MIR typeck: invariant ctxt fast path

This removes variance information from some diagnostics. However,
that variance information is not actually relevant here. Casting
`*const dyn Cat<'a>` to `*const S<dyn Cat<'static>>` is an error
regardless of whether `S` requires its argument to be invariant.
Wide-pointer casts always require the trait object arguments to be
invariant.

Author: lcnr

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -11,6 +11,7 @@ use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::ty::relate::combine::{combine_ty_args, super_combine_consts, super_combine_tys};
+use rustc_middle::ty::relate::relate_args_invariantly;
 use rustc_middle::ty::{self, FnMutDelegate, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::{bug, span_bug};
 use rustc_span::{Span, Symbol, sym};
@@ -313,17 +314,24 @@ impl<'b, 'tcx> TypeRelation<TyCtxt<'tcx>> for NllTypeRelating<'_, 'b, 'tcx> {
         b_args: ty::GenericArgsRef<'tcx>,
         _: impl FnOnce(ty::GenericArgsRef<'tcx>) -> Ty<'tcx>,
     ) -> RelateResult<'tcx, Ty<'tcx>> {
-        let variances = self.cx().variances_of(def_id);
-        combine_ty_args(
-            &self.type_checker.infcx.infcx,
-            self,
-            a_ty,
-            b_ty,
-            variances,
-            a_args,
-            b_args,
-            |_| a_ty,
-        )
+        if self.ambient_variance == ty::Invariant {
+            // Avoid fetching the variance if we are in an invariant context,
+            // slightly improves perf.
+            relate_args_invariantly(self, a_args, b_args)?;
+            Ok(a_ty)
+        } else {
+            let variances = self.cx().variances_of(def_id);
+            combine_ty_args(
+                &self.type_checker.infcx.infcx,
+                self,
+                a_ty,
+                b_ty,
+                variances,
+                a_args,
+                b_args,
+                |_| a_ty,
+            )
+        }
     }
 
     #[instrument(skip(self, info), level = "trace", ret)]

tests/ui/cast/ptr-to-trait-obj-different-regions-id-trait.current.stderr

@@ -6,10 +6,6 @@ LL | fn m<'a>() {
 LL |     let unsend: *const dyn Cat<'a> = &();
 LL |     let _send = unsend as *const S<dyn Cat<'static>>;
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
-   |
-   = note: requirement occurs because of the type `S<dyn Cat<'_>>`, which makes the generic argument `dyn Cat<'_>` invariant
-   = note: the struct `S<T>` is invariant over the parameter `T`
-   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 1 previous error
 

tests/ui/cast/ptr-to-trait-obj-different-regions-id-trait.next.stderr

@@ -6,10 +6,6 @@ LL | fn m<'a>() {
 LL |     let unsend: *const dyn Cat<'a> = &();
 LL |     let _send = unsend as *const S<dyn Cat<'static>>;
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static`
-   |
-   = note: requirement occurs because of the type `S<dyn Cat<'_>>`, which makes the generic argument `dyn Cat<'_>` invariant
-   = note: the struct `S<T>` is invariant over the parameter `T`
-   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 1 previous error
 

tests/ui/fn/implied-bounds-unnorm-associated-type-2.stderr

@@ -9,9 +9,6 @@ LL |     f::<'a, 'b>(());
    |     ^^^^^^^^^^^^^^^ generic argument requires that `'b` must outlive `'a`
    |
    = help: consider adding the following bound: `'b: 'a`
-   = note: requirement occurs because of a function pointer to `f`
-   = note: the function `f` is invariant over the parameter `'a`
-   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
 
 error: aborting due to 1 previous error