rustc_mir_build: Enforce rustc::potential_query_instability lint

Stop allowing `rustc::potential_query_instability` on all of
`rustc_mir_build` and instead allow it on a case-by-case basis if it is
safe to do so. In this crate there was only one instance of the lint,
and it was safe to allow.

Author: Enselic

compiler/rustc_mir_build/src/lib.rs

@@ -1,7 +1,6 @@
 //! Construction of MIR from HIR.
 //!
 //! This crate also contains the match exhaustiveness and usefulness checking.
-#![allow(rustc::potential_query_instability)]
 #![feature(assert_matches)]
 #![feature(associated_type_bounds)]
 #![feature(box_patterns)]

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -952,6 +952,9 @@ fn report_non_exhaustive_match<'p, 'tcx>(
         // Look at the first witness.
         collect_non_exhaustive_tys(cx, &witnesses[0], &mut non_exhaustive_tys);
 
+        // The items are only used to add notes to an err, so the order does not
+        // affect query stability.
+        #[allow(rustc::potential_query_instability)]
         for ty in non_exhaustive_tys {
             if ty.is_ptr_sized_integral() {
                 if ty == cx.tcx.types.usize {