Auto merge of rust-lang#109750 - compiler-errors:deep-reject-less-permissive-num-var, r=lcnr

numeric vars can only be unified with numerical types in deep reject

Don't consider numeric vars (int and float vars) to unify with non-numeric types during deep reject. This helps us reject incompatible impls sooner.

Author: bors

Diff for: compiler/rustc_middle/src/ty/fast_reject.rs

@@ -322,6 +322,10 @@ impl DeepRejectCtxt {
                 TreatParams::AsCandidateKey => true,
             },
 
+            ty::Infer(ty::IntVar(_)) => impl_ty.is_integral(),
+
+            ty::Infer(ty::FloatVar(_)) => impl_ty.is_floating_point(),
+
             ty::Infer(_) => true,
 
             // As we're walking the whole type, it may encounter projections