default binding modes: add pat_binding_modes

This PR kicks off the implementation of the [default binding modes RFC][1] by
introducing the `pat_binding_modes` typeck table mentioned in the [mentoring
instructions][2].

`pat_binding_modes` is populated in `librustc_typeck/check/_match.rs` and
used wherever the HIR would be scraped prior to this PR. Unfortunately, one
blemish, namely a two callers to `contains_explicit_ref_binding`, remains.
This will likely have to be removed when the second part of [1], the
`pat_adjustments` table, is tackled. Appropriate comments have been added.

See #42640.

[1]: rust-lang/rfcs#2005
[2]: #42640 (comment)

Author: tbg

Diff for: src/librustc/hir/lowering.rs

@@ -2191,7 +2191,7 @@ impl<'a> LoweringContext<'a> {
                 let next_ident = self.str_to_ident("__next");
                 let next_pat = self.pat_ident_binding_mode(e.span,
                                                            next_ident,
-                                                           hir::BindByValue(hir::MutMutable));
+                                                           hir::BindingAnnotation::Mutable);
 
                 // `::std::option::Option::Some(val) => next = val`
                 let pat_arm = {
@@ -2215,8 +2215,9 @@ impl<'a> LoweringContext<'a> {
                 };
 
                 // `mut iter`
-                let iter_pat = self.pat_ident_binding_mode(e.span, iter,
-                                                           hir::BindByValue(hir::MutMutable));
+                let iter_pat = self.pat_ident_binding_mode(e.span,
+                                                           iter,
+                                                           hir::BindingAnnotation::Mutable);
 
                 // `match ::std::iter::Iterator::next(&mut iter) { ... }`
                 let match_expr = {
@@ -2503,10 +2504,13 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    fn lower_binding_mode(&mut self, b: &BindingMode) -> hir::BindingMode {
+    fn lower_binding_mode(&mut self, b: &BindingMode) -> hir::BindingAnnotation {
         match *b {
-            BindingMode::ByRef(m) => hir::BindByRef(self.lower_mutability(m)),
-            BindingMode::ByValue(m) => hir::BindByValue(self.lower_mutability(m)),
+            BindingMode::ByValue(Mutability::Immutable) =>
+                hir::BindingAnnotation::Unannotated,
+            BindingMode::ByRef(Mutability::Immutable) => hir::BindingAnnotation::Ref,
+            BindingMode::ByValue(Mutability::Mutable) => hir::BindingAnnotation::Mutable,
+            BindingMode::ByRef(Mutability::Mutable) => hir::BindingAnnotation::RefMut,
         }
     }
 
@@ -2647,7 +2651,7 @@ impl<'a> LoweringContext<'a> {
     fn stmt_let(&mut self, sp: Span, mutbl: bool, ident: Name, ex: P<hir::Expr>)
                 -> (hir::Stmt, NodeId) {
         let pat = if mutbl {
-            self.pat_ident_binding_mode(sp, ident, hir::BindByValue(hir::MutMutable))
+            self.pat_ident_binding_mode(sp, ident, hir::BindingAnnotation::Mutable)
         } else {
             self.pat_ident(sp, ident)
         };
@@ -2703,10 +2707,10 @@ impl<'a> LoweringContext<'a> {
     }
 
     fn pat_ident(&mut self, span: Span, name: Name) -> P<hir::Pat> {
-        self.pat_ident_binding_mode(span, name, hir::BindByValue(hir::MutImmutable))
+        self.pat_ident_binding_mode(span, name, hir::BindingAnnotation::Unannotated)
     }
 
-    fn pat_ident_binding_mode(&mut self, span: Span, name: Name, bm: hir::BindingMode)
+    fn pat_ident_binding_mode(&mut self, span: Span, name: Name, bm: hir::BindingAnnotation)
                               -> P<hir::Pat> {
         let id = self.next_id();
         let parent_def = self.parent_def.unwrap();

Diff for: src/librustc/hir/mod.rs

@@ -10,7 +10,6 @@
 
 // The Rust HIR.
 
