Rewrite the untranslatable_diagnostic lint.

Currently it only checks calls to functions marked with
`#[rustc_lint_diagnostics]`. This commit changes it to check calls to
any function with a `impl Into<{D,Subd}iagnosticMessage>` parameter.
This greatly improves its coverage and doesn't rely on people
remembering to add `#[rustc_lint_diagnostics]`.

The commit also adds `#[allow(rustc::untranslatable_diagnostic)`]
attributes to places that need it that are caught by the improved lint.
These places that might be easy to convert to translatable diagnostics.

Finally, it also:
- Expands and corrects some comments.
- Does some minor formatting improvements.
- Adds missing `DecorateLint` cases to
  `tests/ui-fulldeps/internal-lints/diagnostics.rs`.

compiler/rustc_ast_lowering/src/asm.rs

@@ -22,6 +22,7 @@ use std::collections::hash_map::Entry;
 use std::fmt::Write;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     pub(crate) fn lower_inline_asm(
         &mut self,
         sp: Span,

compiler/rustc_ast_lowering/src/expr.rs

@@ -1499,6 +1499,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
         let is_async_gen = match self.coroutine_kind {
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)) => false,

compiler/rustc_ast_lowering/src/lib.rs

@@ -2272,6 +2272,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.expr_block(block)
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen {
         match c.value.kind {
             ExprKind::Underscore => {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -17,6 +17,7 @@ use crate::errors;
 macro_rules! gate {
     ($visitor:expr, $feature:ident, $span:expr, $explain:expr) => {{
         if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
+            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
             feature_err(&$visitor.sess, sym::$feature, $span, $explain).emit();
         }
     }};
@@ -34,6 +35,7 @@ macro_rules! gate {
 macro_rules! gate_alt {
     ($visitor:expr, $has_feature:expr, $name:expr, $span:expr, $explain:expr) => {{
         if !$has_feature && !$span.allows_unstable($name) {
+            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
             feature_err(&$visitor.sess, $name, $span, $explain).emit();
         }
     }};
@@ -73,6 +75,7 @@ struct PostExpansionVisitor<'a> {
 }
 
 impl<'a> PostExpansionVisitor<'a> {
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
         let ast::StrLit { symbol_unescaped, span, .. } = abi;
 
@@ -579,6 +582,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
                 if let Ok(snippet) = sm.span_to_snippet(span)
                     && snippet == "!"
                 {
+                    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
                     feature_err(sess, sym::never_patterns, span, "`!` patterns are experimental")
                         .emit();
                 } else {

compiler/rustc_attr/src/builtin.rs

@@ -516,6 +516,7 @@ pub struct Condition {
 }
 
 /// Tests if a cfg-pattern matches the cfg set
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn cfg_matches(
     cfg: &ast::MetaItem,
     sess: &Session,
@@ -566,6 +567,7 @@ fn try_gate_cfg(name: Symbol, span: Span, sess: &Session, features: Option<&Feat
     }
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn gate_cfg(gated_cfg: &GatedCfg, cfg_span: Span, sess: &Session, features: &Features) {
     let (cfg, feature, has_feature) = gated_cfg;
     if !has_feature(features) && !cfg_span.allows_unstable(*feature) {
@@ -592,6 +594,7 @@ fn parse_version(s: Symbol) -> Option<RustcVersion> {
 
 /// Evaluate a cfg-like condition (with `any` and `all`), using `eval` to
 /// evaluate individual items.
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn eval_condition(
     cfg: &ast::MetaItem,
     sess: &Session,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -1008,6 +1008,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         self.borrow_spans(span, borrow.reserve_location)
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn explain_captures(
         &mut self,
         err: &mut DiagnosticBuilder<'_>,

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -201,6 +201,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     // For generic associated types (GATs) which implied 'static requirement
     // from higher-ranked trait bounds (HRTB). Try to locate span of the trait
     // and the span which bounded to the trait for adding 'static lifetime suggestion
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn suggest_static_lifetime_for_gat_from_hrtb(
         &self,
         diag: &mut DiagnosticBuilder<'_>,
@@ -825,6 +826,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + 'a {
     ///    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     /// ```
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn add_static_impl_trait_suggestion(
         &self,
         diag: &mut DiagnosticBuilder<'_>,
@@ -975,6 +977,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         self.suggest_constrain_dyn_trait_in_impl(diag, &visitor.0, ident, self_ty);
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     #[instrument(skip(self, err), level = "debug")]
     fn suggest_constrain_dyn_trait_in_impl(
         &self,
@@ -1042,6 +1045,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         suggest_adding_lifetime_params(self.infcx.tcx, sub, ty_sup, ty_sub, diag);
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn suggest_move_on_borrowing_closure(&self, diag: &mut DiagnosticBuilder<'_>) {
         let map = self.infcx.tcx.hir();
         let body_id = map.body_owned_by(self.mir_def_id());

