Don't canonicalize ty::Const if its ParamEnv isn't canonicalized yet

Author: ShE3py

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -170,11 +170,18 @@ impl<'tcx> InferCtxt<'tcx> {
                 return;
             }
 
+            let param_env = relation.param_env();
+            if !self.tcx.canonical_param_env_cache.contains_key(param_env) {
+                // Canonicalizing `param_env` may ICE as `CanonicalParamEnvCache::get_or_insert` requires that `InferCtxt`
+                // isn't used, see #119381.
+                return;
+            }
+
             // We don't have access to trait solving machinery in `rustc_infer` so the logic for determining if the
             // two const param's types are able to be equal has to go through a canonical query with the actual logic
             // in `rustc_trait_selection`.
             let canonical = self.canonicalize_query(
-                relation.param_env().and((a.ty(), b.ty())),
+                param_env.and((a.ty(), b.ty())),
                 &mut OriginalQueryValues::default(),
             );
             self.tcx.check_tys_might_be_eq(canonical).unwrap_or_else(|_| {

compiler/rustc_middle/src/infer/canonical.rs

@@ -356,4 +356,8 @@ impl<'tcx> CanonicalParamEnvCache<'tcx> {
             }
         }
     }
+
+    pub fn contains_key(&self, key: ty::ParamEnv<'tcx>) -> bool {
+        self.map.borrow().contains_key(&key)
+    }
 }

tests/ui/inference/combine-consts-with-negative-coherence.rs

@@ -0,0 +1,10 @@
+//! Issue #119381: `InferCtxt::super_combine_consts` shouldn't canonicalize its `ty::Const` if its
+//! `ParamEnv` isn't canonicalized yet.
+
+#![feature(with_negative_coherence)]
+
+impl<const N: u8> Copy for [(); N] {}
+//~^ only traits defined in the current crate can be implemented for arbitrary types
+//~| mismatched types
+
+fn main() {}

tests/ui/inference/combine-consts-with-negative-coherence.stderr

@@ -0,0 +1,21 @@
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/combine-consts-with-negative-coherence.rs:6:1
+   |
+LL | impl<const N: u8> Copy for [(); N] {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^-------
+   | |                          |
+   | |                          this is not defined in the current crate because arrays are always foreign
+   | impl doesn't use only types from inside the current crate
+   |
+   = note: define and implement a trait or new type instead
+
+error[E0308]: mismatched types
+  --> $DIR/combine-consts-with-negative-coherence.rs:6:33
+   |
+LL | impl<const N: u8> Copy for [(); N] {}
+   |                                 ^ expected `usize`, found `u8`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0117, E0308.
+For more information about an error, try `rustc --explain E0117`.