typeck::check::coercion - roll back failed unsizing type vars

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

src/librustc_typeck/check/coercion.rs

@@ -187,7 +187,11 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
         }
 
         // Consider coercing the subtype to a DST
-        let unsize = self.coerce_unsized(a, b);
+        //
+        // NOTE: this is wrapped in a `commit_if_ok` because it creates
+        // a "spurious" type variable, and we don't want to have that
+        // type variable in memory if the coercion fails.
+        let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
         if unsize.is_ok() {
             debug!("coerce: unsize successful");
             return unsize;