs = &ast.attrs;
+
+        let implementation = {
+            if let syn::Data::Struct(..) = ast.data {
+                let preamble = {
+                    let preamble = attrs.iter().map(|attr| {
+                        builder
+                            .generate_structure_code(attr)
+                            .unwrap_or_else(|v| v.to_compile_error())
+                    });
+                    quote! {
+                        #(#preamble)*;
+                    }
+                };
+
+                let body = structure.each(|field_binding| {
+                    let field = field_binding.ast();
+                    let result = field.attrs.iter().map(|attr| {
+                        builder
+                            .generate_field_code(
+                                attr,
+                                FieldInfo {
+                                    vis: &field.vis,
+                                    binding: field_binding,
+                                    ty: &field.ty,
+                                    span: &field.span(),
+                                },
+                            )
+                            .unwrap_or_else(|v| v.to_compile_error())
+                    });
+                    return quote! {
+                        #(#result);*
+                    };
+                });
+                // Finally, putting it altogether.
+                match builder.kind {
+                    None => {
+                        span_err(ast.span().unwrap(), "`code` not specified")
+                        .help("use the [code = \"...\"] attribute to set this diagnostic's error code ")
+                        .emit();
+                        SessionDiagnosticDeriveError::ErrorHandled.to_compile_error()
+                    }
+                    Some((kind, _)) => match kind {
+                        DiagnosticId::Lint(_lint) => todo!(),
+                        DiagnosticId::Error(code) => {
+                            let (diag, sess) = (&builder.diag, &builder.sess);
+                            quote! {
+                                let mut #diag = #sess.struct_err_with_code("", rustc_errors::DiagnosticId::Error(#code));
+                                #preamble
+                                match self {
+                                    #body
+                                }
+                                #diag
+                            }
+                        }
+                    },
+                }
+            } else {
+                span_err(
+                    ast.span().unwrap(),
+                    "`#[derive(SessionDiagnostic)]` can only be used on structs",
+                )
+                .emit();
+                SessionDiagnosticDeriveError::ErrorHandled.to_compile_error()
+            }
+        };
+
+        let sess = &builder.sess;
+        structure.gen_impl(quote! {
+            gen impl<'__session_diagnostic_sess> rustc_session::SessionDiagnostic<'__session_diagnostic_sess>
+                    for @Self
+            {
+                fn into_diagnostic(
+                    self,
+                    #sess: &'__session_diagnostic_sess rustc_session::Session
+                ) -> rustc_errors::DiagnosticBuilder<'__session_diagnostic_sess> {
+                    #implementation
+                }
+            }
+        })
+    }
+}
+
+/// Field information passed to the builder. Deliberately omits attrs to discourage the generate_*
+/// methods from walking the attributes themselves.
+struct FieldInfo<'a> {
+    vis: &'a syn::Visibility,
+    binding: &'a synstructure::BindingInfo<'a>,
+    ty: &'a syn::Type,
+    span: &'a proc_macro2::Span,
+}
+
+/// Tracks persistent information required for building up the individual calls to diagnostic
+/// methods for the final generated method. This is a separate struct to SessionDerive 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<'a> {
+    /// Name of the session parameter that's passed in to the as_error method.
+    sess: syn::Ident,
+
+    /// Store a map of field name to its corresponding field. This is built on construction of the
+    /// derive builder.
+    fields: HashMap<String, &'a syn::Field>,
+
+    /// The identifier to use for the generated DiagnosticBuilder instance.
+    diag: syn::Ident,
+
+    /// Whether this is a lint or an error. This dictates how the diag will be initialised. Span
+    /// stores at what Span the kind was first set at (for error reporting purposes, if the kind
+    /// was multiply specified).
