suggest adding/removing ref for binding patterns

Author: TaKO8Ki

compiler/rustc_typeck/src/check/pat.rs

@@ -617,7 +617,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // If there are multiple arms, make sure they all agree on
         // what the type of the binding `x` ought to be.
         if var_id != pat.hir_id {
-            self.check_binding_alt_eq_ty(pat.span, var_id, local_ty, ti);
+            self.check_binding_alt_eq_ty(ba, pat.span, var_id, local_ty, ti);
         }
 
         if let Some(p) = sub {
@@ -627,7 +627,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         local_ty
     }
 
-    fn check_binding_alt_eq_ty(&self, span: Span, var_id: HirId, ty: Ty<'tcx>, ti: TopInfo<'tcx>) {
+    fn check_binding_alt_eq_ty(
+        &self,
+        ba: hir::BindingAnnotation,
+        span: Span,
+        var_id: HirId,
+        ty: Ty<'tcx>,
+        ti: TopInfo<'tcx>,
+    ) {
         let var_ty = self.local_ty(span, var_id).decl_ty;
         if let Some(mut err) = self.demand_eqtype_pat_diag(span, var_ty, ty, ti) {
             let hir = self.tcx.hir();
@@ -645,12 +652,50 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             });
             let pre = if in_match { "in the same arm, " } else { "" };
             err.note(&format!("{}a binding must have the same type in all alternatives", pre));
-            // FIXME: check if `var_ty` and `ty` can be made the same type by adding or removing
-            // `ref` or `&` to the pattern.
+            self.suggest_adding_missing_ref_or_removing_ref(
+                &mut err,
+                span,
+                var_ty,
+                self.resolve_vars_with_obligations(ty),
+                ba,
+            );
             err.emit();
         }
     }
 
+    fn suggest_adding_missing_ref_or_removing_ref(
+        &self,
+        err: &mut Diagnostic,
+        span: Span,
+        expected: Ty<'tcx>,
+        actual: Ty<'tcx>,
+        ba: hir::BindingAnnotation,
+    ) {
+        match (expected.kind(), actual.kind(), ba) {
+            (ty::Ref(_, inner_ty, _), _, hir::BindingAnnotation::Unannotated)
+                if self.can_eq(self.param_env, *inner_ty, actual).is_ok() =>
+            {
+                err.span_suggestion_verbose(
+                    span.shrink_to_lo(),
+                    "consider adding `ref`",
+                    "ref ",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            (_, ty::Ref(_, inner_ty, _), hir::BindingAnnotation::Ref)
+                if self.can_eq(self.param_env, expected, *inner_ty).is_ok() =>
+            {
+                err.span_suggestion_verbose(
+                    span.with_hi(span.lo() + BytePos(4)),
+                    "consider removing `ref`",
+                    "",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            _ => (),
+        }
+    }
+
     // Precondition: pat is a Ref(_) pattern
     fn borrow_pat_suggestion(&self, err: &mut Diagnostic, pat: &Pat<'_>) {
         let tcx = self.tcx;

src/test/ui/mismatched_types/E0409.stderr

@@ -17,6 +17,10 @@ LL |         (0, ref y) | (y, 0) => {}
    |             first introduced with type `&{integer}` here
    |
    = note: in the same arm, a binding must have the same type in all alternatives
+help: consider adding `ref`
+   |
+LL |         (0, ref y) | (ref y, 0) => {}
+   |                       +++
 
 error: aborting due to 2 previous errors
 

src/test/ui/mismatched_types/suggest-adding-or-removing-ref-for-binding-pattern.fixed

@@ -0,0 +1,21 @@
+// run-rustfix
+#![allow(dead_code, unused_variables)]
+
+fn main() {
+    enum Blah {
+        A(isize, isize, usize),
+        B(isize, usize),
+    }
+
+    match Blah::A(1, 1, 2) {
+        Blah::A(_, x, ref y) | Blah::B(x, ref y) => {}
+        //~^ ERROR mismatched types
+        //~| ERROR variable `y` is bound inconsistently across alternatives separated by `|`
+    }
+
+    match Blah::A(1, 1, 2) {
+        Blah::A(_, x, y) | Blah::B(x, y) => {}
+        //~^ ERROR mismatched types
+        //~| variable `y` is bound inconsistently across alternatives separated by `|`
+    }
+}

src/test/ui/mismatched_types/suggest-adding-or-removing-ref-for-binding-pattern.rs

@@ -0,0 +1,21 @@
+// run-rustfix
+#![allow(dead_code, unused_variables)]
+
+fn main() {
+    enum Blah {
+        A(isize, isize, usize),
+        B(isize, usize),
+    }
+
+    match Blah::A(1, 1, 2) {
+        Blah::A(_, x, ref y) | Blah::B(x, y) => {}
+        //~^ ERROR mismatched types
+        //~| ERROR variable `y` is bound inconsistently across alternatives separated by `|`
+    }
+
+    match Blah::A(1, 1, 2) {
+        Blah::A(_, x, y) | Blah::B(x, ref y) => {}
+        //~^ ERROR mismatched types
+        //~| variable `y` is bound inconsistently across alternatives separated by `|`
+    }
+}

src/test/ui/mismatched_types/suggest-adding-or-removing-ref-for-binding-pattern.stderr

