diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index 7faf14a2472..02d076c95ca 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -234,6 +234,48 @@ pub fn fallback_fluent_bundle(
 /// Identifier for the Fluent message/attribute corresponding to a diagnostic message.
 type FluentId = Cow<'static, str>;
 
+/// Abstraction over a message in a subdiagnostic (i.e. label, note, help, etc) to support both
+/// translatable and non-translatable diagnostic messages.
+///
+/// Translatable messages for subdiagnostics are typically attributes attached to a larger Fluent
+/// message so messages of this type must be combined with a `DiagnosticMessage` (using
+/// `DiagnosticMessage::with_subdiagnostic_message`) before rendering. However, subdiagnostics from
+/// the `SessionSubdiagnostic` derive refer to Fluent identifiers directly.
+pub enum SubdiagnosticMessage {
+    /// Non-translatable diagnostic message.
+    // FIXME(davidtwco): can a `Cow<'static, str>` be used here?
+    Str(String),
+    /// Identifier of a Fluent message. Instances of this variant are generated by the
+    /// `SessionSubdiagnostic` derive.
+    FluentIdentifier(FluentId),
+    /// Attribute of a Fluent message. Needs to be combined with a Fluent identifier to produce an
+    /// actual translated message. Instances of this variant are generated by the `fluent_messages`
+    /// macro.
+    ///
+    /// <https://projectfluent.org/fluent/guide/attributes.html>
+    FluentAttr(FluentId),
+}
+
+impl SubdiagnosticMessage {
+    /// Create a `SubdiagnosticMessage` for the provided Fluent attribute.
+    pub fn attr(id: impl Into<FluentId>) -> Self {
+        SubdiagnosticMessage::FluentAttr(id.into())
+    }
+
+    /// Create a `SubdiagnosticMessage` for the provided Fluent identifier.
+    pub fn message(id: impl Into<FluentId>) -> Self {
+        SubdiagnosticMessage::FluentIdentifier(id.into())
+    }
+}
+
+/// `From` impl that enables existing diagnostic calls to functions which now take
+/// `impl Into<SubdiagnosticMessage>` to continue to work as before.
+impl<S: Into<String>> From<S> for SubdiagnosticMessage {
+    fn from(s: S) -> Self {
+        SubdiagnosticMessage::Str(s.into())
+    }
+}
+
 /// Abstraction over a message in a diagnostic to support both translatable and non-translatable
 /// diagnostic messages.
 ///
@@ -252,6 +294,29 @@ pub enum DiagnosticMessage {
 }
 
 impl DiagnosticMessage {
+    /// Given a `SubdiagnosticMessage` which may contain a Fluent attribute, create a new
+    /// `DiagnosticMessage` that combines that attribute with the Fluent identifier of `self`.
+    ///
+    /// - If the `SubdiagnosticMessage` is non-translatable then return the message as a
+    /// `DiagnosticMessage`.
+    /// - If `self` is non-translatable then return `self`'s message.
+    pub fn with_subdiagnostic_message(&self, sub: SubdiagnosticMessage) -> Self {
+        let attr = match sub {
+            SubdiagnosticMessage::Str(s) => return DiagnosticMessage::Str(s.clone()),
+            SubdiagnosticMessage::FluentIdentifier(id) => {
+                return DiagnosticMessage::FluentIdentifier(id, None);
+            }
+            SubdiagnosticMessage::FluentAttr(attr) => attr,
+        };
+
+        match self {
+            DiagnosticMessage::Str(s) => DiagnosticMessage::Str(s.clone()),
+            DiagnosticMessage::FluentIdentifier(id, _) => {
+                DiagnosticMessage::FluentIdentifier(id.clone(), Some(attr))
+            }
+        }
+    }
+
     /// Returns the `String` contained within the `DiagnosticMessage::Str` variant, assuming that
     /// this diagnostic message is of the legacy, non-translatable variety. Panics if this
     /// assumption does not hold.
@@ -266,14 +331,9 @@ impl DiagnosticMessage {
     }
 
