diff --git a/Cargo.lock b/Cargo.lock
index 3110f32ade968..0b9eb88ec2b39 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4440,6 +4440,8 @@ dependencies = [
  "rustc_target",
  "smallvec",
  "tracing",
+ "tracing-subscriber",
+ "tracing-tree",
 ]
 
 [[package]]
diff --git a/compiler/rustc_pattern_analysis/Cargo.toml b/compiler/rustc_pattern_analysis/Cargo.toml
index b9bdcb41929dc..6357d18b9da84 100644
--- a/compiler/rustc_pattern_analysis/Cargo.toml
+++ b/compiler/rustc_pattern_analysis/Cargo.toml
@@ -22,6 +22,10 @@ smallvec = { version = "1.8.1", features = ["union"] }
 tracing = "0.1"
 # tidy-alphabetical-end
 
+[dev-dependencies]
+tracing-subscriber = { version = "0.3.3", default-features = false, features = ["fmt", "env-filter", "ansi"] }
+tracing-tree = "0.2.0"
+
 [features]
 default = ["rustc"]
 rustc = [
diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index 5b58d7f43b2b7..95c5556410d34 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -819,6 +819,81 @@ impl<Cx: PatCx> Constructor<Cx> {
             }
         })
     }
+
+    pub(crate) fn fmt_fields(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        ty: &Cx::Ty,
+        mut fields: impl Iterator<Item = impl fmt::Debug>,
+    ) -> fmt::Result {
+        let mut first = true;
+        let mut start_or_continue = |s| {
+            if first {
+                first = false;
+                ""
+            } else {
+                s
+            }
+        };
+        let mut start_or_comma = || start_or_continue(", ");
+
+        match self {
+            Struct | Variant(_) | UnionField => {
+                Cx::write_variant_name(f, self, ty)?;
+                // Without `cx`, we can't know which field corresponds to which, so we can't
+                // get the names of the fields. Instead we just display everything as a tuple
+                // struct, which should be good enough.
+                write!(f, "(")?;
+                for p in fields {
+                    write!(f, "{}{:?}", start_or_comma(), p)?;
+                }
+                write!(f, ")")?;
+            }
+            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
+            // be careful to detect strings here. However a string literal pattern will never
+            // be reported as a non-exhaustiveness witness, so we can ignore this issue.
+            Ref => {
+                write!(f, "&{:?}", &fields.next().unwrap())?;
+            }
+            Slice(slice) => {
+                write!(f, "[")?;
+                match slice.kind {
+                    SliceKind::FixedLen(_) => {
+                        for p in fields {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                    }
+                    SliceKind::VarLen(prefix_len, _) => {
+                        for p in fields.by_ref().take(prefix_len) {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                        write!(f, "{}..", start_or_comma())?;
+                        for p in fields {
+                            write!(f, "{}{:?}", start_or_comma(), p)?;
+                        }
+                    }
+                }
+                write!(f, "]")?;
+            }
+            Bool(b) => write!(f, "{b}")?,
+            // Best-effort, will render signed ranges incorrectly
+            IntRange(range) => write!(f, "{range:?}")?,
+            F32Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
+            F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
+            Str(value) => write!(f, "{value:?}")?,
+            Opaque(..) => write!(f, "<constant pattern>")?,
+            Or => {
+                for pat in fields {
+                    write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
+                }
+            }
+            Never => write!(f, "!")?,
+            Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
+                write!(f, "_ : {:?}", ty)?
+            }
+        }
+        Ok(())
+    }
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index 5c57c990323c8..1a1da5c55f608 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -49,6 +49,12 @@ pub mod index {
         }
     }
 
+    impl<V> FromIterator<V> for IdxContainer<usize, V> {
+        fn from_iter<T: IntoIterator<Item = V>>(iter: T) -> Self {
+            Self(iter.into_iter().enumerate().collect())
+        }
+    }
+
     #[derive(Debug)]
     pub struct IdxSet<T>(pub rustc_hash::FxHashSet<T>);
     impl<T: Idx> IdxSet<T> {
@@ -120,7 +126,8 @@ pub trait PatCx: Sized + fmt::Debug {
     /// `DeconstructedPat`. Only invoqued when `pat.ctor()` is `Struct | Variant(_) | UnionField`.
