3 files changed, 124 insertions, 6 deletions

diff --git a/crates/hir_ty/src/diagnostics/pattern.rs b/crates/hir_ty/src/diagnostics/pattern.rs
index 4dcbd7f9f..3e90461cc 100644
--- a/crates/hir_ty/src/diagnostics/pattern.rs
+++ b/crates/hir_ty/src/diagnostics/pattern.rs
@@ -76,4 +76,49 @@ fn main(v: S) {
 "#,
         );
     }
+
+    #[test]
+    fn c_enum() {
+        check_diagnostics(
+            r#"
+enum E { A, B }
+fn main(v: E) {
+    match v { E::A | E::B => {} }
+    match v { _           => {} }
+    match v { E::A        => {} }
+        //^ Missing match arm
+    match v {                   }
+        //^ Missing match arm
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_() {
+        check_diagnostics(
+            r#"
+struct A; struct B;
+enum E { Tuple(A, B), Struct{ a: A, b: B } }
+fn main(v: E) {
+    match v {
+        E::Tuple(a, b)    => {}
+        E::Struct{ a, b } => {}
+    }
+    match v {
+        E::Tuple(_, _) => {}
+        E::Struct{..}  => {}
+    }
+    match v {
+        E::Tuple(..) => {}
+        _ => {}
+    }
+    match v { E::Tuple(..) => {} }
+        //^ Missing match arm
+    match v { }
+        //^ Missing match arm
+}
+"#,
+        );
+    }
 }
diff --git a/crates/hir_ty/src/diagnostics/pattern/deconstruct_pat.rs b/crates/hir_ty/src/diagnostics/pattern/deconstruct_pat.rs
index 60323aea3..3c1811f95 100644
--- a/crates/hir_ty/src/diagnostics/pattern/deconstruct_pat.rs
+++ b/crates/hir_ty/src/diagnostics/pattern/deconstruct_pat.rs
@@ -135,6 +135,7 @@ impl Constructor {
             Pat::Bind { .. } | Pat::Wild => Wildcard,
             Pat::Tuple { .. } | Pat::Ref { .. } | Pat::Box { .. } => Single,
             Pat::Record { .. } | Pat::Path(_) | Pat::TupleStruct { .. } => {
+                // TODO: path to const
                 let variant_id =
                     cx.infer.variant_resolution_for_pat(pat).unwrap_or_else(|| todo!());
                 match variant_id {
@@ -144,8 +145,8 @@ impl Constructor {
             }
 
             Pat::Or(..) => panic!("bug: Or-pattern should have been expanded earlier on."),
+            Pat::Missing => todo!("Fail gracefully when there is an error in a pattern"),
             pat => todo!("Constructor::from_pat {:?}", pat),
-            // Pat::Missing => {}
             // Pat::Range { start, end } => {}
             // Pat::Slice { prefix, slice, suffix } => {}
             // Pat::Lit(_) => {}
@@ -280,7 +281,7 @@ pub(super) struct SplitWildcard {
 
 impl SplitWildcard {
     pub(super) fn new(pcx: PatCtxt<'_>) -> Self {
-        // let cx = pcx.cx;
+        let cx = pcx.cx;
         // let make_range = |start, end| IntRange(todo!());
 
