missing match arms diagnostic

1 files changed, 944 insertions, 0 deletions

diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
new file mode 100644
index 000000000..ac66dd415
--- /dev/null
+++ b/crates/ra_hir_ty/src/_match.rs
@@ -0,0 +1,944 @@
+//! This module implements match statement exhaustiveness checking and usefulness checking
+//! for match arms.
+//!
+//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which
+//! contains very detailed documentation about the match checking algorithm.
+use std::sync::Arc;
+
+use smallvec::{smallvec, SmallVec};
+
+use crate::{
+    db::HirDatabase,
+    expr::{Body, Expr, Literal, Pat, PatId},
+    InferenceResult,
+};
+use hir_def::{adt::VariantData, EnumVariantId, VariantId};
+
+#[derive(Debug, Clone, Copy)]
+enum PatIdOrWild {
+    PatId(PatId),
+    Wild,
+}
+
+impl PatIdOrWild {
+    fn as_pat(self, cx: &MatchCheckCtx) -> Pat {
+        match self {
+            PatIdOrWild::PatId(id) => cx.body.pats[id].clone(),
+            PatIdOrWild::Wild => Pat::Wild,
+        }
+    }
+
+    fn as_id(self) -> Option<PatId> {
+        match self {
+            PatIdOrWild::PatId(id) => Some(id),
+            PatIdOrWild::Wild => None,
+        }
+    }
+}
+
+impl From<PatId> for PatIdOrWild {
+    fn from(pat_id: PatId) -> Self {
+        Self::PatId(pat_id)
+    }
+}
+
+type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+#[derive(Debug)]
+pub(crate) struct PatStack(PatStackInner);
+
+impl PatStack {
+    pub(crate) fn from_pattern(pat_id: PatId) -> PatStack {
+        Self(smallvec!(pat_id.into()))
+    }
+
+    pub(crate) fn from_wild() -> PatStack {
+        Self(smallvec!(PatIdOrWild::Wild))
+    }
+
+    fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
+        Self(SmallVec::from_slice(slice))
+    }
+
+    fn from_vec(v: PatStackInner) -> PatStack {
+        Self(v)
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn head(&self) -> PatIdOrWild {
+        self.0[0]
+    }
+
+    fn get_head(&self) -> Option<PatIdOrWild> {
+        self.0.first().copied()
+    }
+
+    fn to_tail(&self) -> PatStack {
+        Self::from_slice(&self.0[1..])
+    }
+
+    fn replace_head_with(&self, pat_ids: &[PatId]) -> PatStack {
+        let mut patterns: PatStackInner = smallvec![];
+        for pat in pat_ids {
+            patterns.push((*pat).into());
+        }
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+        PatStack::from_vec(patterns)
+    }
+
+    // Computes `D(self)`.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
+        if matches!(self.head().as_pat(cx), Pat::Wild) {
+            Some(self.to_tail())
+        } else {
+            None
+        }
+    }
+
+    // Computes `S(constructor, self)`.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> Option<PatStack> {
+        match (self.head().as_pat(cx), constructor) {
+            (Pat::Tuple(ref pat_ids), Constructor::Tuple { arity }) => {
+                if pat_ids.len() != *arity {
+                    return None;
+                }
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (Pat::Lit(_), Constructor::Bool(_)) => {
+                // for now we only support bool literals
+                Some(self.to_tail())
+            }
+            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)),
+            (Pat::Path(_), Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    return None;
+                }
+                // enums with no associated data become `Pat::Path`
+                Some(self.to_tail())
+            }
+            (Pat::TupleStruct { args: ref pat_ids, .. }, Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    return None;
+                }
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (Pat::Or(_), _) => unreachable!("we desugar or patterns so this should never happen"),
+            (a, b) => unimplemented!("{:?}, {:?}", a, b),
+        }
+    }
+
+    fn expand_wildcard(&self, cx: &MatchCheckCtx, constructor: &Constructor) -> PatStack {
+        assert_eq!(
+            Pat::Wild,
+            self.head().as_pat(cx),
+            "expand_wildcard must only be called on PatStack with wild at head",
+        );
+
+        let mut patterns: PatStackInner = smallvec![];
+        let arity = match constructor {
+            Constructor::Bool(_) => 0,
+            Constructor::Tuple { arity } => *arity,
+            Constructor::Enum(e) => {
+                match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
+                    VariantData::Tuple(struct_field_data) => struct_field_data.len(),
+                    VariantData::Unit => 0,
+                    x => unimplemented!("{:?}", x),
+                }
+            }
+        };
+
+        for _ in 0..arity {
+            patterns.push(PatIdOrWild::Wild);
+        }
+
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+
+        PatStack::from_vec(patterns)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Matrix(Vec<PatStack>);
+
+impl Matrix {
+    pub(crate) fn empty() -> Self {
+        Self(vec![])
+    }
+
+    pub(crate) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
+        // if the pattern is an or pattern it should be expanded
+        if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
+            for pat_id in pat_ids {
+                self.0.push(PatStack::from_pattern(pat_id));
+            }
+        } else {
+            self.0.push(row);
+        }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn heads(&self) -> Vec<PatIdOrWild> {
+        self.0.iter().map(|p| p.head()).collect()
+    }
+
+    // Computes `D(self)`.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
+    }
+
+    // Computes `S(constructor, self)`.
