rollup merge of #23119: nikomatsakis/issue-23116-ref-mut

Don't allow upcasting to a supertype in the type of the match discriminant. Fixes #23116.

This is a [breaking-change] in that it closes a type hole that previously existed.

r? @pnkfelix

src/librustc/middle/pat_util.rs

@@ -119,6 +119,24 @@ pub fn pat_contains_bindings(dm: &DefMap, pat: &ast::Pat) -> bool {
     contains_bindings
 }
 
+/// Checks if the pattern contains any `ref` or `ref mut` bindings.
+pub fn pat_contains_ref_binding(dm: &DefMap, pat: &ast::Pat) -> bool {
+    let mut result = false;
+    pat_bindings(dm, pat, |mode, _, _, _| {
+        match mode {
+            ast::BindingMode::BindByRef(_) => { result = true; }
+            ast::BindingMode::BindByValue(_) => { }
+        }
+    });
+    result
+}
+
+/// Checks if the patterns for this arm contain any `ref` or `ref mut`
+/// bindings.
+pub fn arm_contains_ref_binding(dm: &DefMap, arm: &ast::Arm) -> bool {
+    arm.pats.iter().any(|pat| pat_contains_ref_binding(dm, pat))
+}
+
 /// Checks if the pattern contains any patterns that bind something to
 /// an ident or wildcard, e.g. `foo`, or `Foo(_)`, `foo @ Bar(..)`,
 pub fn pat_contains_bindings_or_wild(dm: &DefMap, pat: &ast::Pat) -> bool {

src/librustc/middle/ty.rs

@@ -52,6 +52,7 @@ use middle::mem_categorization as mc;
 use middle::region;
 use middle::resolve_lifetime;
 use middle::infer;
+use middle::pat_util;
 use middle::stability;
 use middle::subst::{self, ParamSpace, Subst, Substs, VecPerParamSpace};
 use middle::traits;
@@ -2684,6 +2685,14 @@ impl<'tcx> ctxt<'tcx> {
     {
         self.ty_param_defs.borrow()[node_id].clone()
     }
+
+    pub fn pat_contains_ref_binding(&self, pat: &ast::Pat) -> bool {
+        pat_util::pat_contains_ref_binding(&self.def_map, pat)
+    }
+
+    pub fn arm_contains_ref_binding(&self, arm: &ast::Arm) -> bool {
+        pat_util::arm_contains_ref_binding(&self.def_map, arm)
+    }
 }
 
 // Interns a type/name combination, stores the resulting box in cx.interner,