Rollup merge of #120933 - RalfJung:const-check-misc, r=oli-obk

check_consts: fix duplicate errors, make importance consistent

This is stuff I noticed while working on https://github.com/rust-lang/rust/pull/120932, but it's orthogonal to that PR.

r? ``@oli-obk``

2 files changed, 12 insertions, 10 deletions

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 28dc69859fd..43048dc41d3 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -619,9 +619,11 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 if base_ty.is_unsafe_ptr() {
                     if place_ref.projection.is_empty() {
                         let decl = &self.body.local_decls[place_ref.local];
-                        if let LocalInfo::StaticRef { def_id, .. } = *decl.local_info() {
-                            let span = decl.source_info.span;
-                            self.check_static(def_id, span);
+                        // If this is a static, then this is not really dereferencing a pointer,
+                        // just directly accessing a static. That is not subject to any feature
+                        // gates (except for the one about whether statics can even be used, but
+                        // that is checked already by `visit_operand`).
+                        if let LocalInfo::StaticRef { .. } = *decl.local_info() {
                             return;
                         }
                     }
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/ops.rs b/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
index a9d472d377c..0c93cfaa546 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/ops.rs
@@ -409,11 +409,6 @@ impl<'tcx> NonConstOp<'tcx> for TransientCellBorrow {
     fn status_in_item(&self, _: &ConstCx<'_, 'tcx>) -> Status {
         Status::Unstable(sym::const_refs_to_cell)
     }
-    fn importance(&self) -> DiagnosticImportance {
-        // The cases that cannot possibly work will already emit a `CellBorrow`, so we should
-        // not additionally emit a feature gate error if activating the feature gate won't work.
-        DiagnosticImportance::Secondary
-    }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         ccx.tcx
             .sess
@@ -427,6 +422,11 @@ impl<'tcx> NonConstOp<'tcx> for TransientCellBorrow {
 /// it in the future for static items.
 pub struct CellBorrow;
 impl<'tcx> NonConstOp<'tcx> for CellBorrow {
+    fn importance(&self) -> DiagnosticImportance {
+        // Most likely the code will try to do mutation with these borrows, which
+        // triggers its own errors. Only show this one if that does not happen.
+        DiagnosticImportance::Secondary
+    }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         // FIXME: Maybe a more elegant solution to this if else case
         if let hir::ConstContext::Static(_) = ccx.const_kind() {
@@ -459,8 +459,8 @@ impl<'tcx> NonConstOp<'tcx> for MutBorrow {
     }
 
     fn importance(&self) -> DiagnosticImportance {
-        // If there were primary errors (like non-const function calls), do not emit further
-        // errors about mutable references.
+        // Most likely the code will try to do mutation with these borrows, which
+        // triggers its own errors. Only show this one if that does not happen.
         DiagnosticImportance::Secondary
     }