Implement BindingMode for pattern matching.

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.

2 files changed, 154 insertions, 62 deletions

diff --git a/crates/ra_hir/src/ty/infer.rs b/crates/ra_hir/src/ty/infer.rs
index 268d2c110..7200446ba 100644
--- a/crates/ra_hir/src/ty/infer.rs
+++ b/crates/ra_hir/src/ty/infer.rs
@@ -63,6 +63,24 @@ 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),
+        }
+    }
+}
+
 /// The result of type inference: A mapping from expressions and patterns to types.
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct InferenceResult {
@@ -530,6 +548,7 @@ 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 +561,19 @@ 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 +584,42 @@ 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] {
+            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 +631,7 @@ 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 +647,14 @@ 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 +662,26 @@ 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::Shared) => {
+                        Ty::Ref(inner_ty.clone().into(), Mutability::Shared)
+                    }
+                    BindingMode::Ref(Mutability::Mut) => {
+                        Ty::Ref(inner_ty.clone().into(), Mutability::Mut)
+                    }
+                    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 +761,7 @@ 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::Move);
                 self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
                 Ty::unit()
             }
@@ -714,7 +775,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     } else {
                         Ty::Unknown
                     };
-                    self.infer_pat(*arg_pat, &expected);
+                    self.infer_pat(*arg_pat, &expected, BindingMode::Move);
                 }
 
                 // TODO: infer lambda type etc.
@@ -804,7 +865,7 @@ 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::Move);
                     }
                     if let Some(guard_expr) = arm.guard {
                         self.infer_expr(guard_expr, &Expectation::has_type(Ty::Bool));
@@ -1004,7 +1065,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         decl_ty
                     };
 
-                    self.infer_pat(*pat, &ty);
+                    self.infer_pat(*pat, &ty, BindingMode::Move);
                 }
                 Statement::Expr(expr) => {
                     self.infer_expr(*expr, &Expectation::none());
@@ -1020,7 +1081,7 @@ 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::Move);
         }
         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 c7d409e6d..b12ea4334 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -756,6 +756,8 @@ fn test(x: &str, y: isize) {
 fn infer_pattern() {
     assert_snapshot_matches!(
         infer(r#"
+struct A<T>(T);
+
 fn test(x: &i32) {
     let y = x;
     let &z = x;
@@ -772,6 +774,12 @@ fn test(x: &i32) {
 
     let lambda = |a: u64, b, c: i32| { a + b; c };
 
+    let A(n) = &A(1);
+    let A(n) = &mut A(1);
+
+    let v = &(1, &2);
+    let (_, &w) = v;
+
     let ref ref_to_x = x;
     let mut mut_x = x;
     let ref mut mut_ref_to_x = x;
@@ -779,53 +787,76 @@ fn test(x: &i32) {
 }
 "#),
         @r###"
-[9; 10) 'x': &i32
-[18; 369) '{     ...o_x; }': ()
-[28; 29) 'y': &i32
-[32; 33) 'x': &i32
-[43; 45) '&z': &i32
-[44; 45) 'z': i32
-[48; 49) 'x': &i32
-[59; 60) 'a': i32
-[63; 64) 'z': i32
-[74; 80) '(c, d)': (i32, &str)
-[75; 76) 'c': i32
-[78; 79) 'd': &str
-[83; 95) '(1, "hello")': (i32, &str)
-[84; 85) '1': i32
-[87; 94) '"hello"': &str
-[102; 152) 'for (e...     }': ()
-[106; 112) '(e, f)': ({unknown}, {unknown})
-[107; 108) 'e': {unknown}
-[110; 111) 'f': {unknown}
-[116; 125) 'some_iter': {unknown}
-[126; 152) '{     ...     }': ()
-[140; 141) 'g': {unknown}
-[144; 145) 'e': {unknown}
-[158; 205) 'if let...     }': ()
-[165; 170) '[val]': {unknown}
-[173; 176) 'opt': {unknown}
-[177; 205) '{     ...     }': ()
-[191; 192) 'h': {unknown}
-[195; 198) 'val': {unknown}
-[215; 221) 'lambda': {unknown}
-[224; 256) '|a: u6...b; c }': {unknown}
-[225; 226) 'a': u64
-[233; 234) 'b': u64
-[236; 237) 'c': i32
-[244; 256) '{ a + b; c }': i32
-[246; 247) 'a': u64
-[246; 251) 'a + b': u64
+[26; 27) 'x': &i32
+[35; 479) '{     ...o_x; }': ()
+[45; 46) 'y': &i32
+[49; 50) 'x': &i32
+[60; 62) '&z': &i32
+[61; 62) 'z': i32
+[65; 66) 'x': &i32
+[76; 77) 'a': i32
+[80; 81) 'z': i32
+[91; 97) '(c, d)': (i32, &str)
+[92; 93) 'c': i32
+[95; 96) 'd': &str
+[100; 112) '(1, "hello")': (i32, &str)
+[101; 102) '1': i32
+[104; 111) '"hello"': &str
+[119; 169) 'for (e...     }': ()
+[123; 129) '(e, f)': ({unknown}, {unknown})
+[124; 125) 'e': {unknown}
+[127; 128) 'f': {unknown}
+[133; 142) 'some_iter': {unknown}
+[143; 169) '{     ...     }': ()
+[157; 158) 'g': {unknown}
+[161; 162) 'e': {unknown}
+[175; 222) 'if let...     }': ()
+[182; 187) '[val]': {unknown}
+[190; 193) 'opt': {unknown}
+[194; 222) '{     ...     }': ()
+[208; 209) 'h': {unknown}
+[212; 215) 'val': {unknown}
+[232; 238) 'lambda': {unknown}
+[241; 273) '|a: u6...b; c }': {unknown}
+[242; 243) 'a': u64
 [250; 251) 'b': u64
 [253; 254) 'c': i32
-[267; 279) 'ref ref_to_x': &&i32
-[282; 283) 'x': &i32
-[293; 302) 'mut mut_x': &i32
-[305; 306) 'x': &i32
-[316; 336) 'ref mu...f_to_x': &mut &i32
-[339; 340) 'x': &i32
-[350; 351) 'k': &mut &i32
-[354; 366) 'mut_ref_to_x': &mut &i32"###
+[261; 273) '{ a + b; c }': i32
+[263; 264) 'a': u64
+[263; 268) 'a + b': u64
+[267; 268) 'b': u64
+[270; 271) 'c': i32
+[284; 288) 'A(n)': A<i32>
+[286; 287) 'n': &i32
+[291; 296) '&A(1)': &A<i32>
+[292; 293) 'A': A<i32>(T) -> A<T>
+[292; 296) 'A(1)': A<i32>
+[294; 295) '1': i32
+[306; 310) 'A(n)': A<i32>
+[308; 309) 'n': &mut i32
+[313; 322) '&mut A(1)': &mut A<i32>
+[318; 319) 'A': A<i32>(T) -> A<T>
+[318; 322) 'A(1)': A<i32>
+[320; 321) '1': i32
+[333; 334) 'v': &(i32, &i32)
+[337; 345) '&(1, &2)': &(i32, &i32)
+[338; 345) '(1, &2)': (i32, &i32)
+[339; 340) '1': i32
+[342; 344) '&2': &i32
+[343; 344) '2': i32
+[355; 362) '(_, &w)': (i32, &i32)
+[356; 357) '_': i32
+[359; 361) '&w': &i32
+[360; 361) 'w': i32
+[365; 366) 'v': &(i32, &i32)
+[377; 389) 'ref ref_to_x': &&i32
+[392; 393) 'x': &i32
+[403; 412) 'mut mut_x': &i32
+[415; 416) 'x': &i32
+[426; 446) 'ref mu...f_to_x': &mut &i32
+[449; 450) 'x': &i32
+[460; 461) 'k': &mut &i32
+[464; 476) 'mut_ref_to_x': &mut &i32"###
     );
 }