+    kind: Option<(DiagnosticId, proc_macro2::Span)>,
+}
+
+impl<'a> SessionDiagnosticDeriveBuilder<'a> {
+    fn generate_structure_code(
+        &mut self,
+        attr: &syn::Attribute,
+    ) -> Result<proc_macro2::TokenStream, SessionDiagnosticDeriveError> {
+        Ok(match attr.parse_meta()? {
+            syn::Meta::NameValue(syn::MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                let formatted_str = self.build_format(&s.value(), attr.span());
+                let name = attr.path.segments.last().unwrap().ident.to_string();
+                let name = name.as_str();
+                match name {
+                    "message" => {
+                        let diag = &self.diag;
+                        quote! {
+                            #diag.set_primary_message(#formatted_str);
+                        }
+                    }
+                    attr @ "error" | attr @ "lint" => {
+                        self.set_kind_once(
+                            if attr == "error" {
+                                DiagnosticId::Error(formatted_str)
+                            } else if attr == "lint" {
+                                DiagnosticId::Lint(formatted_str)
+                            } else {
+                                unreachable!()
+                            },
+                            s.span(),
+                        )?;
+                        // This attribute is only allowed to be applied once, and the attribute
+                        // will be set in the initialisation code.
+                        quote! {}
+                    }
+                    other => throw_span_err!(
+                        attr.span().unwrap(),
+                        &format!(
+                            "`#[{} = ...]` is not a valid SessionDiagnostic struct attribute",
+                            other
+                        )
+                    ),
+                }
+            }
+            _ => todo!("unhandled meta kind"),
+        })
+    }
+
+    #[must_use]
+    fn set_kind_once(
+        &mut self,
+        kind: DiagnosticId,
+        span: proc_macro2::Span,
+    ) -> Result<(), SessionDiagnosticDeriveError> {
+        if self.kind.is_none() {
+            self.kind = Some((kind, span));
+            Ok(())
+        } else {
+            let kind_str = |kind: &DiagnosticId| match kind {
+                DiagnosticId::Lint(..) => "lint",
+                DiagnosticId::Error(..) => "error",
+            };
+
+            let existing_kind = kind_str(&self.kind.as_ref().unwrap().0);
+            let this_kind = kind_str(&kind);
+
+            let msg = if this_kind == existing_kind {
+                format!("`{}` specified multiple times", existing_kind)
+            } else {
+                format!("`{}` specified when `{}` was already specified", this_kind, existing_kind)
+            };
+            throw_span_err!(span.unwrap(), &msg);
+        }
+    }
+
+    fn generate_field_code(
+        &mut self,
+        attr: &syn::Attribute,
+        info: FieldInfo<'_>,
+    ) -> Result<proc_macro2::TokenStream, SessionDiagnosticDeriveError> {
+        let field_binding = &info.binding.binding;
+
+        let option_ty = option_inner_ty(&info.ty);
+
+        let generated_code = self.generate_non_option_field_code(
+            attr,
+            FieldInfo {
+                vis: info.vis,
+                binding: info.binding,
+                ty: option_ty.unwrap_or(&info.ty),
+                span: info.span,
+            },
+        )?;
+        Ok(if option_ty.is_none() {
+            quote! { #generated_code }
+        } else {
+            quote! {
+                if let Some(#field_binding) = #field_binding {
+                    #generated_code
+                }
+            }
+        })
+    }
+
+    fn generate_non_option_field_code(
+        &mut self,
+        attr: &syn::Attribute,
+        info: FieldInfo<'_>,
+    ) -> Result<proc_macro2::TokenStream, SessionDiagnosticDeriveError> {
+        let diag = &self.diag;
+        let field_binding = &info.binding.binding;
+        let name = attr.path.segments.last().unwrap().ident.to_string();
+        let name = name.as_str();
+        // At this point, we need to dispatch based on the attribute key + the
+        // type.
