Merge #982

982: Implement BindingMode for pattern matching. r=flodiebold a=mjkillough

Implement `BindingMode` for pattern matching, so that types can be
correctly inferred using match ergonomics. The binding mode defaults to
`Move` (referred to as 'BindingMode::BindByValue` in rustc), and is
updated by automatic dereferencing of the value being matched.

Fixes #888.

 - [Binding modes in The Reference](https://doc.rust-lang.org/reference/patterns.html#binding-modes)
 - [`rustc` implementation](https://github.com/rust-lang/rust/blob/e17c48e2f21eefd59748e364234efc7037a3ec96/src/librustc_typeck/check/_match.rs#L77) (and [definition of `BindingMode`](https://github.com/rust-lang/rust/blob/e957ed9d10ec589bdd523b88b4b44c41b1ecf763/src/librustc/ty/binding.rs))
 - [Match Ergonomics RFC](https://github.com/rust-lang/rfcs/blob/master/text/2005-match-ergonomics.md#binding-mode-rules)

Co-authored-by: Michael Killough <[email protected]>

3 files changed, 137 insertions, 17 deletions

diff --git a/crates/ra_hir/src/marks.rs b/crates/ra_hir/src/marks.rs
index bbf57004d..5b6400042 100644
--- a/crates/ra_hir/src/marks.rs
+++ b/crates/ra_hir/src/marks.rs
@@ -8,4 +8,5 @@ test_utils::marks!(
     glob_enum
     glob_across_crates
     std_prelude
+    match_ergonomics_ref
 );
diff --git a/crates/ra_hir/src/ty/infer.rs b/crates/ra_hir/src/ty/infer.rs
index c9a5bc7a1..735cdecb9 100644
--- a/crates/ra_hir/src/ty/infer.rs
+++ b/crates/ra_hir/src/ty/infer.rs
@@ -63,6 +63,30 @@ enum ExprOrPatId {
 
 impl_froms!(ExprOrPatId: ExprId, PatId);
 
+/// Binding modes inferred for patterns.
+/// https://doc.rust-lang.org/reference/patterns.html#binding-modes
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum BindingMode {
+    Move,
+    Ref(Mutability),
+}
+
+impl BindingMode {
+    pub fn convert(annotation: &BindingAnnotation) -> BindingMode {
+        match annotation {
+            BindingAnnotation::Unannotated | BindingAnnotation::Mutable => BindingMode::Move,
+            BindingAnnotation::Ref => BindingMode::Ref(Mutability::Shared),
+            BindingAnnotation::RefMut => BindingMode::Ref(Mutability::Mut),
+        }
+    }
+}
+
+impl Default for BindingMode {
+    fn default() -> Self {
+        BindingMode::Move
+    }
+}
+
 /// The result of type inference: A mapping from expressions and patterns to types.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct InferenceResult {
@@ -530,6 +554,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         path: Option<&Path>,
         subpats: &[PatId],
         expected: &Ty,
+        default_bm: BindingMode,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
 
@@ -542,13 +567,19 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 .and_then(|d| d.field(self.db, &Name::tuple_field_name(i)))
                 .map_or(Ty::Unknown, |field| field.ty(self.db))
                 .subst(&substs);
-            self.infer_pat(subpat, &expected_ty);
+            self.infer_pat(subpat, &expected_ty, default_bm);
         }
 
         ty
     }
 
-    fn infer_struct_pat(&mut self, path: Option<&Path>, subpats: &[FieldPat], expected: &Ty) -> Ty {
+    fn infer_struct_pat(
+        &mut self,
+        path: Option<&Path>,
+        subpats: &[FieldPat],
+        expected: &Ty,
+        default_bm: BindingMode,
+    ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
 
         self.unify(&ty, expected);
@@ -559,15 +590,45 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             let matching_field = def.and_then(|it| it.field(self.db, &subpat.name));
             let expected_ty =
                 matching_field.map_or(Ty::Unknown, |field| field.ty(self.db)).subst(&substs);
-            self.infer_pat(subpat.pat, &expected_ty);
+            self.infer_pat(subpat.pat, &expected_ty, default_bm);
         }
 
         ty
     }
 
-    fn infer_pat(&mut self, pat: PatId, expected: &Ty) -> Ty {
+    fn infer_pat(&mut self, pat: PatId, mut expected: &Ty, mut default_bm: BindingMode) -> Ty {
         let body = Arc::clone(&self.body); // avoid borrow checker problem
 
