Cleanup constant matching in exhaustiveness checking

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -140,7 +140,7 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
         patcx.include_lint_checks();
         let pattern = patcx.lower_pattern(pat);
         let pattern_ty = pattern.ty;
-        let pattern: &_ = cx.pattern_arena.alloc(expand_pattern(cx, pattern));
+        let pattern: &_ = cx.pattern_arena.alloc(expand_pattern(pattern));
         if !patcx.errors.is_empty() {
             *have_errors = true;
             patcx.report_inlining_errors(pat.span);

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -387,14 +387,16 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
                 // `&str` and `&[u8]` are represented as `ConstValue::Slice`, let's keep using this
                 // optimization for now.
                 ty::Str => PatKind::Constant { value: cv },
-                ty::Slice(elem_ty) if elem_ty == tcx.types.u8 => PatKind::Constant { value: cv },
                 // `b"foo"` produces a `&[u8; 3]`, but you can't use constants of array type when
                 // matching against references, you can only use byte string literals.
-                // FIXME: clean this up, likely by permitting array patterns when matching on slices
-                ty::Array(elem_ty, _) if elem_ty == tcx.types.u8 => PatKind::Constant { value: cv },
+                // The typechecker has a special case for byte string literals, by treating them
+                // as slices. This means we turn `&[T; N]` constants into slice patterns, which
+                // has no negative effects on pattern matching, even if we're actually matching on
+                // arrays.
+                ty::Array(..) |
                 // Cannot merge this with the catch all branch below, because the `const_deref`
-                // changes the type from slice to array, and slice patterns behave differently from
-                // array patterns.
+                // changes the type from slice to array, we need to keep the original type in the
+                // pattern.
                 ty::Slice(..) => {
                     let old = self.behind_reference.replace(true);
                     let array = tcx.deref_const(self.param_env.and(cv));

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -158,6 +158,13 @@ crate enum PatKind<'tcx> {
         subpattern: Pat<'tcx>,
     },
 
+    /// One of the following:
+    /// * `&str`, which will be handled as a string pattern and thus exhaustiveness
+    ///   checking will detect if you use the same string twice in different patterns.
+    /// * integer, bool, char or float, which will be handled by exhaustivenes to cover exactly
+    ///   its own value, similar to `&str`, but these values are much simpler.
+    /// * Opaque constants, that must not be matched structurally. So anything that does not derive
+    ///   `PartialEq` and `Eq`.
     Constant {
         value: &'tcx ty::Const<'tcx>,
     },

src/test/ui/consts/const_in_pattern/issue-78057.rs

@@ -0,0 +1,17 @@
+#![deny(unreachable_patterns)]
+
+#[derive(PartialEq)]
+struct Opaque(i32);
+
+impl Eq for Opaque {}
+
+const FOO: Opaque = Opaque(42);
+
+fn main() {
+    match FOO {
+        FOO => {},
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+}

src/test/ui/consts/const_in_pattern/issue-78057.stderr

@@ -0,0 +1,20 @@
+error: to use a constant of type `Opaque` in a pattern, `Opaque` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/issue-78057.rs:12:9
+   |
+LL |         FOO => {},
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/issue-78057.rs:14:9
+   |
+LL |         _ => {}
+   |         ^
+   |
+note: the lint level is defined here
+  --> $DIR/issue-78057.rs:1:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/pattern/usefulness/consts-opaque.rs

@@ -0,0 +1,112 @@
+// This file tests the exhaustiveness algorithm on opaque constants. Most of the examples give
+// unnecessary warnings because const_to_pat.rs converts a constant pattern to a wildcard when the
+// constant is not allowed as a pattern. This is an edge case so we may not care to fix it.
+// See also https://github.com/rust-lang/rust/issues/78057
+
+#![deny(unreachable_patterns)]
+
+#[derive(PartialEq)]
+struct Foo(i32);
+impl Eq for Foo {}
+const FOO: Foo = Foo(42);
+const FOO_REF: &Foo = &Foo(42);
+const FOO_REF_REF: &&Foo = &&Foo(42);
+
+#[derive(PartialEq)]
+struct Bar;
+impl Eq for Bar {}
+const BAR: Bar = Bar;
+
+#[derive(PartialEq)]
+enum Baz {
+    Baz1,
+    Baz2
+}
+impl Eq for Baz {}
+const BAZ: Baz = Baz::Baz1;
+
+type Quux = fn(usize, usize) -> usize;
+fn quux(a: usize, b: usize) -> usize { a + b }
+const QUUX: Quux = quux;
+
+fn main() {
+    match FOO {
+        FOO => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match FOO_REF {
+        FOO_REF => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Foo(_) => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    // FIXME: this causes an ICE (https://github.com/rust-lang/rust/issues/78071)
+    //match FOO_REF_REF {
+    //    FOO_REF_REF => {}
+    //    Foo(_) => {}
+    //}
+
+    match BAR {
+        Bar => {}
+        BAR => {} // should not be emitting unreachable warning
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAR {
+        BAR => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Bar => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAR {
+        BAR => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        BAR => {} // should not be emitting unreachable warning
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| ERROR unreachable pattern
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Baz::Baz1 => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        Baz::Baz1 => {}
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        _ => {}
+        //~^ ERROR unreachable pattern
+    }
+
+    match BAZ {
+        BAZ => {}
+        //~^ ERROR must be annotated with `#[derive(PartialEq, Eq)]`
+        Baz::Baz2 => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+        _ => {} // should not be emitting unreachable warning
+        //~^ ERROR unreachable pattern
+    }
+
+    match QUUX {
+        QUUX => {}
+        QUUX => {}
+        _ => {}
+    }
+}