+        let meta = attr.parse_meta()?;
+        Ok(match meta {
+            syn::Meta::NameValue(syn::MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
+                let formatted_str = self.build_format(&s.value(), attr.span());
+                match name {
+                    "message" => {
+                        if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
+                            quote! {
+                                #diag.set_span(*#field_binding);
+                                #diag.set_primary_message(#formatted_str);
+                            }
+                        } else {
+                            throw_span_err!(
+                                attr.span().unwrap(),
+                                "the `#[message = \"...\"]` attribute can only be applied to fields of type Span"
+                            );
+                        }
+                    }
+                    "label" => {
+                        if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
+                            quote! {
+                                #diag.span_label(*#field_binding, #formatted_str);
+                            }
+                        } else {
+                            throw_span_err!(
+                                attr.span().unwrap(),
+                                "The `#[label = ...]` attribute can only be applied to fields of type Span"
+                            );
+                        }
+                    }
+                    other => throw_span_err!(
+                        attr.span().unwrap(),
+                        &format!(
+                            "`#[{} = ...]` is not a valid SessionDiagnostic field attribute",
+                            other
+                        )
+                    ),
+                }
+            }
+            syn::Meta::List(list) => {
+                match list.path.segments.iter().last().unwrap().ident.to_string().as_str() {
+                    suggestion_kind @ "suggestion"
+                    | suggestion_kind @ "suggestion_short"
+                    | suggestion_kind @ "suggestion_hidden"
+                    | suggestion_kind @ "suggestion_verbose" => {
+                        // For suggest, we need to ensure we are running on a (Span,
+                        // Applicability) pair.
+                        let (span, applicability) = (|| match &info.ty {
+                            ty @ syn::Type::Path(..)
+                                if type_matches_path(ty, &["rustc_span", "Span"]) =>
+                            {
+                                let binding = &info.binding.binding;
+                                Ok((
+                                    quote!(*#binding),
+                                    quote!(rustc_errors::Applicability::Unspecified),
+                                ))
+                            }
+                            syn::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.clone().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.clone().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(quote!(
+                                            rustc_errors::Applicability::Unspecified
+                                        ));
+                                    return Ok((span, applicability));
+                                }
+                                throw_span_err!(
+                                    info.span.clone().unwrap(),
+                                    "wrong types for suggestion",
+                                    |diag| {
+                                        diag.help("#[suggestion(...)] on a tuple field must be applied to fields of type (Span, Applicability)")
+                                    }
+                                );
+                            }
+                            _ => throw_span_err!(
+                                info.span.clone().unwrap(),
+                                "wrong field type for suggestion",
+                                |diag| {
+                                    diag.help("#[suggestion(...)] should be applied to fields of type Span or (Span, Applicability)")
+                                }
+                            ),
+                        })()?;
+                        // Now read the key-value pairs.
+                        let mut msg = None;
+                        let mut code = None;
+
+                        for arg in list.nested.iter() {
+                            if let syn::NestedMeta::Meta(syn::Meta::NameValue(arg_name_value)) = arg
+                            {
+                                if let syn::MetaNameValue { lit: syn::Lit::Str(s), .. } =
+                                    arg_name_value
+                                {
+                                    let name = arg_name_value
+                                        .path
+                                        .segments
+                                        .last()
+                                        .unwrap()
+                                        .ident
+                                        .to_string();
+                                    let name = name.as_str();
+                                    let formatted_str = self.build_format(&s.value(), arg.span());
+                                    match name {
+                                        "message" => {
+                                            msg = Some(formatted_str);
+                                        }
+                                        "code" => {
+                                            code = Some(formatted_str);
