From c5b16784046bf69e11bab5b2777aaed8d1fad296 Mon Sep 17 00:00:00 2001
From: TheWastl <36932506+TheWastl@users.noreply.github.com>
Date: Fri, 24 Jun 2022 22:42:47 +0200
Subject: [PATCH] const_eval: Consider array length constant even if array is
 not

---
 compiler/rustc_const_eval/src/interpret/step.rs      | 12 +++++++++---
 .../issue-98444-const-index-out-of-bounds.rs         |  8 ++++++++
 .../issue-98444-const-index-out-of-bounds.stderr     | 10 ++++++++++
 src/test/ui/consts/issue-65348.rs                    |  1 +
 4 files changed, 28 insertions(+), 3 deletions(-)
 create mode 100644 src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.rs
 create mode 100644 src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.stderr

diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index fea158a9fe450..5f7fd857078c5 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/compiler/rustc_const_eval/src/interpret/step.rs
@@ -4,6 +4,7 @@
 
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{InterpResult, Scalar};
+use rustc_middle::ty;
 use rustc_middle::ty::layout::LayoutOf;
 
 use super::{InterpCx, Machine};
@@ -251,9 +252,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Len(place) => {
                 let src = self.eval_place(place)?;
-                let mplace = self.force_allocation(&src)?;
-                let len = mplace.len(self)?;
-                self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
+                if let &ty::Array(_, len) = src.layout.ty.kind() {
+                    let len = self.const_to_op(len, Some(dest.layout))?;
+                    self.copy_op(&len, &dest, /*allow_transmute*/ false)?;
+                } else {
+                    let mplace = self.force_allocation(&src)?;
+                    let len = mplace.len(self)?;
+                    self.write_scalar(Scalar::from_machine_usize(len, self), &dest)?;
+                }
             }
 
             AddressOf(_, place) | Ref(_, _, place) => {
diff --git a/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.rs b/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.rs
new file mode 100644
index 0000000000000..a6e0ce2103856
--- /dev/null
+++ b/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.rs
@@ -0,0 +1,8 @@
+// build-fail
+// Need to use build-fail because check doesn't run constant propagation.
+
+fn main() {
+    let xs: [i32; 5] = [1, 2, 3, 4, 5];
+    let _ = &xs;
+    let _ = xs[7]; //~ ERROR this operation will panic at runtime
+}
diff --git a/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.stderr b/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.stderr
new file mode 100644
index 0000000000000..75580bbd6816e
--- /dev/null
+++ b/src/test/ui/const_prop/issue-98444-const-index-out-of-bounds.stderr
@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+  --> $DIR/issue-98444-const-index-out-of-bounds.rs:7:13
+   |
+LL |     let _ = xs[7];
+   |             ^^^^^ index out of bounds: the length is 5 but the index is 7
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/consts/issue-65348.rs b/src/test/ui/consts/issue-65348.rs
index 5eafa831d6317..e3a8c2c922bb1 100644
--- a/src/test/ui/consts/issue-65348.rs
+++ b/src/test/ui/consts/issue-65348.rs
@@ -1,4 +1,5 @@
 // check-pass
+#![allow(unconditional_panic)]
 
 struct Generic<T>(T);