     /// Create a `DiagnosticMessage` for the provided Fluent identifier.
-    pub fn fluent(id: impl Into<FluentId>) -> Self {
+    pub fn new(id: impl Into<FluentId>) -> Self {
         DiagnosticMessage::FluentIdentifier(id.into(), None)
     }
-
-    /// Create a `DiagnosticMessage` for the provided Fluent identifier and attribute.
-    pub fn fluent_attr(id: impl Into<FluentId>, attr: impl Into<FluentId>) -> Self {
-        DiagnosticMessage::FluentIdentifier(id.into(), Some(attr.into()))
-    }
 }
 
 /// `From` impl that enables existing diagnostic calls to functions which now take
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index f130b5aa9a6..643f3c12134 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -1,7 +1,7 @@
 use crate::snippet::Style;
 use crate::{
-    CodeSuggestion, DiagnosticMessage, Level, MultiSpan, Substitution, SubstitutionPart,
-    SuggestionStyle,
+    CodeSuggestion, DiagnosticMessage, Level, MultiSpan, SubdiagnosticMessage, Substitution,
+    SubstitutionPart, SuggestionStyle,
 };
 use rustc_data_structures::stable_map::FxHashMap;
 use rustc_error_messages::FluentValue;
@@ -283,8 +283,8 @@ impl Diagnostic {
     ///
     /// This span is *not* considered a ["primary span"][`MultiSpan`]; only
     /// the `Span` supplied when creating the diagnostic is primary.
-    pub fn span_label(&mut self, span: Span, label: impl Into<DiagnosticMessage>) -> &mut Self {
-        self.span.push_span_label(span, label.into());
+    pub fn span_label(&mut self, span: Span, label: impl Into<SubdiagnosticMessage>) -> &mut Self {
+        self.span.push_span_label(span, self.subdiagnostic_message_to_diagnostic_message(label));
         self
     }
 
@@ -401,12 +401,12 @@ impl Diagnostic {
     }
 
     /// Add a note attached to this diagnostic.
-    pub fn note(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
+    pub fn note(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Note, msg, MultiSpan::new(), None);
         self
     }
 
-    pub fn highlighted_note<M: Into<DiagnosticMessage>>(
+    pub fn highlighted_note<M: Into<SubdiagnosticMessage>>(
         &mut self,
         msg: Vec<(M, Style)>,
     ) -> &mut Self {
@@ -416,7 +416,7 @@ impl Diagnostic {
 
     /// Prints the span with a note above it.
     /// This is like [`Diagnostic::note()`], but it gets its own span.
-    pub fn note_once(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
+    pub fn note_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::OnceNote, msg, MultiSpan::new(), None);
         self
     }
@@ -426,7 +426,7 @@ impl Diagnostic {
     pub fn span_note<S: Into<MultiSpan>>(
         &mut self,
         sp: S,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
         self.sub(Level::Note, msg, sp.into(), None);
         self
@@ -437,14 +437,14 @@ impl Diagnostic {
     pub fn span_note_once<S: Into<MultiSpan>>(
         &mut self,
         sp: S,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
         self.sub(Level::OnceNote, msg, sp.into(), None);
         self
     }
 
     /// Add a warning attached to this diagnostic.
-    pub fn warn(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
+    pub fn warn(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Warning, msg, MultiSpan::new(), None);
         self
     }
@@ -454,14 +454,14 @@ impl Diagnostic {
     pub fn span_warn<S: Into<MultiSpan>>(
         &mut self,
         sp: S,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
         self.sub(Level::Warning, msg, sp.into(), None);
         self
     }
 
     /// Add a help message attached to this diagnostic.
-    pub fn help(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
+    pub fn help(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Help, msg, MultiSpan::new(), None);
         self
     }
@@ -477,7 +477,7 @@ impl Diagnostic {
     pub fn span_help<S: Into<MultiSpan>>(
         &mut self,
         sp: S,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
         self.sub(Level::Help, msg, sp.into(), None);
         self
@@ -514,7 +514,7 @@ impl Diagnostic {
     /// In other words, multiple changes need to be applied as part of this suggestion.
     pub fn multipart_suggestion(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
@@ -530,7 +530,7 @@ impl Diagnostic {
     /// In other words, multiple changes need to be applied as part of this suggestion.
     pub fn multipart_suggestion_verbose(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
@@ -544,7 +544,7 @@ impl Diagnostic {
     /// [`Diagnostic::multipart_suggestion()`] but you can set the [`SuggestionStyle`].
     pub fn multipart_suggestion_with_style(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
         style: SuggestionStyle,
@@ -557,7 +557,7 @@ impl Diagnostic {
                     .map(|(span, snippet)| SubstitutionPart { snippet, span })
                     .collect(),
             }],
-            msg: msg.into(),
+            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
             style,
             applicability,
         });
@@ -572,7 +572,7 @@ impl Diagnostic {
     /// improve understandability.
     pub fn tool_only_multipart_suggestion(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
@@ -584,7 +584,7 @@ impl Diagnostic {
                     .map(|(span, snippet)| SubstitutionPart { snippet, span })
                     .collect(),
             }],
-            msg: msg.into(),
+            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
             style: SuggestionStyle::CompletelyHidden,
             applicability,
         });
@@ -611,7 +611,7 @@ impl Diagnostic {
     pub fn span_suggestion(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
@@ -629,7 +629,7 @@ impl Diagnostic {
     pub fn span_suggestion_with_style(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
         style: SuggestionStyle,
@@ -638,7 +638,7 @@ impl Diagnostic {
             substitutions: vec![Substitution {
                 parts: vec![SubstitutionPart { snippet: suggestion.to_string(), span: sp }],
             }],
-            msg: msg.into(),
+            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
             style,
             applicability,
         });
@@ -649,7 +649,7 @@ impl Diagnostic {
     pub fn span_suggestion_verbose(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
@@ -668,7 +668,7 @@ impl Diagnostic {
     pub fn span_suggestions(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestions: impl Iterator<Item = String>,
         applicability: Applicability,
     ) -> &mut Self {
@@ -680,7 +680,7 @@ impl Diagnostic {
             .collect();
         self.push_suggestion(CodeSuggestion {
             substitutions,
-            msg: msg.into(),
+            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
             style: SuggestionStyle::ShowCode,
             applicability,
         });
@@ -691,7 +691,7 @@ impl Diagnostic {
     /// See also [`Diagnostic::span_suggestion()`].
     pub fn multipart_suggestions(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestions: impl Iterator<Item = Vec<(Span, String)>>,
         applicability: Applicability,
     ) -> &mut Self {
@@ -704,7 +704,7 @@ impl Diagnostic {
                         .collect(),
                 })
                 .collect(),
-            msg: msg.into(),
+            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
             style: SuggestionStyle::ShowCode,
             applicability,
         });
@@ -717,7 +717,7 @@ impl Diagnostic {
     pub fn span_suggestion_short(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
@@ -740,7 +740,7 @@ impl Diagnostic {
     pub fn span_suggestion_hidden(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
@@ -761,7 +761,7 @@ impl Diagnostic {
     pub fn tool_only_span_suggestion(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self {
@@ -831,6 +831,18 @@ impl Diagnostic {
         &self.message
     }
 
+    /// Helper function that takes a `SubdiagnosticMessage` and returns a `DiagnosticMessage` by
+    /// combining it with the primary message of the diagnostic (if translatable, otherwise it just
+    /// passes the user's string along).
+    fn subdiagnostic_message_to_diagnostic_message(
+        &self,
+        attr: impl Into<SubdiagnosticMessage>,
+    ) -> DiagnosticMessage {
+        let msg =
+            self.message.iter().map(|(msg, _)| msg).next().expect("diagnostic with no messages");
+        msg.with_subdiagnostic_message(attr.into())
+    }
+
     /// Convenience function for internal use, clients should use one of the
     /// public methods above.
     ///
@@ -838,13 +850,16 @@ impl Diagnostic {
     pub fn sub(
         &mut self,
         level: Level,
-        message: impl Into<DiagnosticMessage>,
+        message: impl Into<SubdiagnosticMessage>,
         span: MultiSpan,
         render_span: Option<MultiSpan>,
     ) {
         let sub = SubDiagnostic {
             level,
-            message: vec![(message.into(), Style::NoStyle)],
+            message: vec![(
+                self.subdiagnostic_message_to_diagnostic_message(message),
+                Style::NoStyle,
+            )],
             span,
             render_span,
         };
@@ -853,14 +868,17 @@ impl Diagnostic {
 