     fn write_variant_name(
         f: &mut fmt::Formatter<'_>,
-        pat: &crate::pat::DeconstructedPat<Self>,
+        ctor: &crate::constructor::Constructor<Self>,
+        ty: &Self::Ty,
     ) -> fmt::Result;
 
     /// Raise a bug.
diff --git a/compiler/rustc_pattern_analysis/src/pat.rs b/compiler/rustc_pattern_analysis/src/pat.rs
index e7eed673c9441..5f388ee9f8979 100644
--- a/compiler/rustc_pattern_analysis/src/pat.rs
+++ b/compiler/rustc_pattern_analysis/src/pat.rs
@@ -138,81 +138,11 @@ impl<Cx: PatCx> DeconstructedPat<Cx> {
 /// This is best effort and not good enough for a `Display` impl.
 impl<Cx: PatCx> fmt::Debug for DeconstructedPat<Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let pat = self;
-        let mut first = true;
-        let mut start_or_continue = |s| {
-            if first {
-                first = false;
-                ""
-            } else {
-                s
-            }
-        };
-        let mut start_or_comma = || start_or_continue(", ");
-
         let mut fields: Vec<_> = (0..self.arity).map(|_| PatOrWild::Wild).collect();
         for ipat in self.iter_fields() {
             fields[ipat.idx] = PatOrWild::Pat(&ipat.pat);
         }
-
-        match pat.ctor() {
-            Struct | Variant(_) | UnionField => {
-                Cx::write_variant_name(f, pat)?;
-                // Without `cx`, we can't know which field corresponds to which, so we can't
-                // get the names of the fields. Instead we just display everything as a tuple
-                // struct, which should be good enough.
-                write!(f, "(")?;
-                for p in fields {
-                    write!(f, "{}", start_or_comma())?;
-                    write!(f, "{p:?}")?;
-                }
-                write!(f, ")")
-            }
-            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
-            // be careful to detect strings here. However a string literal pattern will never
-            // be reported as a non-exhaustiveness witness, so we can ignore this issue.
-            Ref => {
-                write!(f, "&{:?}", &fields[0])
-            }
-            Slice(slice) => {
-                write!(f, "[")?;
-                match slice.kind {
-                    SliceKind::FixedLen(_) => {
-                        for p in fields {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                    }
-                    SliceKind::VarLen(prefix_len, _) => {
-                        for p in &fields[..prefix_len] {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                        write!(f, "{}", start_or_comma())?;
-                        write!(f, "..")?;
-                        for p in &fields[prefix_len..] {
-                            write!(f, "{}{:?}", start_or_comma(), p)?;
-                        }
-                    }
-                }
-                write!(f, "]")
-            }
-            Bool(b) => write!(f, "{b}"),
-            // Best-effort, will render signed ranges incorrectly
-            IntRange(range) => write!(f, "{range:?}"),
-            F32Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
-            F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}"),
-            Str(value) => write!(f, "{value:?}"),
-            Opaque(..) => write!(f, "<constant pattern>"),
-            Or => {
-                for pat in fields {
-                    write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
-                }
-                Ok(())
-            }
-            Never => write!(f, "!"),
-            Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
-                write!(f, "_ : {:?}", pat.ty())
-            }
-        }
+        self.ctor().fmt_fields(f, self.ty(), fields.into_iter())
     }
 }
 
@@ -295,7 +225,6 @@ impl<'p, Cx: PatCx> fmt::Debug for PatOrWild<'p, Cx> {
 
 /// Same idea as `DeconstructedPat`, except this is a fictitious pattern built up for diagnostics
 /// purposes. As such they don't use interning and can be cloned.
-#[derive(Debug)]
 pub struct WitnessPat<Cx: PatCx> {
     ctor: Constructor<Cx>,
     pub(crate) fields: Vec<WitnessPat<Cx>>,
@@ -353,3 +282,10 @@ impl<Cx: PatCx> WitnessPat<Cx> {
         self.fields.iter()
     }
 }
+
+/// This is best effort and not good enough for a `Display` impl.
+impl<Cx: PatCx> fmt::Debug for WitnessPat<Cx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.ctor().fmt_fields(f, self.ty(), self.fields.iter())
+    }
+}
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index eedc00a561304..f600c20d38470 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -874,13 +874,14 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
 
     fn write_variant_name(
         f: &mut fmt::Formatter<'_>,
-        pat: &crate::pat::DeconstructedPat<Self>,
+        ctor: &crate::constructor::Constructor<Self>,
+        ty: &Self::Ty,
     ) -> fmt::Result {
-        if let ty::Adt(adt, _) = pat.ty().kind() {
+        if let ty::Adt(adt, _) = ty.kind() {
             if adt.is_box() {
                 write!(f, "Box")?