         // This determines the set of all possible constructors for the type `pcx.ty`. For numbers,
@@ -292,9 +293,62 @@ impl SplitWildcard {
         // Invariant: this is empty if and only if the type is uninhabited (as determined by
         // `cx.is_uninhabited()`).
         let all_ctors = match pcx.ty.kind(&Interner) {
-            TyKind::Adt(AdtId(hir_def::AdtId::EnumId(_)), _) => todo!(),
+            TyKind::Scalar(Scalar::Bool) => todo!(),
+            // TyKind::Array(..) if ... => todo!(),
+            TyKind::Array(..) | TyKind::Slice(..) => todo!(),
+            &TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ref _substs) => {
+                let enum_data = cx.db.enum_data(enum_id);
+
+                // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
+                // additional "unknown" constructor.
+                // There is no point in enumerating all possible variants, because the user can't
+                // actually match against them all themselves. So we always return only the fictitious
+                // constructor.
+                // E.g., in an example like:
+                //
+                // ```
+                //     let err: io::ErrorKind = ...;
+                //     match err {
+                //         io::ErrorKind::NotFound => {},
+                //     }
+                // ```
+                //
+                // we don't want to show every possible IO error, but instead have only `_` as the
+                // witness.
+                let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(enum_id);
+
+                // If `exhaustive_patterns` is disabled and our scrutinee is an empty enum, we treat it
+                // as though it had an "unknown" constructor to avoid exposing its emptiness. The
+                // exception is if the pattern is at the top level, because we want empty matches to be
+                // considered exhaustive.
+                let is_secretly_empty = enum_data.variants.is_empty()
+                    && !cx.feature_exhaustive_patterns()
+                    && !pcx.is_top_level;
+
+                if is_secretly_empty || is_declared_nonexhaustive {
+                    smallvec![NonExhaustive]
+                } else if cx.feature_exhaustive_patterns() {
+                    // If `exhaustive_patterns` is enabled, we exclude variants known to be
+                    // uninhabited.
+                    todo!()
+                } else {
+                    enum_data
+                        .variants
+                        .iter()
+                        .map(|(local_id, ..)| Variant(EnumVariantId { parent: enum_id, local_id }))
+                        .collect()
+                }
+            }
+            TyKind::Scalar(Scalar::Char) => todo!(),
+            TyKind::Scalar(Scalar::Int(..)) | TyKind::Scalar(Scalar::Uint(..)) => todo!(),
+            TyKind::Never if !cx.feature_exhaustive_patterns() && !pcx.is_top_level => {
+                smallvec![NonExhaustive]
+            }
+            TyKind::Never => SmallVec::new(),
+            _ if cx.is_uninhabited(&pcx.ty) => SmallVec::new(),
             TyKind::Adt(..) | TyKind::Tuple(..) | TyKind::Ref(..) => smallvec![Single],
-            _ => todo!(),
+            // This type is one for which we cannot list constructors, like `str` or `f64`.
+            _ => smallvec![NonExhaustive],
         };
         SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
     }
@@ -496,7 +550,7 @@ impl Fields {
     pub(super) fn apply(self, pcx: PatCtxt<'_>, ctor: &Constructor) -> Pat {
         let subpatterns_and_indices = self.patterns_and_indices();
         let mut subpatterns = subpatterns_and_indices.iter().map(|&(_, p)| p);
-        // TODO witnesses are not yet used 
+        // TODO witnesses are not yet used
         const TODO: Pat = Pat::Wild;
 
         match ctor {
diff --git a/crates/hir_ty/src/diagnostics/pattern/usefulness.rs b/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
index 4b55aee97..2df87ccea 100644
--- a/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
+++ b/crates/hir_ty/src/diagnostics/pattern/usefulness.rs
@@ -3,7 +3,11 @@
 
 use std::{cell::RefCell, iter::FromIterator, ops::Index, sync::Arc};
 
-use hir_def::{ModuleId, body::Body, expr::{ExprId, Pat, PatId}};
+use hir_def::{
+    body::Body,
+    expr::{ExprId, Pat, PatId},
+    HasModule, ModuleId,
+};
 use la_arena::Arena;
 use once_cell::unsync::OnceCell;
 use rustc_hash::FxHashMap;
@@ -36,6 +40,21 @@ impl<'a> MatchCheckCtx<'a> {
         false
     }
 
+    /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
+    pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool {
+        let has_non_exhaustive_attr =
+            self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists();
+        let is_local =
+            hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
+        has_non_exhaustive_attr && !is_local
+    }
+
+    // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
+    pub(super) fn feature_exhaustive_patterns(&self) -> bool {
+        // TODO
+        false
+    }
+
     pub(super) fn alloc_pat(&self, pat: Pat, ty: &Ty) -> PatId {
         self.pattern_arena.borrow_mut().alloc(pat, ty)
     }