@@ -0,0 +1,49 @@
+error[E0409]: variable `y` is bound inconsistently across alternatives separated by `|`
+  --> $DIR/suggest-adding-or-removing-ref-for-binding-pattern.rs:11:43
+   |
+LL |         Blah::A(_, x, ref y) | Blah::B(x, y) => {}
+   |                           - first binding ^ bound in different ways
+
+error[E0409]: variable `y` is bound inconsistently across alternatives separated by `|`
+  --> $DIR/suggest-adding-or-removing-ref-for-binding-pattern.rs:17:43
+   |
+LL |         Blah::A(_, x, y) | Blah::B(x, ref y) => {}
+   |                       - first binding     ^ bound in different ways
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-adding-or-removing-ref-for-binding-pattern.rs:11:43
+   |
+LL |     match Blah::A(1, 1, 2) {
+   |           ---------------- this expression has type `Blah`
+LL |         Blah::A(_, x, ref y) | Blah::B(x, y) => {}
+   |                       -----               ^ expected `&usize`, found `usize`
+   |                       |
+   |                       first introduced with type `&usize` here
+   |
+   = note: in the same arm, a binding must have the same type in all alternatives
+help: consider adding `ref`
+   |
+LL |         Blah::A(_, x, ref y) | Blah::B(x, ref y) => {}
+   |                                           +++
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-adding-or-removing-ref-for-binding-pattern.rs:17:39
+   |
+LL |     match Blah::A(1, 1, 2) {
+   |           ---------------- this expression has type `Blah`
+LL |         Blah::A(_, x, y) | Blah::B(x, ref y) => {}
+   |                       -               ^^^^^ expected `usize`, found `&usize`
+   |                       |
+   |                       first introduced with type `usize` here
+   |
+   = note: in the same arm, a binding must have the same type in all alternatives
+help: consider removing `ref`
+   |
+LL -         Blah::A(_, x, y) | Blah::B(x, ref y) => {}
+LL +         Blah::A(_, x, y) | Blah::B(x, y) => {}
+   |
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0308, E0409.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/resolve/resolve-inconsistent-binding-mode.stderr

@@ -31,6 +31,10 @@ LL |         Opts::A(ref i) | Opts::B(i) => {}
    |                 first introduced with type `&isize` here
    |
    = note: in the same arm, a binding must have the same type in all alternatives
+help: consider adding `ref`
+   |
+LL |         Opts::A(ref i) | Opts::B(ref i) => {}
+   |                                  +++
 
 error[E0308]: mismatched types
   --> $DIR/resolve-inconsistent-binding-mode.rs:18:34
@@ -43,6 +47,10 @@ LL |         Opts::A(ref i) | Opts::B(i) => {}
    |                 first introduced with type `&isize` here
    |
    = note: in the same arm, a binding must have the same type in all alternatives
+help: consider adding `ref`
+   |
+LL |         Opts::A(ref i) | Opts::B(ref i) => {}
+   |                                  +++
 
 error[E0308]: mismatched types
   --> $DIR/resolve-inconsistent-binding-mode.rs:27:38

src/test/ui/resolve/resolve-inconsistent-names.rs

@@ -23,6 +23,7 @@ fn main() {
         //~| ERROR mismatched types
         //~| ERROR variable `c` is not bound in all patterns
         //~| HELP if you meant to match on unit variant `E::A`, use the full path in the pattern
+        //~| HELP consider removing `ref`
     }
 
     let z = (10, 20);

src/test/ui/resolve/resolve-inconsistent-names.stderr

@@ -55,7 +55,7 @@ LL |         (A, B) | (ref B, c) | (c, A) => ()
    |             first binding
 
 error[E0408]: variable `CONST1` is not bound in all patterns
-  --> $DIR/resolve-inconsistent-names.rs:30:23
+  --> $DIR/resolve-inconsistent-names.rs:31:23
    |
 LL |         (CONST1, _) | (_, Const2) => ()
    |          ------       ^^^^^^^^^^^ pattern doesn't bind `CONST1`
@@ -69,7 +69,7 @@ LL |     const CONST1: usize = 10;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ not accessible
 
 error[E0408]: variable `Const2` is not bound in all patterns
-  --> $DIR/resolve-inconsistent-names.rs:30:9
+  --> $DIR/resolve-inconsistent-names.rs:31:9
    |
 LL |         (CONST1, _) | (_, Const2) => ()
    |         ^^^^^^^^^^^       ------ variable not in all patterns
@@ -92,6 +92,11 @@ LL |         (A, B) | (ref B, c) | (c, A) => ()
    |             first introduced with type `E` here
    |
    = note: in the same arm, a binding must have the same type in all alternatives
+help: consider removing `ref`
+   |
+LL -         (A, B) | (ref B, c) | (c, A) => ()
+LL +         (A, B) | (B, c) | (c, A) => ()
+   |
 
 error: aborting due to 9 previous errors