             } else {
-                let variant = adt.variant(Self::variant_index_for_adt(pat.ctor(), *adt));
+                let variant = adt.variant(Self::variant_index_for_adt(ctor, *adt));
                 write!(f, "{}", variant.name)?;
             }
         }
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index 3760db8b68894..cdc03eaeb37c1 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -1042,7 +1042,7 @@ struct MatrixRow<'p, Cx: PatCx> {
     is_under_guard: bool,
     /// When we specialize, we remember which row of the original matrix produced a given row of the
     /// specialized matrix. When we unspecialize, we use this to propagate usefulness back up the
-    /// callstack.
+    /// callstack. On creation, this stores the index of the original match arm.
     parent_row: usize,
     /// False when the matrix is just built. This is set to `true` by
     /// [`compute_exhaustiveness_and_usefulness`] if the arm is found to be useful.
@@ -1163,10 +1163,10 @@ impl<'p, Cx: PatCx> Matrix<'p, Cx> {
             place_info: smallvec![place_info],
             wildcard_row_is_relevant: true,
         };
-        for (row_id, arm) in arms.iter().enumerate() {
+        for (arm_id, arm) in arms.iter().enumerate() {
             let v = MatrixRow {
                 pats: PatStack::from_pattern(arm.pat),
-                parent_row: row_id, // dummy, we don't read it
+                parent_row: arm_id,
                 is_under_guard: arm.has_guard,
                 useful: false,
                 intersects: BitSet::new_empty(0), // Initialized in `Matrix::expand_and_push`.
@@ -1738,6 +1738,9 @@ pub struct UsefulnessReport<'p, Cx: PatCx> {
     /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
     /// exhaustiveness.
     pub non_exhaustiveness_witnesses: Vec<WitnessPat<Cx>>,
+    /// For each arm, a set of indices of arms above it that have non-empty intersection, i.e. there
+    /// is a value matched by both arms. This may miss real intersections.
+    pub arm_intersections: Vec<BitSet<usize>>,
 }
 
 /// Computes whether a match is exhaustive and which of its arms are useful.
@@ -1769,5 +1772,19 @@ pub fn compute_match_usefulness<'p, Cx: PatCx>(
         })
         .collect();
 
-    Ok(UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses })
+    let mut arm_intersections: Vec<_> =
+        arms.iter().enumerate().map(|(i, _)| BitSet::new_empty(i)).collect();
+    for row in matrix.rows() {
+        let arm_id = row.parent_row;
+        for intersection in row.intersects.iter() {
+            // Convert the matrix row ids into arm ids (they can differ because we expand or-patterns).
+            let arm_intersection = matrix.rows[intersection].parent_row;
+            // Note: self-intersection can happen with or-patterns.
+            if arm_intersection != arm_id {
+                arm_intersections[arm_id].insert(arm_intersection);
+            }
+        }
+    }
+
+    Ok(UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses, arm_intersections })
 }
diff --git a/compiler/rustc_pattern_analysis/tests/common/mod.rs b/compiler/rustc_pattern_analysis/tests/common/mod.rs
new file mode 100644
index 0000000000000..e72fddb9e9a81
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/tests/common/mod.rs
@@ -0,0 +1,315 @@
+use rustc_pattern_analysis::{
+    constructor::{
+        Constructor, ConstructorSet, IntRange, MaybeInfiniteInt, RangeEnd, VariantVisibility,
+    },
+    usefulness::{PlaceValidity, UsefulnessReport},
+    Captures, MatchArm, PatCx, PrivateUninhabitedField,
+};
+
+/// Sets up `tracing` for easier debugging. Tries to look like the `rustc` setup.
+pub fn init_tracing() {
+    use tracing_subscriber::layer::SubscriberExt;
+    use tracing_subscriber::util::SubscriberInitExt;
+    use tracing_subscriber::Layer;
+    let _ = tracing_tree::HierarchicalLayer::default()
+        .with_writer(std::io::stderr)
+        .with_indent_lines(true)
+        .with_ansi(true)
+        .with_targets(true)
+        .with_indent_amount(2)
+        .with_subscriber(
+            tracing_subscriber::Registry::default()
+                .with(tracing_subscriber::EnvFilter::from_default_env()),
+        )
+        .try_init();
+}
+
+/// A simple set of types.
