2 files changed, 110 insertions, 6 deletions

diff --git a/crates/hir_ty/src/diagnostics/pattern.rs b/crates/hir_ty/src/diagnostics/pattern.rs
index 9f65e38d6..d98fb0867 100644
--- a/crates/hir_ty/src/diagnostics/pattern.rs
+++ b/crates/hir_ty/src/diagnostics/pattern.rs
@@ -239,6 +239,101 @@ impl<'a> PatCtxt<'a> {
     }
 }
 
+pub(crate) trait PatternFoldable: Sized {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.super_fold_with(folder)
+    }
+
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self;
+}
+
+pub(crate) trait PatternFolder: Sized {
+    fn fold_pattern(&mut self, pattern: &Pat) -> Pat {
+        pattern.super_fold_with(self)
+    }
+
+    fn fold_pattern_kind(&mut self, kind: &PatKind) -> PatKind {
+        kind.super_fold_with(self)
+    }
+}
+
+impl<T: PatternFoldable> PatternFoldable for Box<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        let content: T = (**self).fold_with(folder);
+        Box::new(content)
+    }
+}
+
+impl<T: PatternFoldable> PatternFoldable for Vec<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.iter().map(|t| t.fold_with(folder)).collect()
+    }
+}
+
+impl<T: PatternFoldable> PatternFoldable for Option<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.as_ref().map(|t| t.fold_with(folder))
+    }
+}
+
+macro_rules! clone_impls {
+    ($($ty:ty),+) => {
+        $(
+            impl PatternFoldable for $ty {
+                fn super_fold_with<F: PatternFolder>(&self, _: &mut F) -> Self {
+                    Clone::clone(self)
+                }
+            }
+        )+
+    }
+}
+
+clone_impls! { LocalFieldId, Ty, Substitution, EnumVariantId }
+
+impl PatternFoldable for FieldPat {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        FieldPat { field: self.field.fold_with(folder), pattern: self.pattern.fold_with(folder) }
+    }
+}
+
+impl PatternFoldable for Pat {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        folder.fold_pattern(self)
+    }
+
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        Pat { ty: self.ty.fold_with(folder), kind: self.kind.fold_with(folder) }
+    }
+}
+
+impl PatternFoldable for PatKind {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        folder.fold_pattern_kind(self)
+    }
+
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        match self {
+            PatKind::Wild => PatKind::Wild,
+            PatKind::Binding { subpattern } => {
+                PatKind::Binding { subpattern: subpattern.fold_with(folder) }
+            }
+            PatKind::Variant { substs, enum_variant, subpatterns } => PatKind::Variant {
+                substs: substs.fold_with(folder),
+                enum_variant: enum_variant.fold_with(folder),
+                subpatterns: subpatterns.fold_with(folder),
+            },
+            PatKind::Leaf { subpatterns } => {
+                PatKind::Leaf { subpatterns: subpatterns.fold_with(folder) }
+            }
+            PatKind::Deref { subpattern } => {
+                PatKind::Deref { subpattern: subpattern.fold_with(folder) }
+            }
+            &PatKind::LiteralBool { value } => PatKind::LiteralBool { value },
+            PatKind::Or { pats } => PatKind::Or { pats: pats.fold_with(folder) },
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::diagnostics::tests::check_diagnostics;
@@ -410,9 +505,8 @@ fn main() {
         _x @ true => {}
         false     => {}
     }
-    //FIXME: false negative. 
-    // Binding patterns should be expanded in `usefulness::expand_pattern()`
     match true { _x @ true => {} }
+        //^^^^ Missing match arm
 }
 "#,
         );
diff --git a/crates/hir_ty/src/diagnostics/pattern/usefulness.rs b/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
index 76dc4412f..ef2be7530 100644
--- a/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
+++ b/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
@@ -9,11 +9,11 @@ use once_cell::unsync::OnceCell;
 use rustc_hash::FxHashMap;
 use smallvec::{smallvec, SmallVec};
 
-use crate::{db::HirDatabase, InferenceResult, Ty};
+use crate::{db::HirDatabase, InferenceResult, Interner, Ty};
 
 use super::{
     deconstruct_pat::{Constructor, Fields, SplitWildcard},
-    Pat, PatId, PatKind,
+    Pat, PatId, PatKind, PatternFoldable, PatternFolder,
 };
 
 use self::{
@@ -75,8 +75,18 @@ pub(super) struct PatCtxt<'a> {
 }
 
 pub(crate) fn expand_pattern(pat: Pat) -> Pat {
-    // TODO: LiteralExpander, it is about string literal patterns
-    pat
+    LiteralExpander.fold_pattern(&pat)
+}
+
+struct LiteralExpander;
+
+impl PatternFolder for LiteralExpander {
+    fn fold_pattern(&mut self, pat: &Pat) -> Pat {
+        match (pat.ty.kind(&Interner), pat.kind.as_ref()) {
+            (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self),
+            _ => pat.super_fold_with(self),
+        }
+    }
 }
 
 impl Pat {