-pub use self::BindingMode::*;
 pub use self::BinOp_::*;
 pub use self::BlockCheckMode::*;
 pub use self::CaptureClause::*;
@@ -628,10 +627,28 @@ pub struct FieldPat {
     pub is_shorthand: bool,
 }
 
+/// Explicit binding annotations given in the HIR for a binding. Note
+/// that this is not the final binding *mode* that we infer after type
+/// inference.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
-pub enum BindingMode {
-    BindByRef(Mutability),
-    BindByValue(Mutability),
+pub enum BindingAnnotation {
+  /// No binding annotation given: this means that the final binding mode
+  /// will depend on whether we have skipped through a `&` reference
+  /// when matching. For example, the `x` in `Some(x)` will have binding
+  /// mode `None`; if you do `let Some(x) = &Some(22)`, it will
+  /// ultimately be inferred to be by-reference.
+  ///
+  /// Note that implicit reference skipping is not implemented yet (#42640).
+  Unannotated,
+
+  /// Annotated with `mut x` -- could be either ref or not, similar to `None`.
+  Mutable,
+
+  /// Annotated as `ref`, like `ref x`
+  Ref,
+
+  /// Annotated as `ref mut x`.
+  RefMut,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
@@ -647,7 +664,7 @@ pub enum PatKind {
 
     /// A fresh binding `ref mut binding @ OPT_SUBPATTERN`.
     /// The `DefId` is for the definition of the variable being bound.
-    Binding(BindingMode, DefId, Spanned<Name>, Option<P<Pat>>),
+    Binding(BindingAnnotation, DefId, Spanned<Name>, Option<P<Pat>>),
 
     /// A struct or struct variant pattern, e.g. `Variant {x, y, ..}`.
     /// The `bool` is `true` in the presence of a `..`.

Diff for: src/librustc/hir/pat_util.rs

@@ -87,7 +87,7 @@ 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<F>(&self, mut f: F)
-        where F: FnMut(hir::BindingMode, ast::NodeId, Span, &Spanned<ast::Name>),
+        where F: FnMut(hir::BindingAnnotation, ast::NodeId, Span, &Spanned<ast::Name>),
     {
         self.walk(|p| {
             if let PatKind::Binding(binding_mode, _, ref pth, _) = p.node {
@@ -130,12 +130,10 @@ impl hir::Pat {
 
     pub fn simple_name(&self) -> Option<ast::Name> {
         match self.node {
-            PatKind::Binding(hir::BindByValue(..), _, ref path1, None) => {
-                Some(path1.node)
-            }
-            _ => {
-                None
-            }
+            PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ref path1, None) |
+            PatKind::Binding(hir::BindingAnnotation::Mutable, _, ref path1, None) =>
+                Some(path1.node),
+            _ => None,
         }
     }
 
@@ -163,16 +161,22 @@ impl hir::Pat {
     }
 
     /// Checks if the pattern contains any `ref` or `ref mut` bindings,
-    /// and if yes whether its containing mutable ones or just immutables ones.
-    pub fn contains_ref_binding(&self) -> Option<hir::Mutability> {
+    /// and if yes whether it contains mutable or just immutables ones.
+    ///
+    /// FIXME(tschottdorf): this is problematic as the HIR is being scraped,
+    /// but ref bindings may be implicit after #42640.
+    pub fn contains_explicit_ref_binding(&self) -> Option<hir::Mutability> {
         let mut result = None;
-        self.each_binding(|mode, _, _, _| {
-            if let hir::BindingMode::BindByRef(m) = mode {
-                // Pick Mutable as maximum
-                match result {
-                    None | Some(hir::MutImmutable) => result = Some(m),
-                    _ => (),
+        self.each_binding(|annotation, _, _, _| {
+            match annotation {
+                hir::BindingAnnotation::Ref => {
+                    match result {
+                        None | Some(hir::MutImmutable) => result = Some(hir::MutImmutable),
+                        _ => (),
+                    }
                 }
+                hir::BindingAnnotation::RefMut => result = Some(hir::MutMutable),
+                _ => (),
             }
         });
         result
@@ -182,9 +186,11 @@ impl hir::Pat {
 impl hir::Arm {
     /// Checks if the patterns for this arm contain any `ref` or `ref mut`
     /// bindings, and if yes whether its containing mutable ones or just immutables ones.
-    pub fn contains_ref_binding(&self) -> Option<hir::Mutability> {
+    pub fn contains_explicit_ref_binding(&self) -> Option<hir::Mutability> {
+        // FIXME(tschottdorf): contains_explicit_ref_binding() must be removed
+        // for #42640.
         self.pats.iter()
-                 .filter_map(|pat| pat.contains_ref_binding())
+                 .filter_map(|pat| pat.contains_explicit_ref_binding())
                  .max_by_key(|m| match *m {
                     hir::MutMutable => 1,
                     hir::MutImmutable => 0,

