compiler: gate `extern "{abi}"` in ast_lowering

By moving this stability check into AST lowering, we effectively make
it impossible to accidentally miss, as it must happen to generate HIR.
Also, we put the ABI-stability code next to code that actually uses it!
This allows code that wants to reason about backend ABI implementations
to stop worrying about high-level concerns like syntax stability,
while still leaving it as the authority on what ABIs actually exist.

It also makes it easy to refactor things to have more consistent errors.
For now, we only apply this to generalize the existing messages a bit.

1 files changed, 5 insertions, 39 deletions

diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 13294efdaf6..e5d8013058f 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -1,9 +1,9 @@
 use rustc_ast as ast;
 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
 use rustc_ast::{NodeId, PatKind, attr, token};
-use rustc_feature::{AttributeGate, BUILTIN_ATTRIBUTE_MAP, BuiltinAttribute, Features, GateIssue};
+use rustc_feature::{AttributeGate, BUILTIN_ATTRIBUTE_MAP, BuiltinAttribute, Features};
 use rustc_session::Session;
-use rustc_session::parse::{feature_err, feature_err_issue, feature_warn};
+use rustc_session::parse::{feature_err, feature_warn};
 use rustc_span::source_map::Spanned;
 use rustc_span::{Span, Symbol, sym};
 use thin_vec::ThinVec;
@@ -72,35 +72,6 @@ struct PostExpansionVisitor<'a> {
 }
 
 impl<'a> PostExpansionVisitor<'a> {
-    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
-    fn check_abi(&self, abi: ast::StrLit) {
-        let ast::StrLit { symbol_unescaped, span, .. } = abi;
-
-        match rustc_abi::is_enabled(self.features, span, symbol_unescaped.as_str()) {
-            Ok(()) => (),
-            Err(rustc_abi::AbiDisabled::Unstable { feature, explain }) => {
-                feature_err_issue(&self.sess, feature, span, GateIssue::Language, explain).emit();
-            }
-            Err(rustc_abi::AbiDisabled::Unrecognized) => {
-                if self.sess.opts.pretty.is_none_or(|ppm| ppm.needs_hir()) {
-                    self.sess.dcx().span_delayed_bug(
-                        span,
-                        format!(
-                            "unrecognized ABI not caught in lowering: {}",
-                            symbol_unescaped.as_str()
-                        ),
-                    );
-                }
-            }
-        }
-    }
-
-    fn check_extern(&self, ext: ast::Extern) {
-        if let ast::Extern::Explicit(abi, _) = ext {
-            self.check_abi(abi);
-        }
-    }
-
     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
     fn check_impl_trait(&self, ty: &ast::Ty, in_associated_ty: bool) {
         struct ImplTraitVisitor<'a> {
@@ -223,12 +194,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_item(&mut self, i: &'a ast::Item) {
         match &i.kind {
-            ast::ItemKind::ForeignMod(foreign_module) => {
-                if let Some(abi) = foreign_module.abi {
-                    self.check_abi(abi);
-                }
+            ast::ItemKind::ForeignMod(_foreign_module) => {
+                // handled during lowering
             }
-
             ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..) => {
                 for attr in attr::filter_by_name(&i.attrs, sym::repr) {
                     for item in attr.meta_item_list().unwrap_or_else(ThinVec::new) {
@@ -315,7 +283,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         match &ty.kind {
             ast::TyKind::BareFn(bare_fn_ty) => {
                 // Function pointers cannot be `const`
-                self.check_extern(bare_fn_ty.ext);
                 self.check_late_bound_lifetime_defs(&bare_fn_ty.generic_params);
             }
             ast::TyKind::Never => {
@@ -418,9 +385,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     }
 
     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
-        if let Some(header) = fn_kind.header() {
+        if let Some(_header) = fn_kind.header() {
             // Stability of const fn methods are covered in `visit_assoc_item` below.
-            self.check_extern(header.ext);
         }
 
         if let FnKind::Closure(ast::ClosureBinder::For { generic_params, .. }, ..) = fn_kind {