     /// Convenience function for internal use, clients should use one of the
     /// public methods above.
-    fn sub_with_highlights<M: Into<DiagnosticMessage>>(
+    fn sub_with_highlights<M: Into<SubdiagnosticMessage>>(
         &mut self,
         level: Level,
         mut message: Vec<(M, Style)>,
         span: MultiSpan,
         render_span: Option<MultiSpan>,
     ) {
-        let message = message.drain(..).map(|m| (m.0.into(), m.1)).collect();
+        let message = message
+            .drain(..)
+            .map(|m| (self.subdiagnostic_message_to_diagnostic_message(m.0), m.1))
+            .collect();
         let sub = SubDiagnostic { level, message, span, render_span };
         self.children.push(sub);
     }
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 6ef2c832c65..9e0a99849a3 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -1,5 +1,8 @@
 use crate::diagnostic::IntoDiagnosticArg;
-use crate::{Diagnostic, DiagnosticId, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed};
+use crate::{
+    Diagnostic, DiagnosticId, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed,
+    SubdiagnosticMessage,
+};
 use crate::{Handler, Level, MultiSpan, StashKey};
 use rustc_lint_defs::Applicability;
 
@@ -395,7 +398,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     /// the diagnostic was constructed. However, the label span is *not* considered a
     /// ["primary span"][`MultiSpan`]; only the `Span` supplied when creating the diagnostic is
     /// primary.
-    pub fn span_label(&mut self, span: Span, label: impl Into<DiagnosticMessage>) -> &mut Self);
+    pub fn span_label(&mut self, span: Span, label: impl Into<SubdiagnosticMessage>) -> &mut Self);
 
     forward!(
     /// Labels all the given spans with the provided label.
@@ -430,25 +433,29 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         found: DiagnosticStyledString,
     ) -> &mut Self);
 
-    forward!(pub fn note(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self);
-    forward!(pub fn note_once(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self);
+    forward!(pub fn note(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
+    forward!(pub fn note_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
     forward!(pub fn span_note(
         &mut self,
         sp: impl Into<MultiSpan>,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self);
     forward!(pub fn span_note_once(
         &mut self,
         sp: impl Into<MultiSpan>,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self);
-    forward!(pub fn warn(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self);
-    forward!(pub fn span_warn(&mut self, sp: impl Into<MultiSpan>, msg: &str) -> &mut Self);
-    forward!(pub fn help(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self);
+    forward!(pub fn warn(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
+    forward!(pub fn span_warn(
+        &mut self,
+        sp: impl Into<MultiSpan>,
+        msg: impl Into<SubdiagnosticMessage>,
+    ) -> &mut Self);
+    forward!(pub fn help(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self);
     forward!(pub fn span_help(
         &mut self,
         sp: impl Into<MultiSpan>,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self);
     forward!(pub fn help_use_latest_edition(&mut self,) -> &mut Self);
     forward!(pub fn set_is_lint(&mut self,) -> &mut Self);
@@ -457,67 +464,67 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
 
     forward!(pub fn multipart_suggestion(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn multipart_suggestion_verbose(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn tool_only_multipart_suggestion(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestions(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestions: impl Iterator<Item = String>,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn multipart_suggestions(
         &mut self,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestions: impl Iterator<Item = Vec<(Span, String)>>,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion_short(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion_verbose(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn span_suggestion_hidden(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
     forward!(pub fn tool_only_span_suggestion(
         &mut self,
         sp: Span,
-        msg: impl Into<DiagnosticMessage>,
+        msg: impl Into<SubdiagnosticMessage>,
         suggestion: impl ToString,
         applicability: Applicability,
     ) -> &mut Self);
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 5b9b65da343..fb02f1d68eb 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -32,7 +32,8 @@ use rustc_data_structures::sync::{self, Lock, Lrc};
 use rustc_data_structures::AtomicRef;
 pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent, fluent_bundle, DiagnosticMessage, FluentBundle,
-    LanguageIdentifier, LazyFallbackBundle, MultiSpan, SpanLabel, DEFAULT_LOCALE_RESOURCES,
+    LanguageIdentifier, LazyFallbackBundle, MultiSpan, SpanLabel, SubdiagnosticMessage,
+    DEFAULT_LOCALE_RESOURCES,
 };
 pub use rustc_lint_defs::{pluralize, Applicability};
 use rustc_span::source_map::SourceMap;
diff --git a/compiler/rustc_macros/src/diagnostics/diagnostic.rs b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
index d7daee64d79..95ee0d4a060 100644
--- a/compiler/rustc_macros/src/diagnostics/diagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
@@ -126,14 +126,14 @@ impl<'a> SessionDiagnosticDerive<'a> {
                     (Some((SessionDiagnosticKind::Error, _)), Some((slug, _))) => {
                         quote! {
                             let mut #diag = #sess.struct_err(
-                                rustc_errors::DiagnosticMessage::fluent(#slug),
+                                rustc_errors::DiagnosticMessage::new(#slug),
                             );
                         }
                     }
                     (Some((SessionDiagnosticKind::Warn, _)), Some((slug, _))) => {
                         quote! {
                             let mut #diag = #sess.struct_warn(
-                                rustc_errors::DiagnosticMessage::fluent(#slug),
+                                rustc_errors::DiagnosticMessage::new(#slug),
                             );
                         }
                     }
@@ -254,21 +254,6 @@ impl SessionDiagnosticDeriveBuilder {
 