Diff for: src/librustc/hir/print.rs

@@ -1651,12 +1651,16 @@ impl<'a> State<'a> {
             PatKind::Wild => self.s.word("_")?,
             PatKind::Binding(binding_mode, _, ref path1, ref sub) => {
                 match binding_mode {
-                    hir::BindByRef(mutbl) => {
+                    hir::BindingAnnotation::Ref => {
                         self.word_nbsp("ref")?;
-                        self.print_mutability(mutbl)?;
+                        self.print_mutability(hir::MutImmutable)?;
                     }
-                    hir::BindByValue(hir::MutImmutable) => {}
-                    hir::BindByValue(hir::MutMutable) => {
+                    hir::BindingAnnotation::RefMut => {
+                        self.word_nbsp("ref")?;
+                        self.print_mutability(hir::MutMutable)?;
+                    }
+                    hir::BindingAnnotation::Unannotated => {}
+                    hir::BindingAnnotation::Mutable => {
                         self.word_nbsp("mut")?;
                     }
                 }

Diff for: src/librustc/ich/impls_hir.rs

@@ -442,9 +442,11 @@ impl_stable_hash_for!(struct hir::FieldPat {
     is_shorthand
 });
 
-impl_stable_hash_for!(enum hir::BindingMode {
-    BindByRef(mutability),
-    BindByValue(mutability)
+impl_stable_hash_for!(enum hir::BindingAnnotation {
+    Unannotated,
+    Mutable,
+    Ref,
+    RefMut
 });
 
 impl_stable_hash_for!(enum hir::RangeEnd {

Diff for: src/librustc/ich/impls_ty.rs

@@ -617,6 +617,7 @@ for ty::TypeckTables<'tcx> {
             ref node_types,
             ref node_substs,
             ref adjustments,
+            ref pat_binding_modes,
             ref upvar_capture_map,
             ref closure_tys,
             ref closure_kinds,
@@ -637,6 +638,7 @@ for ty::TypeckTables<'tcx> {
             ich::hash_stable_nodemap(hcx, hasher, node_types);
             ich::hash_stable_nodemap(hcx, hasher, node_substs);
             ich::hash_stable_nodemap(hcx, hasher, adjustments);
+            ich::hash_stable_nodemap(hcx, hasher, pat_binding_modes);
             ich::hash_stable_hashmap(hcx, hasher, upvar_capture_map, |hcx, up_var_id| {
                 let ty::UpvarId {
                     var_id,

Diff for: src/librustc/middle/expr_use_visitor.rs

@@ -796,16 +796,19 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
         debug!("determine_pat_move_mode cmt_discr={:?} pat={:?}", cmt_discr,
                pat);
         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |cmt_pat, pat| {
-            match pat.node {
-                PatKind::Binding(hir::BindByRef(..), ..) =>
-                    mode.lub(BorrowingMatch),
-                PatKind::Binding(hir::BindByValue(..), ..) => {
-                    match copy_or_move(&self.mc, self.param_env, &cmt_pat, PatBindingMove) {
-                        Copy => mode.lub(CopyingMatch),
-                        Move(..) => mode.lub(MovingMatch),
+            if let PatKind::Binding(..) = pat.node {
+                let bm = *self.mc.tables.pat_binding_modes.get(&pat.id)
+                                                          .expect("missing binding mode");
+                match bm {
+                    ty::BindByReference(..) =>
+                        mode.lub(BorrowingMatch),
+                    ty::BindByValue(..) => {
+                        match copy_or_move(&self.mc, self.param_env, &cmt_pat, PatBindingMove) {
+                            Copy => mode.lub(CopyingMatch),
+                            Move(..) => mode.lub(MovingMatch),
+                        }
                     }
                 }
-                _ => {}
             }
         }));
     }
@@ -818,8 +821,9 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
 
         let ExprUseVisitor { ref mc, ref mut delegate, param_env } = *self;
         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |cmt_pat, pat| {
-            if let PatKind::Binding(bmode, def_id, ..) = pat.node {
+            if let PatKind::Binding(_, def_id, ..) = pat.node {
                 debug!("binding cmt_pat={:?} pat={:?} match_mode={:?}", cmt_pat, pat, match_mode);
+                let bm = *mc.tables.pat_binding_modes.get(&pat.id).expect("missing binding mode");
 
                 // pat_ty: the type of the binding being produced.
                 let pat_ty = return_if_err!(mc.node_ty(pat.id));
@@ -832,14 +836,14 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                 }
 
                 // It is also a borrow or copy/move of the value being matched.
-                match bmode {
-                    hir::BindByRef(m) => {
+                match bm {
+                    ty::BindByReference(m) => {
                         if let ty::TyRef(r, _) = pat_ty.sty {
                             let bk = ty::BorrowKind::from_mutbl(m);
                             delegate.borrow(pat.id, pat.span, cmt_pat, r, bk, RefBinding);
                         }
                     }
-                    hir::BindByValue(..) => {
+                    ty::BindByValue(..) => {
                         let mode = copy_or_move(mc, param_env, &cmt_pat, PatBindingMove);
                         debug!("walk_pat binding consuming pat");
                         delegate.consume_pat(pat, cmt_pat, mode);

