Auto merge of #50912 - varkor:exhaustive-integer-matching, r=arielb1

Exhaustive integer matching

This adds a new feature flag `exhaustive_integer_patterns` that enables exhaustive matching of integer types by their values. For example, the following is now accepted:
```rust
#![feature(exhaustive_integer_patterns)]
#![feature(exclusive_range_pattern)]

fn matcher(x: u8) {
  match x { // ok
    0 .. 32 => { /* foo */ }
    32 => { /* bar */ }
    33 ..= 255 => { /* baz */ }
  }
}
```
This matching is permitted on all integer (signed/unsigned and char) types. Sensible error messages are also provided. For example:
```rust
fn matcher(x: u8) {
  match x { //~ ERROR
    0 .. 32 => { /* foo */ }
  }
}
```
results in:
```
error[E0004]: non-exhaustive patterns: `32u8...255u8` not covered
 --> matches.rs:3:9
  |
6 |   match x {
  |         ^ pattern `32u8...255u8` not covered
```

This implements https://github.com/rust-lang/rfcs/issues/1550 for https://github.com/rust-lang/rust/issues/50907. While there hasn't been a full RFC for this feature, it was suggested that this might be a feature that obviously complements the existing exhaustiveness checks (e.g. for `bool`) and so a feature gate would be sufficient for now.

10 files changed, 927 insertions, 84 deletions

diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 54fe30e609d..bafeb5dd128 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -2228,7 +2228,7 @@ pub fn fmt_const_val<W: Write>(fmt: &mut W, const_val: &ty::Const) -> fmt::Resul
     }
 }
 
-pub fn print_miri_value<W: Write>(value: Value, ty: Ty, f: &mut W) -> fmt::Result {
+pub fn print_miri_value<'tcx, W: Write>(value: Value, ty: Ty<'tcx>, f: &mut W) -> fmt::Result {
     use ty::TypeVariants::*;
     // print some primitives
     if let Value::Scalar(ScalarMaybeUndef::Scalar(Scalar::Bits { bits, .. })) = value {
diff --git a/src/librustc_mir/hair/pattern/_match.rs b/src/librustc_mir/hair/pattern/_match.rs
index 5d459557711..81f7d439feb 100644
--- a/src/librustc_mir/hair/pattern/_match.rs
+++ b/src/librustc_mir/hair/pattern/_match.rs
@@ -8,6 +8,164 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+/// This file includes the logic for exhaustiveness and usefulness checking for
+/// pattern-matching. Specifically, given a list of patterns for a type, we can
+/// tell whether:
+/// (a) the patterns cover every possible constructor for the type [exhaustiveness]
+/// (b) each pattern is necessary [usefulness]
+///
+/// The algorithm implemented here is a modified version of the one described in:
+/// http://moscova.inria.fr/~maranget/papers/warn/index.html
+/// However, to save future implementors from reading the original paper, I'm going
+/// to summarise the algorithm here to hopefully save time and be a little clearer
+/// (without being so rigorous).
+///
+/// The core of the algorithm revolves about a "usefulness" check. In particular, we
+/// are trying to compute a predicate `U(P, p_{m + 1})` where `P` is a list of patterns
+/// of length `m` for a compound (product) type with `n` components (we refer to this as
+/// a matrix). `U(P, p_{m + 1})` represents whether, given an existing list of patterns
+/// `p_1 ..= p_m`, adding a new pattern will be "useful" (that is, cover previously-
+/// uncovered values of the type).
+///
+/// If we have this predicate, then we can easily compute both exhaustiveness of an
+/// entire set of patterns and the individual usefulness of each one.
+/// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e. adding a wildcard
+/// match doesn't increase the number of values we're matching)
+/// (b) a pattern `p_i` is not useful if `U(P[0..=(i-1), p_i)` is false (i.e. adding a
+/// pattern to those that have come before it doesn't increase the number of values
+/// we're matching).
+///
+/// For example, say we have the following:
+/// ```
+///     // x: (Option<bool>, Result<()>)
+///     match x {
+///         (Some(true), _) => {}
+///         (None, Err(())) => {}
+///         (None, Err(_)) => {}
+///     }
+/// ```
+/// Here, the matrix `P` is 3 x 2 (rows x columns).
+/// [
+///     [Some(true), _],
+///     [None, Err(())],
+///     [None, Err(_)],
+/// ]
+/// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
+/// `[Some(false), _]`, for instance). In addition, row 3 is not useful, because
+/// all the values it covers are already covered by row 2.
+///
+/// To compute `U`, we must have two other concepts.
+///     1. `S(c, P)` is a "specialised matrix", where `c` is a constructor (like `Some` or
+///        `None`). You can think of it as filtering `P` to just the rows whose *first* pattern
+///        can cover `c` (and expanding OR-patterns into distinct patterns), and then expanding
+///        the constructor into all of its components.
+///        The specialisation of a row vector is computed by `specialize`.
+///
+///        It is computed as follows. For each row `p_i` of P, we have four cases:
+///             1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `S(c, P)` has a corresponding row:
+///                     r_1, .., r_a, p_(i,2), .., p_(i,n)
+///             1.2. `p_(i,1) = c'(r_1, .., r_a')` where `c ≠ c'`. Then `S(c, P)` has no
+///                  corresponding row.
+///             1.3. `p_(i,1) = _`. Then `S(c, P)` has a corresponding row:
+///                     _, .., _, p_(i,2), .., p_(i,n)
+///             1.4. `p_(i,1) = r_1 | r_2`. Then `S(c, P)` has corresponding rows inlined from:
+///                     S(c, (r_1, p_(i,2), .., p_(i,n)))
+///                     S(c, (r_2, p_(i,2), .., p_(i,n)))
+///
+///     2. `D(P)` is a "default matrix". This is used when we know there are missing
+///        constructor cases, but there might be existing wildcard patterns, so to check the
+///        usefulness of the matrix, we have to check all its *other* components.
+///        The default matrix is computed inline in `is_useful`.
+///
+///         It is computed as follows. For each row `p_i` of P, we have three cases:
+///             1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `D(P)` has no corresponding row.
+///             1.2. `p_(i,1) = _`. Then `D(P)` has a corresponding row:
+///                     p_(i,2), .., p_(i,n)
+///             1.3. `p_(i,1) = r_1 | r_2`. Then `D(P)` has corresponding rows inlined from:
+///                     D((r_1, p_(i,2), .., p_(i,n)))
+///                     D((r_2, p_(i,2), .., p_(i,n)))
+///
+///     Note that the OR-patterns are not always used directly in Rust, but are used to derive
+///     the exhaustive integer matching rules, so they're written here for posterity.
+///
+/// The algorithm for computing `U`
+/// -------------------------------
+/// The algorithm is inductive (on the number of columns: i.e. components of tuple patterns).
+/// That means we're going to check the components from left-to-right, so the algorithm
+/// operates principally on the first component of the matrix and new pattern `p_{m + 1}`.
+/// This algorithm is realised in the `is_useful` function.
+///
+/// Base case. (`n = 0`, i.e. an empty tuple pattern)
+///     - If `P` already contains an empty pattern (i.e. if the number of patterns `m > 0`),
+///       then `U(P, p_{m + 1})` is false.
+///     - Otherwise, `P` must be empty, so `U(P, p_{m + 1})` is true.
+///
+/// Inductive step. (`n > 0`, i.e. whether there's at least one column
+///                  [which may then be expanded into further columns later])
+///     We're going to match on the new pattern, `p_{m + 1}`.
+///         - If `p_{m + 1} == c(r_1, .., r_a)`, then we have a constructor pattern.
+///           Thus, the usefulness of `p_{m + 1}` can be reduced to whether it is useful when
+///           we ignore all the patterns in `P` that involve other constructors. This is where
+///           `S(c, P)` comes in:
+///           `U(P, p_{m + 1}) := U(S(c, P), S(c, p_{m + 1}))`
+///           This special case is handled in `is_useful_specialized`.
+///         - If `p_{m + 1} == _`, then we have two more cases:
+///             + All the constructors of the first component of the type exist within
+///               all the rows (after having expanded OR-patterns). In this case:
+///               `U(P, p_{m + 1}) := ∨(k ϵ constructors) U(S(k, P), S(k, p_{m + 1}))`
+///               I.e. the pattern `p_{m + 1}` is only useful when all the constructors are
+///               present *if* its later components are useful for the respective constructors
+///               covered by `p_{m + 1}` (usually a single constructor, but all in the case of `_`).
+///             + Some constructors are not present in the existing rows (after having expanded
+///               OR-patterns). However, there might be wildcard patterns (`_`) present. Thus, we
+///               are only really concerned with the other patterns leading with wildcards. This is
+///               where `D` comes in:
+///               `U(P, p_{m + 1}) := U(D(P), p_({m + 1},2), ..,  p_({m + 1},n))`
+///         - If `p_{m + 1} == r_1 | r_2`, then the usefulness depends on each separately:
+///           `U(P, p_{m + 1}) := U(P, (r_1, p_({m + 1},2), .., p_({m + 1},n)))
+///                            || U(P, (r_2, p_({m + 1},2), .., p_({m + 1},n)))`
+///
+/// Modifications to the algorithm
+/// ------------------------------
+/// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
+/// example uninhabited types and variable-length slice patterns. These are drawn attention to
+/// throughout the code below. I'll make a quick note here about how exhaustive integer matching
+/// is accounted for, though.
+///
+/// Exhaustive integer matching
+/// ---------------------------
+/// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
+/// So to support exhaustive integer matching, we can make use of the logic in the paper for
+/// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
+/// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
+/// that we have a constructor *of* constructors (the integers themselves). We then need to work
+/// through all the inductive step rules above, deriving how the ranges would be treated as
+/// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
+/// There are really only four special cases here:
+/// - When we match on a constructor that's actually a range, we have to treat it as if we would
+///   an OR-pattern.
+///     + It turns out that we can simply extend the case for single-value patterns in
+///      `specialize` to either be *equal* to a value constructor, or *contained within* a range
+///      constructor.
+///     + When the pattern itself is a range, you just want to tell whether any of the values in
+///       the pattern range coincide with values in the constructor range, which is precisely
+///       intersection.
+///   Since when encountering a range pattern for a value constructor, we also use inclusion, it
+///   means that whenever the constructor is a value/range and the pattern is also a value/range,
+///   we can simply use intersection to test usefulness.
+/// - When we're testing for usefulness of a pattern and the pattern's first component is a
+///   wildcard.
+///     + If all the constructors appear in the matrix, we have a slight complication. By default,
+///       the behaviour (i.e. a disjunction over specialised matrices for each constructor) is
+///       invalid, because we want a disjunction over every *integer* in each range, not just a
+///       disjunction over every range. This is a bit more tricky to deal with: essentially we need
+///       to form equivalence classes of subranges of the constructor range for which the behaviour
+///       of the matrix `P` and new pattern `p_{m + 1}` are the same. This is described in more
+///       detail in `split_grouped_constructors`.
+///     + If some constructors are missing from the matrix, it turns out we don't need to do
+///       anything special (because we know none of the integers are actually wildcards: i.e. we
+///       can't span wildcards using ranges).
+
 use self::Constructor::*;
 use self::Usefulness::*;
 use self::WitnessPreference::*;
@@ -21,18 +179,22 @@ use super::{PatternFoldable, PatternFolder, compare_const_vals};
 use rustc::hir::def_id::DefId;
 use rustc::hir::RangeEnd;
 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
+use rustc::ty::layout::{Integer, IntegerExt};
 