         if matches!(name, "help" | "note") && matches!(meta, Meta::Path(_) | Meta::NameValue(_)) {
             let diag = &self.diag;
-            let slug = match &self.slug {
-                Some((slug, _)) => slug.as_str(),
-                None => throw_span_err!(
-                    span,
-                    &format!(
-                        "`#[{}{}]` must come after `#[error(..)]` or `#[warn(..)]`",
-                        name,
-                        match meta {
-                            Meta::Path(_) => "",
-                            Meta::NameValue(_) => " = ...",
-                            _ => unreachable!(),
-                        }
-                    )
-                ),
-            };
             let id = match meta {
                 Meta::Path(..) => quote! { #name },
                 Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
@@ -279,7 +264,7 @@ impl SessionDiagnosticDeriveBuilder {
             let fn_name = proc_macro2::Ident::new(name, attr.span());
 
             return Ok(quote! {
-                #diag.#fn_name(rustc_errors::DiagnosticMessage::fluent_attr(#slug, #id));
+                #diag.#fn_name(rustc_errors::SubdiagnosticMessage::attr(#id));
             });
         }
 
@@ -525,13 +510,8 @@ impl SessionDiagnosticDeriveBuilder {
 
                 let method = format_ident!("span_{}", name);
 
-                let slug = self
-                    .slug
-                    .as_ref()
-                    .map(|(slug, _)| slug.as_str())
-                    .unwrap_or_else(|| "missing-slug");
                 let msg = msg.as_deref().unwrap_or("suggestion");
-                let msg = quote! { rustc_errors::DiagnosticMessage::fluent_attr(#slug, #msg) };
+                let msg = quote! { rustc_errors::SubdiagnosticMessage::attr(#msg) };
                 let code = code.unwrap_or_else(|| quote! { String::new() });
 
                 Ok(quote! { #diag.#method(#span_field, #msg, #code, #applicability); })
@@ -549,14 +529,11 @@ impl SessionDiagnosticDeriveBuilder {
         fluent_attr_identifier: &str,
     ) -> TokenStream {
         let diag = &self.diag;
-
-        let slug =
-            self.slug.as_ref().map(|(slug, _)| slug.as_str()).unwrap_or_else(|| "missing-slug");
         let fn_name = format_ident!("span_{}", kind);
         quote! {
             #diag.#fn_name(
                 #field_binding,
-                rustc_errors::DiagnosticMessage::fluent_attr(#slug, #fluent_attr_identifier)
+                rustc_errors::SubdiagnosticMessage::attr(#fluent_attr_identifier)
             );
         }
     }
@@ -565,9 +542,8 @@ impl SessionDiagnosticDeriveBuilder {
     /// and `fluent_attr_identifier`.
     fn add_subdiagnostic(&self, kind: &Ident, fluent_attr_identifier: &str) -> TokenStream {
         let diag = &self.diag;
-        let slug = self.slug.as_ref().map(|(slug, _)| slug.as_str()).unwrap_or("missing-slug");
         quote! {
-            #diag.#kind(rustc_errors::DiagnosticMessage::fluent_attr(#slug, #fluent_attr_identifier));
+            #diag.#kind(rustc_errors::SubdiagnosticMessage::attr(#fluent_attr_identifier));
         }
     }
 
