add assertion and comment about mutability

nikomatsakis · nikomatsakis · commit 43aed96d0da2 · 2016-03-18T18:40:22.000-04:00
diff --git a/src/librustc_typeck/check/coercion.rs b/src/librustc_typeck/check/coercion.rs
@@ -367,6 +367,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             // `self.x` both have `&mut `type would be a move of
             // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
             // which is a borrow.
+            assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
             return self.identity(ty);
         }
         let r_borrow = match ty.sty {

Original file line number	Diff line number	Diff line change
`@@ -367,6 +367,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {`
`367`	`367`	// `self.x` both have `&mut `type would be a move of
`368`	`368`	// `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
`369`	`369`	`// which is a borrow.`
	`370`	`+ assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U`
`370`	`371`	`return self.identity(ty);`
`371`	`372`	`}`
`372`	`373`	`let r_borrow = match ty.sty {`