 use rustc::mir::Field;
 use rustc::mir::interpret::ConstValue;
 use rustc::util::common::ErrorReported;
 
+use syntax::attr::{SignedInt, UnsignedInt};
 use syntax_pos::{Span, DUMMY_SP};
 
 use arena::TypedArena;
 
-use std::cmp::{self, Ordering};
+use std::cmp::{self, Ordering, min, max};
 use std::fmt;
 use std::iter::{FromIterator, IntoIterator};
+use std::ops::RangeInclusive;
+use std::u128;
 
 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pattern<'tcx>)
                                 -> &'a Pattern<'tcx>
@@ -138,7 +300,6 @@ impl<'a, 'tcx> FromIterator<Vec<&'a Pattern<'tcx>>> for Matrix<'a, 'tcx> {
     }
 }
 
-//NOTE: appears to be the only place other then InferCtxt to contain a ParamEnv
 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
     pub tcx: TyCtxt<'a, 'tcx, 'tcx>,
     /// The module in which the match occurs. This is necessary for
@@ -165,7 +326,7 @@ impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
             tcx,
             module,
             pattern_arena: &pattern_arena,
-            byte_array_map: FxHashMap(),
+            byte_array_map: FxHashMap::default(),
         })
     }
 
@@ -273,7 +434,7 @@ impl<'tcx> Constructor<'tcx> {
     }
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub enum Usefulness<'tcx> {
     Useful,
     UsefulWithWitness(Vec<Witness<'tcx>>),
@@ -289,7 +450,7 @@ impl<'tcx> Usefulness<'tcx> {
     }
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub enum WitnessPreference {
     ConstructWitness,
     LeaveOutWitness
@@ -301,8 +462,39 @@ struct PatternContext<'tcx> {
     max_slice_length: u64,
 }
 
