macros: use typed identifiers in diag derive

Using typed identifiers instead of strings with the Fluent identifier
enables the diagnostic derive to benefit from the compile-time
validation that comes with typed identifiers - use of a non-existent
Fluent identifier will not compile.

Signed-off-by: David Wood <david.wood@huawei.com>

4 files changed, 320 insertions, 192 deletions

diff --git a/compiler/rustc_macros/src/diagnostics/diagnostic.rs b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
index 95ee0d4a060..d0c86527189 100644
--- a/compiler/rustc_macros/src/diagnostics/diagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/diagnostic.rs
@@ -12,7 +12,9 @@ use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};
 use std::collections::HashMap;
 use std::str::FromStr;
-use syn::{spanned::Spanned, Attribute, Meta, MetaList, MetaNameValue, Type};
+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.
@@ -118,23 +120,23 @@ impl<'a> SessionDiagnosticDerive<'a> {
                         return SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
                     }
                     (Some((kind, _)), None) => {
-                        span_err(span, "`slug` not specified")
-                            .help(&format!("use the `#[{}(slug = \"...\")]` attribute to set this diagnostic's slug", kind.descr()))
+                        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 SessionDiagnosticDeriveError::ErrorHandled.to_compile_error();
                     }
                     (Some((SessionDiagnosticKind::Error, _)), Some((slug, _))) => {
                         quote! {
-                            let mut #diag = #sess.struct_err(
-                                rustc_errors::DiagnosticMessage::new(#slug),
-                            );
+                            let mut #diag = #sess.struct_err(rustc_errors::fluent::#slug);
                         }
                     }
                     (Some((SessionDiagnosticKind::Warn, _)), Some((slug, _))) => {
                         quote! {
-                            let mut #diag = #sess.struct_warn(
-                                rustc_errors::DiagnosticMessage::new(#slug),
-                            );
+                            let mut #diag = #sess.struct_warn(rustc_errors::fluent::#slug);
                         }
                     }
                 };
@@ -226,7 +228,7 @@ struct SessionDiagnosticDeriveBuilder {
     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<(String, proc_macro::Span)>,
+    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)>,
@@ -240,50 +242,79 @@ impl HasFieldMap for SessionDiagnosticDeriveBuilder {
 
 impl SessionDiagnosticDeriveBuilder {
     /// Establishes state in the `SessionDiagnosticDeriveBuilder` resulting from the struct
-    /// attributes like `#[error(..)#`, such as the diagnostic kind and slug. Generates
+    /// 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()?;
 
-        if matches!(name, "help" | "note") && matches!(meta, Meta::Path(_) | Meta::NameValue(_)) {
-            let diag = &self.diag;
-            let id = match meta {
-                Meta::Path(..) => quote! { #name },
-                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
-                    quote! { #s }
-                }
-                _ => unreachable!(),
-            };
-            let fn_name = proc_macro2::Ident::new(name, attr.span());
-
-            return Ok(quote! {
-                #diag.#fn_name(rustc_errors::SubdiagnosticMessage::attr(#id));
-            });
-        }
+        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),
         };
 
-        let kind = match name {
-            "error" => SessionDiagnosticKind::Error,
-            "warning" => SessionDiagnosticKind::Warn,
+        // 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` and `warning` are valid attributes")
+                diag.help("only `error`, `warning`, `help` and `note` are valid attributes")
             }),
-        };
-        self.kind.set_once((kind, span));
+        }
+
+        // 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 {
+        for nested_attr in nested_iter {
             let meta = match nested_attr {
                 syn::NestedMeta::Meta(meta) => meta,
                 _ => throw_invalid_nested_attr!(attr, &nested_attr),
@@ -291,28 +322,24 @@ impl SessionDiagnosticDeriveBuilder {
 
             let path = meta.path();
             let nested_name = path.segments.last().unwrap().ident.to_string();
-            match &meta {
-                // Struct attributes are only allowed to be applied once, and the diagnostic
-                // changes will be set in the initialisation code.
-                Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(s), .. }) => {
-                    let span = s.span().unwrap();
-                    match nested_name.as_str() {
-                        "slug" => {
-                            self.slug.set_once((s.value(), span));
-                        }
-                        "code" => {
-                            self.code.set_once((s.value(), span));
-                            let (diag, code) = (&self.diag, &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 `slug` and `code` are valid nested attributes")
-                            .emit(),
+            // 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(),
                 }
-                _ => invalid_nested_attr(attr, &nested_attr).emit(),
+            } else {
+                invalid_nested_attr(attr, &nested_attr).emit()
             }
         }
 
