Fully destructure slice and array constants into patterns

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 +158,12 @@ 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. So anything that does not derive `PartialEq` and `Eq`.
     Constant {
         value: &'tcx ty::Const<'tcx>,
     },

src/test/ui/consts/const_in_pattern/ice-regression.rs

@@ -0,0 +1,16 @@
+
+
+#[derive(PartialEq)]
+struct Opaque(i32);
+
+impl Eq for Opaque {}
+
+const FOO: &&Opaque = &&Opaque(42);
+
+fn main() {
+    match FOO {
+        FOO => {}, //~ WARN must be annotated with
+        // The following should not lint about unreachable
+        Opaque(42) => {}
+    }
+}

src/test/ui/consts/const_in_pattern/ice-regression.stderr

@@ -0,0 +1,8 @@
+error: expected item, found keyword `match`
+  --> $DIR/ice-regression.rs:8:1
+   |
+LL | match FOO {
+   | ^^^^^ expected item
+
+error: aborting due to previous error
+

src/test/ui/issues/issue-44333.stderr

@@ -21,5 +21,13 @@ LL |         BAR => println!("bar"),
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/70861>
 
-warning: 2 warnings emitted
+warning: unreachable pattern
+  --> $DIR/issue-44333.rs:21:9
+   |
+LL |         BAR => println!("bar"),
+   |         ^^^
+   |
+   = note: `#[warn(unreachable_patterns)]` on by default
+
+warning: 3 warnings emitted
 

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]`