Auto merge of #119614 - RalfJung:const-refs-to-static, r=oli-obk

unstably allow constants to refer to statics and read from immutable statics

I am not aware of any fundamental reason why we cannot allow constants to mention statics. What we really need is that constants do not *read from* statics that can change their value:
- This would break the principle that "constants behave as-if their expression was inlined everywhere and executed at runtime". This is enforced by halting const-eval interpretation when a read from a mutable global occurs.
- When building a valtree we want to be sure that the constant and everything it refers to is truly immutable. This is enforced by aborting valtree construction when a read from a mutable global occurs.

r? `@oli-obk` -- if you are okay with experimenting with this feature, I will create a tracking issue.
Based on and blocked on https://github.com/rust-lang/rust/pull/119044; only the last commit is new.

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/rustc_mir_transform/src/const_prop_lint.rs b/compiler/rustc_mir_transform/src/const_prop_lint.rs
index 04d0a4f303f..f8e6905282c 100644
--- a/compiler/rustc_mir_transform/src/const_prop_lint.rs
+++ b/compiler/rustc_mir_transform/src/const_prop_lint.rs
@@ -3,8 +3,9 @@
 
 use std::fmt::Debug;
 
-use rustc_const_eval::interpret::{ImmTy, Projectable};
-use rustc_const_eval::interpret::{InterpCx, InterpResult, Scalar};
+use rustc_const_eval::interpret::{
+    format_interp_error, ImmTy, InterpCx, InterpResult, Projectable, Scalar,
+};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::DefKind;
 use rustc_hir::HirId;
@@ -246,7 +247,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 assert!(
                     !error.kind().formatted_string(),
                     "const-prop encountered formatting error: {}",
-                    self.ecx.format_error(error),
+                    format_interp_error(self.ecx.tcx.dcx(), error),
                 );
                 None
             }