-/// A stack of patterns in reverse order of construction
-#[derive(Clone)]
+/// A witness of non-exhaustiveness for error reporting, represented
+/// as a list of patterns (in reverse order of construction) with
+/// wildcards inside to represent elements that can take any inhabitant
+/// of the type as a value.
+///
+/// A witness against a list of patterns should have the same types
+/// and length as the pattern matched against. Because Rust `match`
+/// is always against a single pattern, at the end the witness will
+/// have length 1, but in the middle of the algorithm, it can contain
+/// multiple patterns.
+///
+/// For example, if we are constructing a witness for the match against
+/// ```
+/// struct Pair(Option<(u32, u32)>, bool);
+///
+/// match (p: Pair) {
+///    Pair(None, _) => {}
+///    Pair(_, false) => {}
+/// }
+/// ```
+///
+/// We'll perform the following steps:
+/// 1. Start with an empty witness
+///     `Witness(vec![])`
+/// 2. Push a witness `Some(_)` against the `None`
+///     `Witness(vec![Some(_)])`
+/// 3. Push a witness `true` against the `false`
+///     `Witness(vec![Some(_), true])`
+/// 4. Apply the `Pair` constructor to the witnesses
+///     `Witness(vec![Pair(Some(_), true)])`
+///
+/// The final `Pair(Some(_), true)` is then the resulting witness.
+#[derive(Clone, Debug)]
 pub struct Witness<'tcx>(Vec<Pattern<'tcx>>);
 
 impl<'tcx> Witness<'tcx> {
@@ -353,7 +545,7 @@ impl<'tcx> Witness<'tcx> {
         let arity = constructor_arity(cx, ctor, ty);
         let pat = {
             let len = self.0.len() as u64;
-            let mut pats = self.0.drain((len-arity) as usize..).rev();
+            let mut pats = self.0.drain((len - arity) as usize..).rev();
 
             match ty.sty {
                 ty::TyAdt(..) |
@@ -396,6 +588,7 @@ impl<'tcx> Witness<'tcx> {
                 _ => {
                     match *ctor {
                         ConstantValue(value) => PatternKind::Constant { value },
+                        ConstantRange(lo, hi, end) => PatternKind::Range { lo, hi, end },
                         _ => PatternKind::Wild,
                     }
                 }
@@ -417,10 +610,6 @@ impl<'tcx> Witness<'tcx> {
 /// but is instead bounded by the maximum fixed length of slice patterns in
 /// the column of patterns being analyzed.
 ///
-/// This intentionally does not list ConstantValue specializations for
-/// non-booleans, because we currently assume that there is always a
-/// "non-standard constant" that matches. See issue #12483.
-///
 /// We make sure to omit constructors that are statically impossible. eg for
 /// Option<!> we do not include Some(_) in the returned list of constructors.
 fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
@@ -428,7 +617,8 @@ fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                                   -> Vec<Constructor<'tcx>>
 {
     debug!("all_constructors({:?})", pcx.ty);
-    match pcx.ty.sty {
+    let exhaustive_integer_patterns = cx.tcx.features().exhaustive_integer_patterns;
+    let ctors = match pcx.ty.sty {
         ty::TyBool => {
             [true, false].iter().map(|&b| {
                 ConstantValue(ty::Const::from_bool(cx.tcx, b))
@@ -457,6 +647,36 @@ fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                 .map(|v| Variant(v.did))
                 .collect()
         }
+        ty::TyChar if exhaustive_integer_patterns => {
+            let endpoint = |c: char| {
+                let ty = ty::ParamEnv::empty().and(cx.tcx.types.char);
+                ty::Const::from_bits(cx.tcx, c as u128, ty)
+            };
+            vec![
+                // The valid Unicode Scalar Value ranges.
+                ConstantRange(endpoint('\u{0000}'), endpoint('\u{D7FF}'), RangeEnd::Included),
+                ConstantRange(endpoint('\u{E000}'), endpoint('\u{10FFFF}'), RangeEnd::Included),
+            ]
+        }
+        ty::TyInt(ity) if exhaustive_integer_patterns => {
+            // FIXME(49937): refactor these bit manipulations into interpret.
+            let bits = Integer::from_attr(cx.tcx, SignedInt(ity)).size().bits() as u128;
+            let min = 1u128 << (bits - 1);
+            let max = (1u128 << (bits - 1)) - 1;
+            let ty = ty::ParamEnv::empty().and(pcx.ty);
+            vec![ConstantRange(ty::Const::from_bits(cx.tcx, min as u128, ty),
+                               ty::Const::from_bits(cx.tcx, max as u128, ty),
+                               RangeEnd::Included)]
+        }
+        ty::TyUint(uty) if exhaustive_integer_patterns => {
+            // FIXME(49937): refactor these bit manipulations into interpret.
+            let bits = Integer::from_attr(cx.tcx, UnsignedInt(uty)).size().bits() as u128;
+            let max = !0u128 >> (128 - bits);
+            let ty = ty::ParamEnv::empty().and(pcx.ty);
+            vec![ConstantRange(ty::Const::from_bits(cx.tcx, 0, ty),
+                               ty::Const::from_bits(cx.tcx, max, ty),
+                               RangeEnd::Included)]
+        }
         _ => {
             if cx.is_uninhabited(pcx.ty) {
                 vec![]
@@ -464,7 +684,8 @@ fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                 vec![Single]
             }
         }
-    }
+    };
+    ctors
 }
 
 fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
@@ -497,7 +718,7 @@ fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
     //     `[true, ..]`
     //     `[.., false]`
     // Then any slice of length ≥1 that matches one of these two
-    // patterns can be  be trivially turned to a slice of any
+    // patterns can be trivially turned to a slice of any
     // other length ≥1 that matches them and vice-versa - for
     // but the slice from length 2 `[false, true]` that matches neither
     // of these patterns can't be turned to a slice from length 1 that
@@ -569,6 +790,189 @@ fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
     cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
 }
 