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -62,6 +62,7 @@ impl<'tcx> NonConstOp<'tcx> for FloatingPointOp {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         feature_err(
             &ccx.tcx.sess,
@@ -556,6 +557,7 @@ impl<'tcx> NonConstOp<'tcx> for RawMutPtrDeref {
         Status::Unstable(sym::const_mut_refs)
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         feature_err(
             &ccx.tcx.sess,
@@ -589,6 +591,7 @@ impl<'tcx> NonConstOp<'tcx> for StaticAccess {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         let mut err = feature_err(
             &ccx.tcx.sess,
@@ -634,6 +637,7 @@ pub mod ty {
             }
         }
 
+        #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
             feature_err(
                 &ccx.tcx.sess,

compiler/rustc_driver_impl/src/args.rs

@@ -97,6 +97,7 @@ impl Expander {
 /// **Note:** This function doesn't interpret argument 0 in any special way.
 /// If this function is intended to be used with command line arguments,
 /// `argv[0]` must be removed prior to calling it manually.
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn arg_expand_all(early_dcx: &EarlyDiagCtxt, at_args: &[String]) -> Vec<String> {
     let mut expander = Expander::default();
     for arg in at_args {

compiler/rustc_driver_impl/src/lib.rs

@@ -4,6 +4,7 @@
 //!
 //! This API is completely unstable and subject to change.
 
+#![allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]

compiler/rustc_expand/src/base.rs

@@ -1487,6 +1487,8 @@ fn pretty_printing_compatibility_hack(item: &Item, sess: &Session) -> bool {
                             };
 
                             if crate_matches {
+                                // FIXME: make this translatable
+                                #[allow(rustc::untranslatable_diagnostic)]
                                 sess.parse_sess.buffer_lint_with_diagnostic(
                                         PROC_MACRO_BACK_COMPAT,
                                         item.ident.span,

compiler/rustc_expand/src/config.rs

@@ -239,6 +239,7 @@ impl<'a> StripUnconfigured<'a> {
     /// Gives a compiler warning when the `cfg_attr` contains no attributes and
     /// is in the original source file. Gives a compiler error if the syntax of
     /// the attribute is incorrect.
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     pub(crate) fn expand_cfg_attr(&self, attr: &Attribute, recursive: bool) -> Vec<Attribute> {
         let Some((cfg_predicate, expanded_attrs)) =
             rustc_parse::parse_cfg_attr(attr, &self.sess.parse_sess)
@@ -273,6 +274,7 @@ impl<'a> StripUnconfigured<'a> {
         }
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn expand_cfg_attr_item(
         &self,
         attr: &Attribute,
@@ -371,6 +373,7 @@ impl<'a> StripUnconfigured<'a> {
     }
 
     /// If attributes are not allowed on expressions, emit an error for `attr`
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     #[instrument(level = "trace", skip(self))]
     pub(crate) fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
         if self.features.is_some_and(|features| !features.stmt_expr_attributes)
@@ -384,7 +387,6 @@ impl<'a> StripUnconfigured<'a> {
             );
 
             if attr.is_doc_comment() {
-                #[allow(rustc::untranslatable_diagnostic)]
                 err.help("`///` is for documentation comments. For a plain comment, use `//`.");
             }
 

compiler/rustc_expand/src/expand.rs

@@ -788,6 +788,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         })
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn gate_proc_macro_attr_item(&self, span: Span, item: &Annotatable) {
         let kind = match item {
             Annotatable::Item(_)
@@ -830,6 +831,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         }
 
         impl<'ast, 'a> Visitor<'ast> for GateProcMacroInput<'a> {
+            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
             fn visit_item(&mut self, item: &'ast ast::Item) {
                 match &item.kind {
                     ItemKind::Mod(_, mod_kind)
@@ -1694,6 +1696,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
 