Diff for: src/librustc/middle/mem_categorization.rs

@@ -330,11 +330,12 @@ impl MutabilityCategory {
         ret
     }
 
-    fn from_local(tcx: TyCtxt, id: ast::NodeId) -> MutabilityCategory {
+    fn from_local(tcx: TyCtxt, tables: &ty::TypeckTables, id: ast::NodeId) -> MutabilityCategory {
         let ret = match tcx.hir.get(id) {
             hir_map::NodeLocal(p) => match p.node {
-                PatKind::Binding(bind_mode, ..) => {
-                    if bind_mode == hir::BindByValue(hir::MutMutable) {
+                PatKind::Binding(..) => {
+                    let bm = *tables.pat_binding_modes.get(&p.id).expect("missing binding mode");
+                    if bm == ty::BindByValue(hir::MutMutable) {
                         McDeclared
                     } else {
                         McImmutable
@@ -475,16 +476,21 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         // *being borrowed* is.  But ideally we would put in a more
         // fundamental fix to this conflated use of the node id.
         let ret_ty = match pat.node {
-            PatKind::Binding(hir::BindByRef(_), ..) => {
-                // a bind-by-ref means that the base_ty will be the type of the ident itself,
-                // but what we want here is the type of the underlying value being borrowed.
-                // So peel off one-level, turning the &T into T.
-                match base_ty.builtin_deref(false, ty::NoPreference) {
-                    Some(t) => t.ty,
-                    None => {
-                        debug!("By-ref binding of non-derefable type {:?}", base_ty);
-                        return Err(());
+            PatKind::Binding(..) => {
+                let bm = *self.tables.pat_binding_modes.get(&pat.id).expect("missing binding mode");
+                if let ty::BindByReference(_) = bm {
+                    // a bind-by-ref means that the base_ty will be the type of the ident itself,
+                    // but what we want here is the type of the underlying value being borrowed.
+                    // So peel off one-level, turning the &T into T.
+                    match base_ty.builtin_deref(false, ty::NoPreference) {
+                        Some(t) => t.ty,
+                        None => {
+                            debug!("By-ref binding of non-derefable type {:?}", base_ty);
+                            return Err(());
+                        }
                     }
+                } else {
+                    base_ty
                 }
             }
             _ => base_ty,
@@ -659,7 +665,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
                 id,
                 span,
                 cat: Categorization::Local(vid),
-                mutbl: MutabilityCategory::from_local(self.tcx, vid),
+                mutbl: MutabilityCategory::from_local(self.tcx, self.tables, vid),
                 ty: expr_ty,
                 note: NoteNone
             }))
@@ -711,7 +717,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         let var_ty = self.node_ty(var_id)?;
 
         // Mutability of original variable itself
-        let var_mutbl = MutabilityCategory::from_local(self.tcx, var_id);
+        let var_mutbl = MutabilityCategory::from_local(self.tcx, self.tables, var_id);
 
         // Construct the upvar. This represents access to the field
         // from the environment (perhaps we should eventually desugar

Diff for: src/librustc/middle/region.rs

@@ -889,8 +889,32 @@ fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
     ///        | ( ..., P&, ... )
     ///        | box P&
     fn is_binding_pat(pat: &hir::Pat) -> bool {
+        // Note that the code below looks for *explicit* refs only, that is, it won't
+        // know about *implicit* refs as introduced in #42640.
+        //
+        // This is not a problem. For example, consider
+        //
+        //      let (ref x, ref y) = (Foo { .. }, Bar { .. });
+        //
+        // Due to the explicit refs on the left hand side, the below code would signal
+        // that the temporary value on the right hand side should live until the end of
+        // the enclosing block (as opposed to being dropped after the let is complete).
+        //
+        // To create an implicit ref, however, you must have a borrowed value on the RHS
+        // already, as in this example (which won't compile before #42640):
+        //
+        //      let Foo { x, .. } = &Foo { x: ..., ... };
+        //
+        // in place of
+        //
+        //      let Foo { ref x, .. } = Foo { ... };
+        //
+        // In the former case (the implicit ref version), the temporary is created by the
+        // & expression, and its lifetime would be extended to the end of the block (due
+        // to a different rule, not the below code).
         match pat.node {
-            PatKind::Binding(hir::BindByRef(_), ..) => true,
+            PatKind::Binding(hir::BindingAnnotation::Ref, ..) |
+            PatKind::Binding(hir::BindingAnnotation::RefMut, ..) => true,
 
             PatKind::Struct(_, ref field_pats, _) => {
                 field_pats.iter().any(|fp| is_binding_pat(&fp.node.pat))