+                                        }
+                                        other => throw_span_err!(
+                                            arg.span().unwrap(),
+                                            &format!(
+                                                "`{}` is not a valid key for `#[suggestion(...)]`",
+                                                other
+                                            )
+                                        ),
+                                    }
+                                }
+                            }
+                        }
+                        let msg = if let Some(msg) = msg {
+                            quote!(#msg.as_str())
+                        } else {
+                            throw_span_err!(
+                                list.span().unwrap(),
+                                "missing suggestion message",
+                                |diag| {
+                                    diag.help("provide a suggestion message using #[suggestion(message = \"...\")]")
+                                }
+                            );
+                        };
+                        let code = code.unwrap_or_else(|| quote! { String::new() });
+                        // Now build it out:
+                        let suggestion_method = format_ident!("span_{}", suggestion_kind);
+                        quote! {
+                            #diag.#suggestion_method(#span, #msg, #code, #applicability);
+                        }
+                    }
+                    other => throw_span_err!(
+                        list.span().unwrap(),
+                        &format!("invalid annotation list `#[{}(...)]`", other)
+                    ),
+                }
+            }
+            _ => panic!("unhandled meta kind"),
+        })
+    }
+
+    /// In the strings in the attributes supplied to this macro, we want callers to be able to
+    /// reference fields in the format string. Take this, for example:
+    /// ```ignore (not-usage-example)
+    /// struct Point {
+    ///     #[error = "Expected a point greater than ({x}, {y})"]
+    ///     x: i32,
+    ///     y: i32,
+    /// }
+    /// ```
+    /// We want to automatically pick up that {x} refers `self.x` and {y} refers to `self.y`, then
+    /// generate this call to format!:
+    /// ```ignore (not-usage-example)
+    /// format!("Expected a point greater than ({x}, {y})", x = self.x, y = self.y)
+    /// ```
+    /// This function builds the entire call to format!.
+    fn build_format(&self, input: &String, span: proc_macro2::Span) -> proc_macro2::TokenStream {
+        let mut referenced_fields: HashSet<String> = HashSet::new();
+
+        // At this point, we can start parsing the format string.
+        let mut it = input.chars().peekable();
+        // Once the start of a format string has been found, process the format string and spit out
+        // the referenced fields. Leaves `it` sitting on the closing brace of the format string, so the
+        // next call to `it.next()` retrieves the next character.
+        while let Some(c) = it.next() {
+            if c == '{' && *it.peek().unwrap_or(&'\0') != '{' {
+                #[must_use]
+                let mut eat_argument = || -> Option<String> {
+                    let mut result = String::new();
+                    // Format specifiers look like
+                    // format   := '{' [ argument ] [ ':' format_spec ] '}' .
+                    // Therefore, we only need to eat until ':' or '}' to find the argument.
+                    while let Some(c) = it.next() {
+                        result.push(c);
+                        let next = *it.peek().unwrap_or(&'\0');
+                        if next == '}' {
+                            break;
+                        } else if next == ':' {
+                            // Eat the ':' character.
+                            assert_eq!(it.next().unwrap(), ':');
+                            break;
+                        }
+                    }
+                    // Eat until (and including) the matching '}'
+                    while it.next()? != '}' {
+                        continue;
+                    }
+                    Some(result)
+                };
+
+                if let Some(referenced_field) = eat_argument() {
+                    referenced_fields.insert(referenced_field);
+                }
+            }
+        }
+        // At this point, `referenced_fields` contains a set of the unique fields that were
+        // referenced in the format string. Generate the corresponding "x = self.x" format
+        // string parameters:
+        let args = referenced_fields.into_iter().map(|field: String| {
+            let field_ident = format_ident!("{}", field);
+            let value = if self.fields.contains_key(&field) {
+                quote! {
+                    &self.#field_ident
+                }
+            } else {
+                // This field doesn't exist. Emit a diagnostic.
+                Diagnostic::spanned(
+                    span.unwrap(),
+                    proc_macro::Level::Error,
+                    format!("`{}` doesn't refer to a field on this type", field),
+                )
+                .emit();
+                quote! {
+                    "{#field}"
+                }
+            };
+            quote! {
+                #field_ident = #value
+            }
+        });
+        quote! {
+            format!(#input #(,#args)*)
+        }
+    }
+}
+
+/// If `ty` is an Option, returns Some(inner type). Else, returns None.
