fix false positive for enum with no variants

JoshMcguigan · JoshMcguigan · commit 360bdf653b91 · 2020-04-16T05:37:49.000-07:00
diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
@@ -194,9 +194,10 @@ use smallvec::{smallvec, SmallVec};
 use crate::{
     db::HirDatabase,
     expr::{Body, Expr, Literal, Pat, PatId},
-    InferenceResult,
+    ApplicationTy, InferenceResult, Ty, TypeCtor,
 };
-use hir_def::{adt::VariantData, EnumVariantId, VariantId};
+use hir_def::{adt::VariantData, AdtId, EnumVariantId, VariantId};
+use ra_arena::Idx;
 
 #[derive(Debug, Clone, Copy)]
 /// Either a pattern from the source code being analyzed, represented as
@@ -512,6 +513,7 @@ pub enum Usefulness {
 }
 
 pub struct MatchCheckCtx<'a> {
+    pub match_expr: Idx<Expr>,
     pub body: Arc<Body>,
     pub infer: Arc<InferenceResult>,
     pub db: &'a dyn HirDatabase,
@@ -530,6 +532,16 @@ pub(crate) fn is_useful(
     matrix: &Matrix,
     v: &PatStack,
 ) -> MatchCheckResult<Usefulness> {
+    // Handle the special case of enums with no variants. In that case, no match
+    // arm is useful.
+    if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) =
+        cx.infer[cx.match_expr].strip_references()
+    {
+        if cx.db.enum_data(*enum_id).variants.is_empty() {
+            return Ok(Usefulness::NotUseful);
+        }
+    }
+
     if v.is_empty() {
         let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
 
@@ -1618,6 +1630,32 @@ mod tests {
 
         check_no_diagnostic(content);
     }
+
+    #[test]
+    fn enum_never() {
+        let content = r"
+            enum Never {}
+
+            fn test_fn(never: Never) {
+                match never {}
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_never_ref() {
+        let content = r"
+            enum Never {}
+
+            fn test_fn(never: &Never) {
+                match never {}
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
 }
 
 #[cfg(test)]
diff --git a/crates/ra_hir_ty/src/expr.rs b/crates/ra_hir_ty/src/expr.rs
@@ -156,7 +156,7 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             None => return,
         };
 
-        let cx = MatchCheckCtx { body, infer: infer.clone(), db };
+        let cx = MatchCheckCtx { match_expr, body, infer: infer.clone(), db };
         let pats = arms.iter().map(|arm| arm.pat);
 
         let mut seen = Matrix::empty();
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
@@ -680,6 +680,16 @@ impl Ty {
         }
     }
 
+    pub fn strip_references(&self) -> &Ty {
+        let mut t: &Ty = self;
+
+        while let Ty::Apply(ApplicationTy { ctor: TypeCtor::Ref(_mutability), parameters }) = t {
+            t = parameters.as_single();
+        }
+
+        t
+    }
+
     pub fn as_adt(&self) -> Option<(AdtId, &Substs)> {
         match self {
             Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(adt_def), parameters }) => {