+/// An inclusive interval, used for precise integer exhaustiveness checking.
+/// `IntRange`s always store a contiguous range. This means that values are
+/// encoded such that `0` encodes the minimum value for the integer,
+/// regardless of the signedness.
+/// For example, the pattern `-128...127i8` is encoded as `0..=255`.
+/// This makes comparisons and arithmetic on interval endpoints much more
+/// straightforward. See `signed_bias` for details.
+///
+/// `IntRange` is never used to encode an empty range or a "range" that wraps
+/// around the (offset) space: i.e. `range.lo <= range.hi`.
+#[derive(Clone)]
+struct IntRange<'tcx> {
+    pub range: RangeInclusive<u128>,
+    pub ty: Ty<'tcx>,
+}
+
+impl<'tcx> IntRange<'tcx> {
+    fn from_ctor(tcx: TyCtxt<'_, 'tcx, 'tcx>,
+                 ctor: &Constructor<'tcx>)
+                 -> Option<IntRange<'tcx>> {
+        match ctor {
+            ConstantRange(lo, hi, end) => {
+                assert_eq!(lo.ty, hi.ty);
+                let ty = lo.ty;
+                let env_ty = ty::ParamEnv::empty().and(ty);
+                if let Some(lo) = lo.assert_bits(tcx, env_ty) {
+                    if let Some(hi) = hi.assert_bits(tcx, env_ty) {
+                        // Perform a shift if the underlying types are signed,
+                        // which makes the interval arithmetic simpler.
+                        let bias = IntRange::signed_bias(tcx, ty);
+                        let (lo, hi) = (lo ^ bias, hi ^ bias);
+                        // Make sure the interval is well-formed.
+                        return if lo > hi || lo == hi && *end == RangeEnd::Excluded {
+                            None
+                        } else {
+                            let offset = (*end == RangeEnd::Excluded) as u128;
+                            Some(IntRange { range: lo..=(hi - offset), ty })
+                        };
+                    }
+                }
+                None
+            }
+            ConstantValue(val) => {
+                let ty = val.ty;
+                if let Some(val) = val.assert_bits(tcx, ty::ParamEnv::empty().and(ty)) {
+                    let bias = IntRange::signed_bias(tcx, ty);
+                    let val = val ^ bias;
+                    Some(IntRange { range: val..=val, ty })
+                } else {
+                    None
+                }
+            }
+            Single | Variant(_) | Slice(_) => {
+                None
+            }
+        }
+    }
+
+    fn from_pat(tcx: TyCtxt<'_, 'tcx, 'tcx>,
+                pat: &Pattern<'tcx>)
+                -> Option<IntRange<'tcx>> {
+        Self::from_ctor(tcx, &match pat.kind {
+            box PatternKind::Constant { value } => ConstantValue(value),
+            box PatternKind::Range { lo, hi, end } => ConstantRange(lo, hi, end),
+            _ => return None,
+        })
+    }
+
+    // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
+    fn signed_bias(tcx: TyCtxt<'_, 'tcx, 'tcx>, ty: Ty<'tcx>) -> u128 {
+        match ty.sty {
+            ty::TyInt(ity) => {
+                let bits = Integer::from_attr(tcx, SignedInt(ity)).size().bits() as u128;
+                1u128 << (bits - 1)
+            }
+            _ => 0
+        }
+    }
+
+    /// Convert a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
+    fn range_to_ctor(
+        tcx: TyCtxt<'_, 'tcx, 'tcx>,
+        ty: Ty<'tcx>,
+        r: RangeInclusive<u128>,
+    ) -> Constructor<'tcx> {
+        let bias = IntRange::signed_bias(tcx, ty);
+        let ty = ty::ParamEnv::empty().and(ty);
+        let (lo, hi) = r.into_inner();
+        if lo == hi {
+            ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty))
+        } else {
+            ConstantRange(ty::Const::from_bits(tcx, lo ^ bias, ty),
+                          ty::Const::from_bits(tcx, hi ^ bias, ty),
+                          RangeEnd::Included)
+        }
+    }
+
+    /// Return a collection of ranges that spans the values covered by `ranges`, subtracted
+    /// by the values covered by `self`: i.e. `ranges \ self` (in set notation).
+    fn subtract_from(self,
+                     tcx: TyCtxt<'_, 'tcx, 'tcx>,
+                     ranges: Vec<Constructor<'tcx>>)
+                     -> Vec<Constructor<'tcx>> {
+        let ranges = ranges.into_iter().filter_map(|r| {
+            IntRange::from_ctor(tcx, &r).map(|i| i.range)
+        });
+        let mut remaining_ranges = vec![];
+        let ty = self.ty;
+        let (lo, hi) = self.range.into_inner();
+        for subrange in ranges {
+            let (subrange_lo, subrange_hi) = subrange.into_inner();
+            if lo > subrange_hi || subrange_lo > hi  {
+                // The pattern doesn't intersect with the subrange at all,
+                // so the subrange remains untouched.
+                remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=subrange_hi));
+            } else {
+                if lo > subrange_lo {
+                    // The pattern intersects an upper section of the
+                    // subrange, so a lower section will remain.
+                    remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=(lo - 1)));
+                }
+                if hi < subrange_hi {
+                    // The pattern intersects a lower section of the
+                    // subrange, so an upper section will remain.
+                    remaining_ranges.push(Self::range_to_ctor(tcx, ty, (hi + 1)..=subrange_hi));
+                }
+            }
+        }
+        remaining_ranges
+    }
+
+    fn intersection(&self, other: &Self) -> Option<Self> {
+        let ty = self.ty;
+        let (lo, hi) = (*self.range.start(), *self.range.end());
+        let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
+        if lo <= other_hi && other_lo <= hi {
+            Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty })
+        } else {
+            None
+        }
+    }
+}
+
+// Return a set of constructors equivalent to `all_ctors \ used_ctors`.
+fn compute_missing_ctors<'a, 'tcx: 'a>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    all_ctors: &Vec<Constructor<'tcx>>,
+    used_ctors: &Vec<Constructor<'tcx>>,
+) -> Vec<Constructor<'tcx>> {
+    let mut missing_ctors = vec![];
+
+    for req_ctor in all_ctors {
+        let mut refined_ctors = vec![req_ctor.clone()];
+        for used_ctor in used_ctors {
+            if used_ctor == req_ctor {
+                // If a constructor appears in a `match` arm, we can
+                // eliminate it straight away.
+                refined_ctors = vec![]
+            } else if tcx.features().exhaustive_integer_patterns {
+                if let Some(interval) = IntRange::from_ctor(tcx, used_ctor) {
+                    // Refine the required constructors for the type by subtracting
+                    // the range defined by the current constructor pattern.
+                    refined_ctors = interval.subtract_from(tcx, refined_ctors);
+                }
+            }
+
+            // If the constructor patterns that have been considered so far
+            // already cover the entire range of values, then we the
+            // constructor is not missing, and we can move on to the next one.
+            if refined_ctors.is_empty() {
+                break;
+            }
+        }
+        // If a constructor has not been matched, then it is missing.
+        // We add `refined_ctors` instead of `req_ctor`, because then we can
+        // provide more detailed error information about precisely which
+        // ranges have been omitted.
+        missing_ctors.extend(refined_ctors);
+    }
+
+    missing_ctors
+}
+
 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
 /// The algorithm from the paper has been modified to correctly handle empty
 /// types. The changes are:
@@ -637,8 +1041,7 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
         // FIXME: this might lead to "unstable" behavior with macro hygiene
         // introducing uninhabited patterns for inaccessible fields. We
         // need to figure out how to model that.
-        ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error())
-            .unwrap_or(v[0].ty),
+        ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error()).unwrap_or(v[0].ty),
         max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0])))
     };
 
@@ -646,7 +1049,7 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 
     if let Some(constructors) = pat_constructors(cx, v[0], pcx) {
         debug!("is_useful - expanding constructors: {:#?}", constructors);
-        constructors.into_iter().map(|c|
+        split_grouped_constructors(cx.tcx, constructors, matrix, pcx.ty).into_iter().map(|c|
             is_useful_specialized(cx, matrix, v, c.clone(), pcx.ty, witness)
         ).find(|result| result.is_useful()).unwrap_or(NotUseful)
     } else {
@@ -656,11 +1059,10 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
             pat_constructors(cx, row[0], pcx).unwrap_or(vec![])
         }).collect();
         debug!("used_ctors = {:#?}", used_ctors);
+        // `all_ctors` are all the constructors for the given type, which
+        // should all be represented (or caught with the wild pattern `_`).
         let all_ctors = all_constructors(cx, pcx);
         debug!("all_ctors = {:#?}", all_ctors);
-        let missing_ctors: Vec<Constructor> = all_ctors.iter().filter(|c| {
-            !used_ctors.contains(*c)
-        }).cloned().collect();
 
         // `missing_ctors` is the set of constructors from the same type as the
         // first column of `matrix` that are matched only by wildcard patterns
@@ -681,10 +1083,12 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
         // feature flag is not present, so this is only
         // needed for that case.
 