+fn option_inner_ty(ty: &syn::Type) -> Option<&syn::Type> {
+    if type_matches_path(ty, &["std", "option", "Option"]) {
+        if let syn::Type::Path(ty_path) = ty {
+            let path = &ty_path.path;
+            let ty = path.segments.iter().last().unwrap();
+            if let syn::PathArguments::AngleBracketed(bracketed) = &ty.arguments {
+                if bracketed.args.len() == 1 {
+                    if let syn::GenericArgument::Type(ty) = &bracketed.args[0] {
+                        return Some(ty);
+                    }
+                }
+            }
+        }
+    }
+    None
+}

compiler/rustc_session/src/session.rs

@@ -237,6 +237,14 @@ enum DiagnosticBuilderMethod {
                             // Add more variants as needed to support one-time diagnostics.
 }
 
+/// Trait implemented by error types. This should not be implemented manually. Instead, use
+/// `#[derive(SessionDiagnostic)]` -- see [rustc_macros::SessionDiagnostic].
+pub trait SessionDiagnostic<'a> {
+    /// Write out as a diagnostic out of `sess`.
+    #[must_use]
+    fn into_diagnostic(self, sess: &'a Session) -> DiagnosticBuilder<'a>;
+}
+
 /// Diagnostic message ID, used by `Session.one_time_diagnostics` to avoid
 /// emitting the same message more than once.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -392,6 +400,9 @@ impl Session {
     pub fn err(&self, msg: &str) {
         self.diagnostic().err(msg)
     }
+    pub fn emit_err<'a>(&'a self, err: impl SessionDiagnostic<'a>) {
+        err.into_diagnostic(self).emit()
+    }
     pub fn err_count(&self) -> usize {
         self.diagnostic().err_count()
     }

compiler/rustc_typeck/src/collect.rs

@@ -18,6 +18,7 @@ use crate::astconv::{AstConv, SizedByDefault};
 use crate::bounds::Bounds;
 use crate::check::intrinsic::intrinsic_operation_unsafety;
 use crate::constrained_generic_params as cgp;
+use crate::errors;
 use crate::middle::resolve_lifetime as rl;
 use rustc_ast as ast;
 use rustc_ast::MetaItemKind;
@@ -834,16 +835,11 @@ fn convert_variant(
             let fid = tcx.hir().local_def_id(f.hir_id);
             let dup_span = seen_fields.get(&f.ident.normalize_to_macros_2_0()).cloned();
             if let Some(prev_span) = dup_span {
-                struct_span_err!(
-                    tcx.sess,
-                    f.span,
-                    E0124,
-                    "field `{}` is already declared",
-                    f.ident
-                )
-                .span_label(f.span, "field already declared")
-                .span_label(prev_span, format!("`{}` first declared here", f.ident))
-                .emit();
+                tcx.sess.emit_err(errors::FieldAlreadyDeclared {
+                    field_name: f.ident,
+                    span: f.span,
+                    prev_span,
+                });
             } else {
                 seen_fields.insert(f.ident.normalize_to_macros_2_0(), f.span);
             }

compiler/rustc_typeck/src/errors.rs

@@ -0,0 +1,14 @@
+//! Errors emitted by typeck.
