Improve the missing_abi lint.

compiler/rustc_ast/src/ast.rs

@@ -2810,6 +2810,8 @@ pub struct ModSpans {
 /// E.g., `extern { .. }` or `extern "C" { .. }`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct ForeignMod {
+    /// Span of the `extern` keyword.
+    pub extern_span: Span,
     /// `unsafe` keyword accepted syntactically for macro DSLs, but not
     /// semantically by Rust.
     pub safety: Safety,

compiler/rustc_ast/src/mut_visit.rs

@@ -525,7 +525,7 @@ pub fn walk_ty<T: MutVisitor>(vis: &mut T, ty: &mut P<Ty>) {
 }
 
 fn walk_foreign_mod<T: MutVisitor>(vis: &mut T, foreign_mod: &mut ForeignMod) {
-    let ForeignMod { safety, abi: _, items } = foreign_mod;
+    let ForeignMod { extern_span: _, safety, abi: _, items } = foreign_mod;
     visit_safety(vis, safety);
     items.flat_map_in_place(|item| vis.flat_map_foreign_item(item));
 }

compiler/rustc_ast/src/visit.rs

@@ -366,7 +366,7 @@ fn walk<'a, V: Visitor<'a>>(
                 }
                 ModKind::Unloaded => {}
             },
-            ItemKind::ForeignMod(ForeignMod { safety: _, abi: _, items }) => {
+            ItemKind::ForeignMod(ForeignMod { extern_span: _, safety: _, abi: _, items }) => {
                 walk_list!(visitor, visit_foreign_item, items);
             }
             ItemKind::GlobalAsm(asm) => try_visit!(visitor.visit_inline_asm(asm)),

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -677,9 +677,8 @@ fn visit_ty_common(&mut self, ty: &'a Ty) {
                 Self::check_decl_no_pat(&bfty.decl, |span, _, _| {
                     self.dcx().emit_err(errors::PatternFnPointer { span });
                 });
-                if let Extern::Implicit(_) = bfty.ext {
-                    let sig_span = self.sess.source_map().next_point(ty.span.shrink_to_lo());
-                    self.maybe_lint_missing_abi(sig_span, ty.id);
+                if let Extern::Implicit(extern_span) = bfty.ext {
+                    self.maybe_lint_missing_abi(extern_span, ty.id);
                 }
             }
             TyKind::TraitObject(bounds, ..) => {
@@ -953,7 +952,7 @@ fn visit_item(&mut self, item: &'a Item) {
                 walk_list!(self, visit_attribute, &item.attrs);
                 return; // Avoid visiting again.
             }
-            ItemKind::ForeignMod(ForeignMod { abi, safety, .. }) => {
+            ItemKind::ForeignMod(ForeignMod { extern_span, abi, safety, .. }) => {
                 self.with_in_extern_mod(*safety, |this| {
                     let old_item = mem::replace(&mut this.extern_mod, Some(item.span));
                     this.visibility_not_permitted(
@@ -977,7 +976,7 @@ fn visit_item(&mut self, item: &'a Item) {
                     }
 
                     if abi.is_none() {
-                        this.maybe_lint_missing_abi(item.span, item.id);
+                        this.maybe_lint_missing_abi(*extern_span, item.id);
                     }
                     visit::walk_item(this, item);
                     this.extern_mod = old_item;
@@ -1350,13 +1349,13 @@ fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
         if let FnKind::Fn(
             _,
             _,
-            FnSig { span: sig_span, header: FnHeader { ext: Extern::Implicit(_), .. }, .. },
+            FnSig { header: FnHeader { ext: Extern::Implicit(extern_span), .. }, .. },
             _,
             _,
             _,
         ) = fk
         {
-            self.maybe_lint_missing_abi(*sig_span, id);
+            self.maybe_lint_missing_abi(*extern_span, id);
         }
 
         // Functions without bodies cannot have patterns.

compiler/rustc_lint/messages.ftl

@@ -268,7 +268,7 @@ lint_extern_crate_not_idiomatic = `extern crate` is not idiomatic in the new edi
 
 lint_extern_without_abi = extern declarations without an explicit ABI are deprecated
     .label = ABI should be specified here
-    .help = the default ABI is {$default_abi}
+    .suggestion = explicitly specify the {$default_abi} ABI
 
 lint_for_loops_over_fallibles =
     for loop over {$article} `{$ref_prefix}{$ty}`. This is more readably written as an `if let` statement

compiler/rustc_lint/src/lints.rs

@@ -2738,11 +2738,9 @@ pub(crate) struct PatternsInFnsWithoutBodySub {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_extern_without_abi)]
-#[help]
 pub(crate) struct MissingAbi {
-    #[label]
+    #[suggestion(code = "extern \"{default_abi}\"", applicability = "machine-applicable")]
     pub span: Span,
-
     pub default_abi: &'static str,
 }
 

compiler/rustc_parse/src/parser/item.rs

@@ -1194,6 +1194,7 @@ fn parse_item_foreign_mod(
         attrs: &mut AttrVec,
         mut safety: Safety,
     ) -> PResult<'a, ItemInfo> {
+        let extern_span = self.prev_token.uninterpolated_span();
         let abi = self.parse_abi(); // ABI?
         // FIXME: This recovery should be tested better.
         if safety == Safety::Default
@@ -1205,6 +1206,7 @@ fn parse_item_foreign_mod(
             let _ = self.eat_keyword(kw::Unsafe);
         }
         let module = ast::ForeignMod {
+            extern_span,
             safety,
             abi,
             items: self.parse_item_list(attrs, |p| p.parse_foreign_item(ForceCollect::No))?,