From 43a79a0f4bf842c5e6a4ada2e6280fc9535c18ee Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Sun, 15 Dec 2024 22:18:09 -0800
Subject: [PATCH] Check for array lengths that aren't actually `usize`
---
.../src/build/expr/as_place.rs | 32 +++++++++++++++----
.../issues/index_array_bad_type.rs | 15 +++++++++
.../issues/index_array_bad_type.stderr | 8 +++++
3 files changed, 48 insertions(+), 7 deletions(-)
create mode 100644 tests/ui/const-generics/issues/index_array_bad_type.rs
create mode 100644 tests/ui/const-generics/issues/index_array_bad_type.stderr
diff --git a/compiler/rustc_mir_build/src/build/expr/as_place.rs b/compiler/rustc_mir_build/src/build/expr/as_place.rs
index 70a74910a68c6..89a1f06d3d16f 100644
--- a/compiler/rustc_mir_build/src/build/expr/as_place.rs
+++ b/compiler/rustc_mir_build/src/build/expr/as_place.rs
@@ -647,13 +647,31 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
match place_ty.kind() {
ty::Array(_elem_ty, len_const) => {
- // We know how long an array is, so just use that as a constant
- // directly -- no locals needed. We do need one statement so
- // that borrow- and initialization-checking consider it used,
- // though. FIXME: Do we really *need* to count this as a use?
- // Could partial array tracking work off something else instead?
- self.cfg.push_fake_read(block, source_info, FakeReadCause::ForIndex, place);
- let const_ = Const::from_ty_const(*len_const, usize_ty, self.tcx);
+ let ty_const = if let Some((_, len_ty)) = len_const.try_to_valtree()
+ && len_ty != self.tcx.types.usize
+ {
+ // Bad const generics can give us a constant from the type that's
+ // not actually a `usize`, so in that case give an error instead.
+ // FIXME: It'd be nice if the type checker made sure this wasn't
+ // possible, instead.
+ let err = self.tcx.dcx().span_delayed_bug(
+ span,
+ format!(
+ "Array length should have already been a type error, as it's {len_ty:?}"
+ ),
+ );
+ ty::Const::new_error(self.tcx, err)
+ } else {
+ // We know how long an array is, so just use that as a constant
+ // directly -- no locals needed. We do need one statement so
+ // that borrow- and initialization-checking consider it used,
+ // though. FIXME: Do we really *need* to count this as a use?
+ // Could partial array tracking work off something else instead?
+ self.cfg.push_fake_read(block, source_info, FakeReadCause::ForIndex, place);
+ *len_const
+ };
+
+ let const_ = Const::from_ty_const(ty_const, usize_ty, self.tcx);
Operand::Constant(Box::new(ConstOperand { span, user_ty: None, const_ }))
}
ty::Slice(_elem_ty) => {
diff --git a/tests/ui/const-generics/issues/index_array_bad_type.rs b/tests/ui/const-generics/issues/index_array_bad_type.rs
new file mode 100644
index 0000000000000..41e4dba026c2e
--- /dev/null
+++ b/tests/ui/const-generics/issues/index_array_bad_type.rs
@@ -0,0 +1,15 @@
+//@ check-fail
+//@ compile-flags: -C opt-level=0
+
+#![crate_type = "lib"]
+
+// This used to fail in the known-panics lint, as the MIR was ill-typed due to
+// the length constant not actually having type usize.
+// https://github.com/rust-lang/rust/issues/134352
+
+pub struct BadStruct<const N: i64>(pub [u8; N]);
+//~^ ERROR: the constant `N` is not of type `usize`
+
+pub fn bad_array_length_type(value: BadStruct<3>) -> u8 {
+ value.0[0]
+}
diff --git a/tests/ui/const-generics/issues/index_array_bad_type.stderr b/tests/ui/const-generics/issues/index_array_bad_type.stderr
new file mode 100644
index 0000000000000..e4417192150ee
--- /dev/null
+++ b/tests/ui/const-generics/issues/index_array_bad_type.stderr
@@ -0,0 +1,8 @@
+error: the constant `N` is not of type `usize`
+ --> $DIR/index_array_bad_type.rs:10:40
+ |
+LL | pub struct BadStruct<const N: i64>(pub [u8; N]);
+ | ^^^^^^^ expected `usize`, found `i64`
+
+error: aborting due to 1 previous error
+