+        let is_non_ref_pat = match &body[pat] {
+            Pat::Tuple(..)
+            | Pat::TupleStruct { .. }
+            | Pat::Struct { .. }
+            | Pat::Range { .. }
+            | Pat::Slice { .. } => true,
+            // TODO: Path/Lit might actually evaluate to ref, but inference is unimplemented.
+            Pat::Path(..) | Pat::Lit(..) => true,
+            Pat::Wild | Pat::Bind { .. } | Pat::Ref { .. } | Pat::Missing => false,
+        };
+        if is_non_ref_pat {
+            while let Ty::Ref(inner, mutability) = expected {
+                expected = inner;
+                default_bm = match default_bm {
+                    BindingMode::Move => BindingMode::Ref(*mutability),
+                    BindingMode::Ref(Mutability::Shared) => BindingMode::Ref(Mutability::Shared),
+                    BindingMode::Ref(Mutability::Mut) => BindingMode::Ref(*mutability),
+                }
+            }
+        } else if let Pat::Ref { .. } = &body[pat] {
+            tested_by!(match_ergonomics_ref);
+            // When you encounter a `&pat` pattern, reset to Move.
+            // This is so that `w` is by value: `let (_, &w) = &(1, &2);`
+            default_bm = BindingMode::Move;
+        }
+
+        // Lose mutability.
+        let default_bm = default_bm;
+        let expected = expected;
+
         let ty = match &body[pat] {
             Pat::Tuple(ref args) => {
                 let expectations = match *expected {
@@ -579,7 +640,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 let inner_tys = args
                     .iter()
                     .zip(expectations_iter)
-                    .map(|(&pat, ty)| self.infer_pat(pat, ty))
+                    .map(|(&pat, ty)| self.infer_pat(pat, ty, default_bm))
                     .collect::<Vec<_>>()
                     .into();
 
@@ -595,14 +656,14 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     }
                     _ => &Ty::Unknown,
                 };
-                let subty = self.infer_pat(*pat, expectation);
+                let subty = self.infer_pat(*pat, expectation, default_bm);
                 Ty::Ref(subty.into(), *mutability)
             }
             Pat::TupleStruct { path: ref p, args: ref subpats } => {
-                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected)
+                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm)
             }
             Pat::Struct { path: ref p, args: ref fields } => {
-                self.infer_struct_pat(p.as_ref(), fields, expected)
+                self.infer_struct_pat(p.as_ref(), fields, expected, default_bm)
             }
             Pat::Path(path) => {
                 // TODO use correct resolver for the surrounding expression
@@ -610,17 +671,21 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.infer_path_expr(&resolver, &path, pat.into()).unwrap_or(Ty::Unknown)
             }
             Pat::Bind { mode, name: _name, subpat } => {
+                let mode = if mode == &BindingAnnotation::Unannotated {
+                    default_bm
+                } else {
+                    BindingMode::convert(mode)
+                };
                 let inner_ty = if let Some(subpat) = subpat {
-                    self.infer_pat(*subpat, expected)
+                    self.infer_pat(*subpat, expected, default_bm)
                 } else {
                     expected.clone()
                 };
                 let inner_ty = self.insert_type_vars_shallow(inner_ty);
 
