Add hard error for extern without explicit ABI

Author: obeis

compiler/rustc_ast_passes/messages.ftl

@@ -79,6 +79,9 @@ ast_passes_extern_types_cannot = `type`s inside `extern` blocks cannot have {$de
     .suggestion = remove the {$remove_descr}
     .label = `extern` block begins here
 
+ast_passes_extern_without_abi = extern declarations without an explicit ABI are disallowed
+    .help = in previous editions, a default ABI was inferred, but this is no longer the case, so you must specify one explicitly
+
 ast_passes_feature_on_non_nightly = `#![feature]` may not be used on the {$channel} release channel
     .suggestion = remove the attribute
     .stable_since = the feature `{$name}` has been stable since `{$since}` and no longer requires an attribute to enable

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -684,7 +684,7 @@ 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.handle_missing_abi(extern_span, ty.id);
                 }
             }
             TyKind::TraitObject(bounds, ..) => {
@@ -717,10 +717,12 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+    fn handle_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_edition_future() && self.features.explicit_extern_abis() {
+            self.dcx().emit_err(errors::MissingAbi { span });
+        } else if self
             .sess
             .source_map()
             .span_to_snippet(span)
@@ -996,7 +998,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 }
 
                 if abi.is_none() {
-                    self.maybe_lint_missing_abi(*extern_span, item.id);
+                    self.handle_missing_abi(*extern_span, item.id);
                 }
                 self.with_in_extern_mod(*safety, |this| {
                     visit::walk_item(this, item);
@@ -1370,7 +1372,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
             },
         ) = fk
         {
-            self.maybe_lint_missing_abi(*extern_span, id);
+            self.handle_missing_abi(*extern_span, id);
         }
 
         // Functions without bodies cannot have patterns.

compiler/rustc_ast_passes/src/errors.rs

@@ -823,3 +823,11 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_without_abi)]
+#[help]
+pub(crate) struct MissingAbi {
+    #[primary_span]
+    pub span: Span,
+}