+    fn specialize_constructor(&self, cx: &MatchCheckCtx, constructor: &Constructor) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_constructor(cx, constructor)))
+    }
+
+    fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
+        let mut matrix = Matrix::empty();
+
+        for pat in iter {
+            // using push ensures we expand or-patterns
+            matrix.push(cx, pat);
+        }
+
+        matrix
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Usefulness {
+    Useful,
+    NotUseful,
+}
+
+pub struct MatchCheckCtx<'a> {
+    pub body: Arc<Body>,
+    pub match_expr: &'a Expr,
+    pub infer: Arc<InferenceResult>,
+    pub db: &'a dyn HirDatabase,
+}
+
+// see src/librustc_mir_build/hair/pattern/_match.rs
+// It seems the rustc version of this method is able to assume that all the match arm
+// patterns are valid (they are valid given a particular match expression), but I
+// don't think we can make that assumption here. How should that be handled?
+//
+// Perhaps check that validity before passing the patterns into this method?
+pub(crate) fn is_useful(cx: &MatchCheckCtx, matrix: &Matrix, v: &PatStack) -> Usefulness {
+    dbg!(matrix);
+    dbg!(v);
+    if v.is_empty() {
+        if matrix.is_empty() {
+            return Usefulness::Useful;
+        } else {
+            return Usefulness::NotUseful;
+        }
+    }
+
+    if let Pat::Or(pat_ids) = v.head().as_pat(cx) {
+        let any_useful = pat_ids.iter().any(|&pat_id| {
+            let v = PatStack::from_pattern(pat_id);
+
+            is_useful(cx, matrix, &v) == Usefulness::Useful
+        });
+
+        return if any_useful { Usefulness::Useful } else { Usefulness::NotUseful };
+    }
+
+    if let Some(constructor) = pat_constructor(cx, v.head()) {
+        let matrix = matrix.specialize_constructor(&cx, &constructor);
+        let v = v.specialize_constructor(&cx, &constructor).expect("todo handle this case");
+
+        is_useful(&cx, &matrix, &v)
+    } else {
+        dbg!("expanding wildcard");
+        // expanding wildcard
+        let used_constructors: Vec<Constructor> =
+            matrix.heads().iter().filter_map(|&p| pat_constructor(cx, p)).collect();
+
+        // We assume here that the first constructor is the "correct" type. Since we
+        // only care about the "type" of the constructor (i.e. if it is a bool we
+        // don't care about the value), this assumption should be valid as long as
+        // the match statement is well formed. But potentially a better way to handle
+        // this is to use the match expressions type.
+        match &used_constructors.first() {
+            Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
+                dbg!("all constructors are covered");
+                // If all constructors are covered, then we need to consider whether
+                // any values are covered by this wildcard.
+                //
+                // For example, with matrix '[[Some(true)], [None]]', all
+                // constructors are covered (`Some`/`None`), so we need
+                // to perform specialization to see that our wildcard will cover
+                // the `Some(false)` case.
+                let constructor =
+                    matrix.heads().iter().filter_map(|&pat| pat_constructor(cx, pat)).next();
+
+                if let Some(constructor) = constructor {
+                    dbg!("found constructor {:?}, specializing..", &constructor);
+                    if let Constructor::Enum(e) = constructor {
+                        // For enums we handle each variant as a distinct constructor, so
+                        // here we create a constructor for each variant and then check
+                        // usefulness after specializing for that constructor.
+                        let any_useful = cx
+                            .db
+                            .enum_data(e.parent)
+                            .variants
+                            .iter()
+                            .map(|(local_id, _)| {
+                                Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
+                            })
+                            .any(|constructor| {
+                                let matrix = matrix.specialize_constructor(&cx, &constructor);
+                                let v = v.expand_wildcard(&cx, &constructor);
+
+                                is_useful(&cx, &matrix, &v) == Usefulness::Useful
+                            });
+
+                        if any_useful {
+                            Usefulness::Useful
+                        } else {
+                            Usefulness::NotUseful
+                        }
+                    } else {
+                        let matrix = matrix.specialize_constructor(&cx, &constructor);
+                        let v = v.expand_wildcard(&cx, &constructor);
+
+                        is_useful(&cx, &matrix, &v)
+                    }
+                } else {
+                    Usefulness::NotUseful
+                }
+            }
+            _ => {
+                // Either not all constructors are covered, or the only other arms
+                // are wildcards. Either way, this pattern is useful if it is useful
+                // when compared to those arms with wildcards.
