Respect binding mode of a binding pattern for exhaustiveness check

Author: iDawer

crates/hir_ty/src/diagnostics/match_check.rs

@@ -11,8 +11,11 @@ pub(crate) mod deconstruct_pat;
 pub(crate) mod usefulness;
 
 use hir_def::{body::Body, expr::PatId, EnumVariantId, LocalFieldId, VariantId};
+use stdx::never;
 
-use crate::{db::HirDatabase, InferenceResult, Interner, Substitution, Ty, TyKind};
+use crate::{
+    db::HirDatabase, infer::BindingMode, InferenceResult, Interner, Substitution, Ty, TyKind,
+};
 
 use self::pat_util::EnumerateAndAdjustIterator;
 
@@ -21,6 +24,7 @@ pub(crate) use self::usefulness::MatchArm;
 #[derive(Clone, Debug)]
 pub(crate) enum PatternError {
     Unimplemented,
+    UnexpectedType,
     UnresolvedVariant,
     MissingField,
     ExtraFields,
@@ -129,9 +133,16 @@ impl<'a> PatCtxt<'a> {
                 PatKind::Leaf { subpatterns }
             }
 
-            hir_def::expr::Pat::Bind { subpat, .. } => {
-                if let TyKind::Ref(.., rty) = ty.kind(Interner) {
-                    ty = rty;
+            hir_def::expr::Pat::Bind { ref name, subpat, .. } => {
+                let bm = self.infer.pat_binding_modes[&pat];
+                match (bm, ty.kind(Interner)) {
+                    (BindingMode::Ref(_), TyKind::Ref(.., rty)) => ty = rty,
+                    (BindingMode::Ref(_), _) => {
+                        never!("`ref {}` has wrong type {:?}", name, ty);
+                        self.errors.push(PatternError::UnexpectedType);
+                        return Pat { ty: ty.clone(), kind: PatKind::Wild.into() };
+                    }
+                    _ => (),
                 }
                 PatKind::Binding { subpattern: self.lower_opt_pattern(subpat) }
             }

crates/hir_ty/src/infer.rs

@@ -100,7 +100,7 @@ impl_from!(ExprId, PatId for ExprOrPatId);
 /// Binding modes inferred for patterns.
 /// <https://doc.rust-lang.org/reference/patterns.html#binding-modes>
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
-enum BindingMode {
+pub enum BindingMode {
     Move,
     Ref(Mutability),
 }
@@ -292,6 +292,7 @@ pub struct InferenceResult {
     standard_types: InternedStandardTypes,
     /// Stores the types which were implicitly dereferenced in pattern binding modes.
     pub pat_adjustments: FxHashMap<PatId, Vec<Adjustment>>,
+    pub pat_binding_modes: FxHashMap<PatId, BindingMode>,
     pub expr_adjustments: FxHashMap<ExprId, Vec<Adjustment>>,
 }
 

crates/hir_ty/src/infer/pat.rs

@@ -204,6 +204,8 @@ impl<'a> InferenceContext<'a> {
                 } else {
                     BindingMode::convert(*mode)
                 };
+                self.result.pat_binding_modes.insert(pat, mode);
+
                 let inner_ty = match subpat {
                     Some(subpat) => self.infer_pat(*subpat, &expected, default_bm),
                     None => expected,

crates/ide_diagnostics/src/handlers/missing_match_arms.rs

@@ -883,7 +883,20 @@ static __: () = {
     match &n { Next(E::Foo | E::Bar) => {} }
     match &n {      _ | Next(E::Bar) => {} }
 };",
+        );
+    }
 
+    #[test]
+    fn binding_mode_by_ref() {
+        check_diagnostics_no_bails(
+            r"
+enum E{ A, B }
+fn foo() {
+    match &E::A {
+        E::A => {}
+        x => {}
+    }
+}",
         );
     }