diff --git a/compiler/rustc_macros/src/diagnostics/fluent.rs b/compiler/rustc_macros/src/diagnostics/fluent.rs
index 8523d7fa9f9..42a9bf477a4 100644
--- a/compiler/rustc_macros/src/diagnostics/fluent.rs
+++ b/compiler/rustc_macros/src/diagnostics/fluent.rs
@@ -11,7 +11,7 @@ use proc_macro::{Diagnostic, Level, Span};
 use proc_macro2::TokenStream;
 use quote::quote;
 use std::{
-    collections::HashMap,
+    collections::{HashMap, HashSet},
     fs::File,
     io::Read,
     path::{Path, PathBuf},
@@ -100,6 +100,10 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
         let ident_span = res.ident.span().unwrap();
         let path_span = res.resource.span().unwrap();
 
+        // Set of Fluent attribute names already output, to avoid duplicate type errors - any given
+        // constant created for a given attribute is the same.
+        let mut previous_attrs = HashSet::new();
+
         let relative_ftl_path = res.resource.value();
         let absolute_ftl_path =
             invocation_relative_path_to_absolute(ident_span, &relative_ftl_path);
@@ -199,13 +203,15 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
                 });
 
                 for Attribute { id: Identifier { name: attr_name }, .. } in attributes {
-                    let attr_snake_name = attr_name.replace("-", "_");
-                    let snake_name = Ident::new(&format!("{snake_name}_{attr_snake_name}"), span);
+                    let snake_name = Ident::new(&attr_name.replace("-", "_"), span);
+                    if !previous_attrs.insert(snake_name.clone()) {
+                        continue;
+                    }
+
                     constants.extend(quote! {
-                        pub const #snake_name: crate::DiagnosticMessage =
-                            crate::DiagnosticMessage::FluentIdentifier(
-                                std::borrow::Cow::Borrowed(#name),
-                                Some(std::borrow::Cow::Borrowed(#attr_name))
+                        pub const #snake_name: crate::SubdiagnosticMessage =
+                            crate::SubdiagnosticMessage::FluentAttr(
+                                std::borrow::Cow::Borrowed(#attr_name)
                             );
                     });
                 }
diff --git a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
index df01419c82a..9aeb484bfd5 100644
--- a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
@@ -397,7 +397,7 @@ impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
 
         let diag = &self.diag;
         let name = format_ident!("{}{}", if span_field.is_some() { "span_" } else { "" }, kind);
-        let message = quote! { rustc_errors::DiagnosticMessage::fluent(#slug) };
+        let message = quote! { rustc_errors::SubdiagnosticMessage::message(#slug) };
         let call = if matches!(kind, SubdiagnosticKind::Suggestion(..)) {
             if let Some(span) = span_field {
                 quote! { #diag.#name(#span, #message, #code, #applicability); }
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index c9da58aae5c..12302315e90 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -17,10 +17,10 @@ use rustc_ast::{
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{pluralize, struct_span_err, Diagnostic, EmissionGuarantee, ErrorGuaranteed};
 use rustc_errors::{
-    Applicability, DiagnosticBuilder, DiagnosticMessage, Handler, MultiSpan, PResult,
+    fluent, Applicability, DiagnosticBuilder, DiagnosticMessage, Handler, MultiSpan, PResult,
 };
+use rustc_errors::{pluralize, struct_span_err, Diagnostic, EmissionGuarantee, ErrorGuaranteed};
 use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, Ident};
@@ -658,13 +658,10 @@ impl<'a> Parser<'a> {
                     err.delay_as_bug();
                     self.struct_span_err(
                         expr.span,
-                        DiagnosticMessage::fluent("parser-struct-literal-body-without-path"),
+                        fluent::parser::struct_literal_body_without_path,
                     )
                     .multipart_suggestion(
-                        DiagnosticMessage::fluent_attr(
-                            "parser-struct-literal-body-without-path",
-                            "suggestion",
-                        ),
+                        fluent::parser::suggestion,
                         vec![
                             (expr.span.shrink_to_lo(), "{ SomeStruct ".to_string()),
                             (expr.span.shrink_to_hi(), " }".to_string()),
diff --git a/compiler/rustc_typeck/src/errors.rs b/compiler/rustc_typeck/src/errors.rs
index d9c9f2920b0..a7f736fed14 100644
--- a/compiler/rustc_typeck/src/errors.rs
+++ b/compiler/rustc_typeck/src/errors.rs
@@ -277,7 +277,7 @@ impl<'a> SessionDiagnostic<'a> for MissingTypeParams {
                 .join(", "),
         );
 
