Fully destructure slice and array constants into patterns

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

@@ -304,12 +304,11 @@ use std::iter::{FromIterator, IntoIterator};
 use std::ops::RangeInclusive;
 
 crate fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
-    LiteralExpander { tcx: cx.tcx, param_env: cx.param_env }.fold_pattern(&pat)
+    LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
 }
 
 struct LiteralExpander<'tcx> {
     tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
 }
 
 impl<'tcx> LiteralExpander<'tcx> {
@@ -328,40 +327,17 @@ impl<'tcx> LiteralExpander<'tcx> {
     ) -> ConstValue<'tcx> {
         debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
         match (val, &crty.kind(), &rty.kind()) {
-            // the easy case, deref a reference
-            (ConstValue::Scalar(p), x, y) if x == y => {
-                match p {
-                    Scalar::Ptr(p) => {
-                        let alloc = self.tcx.global_alloc(p.alloc_id).unwrap_memory();
-                        ConstValue::ByRef { alloc, offset: p.offset }
-                    }
-                    Scalar::Raw { .. } => {
-                        let layout = self.tcx.layout_of(self.param_env.and(rty)).unwrap();
-                        if layout.is_zst() {
-                            // Deref of a reference to a ZST is a nop.
-                            ConstValue::Scalar(Scalar::zst())
-                        } else {
-                            // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;`
-                            bug!("cannot deref {:#?}, {} -> {}", val, crty, rty);
-                        }
-                    }
-                }
-            }
             // unsize array to slice if pattern is array but match value or other patterns are slice
             (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
                 assert_eq!(t, u);
+                assert_eq!(p.offset, Size::ZERO);
                 ConstValue::Slice {
                     data: self.tcx.global_alloc(p.alloc_id).unwrap_memory(),
-                    start: p.offset.bytes().try_into().unwrap(),
+                    start: 0,
                     end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
                 }
             }
-            // fat pointers stay the same
-            (ConstValue::Slice { .. }, _, _)
-            | (_, ty::Slice(_), ty::Slice(_))
-            | (_, ty::Str, ty::Str) => val,
-            // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
-            _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
+            _ => val,
         }
     }
 }

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

@@ -390,11 +390,14 @@ impl<'a, 'tcx> ConstToPat<'a, 'tcx> {
                 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));

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]`