+#[allow(dead_code)]
+#[derive(Debug, Copy, Clone)]
+pub enum Ty {
+    /// Booleans
+    Bool,
+    /// 8-bit unsigned integers
+    U8,
+    /// Tuples.
+    Tuple(&'static [Ty]),
+    /// A struct with `arity` fields of type `ty`.
+    BigStruct { arity: usize, ty: &'static Ty },
+    /// A enum with `arity` variants of type `ty`.
+    BigEnum { arity: usize, ty: &'static Ty },
+}
+
+/// The important logic.
+impl Ty {
+    pub fn sub_tys(&self, ctor: &Constructor<Cx>) -> Vec<Self> {
+        use Constructor::*;
+        match (ctor, *self) {
+            (Struct, Ty::Tuple(tys)) => tys.iter().copied().collect(),
+            (Struct, Ty::BigStruct { arity, ty }) => (0..arity).map(|_| *ty).collect(),
+            (Variant(_), Ty::BigEnum { ty, .. }) => vec![*ty],
+            (Bool(..) | IntRange(..) | NonExhaustive | Missing | Wildcard, _) => vec![],
+            _ => panic!("Unexpected ctor {ctor:?} for type {self:?}"),
+        }
+    }
+
+    pub fn ctor_set(&self) -> ConstructorSet<Cx> {
+        match *self {
+            Ty::Bool => ConstructorSet::Bool,
+            Ty::U8 => ConstructorSet::Integers {
+                range_1: IntRange::from_range(
+                    MaybeInfiniteInt::new_finite_uint(0),
+                    MaybeInfiniteInt::new_finite_uint(255),
+                    RangeEnd::Included,
+                ),
+                range_2: None,
+            },
+            Ty::Tuple(..) | Ty::BigStruct { .. } => ConstructorSet::Struct { empty: false },
+            Ty::BigEnum { arity, .. } => ConstructorSet::Variants {
+                variants: (0..arity).map(|_| VariantVisibility::Visible).collect(),
+                non_exhaustive: false,
+            },
+        }
+    }
+
+    pub fn write_variant_name(
+        &self,
+        f: &mut std::fmt::Formatter<'_>,
+        ctor: &Constructor<Cx>,
+    ) -> std::fmt::Result {
+        match (*self, ctor) {
+            (Ty::Tuple(..), _) => Ok(()),
+            (Ty::BigStruct { .. }, _) => write!(f, "BigStruct"),
+            (Ty::BigEnum { .. }, Constructor::Variant(i)) => write!(f, "BigEnum::Variant{i}"),
+            _ => write!(f, "{:?}::{:?}", self, ctor),
+        }
+    }
+}
+
+/// Compute usefulness in our simple context (and set up tracing for easier debugging).
+pub fn compute_match_usefulness<'p>(
+    arms: &[MatchArm<'p, Cx>],
+    ty: Ty,
+    scrut_validity: PlaceValidity,
+    complexity_limit: Option<usize>,
+) -> Result<UsefulnessReport<'p, Cx>, ()> {
+    init_tracing();
+    rustc_pattern_analysis::usefulness::compute_match_usefulness(
+        &Cx,
+        arms,
+        ty,
+        scrut_validity,
+        complexity_limit,
+    )
+}
+
+#[derive(Debug)]
+pub struct Cx;
+
+/// The context for pattern analysis. Forwards anything interesting to `Ty` methods.
+impl PatCx for Cx {
+    type Ty = Ty;
+    type Error = ();
+    type VariantIdx = usize;
+    type StrLit = ();
+    type ArmData = ();
+    type PatData = ();
+
+    fn is_exhaustive_patterns_feature_on(&self) -> bool {
+        false
+    }
+
+    fn is_min_exhaustive_patterns_feature_on(&self) -> bool {
+        false
+    }
+
+    fn ctor_arity(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> usize {
+        ty.sub_tys(ctor).len()
+    }
+
+    fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a Constructor<Self>,
+        ty: &'a Self::Ty,
+    ) -> impl Iterator<Item = (Self::Ty, PrivateUninhabitedField)> + ExactSizeIterator + Captures<'a>
+    {
+        ty.sub_tys(ctor).into_iter().map(|ty| (ty, PrivateUninhabitedField(false)))
+    }
+
+    fn ctors_for_ty(&self, ty: &Self::Ty) -> Result<ConstructorSet<Self>, Self::Error> {
+        Ok(ty.ctor_set())
+    }
+
+    fn write_variant_name(
+        f: &mut std::fmt::Formatter<'_>,
+        ctor: &Constructor<Self>,
+        ty: &Self::Ty,
+    ) -> std::fmt::Result {
+        ty.write_variant_name(f, ctor)
+    }
+
+    fn bug(&self, fmt: std::fmt::Arguments<'_>) -> Self::Error {
+        panic!("{}", fmt)
+    }
+
+    /// Abort when reaching the complexity limit. This is what we'll check in tests.