@@ -382,142 +409,215 @@ impl SessionDiagnosticDeriveBuilder {
         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()?;
-        match meta {
-            Meta::Path(_) => 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, name))
-                }
-                "note" | "help" => {
-                    if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
-                        Ok(self.add_spanned_subdiagnostic(binding, ident, name))
-                    } else if type_is_unit(&info.ty) {
-                        Ok(self.add_subdiagnostic(ident, name))
-                    } 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")
-                }),
-            },
-            Meta::NameValue(MetaNameValue { lit: syn::Lit::Str(ref s), .. }) => match name {
-                "label" => {
-                    report_error_if_not_applied_to_span(attr, &info)?;
-                    Ok(self.add_spanned_subdiagnostic(binding, ident, &s.value()))
-                }
-                "note" | "help" => {
-                    if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
-                        Ok(self.add_spanned_subdiagnostic(binding, ident, &s.value()))
-                    } else if type_is_unit(&info.ty) {
-                        Ok(self.add_subdiagnostic(ident, &s.value()))
-                    } else {
-                        report_type_error(attr, "`Span` or `()`")?;
-                    }
-                }
-                _ => throw_invalid_attr!(attr, &meta, |diag| {
-                    diag.help("only `label`, `note` and `help` are valid field attributes")
-                }),
-            },
-            Meta::List(MetaList { ref path, ref nested, .. }) => {
-                let name = path.segments.last().unwrap().ident.to_string();
-                let name = name.as_ref();
-
-                match name {
-                    "suggestion" | "suggestion_short" | "suggestion_hidden"
-                    | "suggestion_verbose" => (),
-                    _ => throw_invalid_attr!(attr, &meta, |diag| {
-                        diag
-                            .help("only `suggestion{,_short,_hidden,_verbose}` are valid field attributes")
-                    }),
-                };
+        let Meta::List(MetaList { ref path, ref nested, .. }) = meta  else { unreachable!() };
 
-                let (span_field, mut applicability) = self.span_and_applicability_of_ty(info)?;
+        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",
+                )
+            }),
+        }
 
-                let mut msg = None;
-                let mut code = None;
+        // 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),
+        };
 
-                for nested_attr in nested {
-                    let meta = match nested_attr {
-                        syn::NestedMeta::Meta(ref meta) => meta,
-                        syn::NestedMeta::Lit(_) => throw_invalid_nested_attr!(attr, &nested_attr),
-                    };
+        // None of these attributes should have anything following the slug.
+        if nested_iter.next().is_some() {
+            throw_invalid_attr!(attr, &meta);
+        }
 
-                    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 {
-                                "message" => {
-                                    msg = Some(s.value());
-                                }
-                                "code" => {
-                                    let formatted_str = self.build_format(&s.value(), s.span());
-                                    code = Some(formatted_str);
+        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,
+                                        "applicability cannot be set in both the field and \
+                                         attribute",
+                                    )
+                                    .emit();
+                                    Some(v)
                                 }
-                                "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
-                                            }
-                                        },
+                                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),
+                        _ => 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 applicability =
+            applicability.unwrap_or_else(|| quote!(rustc_errors::Applicability::Unspecified));
 
-                let method = format_ident!("span_{}", name);
+        let name = path.segments.last().unwrap().ident.to_string();
+        let method = format_ident!("span_{}", name);
 
-                let msg = msg.as_deref().unwrap_or("suggestion");
-                let msg = quote! { rustc_errors::SubdiagnosticMessage::attr(#msg) };
-                let code = code.unwrap_or_else(|| quote! { String::new() });
+        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); })
-            }
-            _ => throw_invalid_attr!(attr, &meta),
-        }
+        Ok(quote! { #diag.#method(#span_field, #msg, #code, #applicability); })
     }
 