-        err.span_label(self.def_span, rustc_errors::fluent::typeck::missing_type_params_label);
+        err.span_label(self.def_span, rustc_errors::fluent::typeck::label);
 
         let mut suggested = false;
         if let (Ok(snippet), true) = (
@@ -295,7 +295,7 @@ impl<'a> SessionDiagnostic<'a> for MissingTypeParams {
                 // least we can clue them to the correct syntax `Iterator<Type>`.
                 err.span_suggestion(
                     self.span,
-                    rustc_errors::fluent::typeck::missing_type_params_suggestion,
+                    rustc_errors::fluent::typeck::suggestion,
                     format!("{}<{}>", snippet, self.missing_type_params.join(", ")),
                     Applicability::HasPlaceholders,
                 );
@@ -303,13 +303,10 @@ impl<'a> SessionDiagnostic<'a> for MissingTypeParams {
             }
         }
         if !suggested {
-            err.span_label(
-                self.span,
-                rustc_errors::fluent::typeck::missing_type_params_no_suggestion_label,
-            );
+            err.span_label(self.span, rustc_errors::fluent::typeck::no_suggestion_label);
         }
 
-        err.note(rustc_errors::fluent::typeck::missing_type_params_note);
+        err.note(rustc_errors::fluent::typeck::note);
         err
     }
 }
diff --git a/src/test/ui-fulldeps/fluent-messages/test.rs b/src/test/ui-fulldeps/fluent-messages/test.rs
index b05d3d08ccb..0390a07850b 100644
--- a/src/test/ui-fulldeps/fluent-messages/test.rs
+++ b/src/test/ui-fulldeps/fluent-messages/test.rs
@@ -12,6 +12,12 @@ pub enum DiagnosticMessage {
     FluentIdentifier(std::borrow::Cow<'static, str>, Option<std::borrow::Cow<'static, str>>),
 }
 
+/// Copy of the relevant `SubdiagnosticMessage` variant constructed by `fluent_messages` as it
+/// expects `crate::SubdiagnosticMessage` to exist.
+pub enum SubdiagnosticMessage {
+    FluentAttr(std::borrow::Cow<'static, str>),
+}
+
 mod missing_absolute {
     use super::fluent_messages;
 
diff --git a/src/test/ui-fulldeps/fluent-messages/test.stderr b/src/test/ui-fulldeps/fluent-messages/test.stderr
index f88d09bee6e..526bca43f69 100644
--- a/src/test/ui-fulldeps/fluent-messages/test.stderr
+++ b/src/test/ui-fulldeps/fluent-messages/test.stderr
@@ -1,5 +1,5 @@
 error: could not open Fluent resource
-  --> $DIR/test.rs:19:29
+  --> $DIR/test.rs:25:29
    |
 LL |         missing_absolute => "/definitely_does_not_exist.ftl",
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL |         missing_absolute => "/definitely_does_not_exist.ftl",
    = note: os-specific message
 
 error: could not open Fluent resource
-  --> $DIR/test.rs:28:29
+  --> $DIR/test.rs:34:29
    |
 LL |         missing_relative => "../definitely_does_not_exist.ftl",
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -15,7 +15,7 @@ LL |         missing_relative => "../definitely_does_not_exist.ftl",
    = note: os-specific message
 
 error: could not parse Fluent resource
-  --> $DIR/test.rs:37:28
+  --> $DIR/test.rs:43:28
    |
 LL |         missing_message => "./missing-message.ftl",
    |                            ^^^^^^^^^^^^^^^^^^^^^^^
@@ -30,13 +30,13 @@ error: expected a message field for "missing-message"
   |
 
 error: overrides existing message: `key`
-  --> $DIR/test.rs:47:9
+  --> $DIR/test.rs:53:9
    |
 LL |         b => "./duplicate-b.ftl",
    |         ^
    |
 help: previously defined in this resource
-  --> $DIR/test.rs:46:9
+  --> $DIR/test.rs:52:9
    |
 LL |         a => "./duplicate-a.ftl",
    |         ^
diff --git a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
index 1cdc5d18c28..84d5de17309 100644
--- a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
+++ b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs
@@ -404,7 +404,6 @@ struct ErrorWithHelpCustom {
 