-        let is_privately_empty =
-            all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
-        let is_declared_nonexhaustive =
-            cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
+        // Find those constructors that are not matched by any non-wildcard patterns in the
+        // current column.
+        let missing_ctors = compute_missing_ctors(cx.tcx, &all_ctors, &used_ctors);
+
+        let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
+        let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
         debug!("missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
                missing_ctors, is_privately_empty, is_declared_nonexhaustive);
 
@@ -693,7 +1097,7 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
         let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive;
 
         if missing_ctors.is_empty() && !is_non_exhaustive {
-            all_ctors.into_iter().map(|c| {
+            split_grouped_constructors(cx.tcx, all_ctors, matrix, pcx.ty).into_iter().map(|c| {
                 is_useful_specialized(cx, matrix, v, c.clone(), pcx.ty, witness)
             }).find(|result| result.is_useful()).unwrap_or(NotUseful)
         } else {
@@ -753,7 +1157,7 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                     // `used_ctors` is empty.
                     let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
                         // All constructors are unused. Add wild patterns
-                        // rather than each individual constructor
+                        // rather than each individual constructor.
                         pats.into_iter().map(|mut witness| {
                             witness.0.push(Pattern {
                                 ty: pcx.ty,
@@ -765,6 +1169,10 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                     } else {
                         pats.into_iter().flat_map(|witness| {
                             missing_ctors.iter().map(move |ctor| {
+                                // Extends the witness with a "wild" version of this
+                                // constructor, that matches everything that can be built with
+                                // it. For example, if `ctor` is a `Constructor::Variant` for
+                                // `Option::Some`, this pushes the witness for `Some(_)`.
                                 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
                             })
                         }).collect()
@@ -777,14 +1185,16 @@ pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
     }
 }
 
+/// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e. `is_useful` applied
+/// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
 fn is_useful_specialized<'p, 'a:'p, 'tcx: 'a>(
     cx: &mut MatchCheckCtxt<'a, 'tcx>,
     &Matrix(ref m): &Matrix<'p, 'tcx>,
     v: &[&'p Pattern<'tcx>],
     ctor: Constructor<'tcx>,
     lty: Ty<'tcx>,
-    witness: WitnessPreference) -> Usefulness<'tcx>
-{
+    witness: WitnessPreference,
+) -> Usefulness<'tcx> {
     debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
     let sub_pat_tys = constructor_sub_pattern_tys(cx, &ctor, lty);
     let wild_patterns_owned: Vec<_> = sub_pat_tys.iter().map(|ty| {
@@ -806,7 +1216,7 @@ fn is_useful_specialized<'p, 'a:'p, 'tcx: 'a>(
                     .collect()
             ),
             result => result
-        },
+        }
         None => NotUseful
     }
 }
@@ -818,23 +1228,20 @@ fn is_useful_specialized<'p, 'a:'p, 'tcx: 'a>(
 /// Slice patterns, however, can match slices of different lengths. For instance,
 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
 ///
-/// Returns None in case of a catch-all, which can't be specialized.
+/// Returns `None` in case of a catch-all, which can't be specialized.
 fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt,
                           pat: &Pattern<'tcx>,
                           pcx: PatternContext)
                           -> Option<Vec<Constructor<'tcx>>>
 {
     match *pat.kind {
-        PatternKind::Binding { .. } | PatternKind::Wild =>
-            None,
-        PatternKind::Leaf { .. } | PatternKind::Deref { .. } =>
-            Some(vec![Single]),
-        PatternKind::Variant { adt_def, variant_index, .. } =>
-            Some(vec![Variant(adt_def.variants[variant_index].did)]),
-        PatternKind::Constant { value } =>
-            Some(vec![ConstantValue(value)]),
-        PatternKind::Range { lo, hi, end } =>
-            Some(vec![ConstantRange(lo, hi, end)]),
+        PatternKind::Binding { .. } | PatternKind::Wild => None,
+        PatternKind::Leaf { .. } | PatternKind::Deref { .. } => Some(vec![Single]),
+        PatternKind::Variant { adt_def, variant_index, .. } => {
+            Some(vec![Variant(adt_def.variants[variant_index].did)])
+        }
+        PatternKind::Constant { value } => Some(vec![ConstantValue(value)]),
+        PatternKind::Range { lo, hi, end } => Some(vec![ConstantRange(lo, hi, end)]),
         PatternKind::Array { .. } => match pcx.ty.sty {
             ty::TyArray(_, length) => Some(vec![
                 Slice(length.unwrap_usize(cx.tcx))
@@ -970,13 +1377,167 @@ fn slice_pat_covered_by_constructor<'tcx>(
     Ok(true)
 }
 