     /// Adds a spanned subdiagnostic by generating a `diag.span_$kind` call with the current slug
@@ -526,24 +626,24 @@ impl SessionDiagnosticDeriveBuilder {
         &self,
         field_binding: TokenStream,
         kind: &Ident,
-        fluent_attr_identifier: &str,
+        fluent_attr_identifier: Path,
     ) -> TokenStream {
         let diag = &self.diag;
         let fn_name = format_ident!("span_{}", kind);
         quote! {
             #diag.#fn_name(
                 #field_binding,
-                rustc_errors::SubdiagnosticMessage::attr(#fluent_attr_identifier)
+                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: &str) -> TokenStream {
+    fn add_subdiagnostic(&self, kind: &Ident, fluent_attr_identifier: Path) -> TokenStream {
         let diag = &self.diag;
         quote! {
-            #diag.#kind(rustc_errors::SubdiagnosticMessage::attr(#fluent_attr_identifier));
+            #diag.#kind(rustc_errors::fluent::#fluent_attr_identifier);
         }
     }
 
@@ -569,7 +669,8 @@ impl SessionDiagnosticDeriveBuilder {
                         } else {
                             throw_span_err!(
                                 info.span.unwrap(),
-                                "type of field annotated with `#[suggestion(...)]` contains more than one `Span`"
+                                "type of field annotated with `#[suggestion(...)]` contains more \
+                                 than one `Span`"
                             );
                         }
                     } else if type_matches_path(elem, &["rustc_errors", "Applicability"]) {
@@ -578,7 +679,8 @@ impl SessionDiagnosticDeriveBuilder {
                         } else {
                             throw_span_err!(
                                 info.span.unwrap(),
-                                "type of field annotated with `#[suggestion(...)]` contains more than one Applicability"
+                                "type of field annotated with `#[suggestion(...)]` contains more \
+                                 than one Applicability"
                             );
                         }
                     }
@@ -595,12 +697,18 @@ impl SessionDiagnosticDeriveBuilder {
                 }
 
                 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)`")
+                    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)`")
+                diag.help(
+                    "`#[suggestion(...)]` should be applied to fields of type `Span` or \
+                     `(Span, Applicability)`",
+                )
             }),
         }
     }
diff --git a/compiler/rustc_macros/src/diagnostics/error.rs b/compiler/rustc_macros/src/diagnostics/error.rs
index fd1dc2f3073..d088402abc6 100644
--- a/compiler/rustc_macros/src/diagnostics/error.rs
+++ b/compiler/rustc_macros/src/diagnostics/error.rs
@@ -39,6 +39,19 @@ pub(crate) fn _throw_err(
     SessionDiagnosticDeriveError::ErrorHandled
 }
 
+/// Helper function for printing `syn::Path` - doesn't handle arguments in paths and these are
+/// unlikely to come up much in use of the macro.
+fn path_to_string(path: &syn::Path) -> String {
+    let mut out = String::new();
+    for (i, segment) in path.segments.iter().enumerate() {
+        if i > 0 || path.leading_colon.is_some() {
+            out.push_str("::");
+        }
+        out.push_str(&segment.ident.to_string());
+    }
+    out
+}
+
 /// Returns an error diagnostic on span `span` with msg `msg`.
 pub(crate) fn span_err(span: impl MultiSpan, msg: &str) -> Diagnostic {
     Diagnostic::spanned(span, Level::Error, msg)
@@ -61,15 +74,13 @@ pub(crate) use throw_span_err;
 /// Returns an error diagnostic for an invalid attribute.
 pub(crate) fn invalid_attr(attr: &Attribute, meta: &Meta) -> Diagnostic {
     let span = attr.span().unwrap();
-    let name = attr.path.segments.last().unwrap().ident.to_string();
-    let name = name.as_str();
-
+    let path = path_to_string(&attr.path);
     match meta {
-        Meta::Path(_) => span_err(span, &format!("`#[{}]` is not a valid attribute", name)),
+        Meta::Path(_) => span_err(span, &format!("`#[{}]` is not a valid attribute", path)),
         Meta::NameValue(_) => {
-            span_err(span, &format!("`#[{} = ...]` is not a valid attribute", name))
+            span_err(span, &format!("`#[{} = ...]` is not a valid attribute", path))
         }
-        Meta::List(_) => span_err(span, &format!("`#[{}(...)]` is not a valid attribute", name)),
+        Meta::List(_) => span_err(span, &format!("`#[{}(...)]` is not a valid attribute", path)),
     }
 }
 