 #[derive(SessionDiagnostic)]
 #[help]
-//~^ ERROR `#[help]` must come after `#[error(..)]` or `#[warn(..)]`
 #[error(code = "E0123", slug = "foo")]
 struct ErrorWithHelpWrongOrder {
     val: String,
@@ -412,7 +411,6 @@ struct ErrorWithHelpWrongOrder {
 
 #[derive(SessionDiagnostic)]
 #[help = "bar"]
-//~^ ERROR `#[help = ...]` must come after `#[error(..)]` or `#[warn(..)]`
 #[error(code = "E0123", slug = "foo")]
 struct ErrorWithHelpCustomWrongOrder {
     val: String,
@@ -420,7 +418,6 @@ struct ErrorWithHelpCustomWrongOrder {
 
 #[derive(SessionDiagnostic)]
 #[note]
-//~^ ERROR `#[note]` must come after `#[error(..)]` or `#[warn(..)]`
 #[error(code = "E0123", slug = "foo")]
 struct ErrorWithNoteWrongOrder {
     val: String,
@@ -428,7 +425,6 @@ struct ErrorWithNoteWrongOrder {
 
 #[derive(SessionDiagnostic)]
 #[note = "bar"]
-//~^ ERROR `#[note = ...]` must come after `#[error(..)]` or `#[warn(..)]`
 #[error(code = "E0123", slug = "foo")]
 struct ErrorWithNoteCustomWrongOrder {
     val: String,
diff --git a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
index 2583363120a..85ea44ec278 100644
--- a/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
+++ b/src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr
@@ -301,38 +301,14 @@ LL |     #[label("bar")]
    |
    = help: only `suggestion{,_short,_hidden,_verbose}` are valid field attributes
 
-error: `#[help]` must come after `#[error(..)]` or `#[warn(..)]`
-  --> $DIR/diagnostic-derive.rs:406:1
-   |
-LL | #[help]
-   | ^^^^^^^
-
-error: `#[help = ...]` must come after `#[error(..)]` or `#[warn(..)]`
-  --> $DIR/diagnostic-derive.rs:414:1
-   |
-LL | #[help = "bar"]
-   | ^^^^^^^^^^^^^^^
-
-error: `#[note]` must come after `#[error(..)]` or `#[warn(..)]`
-  --> $DIR/diagnostic-derive.rs:422:1
-   |
-LL | #[note]
-   | ^^^^^^^
-
-error: `#[note = ...]` must come after `#[error(..)]` or `#[warn(..)]`
-  --> $DIR/diagnostic-derive.rs:430:1
-   |
-LL | #[note = "bar"]
-   | ^^^^^^^^^^^^^^^
-
 error: applicability cannot be set in both the field and attribute
-  --> $DIR/diagnostic-derive.rs:440:49
+  --> $DIR/diagnostic-derive.rs:436:49
    |
 LL |     #[suggestion(message = "bar", code = "...", applicability = "maybe-incorrect")]
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: invalid applicability
-  --> $DIR/diagnostic-derive.rs:448:49
+  --> $DIR/diagnostic-derive.rs:444:49
    |
 LL |     #[suggestion(message = "bar", code = "...", applicability = "batman")]
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -363,12 +339,12 @@ LL | #[derive(SessionDiagnostic)]
              rustc_middle::ty::Ty<'tcx>
              usize
 note: required by a bound in `DiagnosticBuilder::<'a, G>::set_arg`
-  --> $COMPILER_DIR/rustc_errors/src/diagnostic_builder.rs:531:19
+  --> $COMPILER_DIR/rustc_errors/src/diagnostic_builder.rs:538:19
    |
 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 43 previous errors
+error: aborting due to 39 previous errors
 
 For more information about this error, try `rustc --explain E0277`.