+// Whether to evaluate a constructor using exhaustive integer matching. This is true if the
+// constructor is a range or constant with an integer type.
+fn should_treat_range_exhaustively(tcx: TyCtxt<'_, 'tcx, 'tcx>, ctor: &Constructor<'tcx>) -> bool {
+    if tcx.features().exhaustive_integer_patterns {
+        if let ConstantValue(value) | ConstantRange(value, _, _) = ctor {
+            if let ty::TyChar | ty::TyInt(_) | ty::TyUint(_) = value.ty.sty {
+                return true;
+            }
+        }
+    }
+    false
+}
+
+/// For exhaustive integer matching, some constructors are grouped within other constructors
+/// (namely integer typed values are grouped within ranges). However, when specialising these
+/// constructors, we want to be specialising for the underlying constructors (the integers), not
+/// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
+/// mean creating a separate constructor for every single value in the range, which is clearly
+/// impractical. However, observe that for some ranges of integers, the specialisation will be
+/// identical across all values in that range (i.e. there are equivalence classes of ranges of
+/// constructors based on their `is_useful_specialised` outcome). These classes are grouped by
+/// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
+/// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
+/// change.
+/// Our solution, therefore, is to split the range constructor into subranges at every single point
+/// the group of intersecting patterns changes (using the method described below).
+/// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
+/// on actual integers. The nice thing about this is that the number of subranges is linear in the
+/// number of rows in the matrix (i.e. the number of cases in the `match` statement), so we don't
+/// need to be worried about matching over gargantuan ranges.
+///
+/// Essentially, given the first column of a matrix representing ranges, looking like the following:
+///
+/// |------|  |----------| |-------|    ||
+///    |-------| |-------|            |----| ||
+///       |---------|
+///
+/// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
+///
+/// |--|--|||-||||--||---|||-------|  |-|||| ||
+///
+/// The logic for determining how to split the ranges is fairly straightforward: we calculate
+/// boundaries for each interval range, sort them, then create constructors for each new interval
+/// between every pair of boundary points. (This essentially sums up to performing the intuitive
+/// merging operation depicted above.)
+fn split_grouped_constructors<'p, 'a: 'p, 'tcx: 'a>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    ctors: Vec<Constructor<'tcx>>,
+    &Matrix(ref m): &Matrix<'p, 'tcx>,
+    ty: Ty<'tcx>,
+) -> Vec<Constructor<'tcx>> {
+    let mut split_ctors = Vec::with_capacity(ctors.len());
+
+    for ctor in ctors.into_iter() {
+        match ctor {
+            // For now, only ranges may denote groups of "subconstructors", so we only need to
+            // special-case constant ranges.
+            ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
+                // We only care about finding all the subranges within the range of the constructor
+                // range. Anything else is irrelevant, because it is guaranteed to result in
+                // `NotUseful`, which is the default case anyway, and can be ignored.
+                let ctor_range = IntRange::from_ctor(tcx, &ctor).unwrap();
+
+                /// Represents a border between 2 integers. Because the intervals spanning borders
+                /// must be able to cover every integer, we need to be able to represent
+                /// 2^128 + 1 such borders.
+                #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+                enum Border {
+                    JustBefore(u128),
+                    AfterMax,
+                }
+
+                // A function for extracting the borders of an integer interval.
+                fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
+                    let (lo, hi) = r.range.into_inner();
+                    let from = Border::JustBefore(lo);
+                    let to = match hi.checked_add(1) {
+                        Some(m) => Border::JustBefore(m),
+                        None => Border::AfterMax,
+                    };
+                    vec![from, to].into_iter()
+                }
+
+                // `borders` is the set of borders between equivalence classes: each equivalence
+                // class lies between 2 borders.
+                let row_borders = m.iter()
+                    .flat_map(|row| IntRange::from_pat(tcx, row[0]))
+                    .flat_map(|range| ctor_range.intersection(&range))
+                    .flat_map(|range| range_borders(range));
+                let ctor_borders = range_borders(ctor_range.clone());
+                let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
+                borders.sort_unstable();
+
+                // We're going to iterate through every pair of borders, making sure that each
+                // represents an interval of nonnegative length, and convert each such interval
+                // into a constructor.
+                for IntRange { range, .. } in borders.windows(2).filter_map(|window| {
+                    match (window[0], window[1]) {
+                        (Border::JustBefore(n), Border::JustBefore(m)) => {
+                            if n < m {
+                                Some(IntRange { range: n..=(m - 1), ty })
+                            } else {
+                                None
+                            }
+                        }
+                        (Border::JustBefore(n), Border::AfterMax) => {
+                            Some(IntRange { range: n..=u128::MAX, ty })
+                        }
+                        (Border::AfterMax, _) => None,
+                    }
+                }) {
+                    split_ctors.push(IntRange::range_to_ctor(tcx, ty, range));
+                }
+            }
+            // Any other constructor can be used unchanged.
+            _ => split_ctors.push(ctor),
+        }
+    }
+
+    split_ctors
+}
+
+/// Check whether there exists any shared value in either `ctor` or `pat` by intersecting them.
+fn constructor_intersects_pattern<'p, 'a: 'p, 'tcx: 'a>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    ctor: &Constructor<'tcx>,
+    pat: &'p Pattern<'tcx>,
+) -> Option<Vec<&'p Pattern<'tcx>>> {
+    if should_treat_range_exhaustively(tcx, ctor) {
+        match (IntRange::from_ctor(tcx, ctor), IntRange::from_pat(tcx, pat)) {
+            (Some(ctor), Some(pat)) => {
+                ctor.intersection(&pat).map(|_| {
+                    let (pat_lo, pat_hi) = pat.range.into_inner();
+                    let (ctor_lo, ctor_hi) = ctor.range.into_inner();
+                    assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
+                    vec![]
+                })
+            }
+            _ => None,
+        }
+    } else {
+        // Fallback for non-ranges and ranges that involve floating-point numbers, which are not
+        // conveniently handled by `IntRange`. For these cases, the constructor may not be a range
+        // so intersection actually devolves into being covered by the pattern.
+        match constructor_covered_by_range(tcx, ctor, pat) {
+            Ok(true) => Some(vec![]),
+            Ok(false) | Err(ErrorReported) => None,
+        }
+    }
+}
+
 fn constructor_covered_by_range<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     ctor: &Constructor<'tcx>,
-    from: &'tcx ty::Const<'tcx>, to: &'tcx ty::Const<'tcx>,
-    end: RangeEnd,
-    ty: Ty<'tcx>,
+    pat: &Pattern<'tcx>,
 ) -> Result<bool, ErrorReported> {
+    let (from, to, end, ty) = match pat.kind {
+        box PatternKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
+        box PatternKind::Range { lo, hi, end } => (lo, hi, end, lo.ty),
+        _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
+    };
     trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
     let cmp_from = |c_from| compare_const_vals(tcx, c_from, from, ty::ParamEnv::empty().and(ty))
         .map(|res| res != Ordering::Less);
@@ -1040,15 +1601,14 @@ fn specialize<'p, 'a: 'p, 'tcx: 'a>(
     cx: &mut MatchCheckCtxt<'a, 'tcx>,
     r: &[&'p Pattern<'tcx>],
     constructor: &Constructor<'tcx>,
-    wild_patterns: &[&'p Pattern<'tcx>])
-    -> Option<Vec<&'p Pattern<'tcx>>>
-{
+    wild_patterns: &[&'p Pattern<'tcx>],
+) -> Option<Vec<&'p Pattern<'tcx>>> {
     let pat = &r[0];
 
     let head: Option<Vec<&Pattern>> = match *pat.kind {
         PatternKind::Binding { .. } | PatternKind::Wild => {
             Some(wild_patterns.to_owned())
-        },
+        }
 
         PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
             let ref variant = adt_def.variants[variant_index];
@@ -1062,6 +1622,7 @@ fn specialize<'p, 'a: 'p, 'tcx: 'a>(
         PatternKind::Leaf { ref subpatterns } => {
             Some(patterns_for_variant(subpatterns, wild_patterns))
         }
+
         PatternKind::Deref { ref subpattern } => {
             Some(vec![subpattern])
         }
@@ -1090,30 +1651,21 @@ fn specialize<'p, 'a: 'p, 'tcx: 'a>(
                         span_bug!(pat.span,
                         "unexpected const-val {:?} with ctor {:?}", value, constructor)
                     }
-                },
+                }
                 _ => {
-                    match constructor_covered_by_range(
-                        cx.tcx,
-                        constructor, value, value, RangeEnd::Included,
-                        value.ty,
-                            ) {
-                        Ok(true) => Some(vec![]),
-                        Ok(false) => None,
-                        Err(ErrorReported) => None,
-                    }
+                    // If the constructor is a:
+                    //      Single value: add a row if the constructor equals the pattern.
+                    //      Range: add a row if the constructor contains the pattern.
+                    constructor_intersects_pattern(cx.tcx, constructor, pat)
                 }
             }
         }
 
-        PatternKind::Range { lo, hi, ref end } => {
-            match constructor_covered_by_range(
-                cx.tcx,
-                constructor, lo, hi, end.clone(), lo.ty,
-            ) {
-                Ok(true) => Some(vec![]),
-                Ok(false) => None,
-                Err(ErrorReported) => None,
-            }
+        PatternKind::Range { .. } => {
+            // If the constructor is a:
+            //      Single value: add a row if the pattern contains the constructor.
+            //      Range: add a row if the constructor intersects the pattern.
+            constructor_intersects_pattern(cx.tcx, constructor, pat)
         }
 