+                let matrix = matrix.specialize_wildcard(&cx);
+                let v = v.to_tail();
+
+                is_useful(&cx, &matrix, &v)
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+enum Constructor {
+    Bool(bool),
+    Tuple { arity: usize },
+    Enum(EnumVariantId),
+}
+
+fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> Option<Constructor> {
+    match pat.as_pat(cx) {
+        Pat::Wild => None,
+        Pat::Tuple(pats) => Some(Constructor::Tuple { arity: pats.len() }),
+        Pat::Lit(lit_expr) => {
+            // for now we only support bool literals
+            match cx.body.exprs[lit_expr] {
+                Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
+                _ => unimplemented!(),
+            }
+        }
+        Pat::TupleStruct { .. } | Pat::Path(_) => {
+            let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
+            let variant_id =
+                cx.infer.variant_resolution_for_pat(pat_id).unwrap_or_else(|| unimplemented!());
+            match variant_id {
+                VariantId::EnumVariantId(enum_variant_id) => {
+                    Some(Constructor::Enum(enum_variant_id))
+                }
+                _ => unimplemented!(),
+            }
+        }
+        x => unimplemented!("{:?} not yet implemented", x),
+    }
+}
+
+fn all_constructors_covered(
+    cx: &MatchCheckCtx,
+    constructor: &Constructor,
+    used_constructors: &[Constructor],
+) -> bool {
+    match constructor {
+        Constructor::Tuple { arity } => {
+            used_constructors.iter().any(|constructor| match constructor {
+                Constructor::Tuple { arity: used_arity } => arity == used_arity,
+                _ => false,
+            })
+        }
+        Constructor::Bool(_) => {
+            if used_constructors.is_empty() {
+                return false;
+            }
+
+            let covers_true =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true)));
+            let covers_false =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false)));
+
+            covers_true && covers_false
+        }
+        Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
+            for constructor in used_constructors {
+                if let Constructor::Enum(e) = constructor {
+                    if id == e.local_id {
+                        return true;
+                    }
+                }
+            }
+
+            false
+        }),
+    }
+}
+
+fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool {
+    if let Some(VariantId::EnumVariantId(pat_variant_id)) =
+        cx.infer.variant_resolution_for_pat(pat_id)
+    {
+        if pat_variant_id.local_id == enum_variant_id.local_id {
+            return true;
+        }
+    }
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    pub(super) use insta::assert_snapshot;
+    pub(super) use ra_db::fixture::WithFixture;
+
+    pub(super) use crate::test_db::TestDB;
+
+    pub(super) fn check_diagnostic_message(content: &str) -> String {
+        TestDB::with_single_file(content).0.diagnostics().0
+    }
+
+    pub(super) fn check_diagnostic_with_no_fix(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(1, diagnostic_count, "no diagnotic reported");
+    }
+
+    pub(super) fn check_no_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(0, diagnostic_count, "expected no diagnostic, found one");
+    }
+
+    #[test]
+    fn empty_tuple_no_arms_diagnostic_message() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        assert_snapshot!(
+            check_diagnostic_message(content),
+            @"\"{\\n                }\": Missing match arm\n"
+        );
+    }
+
+    #[test]
+    fn empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    () => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                    (()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                    ((), ()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn bool_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn bool_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                    false => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                    (true, false) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, ((), true)) => {},
+                    (false, ((), false)) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, _) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, _) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::A {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::A(false) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::B => (),
+                    _ => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_2_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(_) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn or_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(true) | Either::A(false) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match (Either::A(false), Either2::C) {
+                    (Either::A(true), _) | (Either::A(false), _) => (),
+                    (Either::B(true, _), Either2::C) => (),
+                    (Either::B(false, _), Either2::C) => (),
+                    (Either::B(_, _), Either2::D) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+}
+
+#[cfg(test)]
+mod false_negatives {
+    //! The implementation of match checking here is a work in progress. As we roll this out, we
+    //! prefer false negatives to false positives (ideally there would be no false positives). This
+    //! test module should document known false negatives. Eventually we will have a complete
+    //! implementation of match checking and this module will be empty.
+    //!
+    //! The reasons for documenting known false negatives:
+    //!
+    //!   1. It acts as a backlog of work that can be done to improve the behavior of the system.
+    //!   2. It ensures the code doesn't panic when handling these cases.
+
+    use super::tests::*;
+
+    #[test]
+    fn mismatched_types() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match Either::A {
+                    Either2::C => (),
+                    Either2::D => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently check that the match arms actually
+        // match the type of the match expression.
+        check_no_diagnostic(content);
+    }
+}