Add hard error for extern without explicit ABIs

Author: obeis

compiler/rustc_ast_passes/messages.ftl

@@ -79,6 +79,8 @@ 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
+
 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

@@ -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.handle_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 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)
@@ -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.handle_missing_abi(*extern_span, item.id);
                     }
                     visit::walk_item(this, item);
                     this.extern_mod = old_item;
@@ -1357,7 +1359,7 @@ 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 +823,10 @@ pub(crate) struct DuplicatePreciseCapturing {
     #[label]
     pub bound2: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_without_abi)]
+pub(crate) struct MissingAbi {
+    #[primary_span]
+    pub span: Span,
+}

tests/ui/extern/explicit_extern_abis.rs

@@ -0,0 +1,8 @@
+//@ compile-flags: -Z unstable-options --edition future
+
+#![feature(explicit_extern_abis)]
+
+extern fn foo() {}
+//~^ ERROR extern declarations without an explicit ABI are disallowed
+
+fn main() {}

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:5:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/feature-gates/feature-gate-explicit-extern-abis.rs

@@ -1,6 +1,6 @@
-#![feature(explicit_extern_abis)]
+//@ run-pass
 
-extern fn foo() {}
-//~^ ERROR extern declarations without an explicit ABI are disallowed
+extern fn _foo() {}
+//~^ WARN extern declarations without an explicit ABI are deprecated
 
 fn main() {}

tests/ui/feature-gates/feature-gate-explicit-extern-abis.stderr

@@ -1,8 +1,10 @@
-error: extern declarations without an explicit ABI are disallowed
+warning: extern declarations without an explicit ABI are deprecated
   --> $DIR/feature-gate-explicit-extern-abis.rs:3:1
    |
-LL | extern fn foo() {}
-   | ^^^^^^
+LL | extern fn _foo() {}
+   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
+   |
+   = note: `#[warn(missing_abi)]` on by default
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted