Add hard error for explicit ABIs in extern

Author: obeis

Diff for: compiler/rustc_ast_passes/messages.ftl

@@ -164,6 +164,8 @@ ast_passes_match_arm_with_no_body =
     `match` arm with no body
     .suggestion = add a body after the pattern
 
+ast_passes_missing_extern_abi = extern declarations without an explicit ABI are disallowed
+
 ast_passes_missing_unsafe_on_extern = extern blocks must be unsafe
 
 ast_passes_module_nonascii = trying to load file for module `{$name}` with non-ascii identifier name

Diff for: compiler/rustc_ast_passes/src/ast_validation.rs

@@ -677,7 +677,7 @@ impl<'a> AstValidator<'a> {
                     self.dcx().emit_err(errors::PatternFnPointer { span });
                 });
                 if let Extern::Implicit(extern_span) = bfty.ext {
-                    self.maybe_lint_missing_abi(extern_span, ty.id);
+                    self.maybe_missing_abi(extern_span, ty.id);
                 }
             }
             TyKind::TraitObject(bounds, ..) => {
@@ -710,10 +710,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+    fn maybe_missing_abi(&mut self, span: Span, id: NodeId) {
         // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
         // call site which do not have a macro backtrace. See #61963.
-        if self
+        if span.edition().at_least_rust_2027() || self.features.explicit_extern_abis() {
+            self.dcx().emit_err(errors::MissingExternABI { span });
+        } else if self
             .sess
             .source_map()
             .span_to_snippet(span)
@@ -976,7 +978,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
 
                     if abi.is_none() {
-                        this.maybe_lint_missing_abi(*extern_span, item.id);
+                        this.maybe_missing_abi(*extern_span, item.id);
                     }
                     visit::walk_item(this, item);
                     this.extern_mod = old_item;
@@ -1356,7 +1358,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             _,
         ) = fk
         {
-            self.maybe_lint_missing_abi(*extern_span, id);
+            self.maybe_missing_abi(*extern_span, id);
         }
 
         // Functions without bodies cannot have patterns.

Diff for: compiler/rustc_ast_passes/src/errors.rs

@@ -824,3 +824,10 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_missing_extern_abi)]
+pub(crate) struct MissingExternABI {
+    #[primary_span]
+    pub span: Span,
+}

Diff for: tests/ui/extern/explicit_extern_abis.rs

@@ -0,0 +1,6 @@
+//@ compile-flags: -Z unstable-options --edition 2027
+
+extern fn foo() {}
+//~^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

Diff for: tests/ui/extern/explicit_extern_abis.stderr

@@ -0,0 +1,8 @@
+error: extern declarations without an explicit ABI are disallowed
+  --> $DIR/explicit_extern_abis.rs:3:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+