@@ -101,18 +112,16 @@ pub(crate) fn invalid_nested_attr(attr: &Attribute, nested: &NestedMeta) -> Diag
     };
 
     let span = meta.span().unwrap();
-    let nested_name = meta.path().segments.last().unwrap().ident.to_string();
-    let nested_name = nested_name.as_str();
+    let path = path_to_string(meta.path());
     match meta {
-        Meta::NameValue(..) => span_err(
-            span,
-            &format!("`#[{}({} = ...)]` is not a valid attribute", name, nested_name),
-        ),
+        Meta::NameValue(..) => {
+            span_err(span, &format!("`#[{}({} = ...)]` is not a valid attribute", name, path))
+        }
         Meta::Path(..) => {
-            span_err(span, &format!("`#[{}({})]` is not a valid attribute", name, nested_name))
+            span_err(span, &format!("`#[{}({})]` is not a valid attribute", name, path))
         }
         Meta::List(..) => {
-            span_err(span, &format!("`#[{}({}(...))]` is not a valid attribute", name, nested_name))
+            span_err(span, &format!("`#[{}({}(...))]` is not a valid attribute", name, path))
         }
     }
 }
diff --git a/compiler/rustc_macros/src/diagnostics/fluent.rs b/compiler/rustc_macros/src/diagnostics/fluent.rs
index 42a9bf477a4..2317186e655 100644
--- a/compiler/rustc_macros/src/diagnostics/fluent.rs
+++ b/compiler/rustc_macros/src/diagnostics/fluent.rs
@@ -254,6 +254,17 @@ pub(crate) fn fluent_messages(input: proc_macro::TokenStream) -> proc_macro::Tok
             ];
 
             #generated
+
+            pub mod _subdiag {
+                pub const note: crate::SubdiagnosticMessage =
+                    crate::SubdiagnosticMessage::FluentAttr(std::borrow::Cow::Borrowed("note"));
+                pub const help: crate::SubdiagnosticMessage =
+                    crate::SubdiagnosticMessage::FluentAttr(std::borrow::Cow::Borrowed("help"));
+                pub const label: crate::SubdiagnosticMessage =
+                    crate::SubdiagnosticMessage::FluentAttr(std::borrow::Cow::Borrowed("label"));
+                pub const suggestion: crate::SubdiagnosticMessage =
+                    crate::SubdiagnosticMessage::FluentAttr(std::borrow::Cow::Borrowed("suggestion"));
+            }
         }
     }
     .into()
diff --git a/compiler/rustc_macros/src/diagnostics/mod.rs b/compiler/rustc_macros/src/diagnostics/mod.rs
index 69573d904d4..88182ed5a12 100644
--- a/compiler/rustc_macros/src/diagnostics/mod.rs
+++ b/compiler/rustc_macros/src/diagnostics/mod.rs
@@ -22,14 +22,14 @@ use synstructure::Structure;
 /// # extern crate rust_middle;
 /// # use rustc_middle::ty::Ty;
 /// #[derive(SessionDiagnostic)]
-/// #[error(code = "E0505", slug = "borrowck-move-out-of-borrow")]
+/// #[error(borrowck::move_out_of_borrow, code = "E0505")]
 /// pub struct MoveOutOfBorrowError<'tcx> {
 ///     pub name: Ident,
 ///     pub ty: Ty<'tcx>,
 ///     #[primary_span]
 ///     #[label]
 ///     pub span: Span,
-///     #[label = "first-borrow-label"]
+///     #[label(borrowck::first_borrow_label)]
 ///     pub first_borrow_span: Span,
 ///     #[suggestion(code = "{name}.clone()")]
 ///     pub clone_sugg: Option<(Span, Applicability)>