         PatternKind::Array { ref prefix, ref slice, ref suffix } |
@@ -1123,14 +1675,12 @@ fn specialize<'p, 'a: 'p, 'tcx: 'a>(
                     let pat_len = prefix.len() + suffix.len();
                     if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
                         if slice_count == 0 || slice.is_some() {
-                            Some(
-                                prefix.iter().chain(
-                                wild_patterns.iter().map(|p| *p)
-                                                    .skip(prefix.len())
-                                                    .take(slice_count)
-                                                    .chain(
-                                suffix.iter()
-                            )).collect())
+                            Some(prefix.iter().chain(
+                                    wild_patterns.iter().map(|p| *p)
+                                                 .skip(prefix.len())
+                                                 .take(slice_count)
+                                                 .chain(suffix.iter())
+                            ).collect())
                         } else {
                             None
                         }
diff --git a/src/librustc_mir/hair/pattern/check_match.rs b/src/librustc_mir/hair/pattern/check_match.rs
index 35f9dcee99f..7f6298fd171 100644
--- a/src/librustc_mir/hair/pattern/check_match.rs
+++ b/src/librustc_mir/hair/pattern/check_match.rs
@@ -272,7 +272,7 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
                                                 self.tables);
             let pattern = patcx.lower_pattern(pat);
             let pattern_ty = pattern.ty;
-            let pats : Matrix = vec![vec![
+            let pats: Matrix = vec![vec![
                 expand_pattern(cx, pattern)
             ]].into_iter().collect();
 
@@ -391,7 +391,7 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                                     printed_if_let_err = true;
                                 }
                             }
-                        },
+                        }
 
                         hir::MatchSource::WhileLetDesugar => {
                             // check which arm we're on.
diff --git a/src/librustc_mir/hair/pattern/mod.rs b/src/librustc_mir/hair/pattern/mod.rs
index d614131c526..7ec93731304 100644
--- a/src/librustc_mir/hair/pattern/mod.rs
+++ b/src/librustc_mir/hair/pattern/mod.rs
@@ -233,7 +233,7 @@ impl<'tcx> fmt::Display for Pattern<'tcx> {
             PatternKind::Range { lo, hi, end } => {
                 fmt_const_val(f, lo)?;
                 match end {
-                    RangeEnd::Included => write!(f, "...")?,
+                    RangeEnd::Included => write!(f, "..=")?,
                     RangeEnd::Excluded => write!(f, "..")?,
                 }
                 fmt_const_val(f, hi)
@@ -368,9 +368,14 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
                                     "lower range bound must be less than upper",
                                 );
                                 PatternKind::Wild
-                            },
-                            (RangeEnd::Included, None) |
-                            (RangeEnd::Included, Some(Ordering::Greater)) => {
+                            }
+                            (RangeEnd::Included, Some(Ordering::Equal)) => {
+                                PatternKind::Constant { value: lo }
+                            }
+                            (RangeEnd::Included, Some(Ordering::Less)) => {
+                                PatternKind::Range { lo, hi, end }
+                            }
+                            (RangeEnd::Included, _) => {
                                 let mut err = struct_span_err!(
                                     self.tcx.sess,
                                     lo_expr.span,
@@ -390,8 +395,7 @@ impl<'a, 'tcx> PatternContext<'a, 'tcx> {
                                 }
                                 err.emit();
                                 PatternKind::Wild
-                            },
-                            (RangeEnd::Included, Some(_)) => PatternKind::Range { lo, hi, end },
+                            }
                         }
                     }
                     _ => PatternKind::Wild
diff --git a/src/librustc_mir/lib.rs b/src/librustc_mir/lib.rs
index 19fc087f128..05494131f32 100644
--- a/src/librustc_mir/lib.rs
+++ b/src/librustc_mir/lib.rs
@@ -36,6 +36,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(unicode_internals)]
 #![feature(step_trait)]
 #![feature(slice_concat_ext)]
+#![feature(if_while_or_patterns)]
 
 #![recursion_limit="256"]
 
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index bc36bde5cba..71ad118ed8e 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -471,6 +471,9 @@ declare_features! (
     // 'a: { break 'a; }
     (active, label_break_value, "1.28.0", Some(48594), None),
 
+    // Integer match exhaustiveness checking
+    (active, exhaustive_integer_patterns, "1.30.0", Some(50907), None),
+
     // #[panic_implementation]
     (active, panic_implementation, "1.28.0", Some(44489), None),
 