     // Detect use of feature-gated or invalid attributes on macro invocations
     // since they will not be detected after macro expansion.
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_attributes(&self, attrs: &[ast::Attribute], call: &ast::MacCall) {
         let features = self.cx.ecfg.features;
         let mut attrs = attrs.iter().peekable();

compiler/rustc_expand/src/lib.rs

@@ -5,6 +5,7 @@
 #![feature(associated_type_defaults)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
+#![feature(lint_reasons)]
 #![feature(macro_metavar_expr)]
 #![feature(map_try_insert)]
 #![feature(proc_macro_diagnostic)]

compiler/rustc_expand/src/proc_macro_server.rs

@@ -517,6 +517,9 @@ impl server::FreeFunctions for Rustc<'_, '_> {
             DiagnosticBuilder::new(&self.sess().dcx, diagnostic.level.to_internal(), message);
         diag.span(MultiSpan::from_spans(diagnostic.spans));
         for child in diagnostic.children {
+            // This message comes from another diagnostic, and we are just reconstructing the
+            // diagnostic, so there's no need for translation.
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.sub(child.level.to_internal(), child.message, MultiSpan::from_spans(child.spans));
         }
         diag.emit();

compiler/rustc_expand/src/tests.rs

@@ -180,6 +180,7 @@ impl<T: Write> Write for Shared<T> {
     }
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // no translation needed for tests
 fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &str) {
     create_default_session_globals_then(|| {
         let (handler, source_map, output) = create_test_handler();
@@ -194,7 +195,6 @@ fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &
             println!("text: {:?}", source_map.span_to_snippet(span));
         }
 
-        #[allow(rustc::untranslatable_diagnostic)]
         handler.span_err(msp, "foo");
 
         assert!(

compiler/rustc_feature/src/builtin_attrs.rs

@@ -650,7 +650,8 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // might not be stable during incremental compilation.
     rustc_attr!(rustc_lint_query_instability, Normal, template!(Word), WarnFollowing, INTERNAL_UNSTABLE),
     // Used by the `rustc::untranslatable_diagnostic` and `rustc::diagnostic_outside_of_impl` lints
-    // to assist in changes to diagnostic APIs.
+    // to assist in changes to diagnostic APIs. Any function with this attribute will be checked by
+    // those lints.
     rustc_attr!(rustc_lint_diagnostics, Normal, template!(Word), WarnFollowing, INTERNAL_UNSTABLE),
     // Used by the `rustc::bad_opt_access` lint to identify `DebuggingOptions` and `CodegenOptions`
     // types (as well as any others in future).

compiler/rustc_interface/src/interface.rs

@@ -318,6 +318,7 @@ pub struct Config {
 
 // JUSTIFICATION: before session exists, only config
 #[allow(rustc::bad_opt_access)]
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Send) -> R {
     trace!("run_compiler");
 

compiler/rustc_interface/src/util.rs

@@ -160,6 +160,7 @@ pub(crate) fn run_in_thread_pool_with_globals<F: FnOnce() -> R + Send, R: Send>(
     })
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn load_backend_from_dylib(early_dcx: &EarlyDiagCtxt, path: &Path) -> MakeBackendFn {
     match unsafe { load_symbol_from_dylib::<MakeBackendFn>(path, "__rustc_codegen_backend") } {
         Ok(backend_sym) => backend_sym,
@@ -227,6 +228,7 @@ fn get_rustc_path_inner(bin_path: &str) -> Option<PathBuf> {
     })
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn get_codegen_sysroot(
     early_dcx: &EarlyDiagCtxt,
     maybe_sysroot: &Option<PathBuf>,
@@ -319,6 +321,7 @@ fn get_codegen_sysroot(
     }
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub(crate) fn check_attr_crate_type(
     sess: &Session,
     attrs: &[ast::Attribute],

compiler/rustc_lint/src/context.rs

@@ -621,12 +621,13 @@ pub trait LintContext {
     /// Note that this function should only be called for [`LintExpectationId`]s
     /// retrieved from the current lint pass. Buffered or manually created ids can
     /// cause ICEs.
-    #[rustc_lint_diagnostics]
     fn fulfill_expectation(&self, expectation: LintExpectationId) {
         // We need to make sure that submitted expectation ids are correctly fulfilled suppressed
         // and stored between compilation sessions. To not manually do these steps, we simply create
-        // a dummy diagnostic and emit is as usual, which will be suppressed and stored like a normal
-        // expected lint diagnostic.
+        // a dummy diagnostic and emit it as usual, which will be suppressed and stored like a
+        // normal expected lint diagnostic.
+        #[allow(rustc::diagnostic_outside_of_impl)]
+        #[allow(rustc::untranslatable_diagnostic)]
         self.sess()
             .dcx()
             .struct_expect(