Auto merge of #66296 - Centril:bindings_after_at-init, r=pnkfelix

Initial implementation of `#![feature(bindings_after_at)]`

Following up on #16053, under the gate `#![feature(bindings_after_at)]`, `x @ Some(y)` is allowed subject to restrictions necessary for soundness.

The implementation and test suite should be fairly complete now.

One aspect that is not covered is the interaction with nested `#![feature(or_patterns)]`.
This is not possible to test at the moment in a good way because that feature has not progressed sufficiently and has fatal errors in MIR building. We should make sure to add such tests before we stabilize both features (but shipping one of them is fine).

r? @pnkfelix
cc @nikomatsakis @matthewjasper @pcwalton
cc #65490

src/librustc/hir/mod.rs

@@ -921,6 +921,16 @@ impl Pat {
     pub fn walk(&self, mut it: impl FnMut(&Pat) -> bool) {
         self.walk_(&mut it)
     }
+
+    /// Walk the pattern in left-to-right order.
+    ///
+    /// If you always want to recurse, prefer this method over `walk`.
+    pub fn walk_always(&self, mut it: impl FnMut(&Pat)) {
+        self.walk(|p| {
+            it(p);
+            true
+        })
+    }
 }
 
 /// A single field in a struct pattern.

src/librustc/hir/pat_util.rs

@@ -79,11 +79,10 @@ impl hir::Pat {
     /// Call `f` on every "binding" in a pattern, e.g., on `a` in
     /// `match foo() { Some(a) => (), None => () }`
     pub fn each_binding(&self, mut f: impl FnMut(hir::BindingAnnotation, HirId, Span, ast::Ident)) {
-        self.walk(|p| {
+        self.walk_always(|p| {
             if let PatKind::Binding(binding_mode, _, ident, _) = p.kind {
                 f(binding_mode, p.hir_id, p.span, ident);
             }
-            true
         });
     }
 

src/librustc/ty/context.rs

@@ -648,6 +648,13 @@ impl<'tcx> TypeckTables<'tcx> {
         }
     }
 
+    pub fn extract_binding_mode(&self, s: &Session, id: HirId, sp: Span) -> Option<BindingMode> {
+        self.pat_binding_modes().get(id).copied().or_else(|| {
+            s.delay_span_bug(sp, "missing binding mode");
+            None
+        })
+    }
+
     pub fn pat_binding_modes(&self) -> LocalTableInContext<'_, BindingMode> {
         LocalTableInContext {
             local_id_root: self.local_id_root,

src/librustc_error_codes/error_codes/E0303.md

@@ -1,10 +1,18 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+Sub-bindings, e.g. `ref x @ Some(ref y)` are now allowed under
+`#![feature(bindings_after_at)]` and checked to make sure that
+memory safety is upheld.
+
+--------------
+
 In certain cases it is possible for sub-bindings to violate memory safety.
 Updates to the borrow checker in a future version of Rust may remove this
 restriction, but for now patterns must be rewritten without sub-bindings.
 
 Before:
 
-```compile_fail,E0303
+```compile_fail
 match Some("hi".to_string()) {
     ref op_string_ref @ Some(s) => {},
     None => {},

src/librustc_feature/active.rs

@@ -539,6 +539,10 @@ declare_features! (
     /// Allows the use of `loop` and `while` in constants.
     (active, const_loop, "1.41.0", Some(52000), None),
 
+    /// Allows bindings in the subpattern of a binding pattern.
+    /// For example, you can write `x @ Some(y)`.
+    (active, bindings_after_at, "1.41.0", Some(65490), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_mir/build/mod.rs

@@ -816,15 +816,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 if let Some(Node::Binding(pat)) = tcx_hir.find(var_id) {
                     if let hir::PatKind::Binding(_, _, ident, _) = pat.kind {
                         name = ident.name;
-
-                        if let Some(&bm) = hir_tables.pat_binding_modes().get(pat.hir_id) {
-                            if bm == ty::BindByValue(hir::Mutability::Mut) {
+                        match hir_tables.extract_binding_mode(tcx.sess, pat.hir_id, pat.span) {
+                            Some(ty::BindByValue(hir::Mutability::Mut)) => {
                                 mutability = Mutability::Mut;
-                            } else {
-                                mutability = Mutability::Not;
                             }
-                        } else {
-                            tcx.sess.delay_span_bug(pat.span, "missing binding mode");
+                            Some(_) => mutability = Mutability::Not,
+                            _ => {}
                         }
                     }
                 }