+use rustc_macros::SessionDiagnostic;
+use rustc_span::{symbol::Ident, Span };
+
+#[derive(SessionDiagnostic)]
+#[error = "E0124"]
+pub struct FieldAlreadyDeclared {
+    pub field_name: Ident,
+    #[message = "field `{field_name}` is already declared"]
+    #[label = "field already declared"]
+    pub span: Span,
+    #[label = "`{field_name}` first declared here"]
+    pub prev_span: Span,
+}

compiler/rustc_typeck/src/lib.rs

@@ -84,6 +84,7 @@ mod check_unused;
 mod coherence;
 mod collect;
 mod constrained_generic_params;
+mod errors;
 mod impl_wf_check;
 mod mem_categorization;
 mod outlives;

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

@@ -0,0 +1,260 @@
+// check-fail
+// Tests error conditions for specifying diagnostics using #[derive(SessionDiagnostic)]
+
+#![feature(rustc_private)]
+#![crate_type = "lib"]
+
+extern crate rustc_span;
+use rustc_span::Span;
+use rustc_span::symbol::Ident;
+
+extern crate rustc_macros;
+use rustc_macros::SessionDiagnostic;
+
+extern crate rustc_middle;
+use rustc_middle::ty::Ty;
+
+extern crate rustc_errors;
+use rustc_errors::Applicability;
+
+extern crate rustc_session;
+
+#[derive(SessionDiagnostic)]
+#[message = "Hello, world!"]
+#[error = "E0123"]
+struct Hello {}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+//~^ ERROR `#[derive(SessionDiagnostic)]` can only be used on structs
+enum SessionDiagnosticOnEnum {
+    Foo,
+    Bar,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[label = "This is in the wrong place"]
+//~^ ERROR `#[label = ...]` is not a valid SessionDiagnostic struct attribute
+struct WrongPlace {}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct WrongPlaceField {
+    #[suggestion = "this is the wrong kind of attribute"]
+//~^ ERROR `#[suggestion = ...]` is not a valid SessionDiagnostic field attribute
+    sp: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[message = "Hello, world!"]
+#[error = "E0123"]
+#[error = "E0456"] //~ ERROR `error` specified multiple times
+struct ErrorSpecifiedTwice {}
+
+#[derive(SessionDiagnostic)]
+#[message = "Hello, world!"]
+#[error = "E0123"]
+#[lint = "some_useful_lint"] //~ ERROR `lint` specified when `error` was already specified
+struct LintSpecifiedAfterError {}
+
+#[derive(SessionDiagnostic)]
+#[message = "Some lint message"]
+#[error = "E0123"]
+struct LintButHasErrorCode {}
+
+#[derive(SessionDiagnostic)]
+struct ErrorCodeNotProvided {} //~ ERROR `code` not specified
+
+// FIXME: Uncomment when emitting lints is supported.
+/*
+#[derive(SessionDiagnostic)]
+#[message = "Hello, world!"]
+#[lint = "clashing_extern_declarations"]
+#[lint = "improper_ctypes"] // FIXME: ERROR `lint` specified multiple times
+struct LintSpecifiedTwice {}
+
+#[derive(SessionDiagnostic)]
+#[lint = "Some lint message"]
+#[message = "Some error message"]
+#[error = "E0123"] // ERROR `error` specified when `lint` was already specified
+struct ErrorSpecifiedAfterLint {}
+*/
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct ErrorWithField {
+    name: String,
+    #[message = "This error has a field, and references {name}"]
+    span: Span
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct ErrorWithMessageAppliedToField {
+    #[message = "this message is applied to a String field"]
+    //~^ ERROR the `#[message = "..."]` attribute can only be applied to fields of type Span
+    name: String,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "This error has a field, and references {name}"]
+//~^ ERROR `name` doesn't refer to a field on this type
+struct ErrorWithNonexistentField {
+    span: Span
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "This is missing a closing brace: {name"]
+//~^ ERROR invalid format string: expected `'}'`
+struct ErrorMissingClosingBrace {
+    name: String,
+    span: Span
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "This is missing an opening brace: name}"]
+//~^ ERROR invalid format string: unmatched `}`
+struct ErrorMissingOpeningBrace {
+    name: String,
+    span: Span
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "Something something"]
+struct LabelOnSpan {
+    #[label = "See here"]
+    sp: Span
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "Something something"]