diff --git a/src/test/ui/exhaustive_integer_patterns.rs b/src/test/ui/exhaustive_integer_patterns.rs
new file mode 100644
index 00000000000..7825aaa2912
--- /dev/null
+++ b/src/test/ui/exhaustive_integer_patterns.rs
@@ -0,0 +1,173 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(exhaustive_integer_patterns)]
+#![feature(exclusive_range_pattern)]
+#![deny(unreachable_patterns)]
+
+use std::{char, usize, u8, u16, u32, u64, u128, isize, i8, i16, i32, i64, i128};
+
+fn main() {
+    let x: u8 = 0;
+
+    // A single range covering the entire domain.
+    match x {
+        0 ..= 255 => {} // ok
+    }
+
+    // A combination of ranges and values.
+    // These are currently allowed to be overlapping.
+    match x {
+        0 ..= 32 => {}
+        33 => {}
+        34 .. 128 => {}
+        100 ..= 200 => {}
+        200 => {} //~ ERROR unreachable pattern
+        201 ..= 255 => {}
+    }
+
+    // An incomplete set of values.
+    match x { //~ ERROR non-exhaustive patterns
+        0 .. 128 => {}
+    }
+
+    // A more incomplete set of values.
+    match x { //~ ERROR non-exhaustive patterns
+        0 ..= 10 => {}
+        20 ..= 30 => {}
+        35 => {}
+        70 .. 255 => {}
+    }
+
+    let x: i8 = 0;
+    match x { //~ ERROR non-exhaustive patterns
+        -7 => {}
+        -5..=120 => {}
+        -2..=20 => {} //~ ERROR unreachable pattern
+        125 => {}
+    }
+
+    // Let's test other types too!
+    let c: char = '\u{0}';
+    match c {
+        '\u{0}' ..= char::MAX => {} // ok
+    }
+
+    // We can actually get away with just covering the
+    // following two ranges, which correspond to all
+    // valid Unicode Scalar Values.
+    match c {
+        '\u{0000}' ..= '\u{D7FF}' => {}
+        '\u{E000}' ..= '\u{10_FFFF}' => {}
+    }
+
+    match 0usize {
+        0 ..= usize::MAX => {} // ok
+    }
+
+    match 0u16 {
+        0 ..= u16::MAX => {} // ok
+    }
+
+    match 0u32 {
+        0 ..= u32::MAX => {} // ok
+    }
+
+    match 0u64 {
+        0 ..= u64::MAX => {} // ok
+    }
+
+    match 0u128 {
+        0 ..= u128::MAX => {} // ok
+    }
+
+    match 0isize {
+        isize::MIN ..= isize::MAX => {} // ok
+    }
+
+    match 0i8 {
+        -128 ..= 127 => {} // ok
+    }
+
+    match 0i8 { //~ ERROR non-exhaustive patterns
+        -127 ..= 127 => {}
+    }
+
+    match 0i16 {
+        i16::MIN ..= i16::MAX => {} // ok
+    }
+
+    match 0i16 { //~ ERROR non-exhaustive patterns
+        i16::MIN ..= -1 => {}
+        1 ..= i16::MAX => {}
+    }
+
+    match 0i32 {
+        i32::MIN ..= i32::MAX => {} // ok
+    }
+
+    match 0i64 {
+        i64::MIN ..= i64::MAX => {} // ok
+    }
+
+    match 0i128 {
+        i128::MIN ..= i128::MAX => {} // ok
+    }
+
+    // Make sure that guards don't factor into the exhaustiveness checks.
+    match 0u8 { //~ ERROR non-exhaustive patterns
+        0 .. 128 => {}
+        128 ..= 255 if true => {}
+    }
+
+    match 0u8 {
+        0 .. 128 => {}
+        128 ..= 255 if false => {}
+        128 ..= 255 => {} // ok, because previous arm was guarded
+    }
+
+    // Now things start getting a bit more interesting. Testing products!
+    match (0u8, Some(())) { //~ ERROR non-exhaustive patterns
+        (1, _) => {}
+        (_, None) => {}
+    }
+
+    match (0u8, true) { //~ ERROR non-exhaustive patterns
+        (0 ..= 125, false) => {}
+        (128 ..= 255, false) => {}
+        (0 ..= 255, true) => {}
+    }
+
+    match (0u8, true) { // ok
+        (0 ..= 125, false) => {}
+        (128 ..= 255, false) => {}
+        (0 ..= 255, true) => {}
+        (125 .. 128, false) => {}
+    }
+
+    match 0u8 { // ok
+        0 .. 2 => {}
+        1 ..= 2 => {}
+        _ => {}
+    }
+
+    const LIM: u128 = u128::MAX - 1;
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        0 ..= LIM => {}
+    }
+
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        0 ..= 4 => {}
+    }
+
+    match 0u128 { //~ ERROR non-exhaustive patterns
+        4 ..= u128::MAX => {}
+    }
+}
diff --git a/src/test/ui/exhaustive_integer_patterns.stderr b/src/test/ui/exhaustive_integer_patterns.stderr
new file mode 100644
index 00000000000..44b05a12aeb
--- /dev/null
+++ b/src/test/ui/exhaustive_integer_patterns.stderr
@@ -0,0 +1,87 @@
+error: unreachable pattern
+  --> $DIR/exhaustive_integer_patterns.rs:32:9
+   |
+LL |         200 => {} //~ ERROR unreachable pattern
+   |         ^^^
+   |
+note: lint level defined here
+  --> $DIR/exhaustive_integer_patterns.rs:13:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:37:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ pattern `128u8..=255u8` not covered
+
+error[E0004]: non-exhaustive patterns: `11u8..=19u8`, `31u8..=34u8`, `36u8..=69u8` and 1 more not covered
+  --> $DIR/exhaustive_integer_patterns.rs:42:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ patterns `11u8..=19u8`, `31u8..=34u8`, `36u8..=69u8` and 1 more not covered
+
+error: unreachable pattern
+  --> $DIR/exhaustive_integer_patterns.rs:53:9
+   |
+LL |         -2..=20 => {} //~ ERROR unreachable pattern
+   |         ^^^^^^^
+
+error[E0004]: non-exhaustive patterns: `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+  --> $DIR/exhaustive_integer_patterns.rs:50:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns
+   |           ^ patterns `-128i8..=-8i8`, `-6i8`, `121i8..=124i8` and 1 more not covered
+
+error[E0004]: non-exhaustive patterns: `-128i8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:99:11
+   |
+LL |     match 0i8 { //~ ERROR non-exhaustive patterns
+   |           ^^^ pattern `-128i8` not covered
+
+error[E0004]: non-exhaustive patterns: `0i16` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:107:11
+   |
+LL |     match 0i16 { //~ ERROR non-exhaustive patterns
+   |           ^^^^ pattern `0i16` not covered
+
+error[E0004]: non-exhaustive patterns: `128u8..=255u8` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:125:11
+   |
+LL |     match 0u8 { //~ ERROR non-exhaustive patterns
+   |           ^^^ pattern `128u8..=255u8` not covered
+
+error[E0004]: non-exhaustive patterns: `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:137:11
+   |
+LL |     match (0u8, Some(())) { //~ ERROR non-exhaustive patterns
+   |           ^^^^^^^^^^^^^^^ patterns `(0u8, Some(_))` and `(2u8..=255u8, Some(_))` not covered
+
+error[E0004]: non-exhaustive patterns: `(126u8..=127u8, false)` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:142:11
+   |
+LL |     match (0u8, true) { //~ ERROR non-exhaustive patterns
+   |           ^^^^^^^^^^^ pattern `(126u8..=127u8, false)` not covered
+
+error[E0004]: non-exhaustive patterns: `340282366920938463463374607431768211455u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:162:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `340282366920938463463374607431768211455u128` not covered
+
+error[E0004]: non-exhaustive patterns: `5u128..=340282366920938463463374607431768211455u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:166:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `5u128..=340282366920938463463374607431768211455u128` not covered
+
+error[E0004]: non-exhaustive patterns: `0u128..=3u128` not covered
+  --> $DIR/exhaustive_integer_patterns.rs:170:11
+   |
+LL |     match 0u128 { //~ ERROR non-exhaustive patterns
+   |           ^^^^^ pattern `0u128..=3u128` not covered
+
+error: aborting due to 13 previous errors
+
+For more information about this error, try `rustc --explain E0004`.
diff --git a/src/test/ui/feature-gate-exhaustive_integer_patterns.rs b/src/test/ui/feature-gate-exhaustive_integer_patterns.rs
new file mode 100644
index 00000000000..3aa15229455
--- /dev/null
+++ b/src/test/ui/feature-gate-exhaustive_integer_patterns.rs
@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let x: u8 = 0;
+    match x { //~ ERROR non-exhaustive patterns: `_` not covered
+        0 ..= 255 => {}
+    }
+}
diff --git a/src/test/ui/feature-gate-exhaustive_integer_patterns.stderr b/src/test/ui/feature-gate-exhaustive_integer_patterns.stderr
new file mode 100644
index 00000000000..63d98f6b5eb
--- /dev/null
+++ b/src/test/ui/feature-gate-exhaustive_integer_patterns.stderr
@@ -0,0 +1,9 @@
+error[E0004]: non-exhaustive patterns: `_` not covered
+  --> $DIR/feature-gate-exhaustive_integer_patterns.rs:13:11
+   |
+LL |     match x { //~ ERROR non-exhaustive patterns: `_` not covered
+   |           ^ pattern `_` not covered
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0004`.