+    fn complexity_exceeded(&self) -> Result<(), Self::Error> {
+        Err(())
+    }
+}
+
+/// Construct a single pattern; see `pats!()`.
+#[allow(unused_macros)]
+macro_rules! pat {
+    ($($rest:tt)*) => {{
+        let mut vec = pats!($($rest)*);
+        vec.pop().unwrap()
+    }};
+}
+
+/// A macro to construct patterns. Called like `pats!(type_expr; pattern, pattern, ..)` and returns
+/// a `Vec<DeconstructedPat>`. A pattern can be nested and looks like `Constructor(pat, pat)` or
+/// `Constructor { .i: pat, .j: pat }`, where `Constructor` is `Struct`, `Variant.i` (with index
+/// `i`), as well as booleans and integer ranges.
+///
+/// The general structure of the macro is a tt-muncher with several stages identified with
+/// `@something(args)`. The args are a key-value list (the keys ensure we don't mix the arguments
+/// around) which is passed down and modified as needed. We then parse token-trees from
+/// left-to-right. Non-trivial recursion happens when we parse the arguments to a pattern: we
+/// recurse to parse the tokens inside `{..}`/`(..)`, and then we continue parsing anything that
+/// follows.
+macro_rules! pats {
+    // Entrypoint
+    // Parse `type; ..`
+    ($ty:expr; $($rest:tt)*) => {{
+        #[allow(unused_imports)]
+        use rustc_pattern_analysis::{
+            constructor::{Constructor, IntRange, MaybeInfiniteInt, RangeEnd},
+            pat::DeconstructedPat,
+        };
+        let ty = $ty;
+        // The heart of the macro is designed to push `IndexedPat`s into a `Vec`, so we work around
+        // that.
+        let sub_tys = ::std::iter::repeat(&ty);
+        let mut vec = Vec::new();
+        pats!(@ctor(vec:vec, sub_tys:sub_tys, idx:0) $($rest)*);
+        vec.into_iter().map(|ipat| ipat.pat).collect::<Vec<_>>()
+    }};
+
+    // Parse `constructor ..`
+
+    (@ctor($($args:tt)*) true $($rest:tt)*) => {{
+        let ctor = Constructor::Bool(true);
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) false $($rest:tt)*) => {{
+        let ctor = Constructor::Bool(false);
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) Struct $($rest:tt)*) => {{
+        let ctor = Constructor::Struct;
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) ( $($fields:tt)* ) $($rest:tt)*) => {{
+        let ctor = Constructor::Struct; // tuples
+        pats!(@pat($($args)*, ctor:ctor) ( $($fields)* ) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) Variant.$variant:ident $($rest:tt)*) => {{
+        let ctor = Constructor::Variant($variant);
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) Variant.$variant:literal $($rest:tt)*) => {{
+        let ctor = Constructor::Variant($variant);
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) _ $($rest:tt)*) => {{
+        let ctor = Constructor::Wildcard;
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+
+    // Integers and int ranges
+    (@ctor($($args:tt)*) $($start:literal)?..$end:literal $($rest:tt)*) => {{
+        let ctor = Constructor::IntRange(IntRange::from_range(
+            pats!(@rangeboundary- $($start)?),
+            pats!(@rangeboundary+ $end),
+            RangeEnd::Excluded,
+        ));
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) $($start:literal)?.. $($rest:tt)*) => {{
+        let ctor = Constructor::IntRange(IntRange::from_range(
+            pats!(@rangeboundary- $($start)?),
+            pats!(@rangeboundary+),
+            RangeEnd::Excluded,
+        ));
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) $($start:literal)?..=$end:literal $($rest:tt)*) => {{
+        let ctor = Constructor::IntRange(IntRange::from_range(
+            pats!(@rangeboundary- $($start)?),
+            pats!(@rangeboundary+ $end),
+            RangeEnd::Included,
+        ));
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    (@ctor($($args:tt)*) $int:literal $($rest:tt)*) => {{
+        let ctor = Constructor::IntRange(IntRange::from_range(
+            pats!(@rangeboundary- $int),
+            pats!(@rangeboundary+ $int),
+            RangeEnd::Included,
+        ));
+        pats!(@pat($($args)*, ctor:ctor) $($rest)*)
+    }};
+    // Utility to manage range boundaries.