+struct LabelOnNonSpan {
+    #[label = "See here"]
+    //~^ ERROR The `#[label = ...]` attribute can only be applied to fields of type Span
+    id: u32,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct Suggest {
+    #[suggestion(message = "This is a suggestion", code = "This is the suggested code")]
+    #[suggestion_short(message = "This is a suggestion", code = "This is the suggested code")]
+    #[suggestion_hidden(message = "This is a suggestion", code = "This is the suggested code")]
+    #[suggestion_verbose(message = "This is a suggestion", code = "This is the suggested code")]
+    suggestion: (Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithoutCode {
+    #[suggestion(message = "This is a suggestion")]
+    suggestion: (Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithBadKey {
+    #[suggestion(nonsense = "This is nonsense")]
+    //~^ ERROR `nonsense` is not a valid key for `#[suggestion(...)]`
+    suggestion: (Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithShorthandMsg {
+    #[suggestion(msg = "This is a suggestion")]
+    //~^ ERROR `msg` is not a valid key for `#[suggestion(...)]`
+    suggestion: (Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithoutMsg {
+    #[suggestion(code = "This is suggested code")]
+    //~^ ERROR missing suggestion message
+    suggestion: (Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithTypesSwapped {
+    #[suggestion(message = "This is a message", code = "This is suggested code")]
+    suggestion: (Applicability, Span),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithWrongTypeApplicabilityOnly {
+    #[suggestion(message = "This is a message", code = "This is suggested code")]
+    //~^ ERROR wrong field type for suggestion
+    suggestion: Applicability,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithSpanOnly{
+    #[suggestion(message = "This is a message", code = "This is suggested code")]
+    suggestion: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithDuplicateSpanAndApplicability {
+    #[suggestion(message = "This is a message", code = "This is suggested code")]
+    //~^ ERROR type of field annotated with `#[suggestion(...)]` contains more than one Span
+    suggestion: (Span, Span, Applicability),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct SuggestWithDuplicateApplicabilityAndSpan {
+    #[suggestion(message = "This is a message", code = "This is suggested code")]
+    //~^ ERROR type of field annotated with `#[suggestion(...)]` contains more than one
+    suggestion: (Applicability, Applicability, Span),
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct WrongKindOfAnnotation {
+    #[label("wrong kind of annotation for label")]
+    //~^ ERROR invalid annotation list `#[label(...)]`
+    z: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+#[message = "Something something else"]
+struct OptionsInErrors {
+    #[label = "Label message"]
+    label: Option<Span>,
+    #[suggestion(message = "suggestion message")]
+    opt_sugg: Option<(Span, Applicability)>,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0456"]
+struct MoveOutOfBorrowError<'tcx> {
+    name: Ident,
+    ty: Ty<'tcx>,
+    #[message = "cannot move {ty} out of borrow"]
+    #[label = "cannot move out of borrow"]
+    span: Span,
+    #[label = "`{ty}` first borrowed here"]
+    other_span: Span,
+    #[suggestion(message = "consider cloning here", code = "{name}.clone()")]
+    opt_sugg: Option<(Span, Applicability)>,
+}
+
+#[derive(SessionDiagnostic)]
+#[error = "E0123"]
+struct ErrorWithLifetime<'a> {
+    #[message = "Some message that references {name}"]
+    span: Span,
+    name: &'a str,
+}

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

@@ -0,0 +1,135 @@
+error: `#[derive(SessionDiagnostic)]` can only be used on structs
+  --> $DIR/session-derive-errors.rs:28:1
+   |
+LL | / #[error = "E0123"]
+LL | |
+LL | | enum SessionDiagnosticOnEnum {
+LL | |     Foo,
+LL | |     Bar,
+LL | | }
+   | |_^
+
+error: `#[label = ...]` is not a valid SessionDiagnostic struct attribute
+  --> $DIR/session-derive-errors.rs:37:1
+   |
