WIP: Add lint for String in diagnostic implementation

compiler/rustc_lint/locales/en-US.ftl

@@ -90,6 +90,9 @@ lint_non_existant_doc_keyword = found non-existing keyword `{$keyword}` used in
 lint_diag_out_of_impl =
     diagnostics should only be created in `IntoDiagnostic`/`AddToDiagnostic` impls
 
+lint_string_in_diag = use of String in diagnostic
+    .note = this could indicate incorrectly eagerly converting to a string
+
 lint_untranslatable_diag = diagnostics should be created using translatable messages
 
 lint_bad_opt_access = {$msg}

compiler/rustc_lint/src/internal.rs

@@ -3,7 +3,7 @@
 
 use crate::lints::{
     BadOptAccessDiag, DefaultHashTypesDiag, DiagOutOfImpl, LintPassByHand, NonExistantDocKeyword,
-    QueryInstability, TyQualified, TykindDiag, TykindKind, UntranslatableDiag,
+    QueryInstability, StringInDiagnostic, TyQualified, TykindDiag, TykindKind, UntranslatableDiag,
 };
 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
 use rustc_ast as ast;
@@ -338,7 +338,14 @@ declare_tool_lint! {
     report_in_external_macro: true
 }
 
-declare_lint_pass!(Diagnostics => [ UNTRANSLATABLE_DIAGNOSTIC, DIAGNOSTIC_OUTSIDE_OF_IMPL ]);
+declare_tool_lint! {
+    pub rustc::STRING_IN_DIAGNOSTIC,
+    Warn,
+    "prevent the use of `String` in diagnostics, which could indicate incorrect eager conversion",
+    report_in_external_macro: true
+}
+
+declare_lint_pass!(Diagnostics => [ UNTRANSLATABLE_DIAGNOSTIC, DIAGNOSTIC_OUTSIDE_OF_IMPL, STRING_IN_DIAGNOSTIC ]);
 
 impl LateLintPass<'_> for Diagnostics {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
@@ -395,6 +402,29 @@ impl LateLintPass<'_> for Diagnostics {
     }
 }
 
+impl EarlyLintPass for Diagnostics {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
+        if cx.sess().find_by_name(&item.attrs, sym::diag).is_none() {
+            return;
+        }
+
+        let variants = match &item.kind {
+            ast::ItemKind::Struct(variant, _) => vec![variant],
+            ast::ItemKind::Enum(enum_def, _) => enum_def.variants.iter().map(|v| &v.data).collect(),
+            _ => vec![],
+        };
+
+        for field in variants.iter().flat_map(|d| d.fields()) {
+            if let ast::TyKind::Path(_, path) = &field.ty.kind
+                && let [path] = path.segments.as_ref()
+                && path.ident.name == sym::String
+            {
+                cx.emit_spanned_lint(STRING_IN_DIAGNOSTIC, path.span(), StringInDiagnostic);
+            }
+        }
+    }
+}
+
 declare_tool_lint! {
     pub rustc::BAD_OPT_ACCESS,
     Deny,

compiler/rustc_lint/src/lib.rs

@@ -514,6 +514,7 @@ fn register_internals(store: &mut LintStore) {
     store.register_lints(&TyTyKind::get_lints());
     store.register_late_pass(|_| Box::new(TyTyKind));
     store.register_lints(&Diagnostics::get_lints());
+    store.register_early_pass(|| Box::new(Diagnostics));
     store.register_late_pass(|_| Box::new(Diagnostics));
     store.register_lints(&BadOptAccess::get_lints());
     store.register_late_pass(|_| Box::new(BadOptAccess));

compiler/rustc_lint/src/lints.rs

@@ -800,6 +800,11 @@ pub struct NonExistantDocKeyword {
 #[diag(lint_diag_out_of_impl)]
 pub struct DiagOutOfImpl;
 
+#[derive(LintDiagnostic)]
+#[diag(lint_string_in_diag)]
+#[note]
+pub struct StringInDiagnostic;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_untranslatable_diag)]
 pub struct UntranslatableDiag;

compiler/rustc_span/src/symbol.rs

@@ -608,6 +608,7 @@ symbols! {
         derive_default_enum,
         destruct,
         destructuring_assignment,
+        diag,
         diagnostic,
         direct,
         discriminant_kind,