Auto merge of #119821 - oli-obk:reveal_all_const_evals, r=lcnr

Always use RevealAll for const eval queries

implements what is described in https://github.com/rust-lang/rust/pull/116803#discussion_r1364089471

Using `UserFacing` for const eval does not make sense anymore, unless we significantly change things like avoiding revealing opaque types.

New tests are copied from https://github.com/rust-lang/rust/pull/101478

1 files changed, 9 insertions, 29 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index 9d22df50d4f..4236117d75b 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -225,17 +225,10 @@ pub fn eval_to_const_value_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToConstValueResult<'tcx> {
-    // see comment in eval_to_allocation_raw_provider for what we're doing here
-    if key.param_env.reveal() == Reveal::All {
-        let mut key = key;
-        key.param_env = key.param_env.with_user_facing();
-        match tcx.eval_to_const_value_raw(key) {
-            // try again with reveal all as requested
-            Err(ErrorHandled::TooGeneric(_)) => {}
-            // deduplicate calls
-            other => return other,
-        }
-    }
+    // Const eval always happens in Reveal::All mode in order to be able to use the hidden types of
+    // opaque types. This is needed for trivial things like `size_of`, but also for using associated
+    // types that are not specified in the opaque type.
+    assert_eq!(key.param_env.reveal(), Reveal::All);
 
     // We call `const_eval` for zero arg intrinsics, too, in order to cache their value.
     // Catch such calls and evaluate them instead of trying to load a constant's MIR.
@@ -265,24 +258,11 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
     tcx: TyCtxt<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc_middle::mir::interpret::EvalToAllocationRawResult<'tcx> {
-    // Because the constant is computed twice (once per value of `Reveal`), we are at risk of
-    // reporting the same error twice here. To resolve this, we check whether we can evaluate the
-    // constant in the more restrictive `Reveal::UserFacing`, which most likely already was
-    // computed. For a large percentage of constants that will already have succeeded. Only
-    // associated constants of generic functions will fail due to not enough monomorphization
-    // information being available.
-
-    // In case we fail in the `UserFacing` variant, we just do the real computation.
-    if key.param_env.reveal() == Reveal::All {
-        let mut key = key;
-        key.param_env = key.param_env.with_user_facing();
-        match tcx.eval_to_allocation_raw(key) {
-            // try again with reveal all as requested
-            Err(ErrorHandled::TooGeneric(_)) => {}
-            // deduplicate calls
-            other => return other,
-        }
-    }
+    // Const eval always happens in Reveal::All mode in order to be able to use the hidden types of
+    // opaque types. This is needed for trivial things like `size_of`, but also for using associated
+    // types that are not specified in the opaque type.
+
+    assert_eq!(key.param_env.reveal(), Reveal::All);
     if cfg!(debug_assertions) {
         // Make sure we format the instance even if we do not print it.
         // This serves as a regression test against an ICE on printing.