+LL | #[label = "This is in the wrong place"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `#[suggestion = ...]` is not a valid SessionDiagnostic field attribute
+  --> $DIR/session-derive-errors.rs:44:5
+   |
+LL |     #[suggestion = "this is the wrong kind of attribute"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `error` specified multiple times
+  --> $DIR/session-derive-errors.rs:52:11
+   |
+LL | #[error = "E0456"]
+   |           ^^^^^^^
+
+error: `lint` specified when `error` was already specified
+  --> $DIR/session-derive-errors.rs:58:10
+   |
+LL | #[lint = "some_useful_lint"]
+   |          ^^^^^^^^^^^^^^^^^^
+
+error: `code` not specified
+  --> $DIR/session-derive-errors.rs:67:1
+   |
+LL | struct ErrorCodeNotProvided {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use the [code = "..."] attribute to set this diagnostic's error code 
+
+error: the `#[message = "..."]` attribute can only be applied to fields of type Span
+  --> $DIR/session-derive-errors.rs:95:5
+   |
+LL |     #[message = "this message is applied to a String field"]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `name` doesn't refer to a field on this type
+  --> $DIR/session-derive-errors.rs:102:1
+   |
+LL | #[message = "This error has a field, and references {name}"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: invalid format string: expected `'}'` but string was terminated
+  --> $DIR/session-derive-errors.rs:110:1
+   |
+LL | #[error = "E0123"]
+   |               - because of this opening brace
+LL | #[message = "This is missing a closing brace: {name"]
+   | ^ expected `'}'` in format string
+   |
+   = note: if you intended to print `{`, you can escape it using `{{`
+   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: invalid format string: unmatched `}` found
+  --> $DIR/session-derive-errors.rs:119:1
+   |
+LL | #[message = "This is missing an opening brace: name}"]
+   | ^ unmatched `}` in format string
+   |
+   = note: if you intended to print `}`, you can escape it using `}}`
+   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: The `#[label = ...]` attribute can only be applied to fields of type Span
+  --> $DIR/session-derive-errors.rs:138:5
+   |
+LL |     #[label = "See here"]
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: `nonsense` is not a valid key for `#[suggestion(...)]`
+  --> $DIR/session-derive-errors.rs:163:18
+   |
+LL |     #[suggestion(nonsense = "This is nonsense")]
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `msg` is not a valid key for `#[suggestion(...)]`
+  --> $DIR/session-derive-errors.rs:171:18
+   |
+LL |     #[suggestion(msg = "This is a suggestion")]
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: missing suggestion message
+  --> $DIR/session-derive-errors.rs:179:7
+   |
+LL |     #[suggestion(code = "This is suggested code")]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: provide a suggestion message using #[suggestion(message = "...")]
+
+error: wrong field type for suggestion
+  --> $DIR/session-derive-errors.rs:194:5
+   |
+LL | /     #[suggestion(message = "This is a message", code = "This is suggested code")]
+LL | |
+LL | |     suggestion: Applicability,
+   | |_____________________________^
+   |
+   = help: #[suggestion(...)] should be applied to fields of type Span or (Span, Applicability)
+
+error: type of field annotated with `#[suggestion(...)]` contains more than one Span
+  --> $DIR/session-derive-errors.rs:209:5
+   |
+LL | /     #[suggestion(message = "This is a message", code = "This is suggested code")]
+LL | |
+LL | |     suggestion: (Span, Span, Applicability),
+   | |___________________________________________^
+
+error: type of field annotated with `#[suggestion(...)]` contains more than one Applicability
+  --> $DIR/session-derive-errors.rs:217:5
+   |
+LL | /     #[suggestion(message = "This is a message", code = "This is suggested code")]
+LL | |
+LL | |     suggestion: (Applicability, Applicability, Span),
+   | |____________________________________________________^
+
+error: invalid annotation list `#[label(...)]`
+  --> $DIR/session-derive-errors.rs:225:7
+   |
+LL |     #[label("wrong kind of annotation for label")]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 18 previous errors
+