                 let bound_ty = match mode {
-                    BindingAnnotation::Ref => Ty::Ref(inner_ty.clone().into(), Mutability::Shared),
-                    BindingAnnotation::RefMut => Ty::Ref(inner_ty.clone().into(), Mutability::Mut),
-                    BindingAnnotation::Mutable | BindingAnnotation::Unannotated => inner_ty.clone(),
+                    BindingMode::Ref(mutability) => Ty::Ref(inner_ty.clone().into(), mutability),
+                    BindingMode::Move => inner_ty.clone(),
                 };
                 let bound_ty = self.resolve_ty_as_possible(&mut vec![], bound_ty);
                 self.write_pat_ty(pat, bound_ty);
@@ -700,7 +765,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             }
             Expr::For { iterable, body, pat } => {
                 let _iterable_ty = self.infer_expr(*iterable, &Expectation::none());
-                self.infer_pat(*pat, &Ty::Unknown);
+                self.infer_pat(*pat, &Ty::Unknown, BindingMode::default());
                 self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
                 Ty::unit()
             }
@@ -714,7 +779,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     } else {
                         Ty::Unknown
                     };
-                    self.infer_pat(*arg_pat, &expected);
+                    self.infer_pat(*arg_pat, &expected, BindingMode::default());
                 }
 
                 // TODO: infer lambda type etc.
@@ -807,7 +872,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
 
                 for arm in arms {
                     for &pat in &arm.pats {
-                        let _pat_ty = self.infer_pat(pat, &input_ty);
+                        let _pat_ty = self.infer_pat(pat, &input_ty, BindingMode::default());
                     }
                     if let Some(guard_expr) = arm.guard {
                         self.infer_expr(guard_expr, &Expectation::has_type(Ty::Bool));
@@ -1007,7 +1072,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         decl_ty
                     };
 
-                    self.infer_pat(*pat, &ty);
+                    self.infer_pat(*pat, &ty, BindingMode::default());
                 }
                 Statement::Expr(expr) => {
                     self.infer_expr(*expr, &Expectation::none());
@@ -1023,7 +1088,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         for (type_ref, pat) in signature.params().iter().zip(body.params()) {
             let ty = self.make_ty(type_ref);
 
-            self.infer_pat(*pat, &ty);
+            self.infer_pat(*pat, &ty, BindingMode::default());
         }
         self.return_ty = self.make_ty(signature.ret_type());
     }
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index acae71c26..0f2172ddf 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -831,6 +831,60 @@ fn test(x: &i32) {
 }
 
 #[test]
+fn infer_pattern_match_ergonomics() {
+    assert_snapshot_matches!(
+        infer(r#"
+struct A<T>(T);
+
+fn test() {
+    let A(n) = &A(1);
+    let A(n) = &mut A(1);
+}
+"#),
+    @r###"
+[28; 79) '{     ...(1); }': ()
+[38; 42) 'A(n)': A<i32>
+[40; 41) 'n': &i32
+[45; 50) '&A(1)': &A<i32>
+[46; 47) 'A': A<i32>(T) -> A<T>
+[46; 50) 'A(1)': A<i32>
+[48; 49) '1': i32
+[60; 64) 'A(n)': A<i32>
+[62; 63) 'n': &mut i32
+[67; 76) '&mut A(1)': &mut A<i32>
+[72; 73) 'A': A<i32>(T) -> A<T>
+[72; 76) 'A(1)': A<i32>
+[74; 75) '1': i32"###
+    );
+}
+
+#[test]
+fn infer_pattern_match_ergonomics_ref() {
+    covers!(match_ergonomics_ref);
+    assert_snapshot_matches!(
+        infer(r#"
+fn test() {
+    let v = &(1, &2);
+    let (_, &w) = v;
+}
+"#),
+    @r###"
+[11; 57) '{     ...= v; }': ()
+[21; 22) 'v': &(i32, &i32)
+[25; 33) '&(1, &2)': &(i32, &i32)
+[26; 33) '(1, &2)': (i32, &i32)
+[27; 28) '1': i32
+[30; 32) '&2': &i32
+[31; 32) '2': i32
+[43; 50) '(_, &w)': (i32, &i32)
+[44; 45) '_': i32
+[47; 49) '&w': &i32
+[48; 49) 'w': i32
+[53; 54) 'v': &(i32, &i32)"###
+    );
+}
+
+#[test]
 fn infer_adt_pattern() {
     assert_snapshot_matches!(
         infer(r#"