src/test/ui/pattern/usefulness/consts-opaque.stderr

@@ -0,0 +1,148 @@
+error: to use a constant of type `Foo` in a pattern, `Foo` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:34:9
+   |
+LL |         FOO => {}
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:36:9
+   |
+LL |         _ => {} // should not be emitting unreachable warning
+   |         ^
+   |
+note: the lint level is defined here
+  --> $DIR/consts-opaque.rs:6:9
+   |
+LL | #![deny(unreachable_patterns)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: to use a constant of type `Foo` in a pattern, `Foo` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:41:9
+   |
+LL |         FOO_REF => {}
+   |         ^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:43:9
+   |
+LL |         Foo(_) => {} // should not be emitting unreachable warning
+   |         ^^^^^^
+
+error: to use a constant of type `Bar` in a pattern, `Bar` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:55:9
+   |
+LL |         BAR => {} // should not be emitting unreachable warning
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:55:9
+   |
+LL |         Bar => {}
+   |         --- matches any value
+LL |         BAR => {} // should not be emitting unreachable warning
+   |         ^^^ unreachable pattern
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:58:9
+   |
+LL |         Bar => {}
+   |         --- matches any value
+...
+LL |         _ => {}
+   |         ^ unreachable pattern
+
+error: to use a constant of type `Bar` in a pattern, `Bar` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:63:9
+   |
+LL |         BAR => {}
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:65:9
+   |
+LL |         Bar => {} // should not be emitting unreachable warning
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:67:9
+   |
+LL |         Bar => {} // should not be emitting unreachable warning
+   |         --- matches any value
+LL |
+LL |         _ => {}
+   |         ^ unreachable pattern
+
+error: to use a constant of type `Bar` in a pattern, `Bar` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:72:9
+   |
+LL |         BAR => {}
+   |         ^^^
+
+error: to use a constant of type `Bar` in a pattern, `Bar` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:74:9
+   |
+LL |         BAR => {} // should not be emitting unreachable warning
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:74:9
+   |
+LL |         BAR => {} // should not be emitting unreachable warning
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:77:9
+   |
+LL |         _ => {} // should not be emitting unreachable warning
+   |         ^
+
+error: to use a constant of type `Baz` in a pattern, `Baz` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:82:9
+   |
+LL |         BAZ => {}
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:84:9
+   |
+LL |         Baz::Baz1 => {} // should not be emitting unreachable warning
+   |         ^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:86:9
+   |
+LL |         _ => {}
+   |         ^
+
+error: to use a constant of type `Baz` in a pattern, `Baz` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:92:9
+   |
+LL |         BAZ => {}
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:94:9
+   |
+LL |         _ => {}
+   |         ^
+
+error: to use a constant of type `Baz` in a pattern, `Baz` must be annotated with `#[derive(PartialEq, Eq)]`
+  --> $DIR/consts-opaque.rs:99:9
+   |
+LL |         BAZ => {}
+   |         ^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:101:9
+   |
+LL |         Baz::Baz2 => {} // should not be emitting unreachable warning
+   |         ^^^^^^^^^
+
+error: unreachable pattern
+  --> $DIR/consts-opaque.rs:103:9
+   |
+LL |         _ => {} // should not be emitting unreachable warning
+   |         ^
+
+error: aborting due to 22 previous errors
+

src/test/ui/pattern/usefulness/match-byte-array-patterns-2.stderr

@@ -7,11 +7,11 @@ LL |     match buf {
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `&[u8; 4]`
 
-error[E0004]: non-exhaustive patterns: `&[]`, `&[_]`, `&[_, _]` and 2 more not covered
+error[E0004]: non-exhaustive patterns: `&[0_u8..=64_u8, _, _, _]` and `&[66_u8..=u8::MAX, _, _, _]` not covered
   --> $DIR/match-byte-array-patterns-2.rs:10:11
    |
 LL |     match buf {
-   |           ^^^ patterns `&[]`, `&[_]`, `&[_, _]` and 2 more not covered
+   |           ^^^ patterns `&[0_u8..=64_u8, _, _, _]` and `&[66_u8..=u8::MAX, _, _, _]` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `&[u8]`