Permit mutable references in all const contexts

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -268,43 +268,41 @@ impl NonConstOp for CellBorrow {
 }
 
 #[derive(Debug)]
+/// This op is for `&mut` borrows in the trailing expression of a constant
+/// which uses the "enclosing scopes rule" to leak its locals into anonymous
+/// static or const items.
 pub struct MutBorrow(pub hir::BorrowKind);
 
 impl NonConstOp for MutBorrow {
     fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-        // Forbid everywhere except in const fn with a feature gate
-        if ccx.const_kind() == hir::ConstContext::ConstFn {
-            Status::Unstable(sym::const_mut_refs)
-        } else {
-            Status::Forbidden
+        match ccx.const_kind() {
+            // Mutable statics can handle mutable references in their final value
+            hir::ConstContext::Static(hir::Mutability::Mut) => Status::Allowed,
+            _ => Status::Forbidden,
         }
     }
 
+    fn importance(&self) -> DiagnosticImportance {
+        // If there were primary errors (like non-const function calls), do not emit further
+        // errors about mutable references.
+        DiagnosticImportance::Secondary
+    }
+
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         let raw = match self.0 {
             hir::BorrowKind::Raw => "raw ",
             hir::BorrowKind::Ref => "",
         };
 
-        let mut err = if ccx.const_kind() == hir::ConstContext::ConstFn {
-            feature_err(
-                &ccx.tcx.sess.parse_sess,
-                sym::const_mut_refs,
-                span,
-                &format!("{}mutable references are not allowed in {}s", raw, ccx.const_kind()),
-            )
-        } else {
-            let mut err = struct_span_err!(
-                ccx.tcx.sess,
-                span,
-                E0764,
-                "{}mutable references are not allowed in {}s",
-                raw,
-                ccx.const_kind(),
-            );
-            err.span_label(span, format!("`&{}mut` is only allowed in `const fn`", raw));
-            err
-        };
+        let mut err = struct_span_err!(
+            ccx.tcx.sess,
+            span,
+            E0764,
+            "{}mutable references are not allowed in final value of {}s",
+            raw,
+            ccx.const_kind(),
+        );
+
         if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
             err.note(
                 "References in statics and constants may only refer \
@@ -321,6 +319,29 @@ impl NonConstOp for MutBorrow {
     }
 }
 
+#[derive(Debug)]
+pub struct TransientMutBorrow(pub hir::BorrowKind);
+
+impl NonConstOp for TransientMutBorrow {
+    fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+        Status::Unstable(sym::const_mut_refs)
+    }
+
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
+        let raw = match self.0 {
+            hir::BorrowKind::Raw => "raw ",
+            hir::BorrowKind::Ref => "",
+        };
+
+        feature_err(
+            &ccx.tcx.sess.parse_sess,
+            sym::const_mut_refs,
+            span,
+            &format!("{}mutable references are not allowed in {}s", raw, ccx.const_kind()),
+        )
+    }
+}
+
 #[derive(Debug)]
 pub struct MutDeref;
 impl NonConstOp for MutDeref {
@@ -329,7 +350,7 @@ impl NonConstOp for MutDeref {
     }
 
     fn importance(&self) -> DiagnosticImportance {
-        // Usually a side-effect of a `MutBorrow` somewhere.
+        // Usually a side-effect of a `TransientMutBorrow` somewhere.
         DiagnosticImportance::Secondary
     }