+    (@rangeboundary $sign:tt $int:literal) => { MaybeInfiniteInt::new_finite_uint($int) };
+    (@rangeboundary -) => { MaybeInfiniteInt::NegInfinity };
+    (@rangeboundary +) => { MaybeInfiniteInt::PosInfinity };
+
+    // Parse subfields: `(..)` or `{..}`
+
+    // Constructor with no fields, e.g. `bool` or `Variant.1`.
+    (@pat($($args:tt)*) $(,)?) => {
+        pats!(@pat($($args)*) {})
+    };
+    (@pat($($args:tt)*) , $($rest:tt)*) => {
+        pats!(@pat($($args)*) {}, $($rest)*)
+    };
+    // `(..)` and `{..}` are treated the same.
+    (@pat($($args:tt)*) ( $($subpat:tt)* ) $($rest:tt)*) => {{
+        pats!(@pat($($args)*) { $($subpat)* } $($rest)*)
+    }};
+    (@pat(vec:$vec:expr, sub_tys:$sub_tys:expr, idx:$idx:expr, ctor:$ctor:expr) { $($fields:tt)* } $($rest:tt)*) => {{
+        let sub_tys = $sub_tys;
+        let index = $idx;
+        // Silly dance to work with both a vec and `iter::repeat()`.
+        let ty = *(&sub_tys).clone().into_iter().nth(index).unwrap();
+        let ctor = $ctor;
+        let ctor_sub_tys = &ty.sub_tys(&ctor);
+        #[allow(unused_mut)]
+        let mut fields = Vec::new();
+        // Parse subpatterns (note the leading comma).
+        pats!(@fields(idx:0, vec:fields, sub_tys:ctor_sub_tys) ,$($fields)*);
+        let arity = ctor_sub_tys.len();
+        let pat = DeconstructedPat::new(ctor, fields, arity, ty, ()).at_index(index);
+        $vec.push(pat);
+
+        // Continue parsing further patterns.
+        pats!(@fields(idx:index+1, vec:$vec, sub_tys:sub_tys) $($rest)*);
+    }};
+
+    // Parse fields one by one.
+
+    // No fields left.
+    (@fields($($args:tt)*) $(,)?) => {};
+    // `.i: pat` sets the current index to `i`.
+    (@fields(idx:$_idx:expr, $($args:tt)*) , .$idx:literal : $($rest:tt)*) => {{
+        pats!(@ctor($($args)*, idx:$idx) $($rest)*);
+    }};
+    (@fields(idx:$_idx:expr, $($args:tt)*) , .$idx:ident : $($rest:tt)*) => {{
+        pats!(@ctor($($args)*, idx:$idx) $($rest)*);
+    }};
+    // Field without an explicit index; we use the current index which gets incremented above.
+    (@fields(idx:$idx:expr, $($args:tt)*) , $($rest:tt)*) => {{
+        pats!(@ctor($($args)*, idx:$idx) $($rest)*);
+    }};
+}
diff --git a/compiler/rustc_pattern_analysis/tests/complexity.rs b/compiler/rustc_pattern_analysis/tests/complexity.rs
new file mode 100644
index 0000000000000..93f455c6257d2
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/tests/complexity.rs
@@ -0,0 +1,109 @@
+//! Test the pattern complexity limit.
+use common::*;
+use rustc_pattern_analysis::{pat::DeconstructedPat, usefulness::PlaceValidity, MatchArm};
+
+#[macro_use]
+mod common;
+
+/// Analyze a match made of these patterns. Ignore the report; we only care whether we exceeded the
+/// limit or not.
