Rollup merge of #108947 - compiler-errors:ct-infer-no-shapeshifting, r=BoxyUwU

Don't even try to combine consts with incompatible types

~I left a more detailed explanation for why this fixes this issue in the UI test, but in general, we should not try to unify const infer vars and rigid consts if they have incompatible types. That's because we don't want something like a `ConstArgHasType` predicate to suddenly go from passing to failing, or vice versa, due to a shallow resolve.~

1. Use the `type_of` for a parameter in `try_eval_lit_or_param`, instead of the "expected" type from a `WithOptConstParam` def id.
2. Don't combine consts that have incompatible types.

Fixes #108781

3 files changed, 24 insertions, 13 deletions

diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index b0613abeb48..a2332797e86 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -34,7 +34,6 @@ use rustc_hir::def_id::DefId;
 use rustc_middle::infer::canonical::OriginalQueryValues;
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
-use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
@@ -171,9 +170,9 @@ impl<'tcx> InferCtxt<'tcx> {
         //
         // This probe is probably not strictly necessary but it seems better to be safe and not accidentally find
         // ourselves with a check to find bugs being required for code to compile because it made inference progress.
-        self.probe(|_| {
+        let compatible_types = self.probe(|_| {
             if a.ty() == b.ty() {
-                return;
+                return Ok(());
             }
 
             // We don't have access to trait solving machinery in `rustc_infer` so the logic for determining if the
@@ -183,15 +182,24 @@ impl<'tcx> InferCtxt<'tcx> {
                 (relation.param_env(), a.ty(), b.ty()),
                 &mut OriginalQueryValues::default(),
             );
-
-            if let Err(NoSolution) = self.tcx.check_tys_might_be_eq(canonical) {
+            self.tcx.check_tys_might_be_eq(canonical).map_err(|_| {
                 self.tcx.sess.delay_span_bug(
                     DUMMY_SP,
                     &format!("cannot relate consts of different types (a={:?}, b={:?})", a, b,),
-                );
-            }
+                )
+            })
         });
 
+        // If the consts have differing types, just bail with a const error with
+        // the expected const's type. Specifically, we don't want const infer vars
+        // to do any type shapeshifting before and after resolution.
+        if let Err(guar) = compatible_types {
+            return Ok(self.tcx.const_error_with_guaranteed(
+                if relation.a_is_expected() { a.ty() } else { b.ty() },
+                guar,
+            ));
+        }
+
         match (a.kind(), b.kind()) {
             (
                 ty::ConstKind::Infer(InferConst::Var(a_vid)),
diff --git a/compiler/rustc_middle/src/ty/consts.rs b/compiler/rustc_middle/src/ty/consts.rs
index 5d0cf23280b..42101f6b931 100644
--- a/compiler/rustc_middle/src/ty/consts.rs
+++ b/compiler/rustc_middle/src/ty/consts.rs
@@ -135,6 +135,9 @@ impl<'tcx> Const<'tcx> {
                 _,
                 &hir::Path { res: Res::Def(DefKind::ConstParam, def_id), .. },
             )) => {
+                // Use the type from the param's definition, since we can resolve it,
+                // not the expected parameter type from WithOptConstParam.
+                let param_ty = tcx.type_of(def_id).subst_identity();
                 match tcx.named_bound_var(expr.hir_id) {
                     Some(rbv::ResolvedArg::EarlyBound(_)) => {
                         // Find the name and index of the const parameter by indexing the generics of
@@ -143,14 +146,14 @@ impl<'tcx> Const<'tcx> {
                         let generics = tcx.generics_of(item_def_id);
                         let index = generics.param_def_id_to_index[&def_id];
                         let name = tcx.item_name(def_id);
-                        Some(tcx.mk_const(ty::ParamConst::new(index, name), ty))
+                        Some(tcx.mk_const(ty::ParamConst::new(index, name), param_ty))
                     }
                     Some(rbv::ResolvedArg::LateBound(debruijn, index, _)) => Some(tcx.mk_const(
                         ty::ConstKind::Bound(debruijn, ty::BoundVar::from_u32(index)),
-                        ty,
+                        param_ty,
                     )),
                     Some(rbv::ResolvedArg::Error(guar)) => {
-                        Some(tcx.const_error_with_guaranteed(ty, guar))
+                        Some(tcx.const_error_with_guaranteed(param_ty, guar))
                     }
                     arg => bug!("unexpected bound var resolution for {:?}: {arg:?}", expr.hir_id),
                 }
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index d1d6a7a90cf..fcfb60b2603 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -99,10 +99,10 @@ pub fn translate_substs<'tcx>(
             }
 
             fulfill_implication(infcx, param_env, source_trait_ref, target_impl).unwrap_or_else(
-                |_| {
+                |()| {
                     bug!(
-                        "When translating substitutions for specialization, the expected \
-                         specialization failed to hold"
+                        "When translating substitutions from {source_impl:?} to {target_impl:?}, \
+                        the expected specialization failed to hold"
                     )
                 },
             )