+fn check(patterns: &[DeconstructedPat<Cx>], complexity_limit: usize) -> Result<(), ()> {
+    let ty = *patterns[0].ty();
+    let arms: Vec<_> =
+        patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
+    compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, Some(complexity_limit))
+        .map(|_report| ())
+}
+
+/// Asserts that analyzing this match takes exactly `complexity` steps.
+#[track_caller]
+fn assert_complexity(patterns: Vec<DeconstructedPat<Cx>>, complexity: usize) {
+    assert!(check(&patterns, complexity).is_ok());
+    assert!(check(&patterns, complexity - 1).is_err());
+}
+
+/// Construct a match like:
+/// ```ignore(illustrative)
+/// match ... {
+///     BigStruct { field01: true, .. } => {}
+///     BigStruct { field02: true, .. } => {}
+///     BigStruct { field03: true, .. } => {}
+///     BigStruct { field04: true, .. } => {}
+///     ...
+///     _ => {}
+/// }
+/// ```
+fn diagonal_match(arity: usize) -> Vec<DeconstructedPat<Cx>> {
+    let struct_ty = Ty::BigStruct { arity, ty: &Ty::Bool };
+    let mut patterns = vec![];
+    for i in 0..arity {
+        patterns.push(pat!(struct_ty; Struct { .i: true }));
+    }
+    patterns.push(pat!(struct_ty; _));
+    patterns
+}
+
+/// Construct a match like:
+/// ```ignore(illustrative)
+/// match ... {
+///     BigStruct { field01: true, .. } => {}
+///     BigStruct { field02: true, .. } => {}
+///     BigStruct { field03: true, .. } => {}
+///     BigStruct { field04: true, .. } => {}
+///     ...
+///     BigStruct { field01: false, .. } => {}
+///     BigStruct { field02: false, .. } => {}
+///     BigStruct { field03: false, .. } => {}
+///     BigStruct { field04: false, .. } => {}
+///     ...
+///     _ => {}
+/// }
+/// ```
+fn diagonal_exponential_match(arity: usize) -> Vec<DeconstructedPat<Cx>> {
+    let struct_ty = Ty::BigStruct { arity, ty: &Ty::Bool };
+    let mut patterns = vec![];
+    for i in 0..arity {
+        patterns.push(pat!(struct_ty; Struct { .i: true }));
+    }
+    for i in 0..arity {
+        patterns.push(pat!(struct_ty; Struct { .i: false }));
+    }
+    patterns.push(pat!(struct_ty; _));
+    patterns
+}
+
+#[test]
+fn test_diagonal_struct_match() {
+    // These cases are nicely linear: we check `arity` patterns with exactly one `true`, matching
+    // in 2 branches each, and a final pattern with all `false`, matching only the `_` branch.
+    assert_complexity(diagonal_match(20), 41);
+    assert_complexity(diagonal_match(30), 61);
+    // This case goes exponential.
+    assert!(check(&diagonal_exponential_match(10), 10000).is_err());
+}
+
+/// Construct a match like:
+/// ```ignore(illustrative)
+/// match ... {
+///     BigEnum::Variant1(_) => {}
+///     BigEnum::Variant2(_) => {}
+///     BigEnum::Variant3(_) => {}
+///     ...
+///     _ => {}
+/// }
+/// ```
+fn big_enum(arity: usize) -> Vec<DeconstructedPat<Cx>> {
+    let enum_ty = Ty::BigEnum { arity, ty: &Ty::Bool };
+    let mut patterns = vec![];
+    for i in 0..arity {
+        patterns.push(pat!(enum_ty; Variant.i));
+    }
+    patterns.push(pat!(enum_ty; _));
+    patterns
+}
+
+#[test]
+fn test_big_enum() {
+    // We try 2 branches per variant.
+    assert_complexity(big_enum(20), 40);
+}
diff --git a/compiler/rustc_pattern_analysis/tests/exhaustiveness.rs b/compiler/rustc_pattern_analysis/tests/exhaustiveness.rs
new file mode 100644
index 0000000000000..4c6c72fa8ecbc
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/tests/exhaustiveness.rs
@@ -0,0 +1,77 @@
+//! Test exhaustiveness checking.
+use common::*;
+use rustc_pattern_analysis::{
+    pat::{DeconstructedPat, WitnessPat},
+    usefulness::PlaceValidity,
+    MatchArm,
+};
+
+#[macro_use]
+mod common;
+
+/// Analyze a match made of these patterns.
+fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<WitnessPat<Cx>> {
+    let ty = *patterns[0].ty();
+    let arms: Vec<_> =
+        patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
+    let report =
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, None).unwrap();
+    report.non_exhaustiveness_witnesses
+}
+
+#[track_caller]
+fn assert_exhaustive(patterns: Vec<DeconstructedPat<Cx>>) {
+    let witnesses = check(patterns);
+    if !witnesses.is_empty() {
+        panic!("non-exaustive match: missing {witnesses:?}");
+    }
+}
+
+#[track_caller]
+fn assert_non_exhaustive(patterns: Vec<DeconstructedPat<Cx>>) {
+    let witnesses = check(patterns);
+    assert!(!witnesses.is_empty())
+}
+
+#[test]
+fn test_int_ranges() {
+    let ty = Ty::U8;
+    assert_exhaustive(pats!(ty;
+        0..=255,
+    ));
+    assert_exhaustive(pats!(ty;
+        0..,
+    ));
+    assert_non_exhaustive(pats!(ty;
+        0..255,
+    ));
+    assert_exhaustive(pats!(ty;
+        0..255,
+        255,
+    ));
+    assert_exhaustive(pats!(ty;
+        ..10,
+        10..
+    ));
+}
+
+#[test]
+fn test_nested() {
+    let ty = Ty::BigStruct { arity: 2, ty: &Ty::BigEnum { arity: 2, ty: &Ty::Bool } };
+    assert_non_exhaustive(pats!(ty;
+        Struct(Variant.0, _),
+    ));
+    assert_exhaustive(pats!(ty;
+        Struct(Variant.0, _),
+        Struct(Variant.1, _),
+    ));
+    assert_non_exhaustive(pats!(ty;
+        Struct(Variant.0, _),
+        Struct(_, Variant.0),
+    ));
+    assert_exhaustive(pats!(ty;
+        Struct(Variant.0, _),
+        Struct(_, Variant.0),
+        Struct(Variant.1, Variant.1),
+    ));
+}
diff --git a/compiler/rustc_pattern_analysis/tests/intersection.rs b/compiler/rustc_pattern_analysis/tests/intersection.rs
new file mode 100644
index 0000000000000..4d8a21506d760
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/tests/intersection.rs
@@ -0,0 +1,69 @@
+//! Test the computation of arm intersections.
+use common::*;
+use rustc_pattern_analysis::{pat::DeconstructedPat, usefulness::PlaceValidity, MatchArm};
+
+#[macro_use]
+mod common;
+
+/// Analyze a match made of these patterns and returns the computed arm intersections.
+fn check(patterns: Vec<DeconstructedPat<Cx>>) -> Vec<Vec<usize>> {
+    let ty = *patterns[0].ty();
+    let arms: Vec<_> =
+        patterns.iter().map(|pat| MatchArm { pat, has_guard: false, arm_data: () }).collect();
+    let report =
+        compute_match_usefulness(arms.as_slice(), ty, PlaceValidity::ValidOnly, None).unwrap();
+    report.arm_intersections.into_iter().map(|bitset| bitset.iter().collect()).collect()
+}
+
+#[track_caller]
+fn assert_intersects(patterns: Vec<DeconstructedPat<Cx>>, intersects: &[&[usize]]) {
+    let computed_intersects = check(patterns);
+    assert_eq!(computed_intersects, intersects);
+}
+
+#[test]
+fn test_int_ranges() {
+    let ty = Ty::U8;
+    assert_intersects(
+        pats!(ty;
+            0..=100,
+            100..,
+        ),
+        &[&[], &[0]],
+    );
+    assert_intersects(
+        pats!(ty;
+            0..=101,
+            100..,
+        ),
+        &[&[], &[0]],
+    );
+    assert_intersects(
+        pats!(ty;
+            0..100,
+            100..,
+        ),
+        &[&[], &[]],
+    );
+}
+
+#[test]
+fn test_nested() {
+    let ty = Ty::Tuple(&[Ty::Bool; 2]);
+    assert_intersects(
+        pats!(ty;
+            (true, true),
+            (true, _),
+            (_, true),
+        ),
+        &[&[], &[0], &[0, 1]],
+    );
+    // Here we shortcut because `(true, true)` is irrelevant, so we fail to detect the intersection.
+    assert_intersects(
+        pats!(ty;
+            (true, _),
+            (_, true),
+        ),
+        &[&[], &[]],
+    );
+}