const_prop_lint: Consider array length constant even if array is not

compiler/rustc_mir_transform/src/const_prop_lint.rs

@@ -501,12 +501,23 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 return None;
             }
 
+            Rvalue::Len(len_place) => {
+                // To get the length of an array, we don't need to know the value.
+                let ty = len_place.ty(self.local_decls, self.tcx).ty;
+                if let &ty::Array(_, len) = ty.kind() {
+                    return self.use_ecx(source_info, |this| {
+                        let const_len = this.ecx.const_to_op(len, None)?;
+                        let ecx_place = this.ecx.eval_place(place)?;
+                        this.ecx.copy_op(&const_len, &ecx_place, /*allow_transmute*/ false)
+                    });
+                }
+            }
+
             // There's no other checking to do at this time.
             Rvalue::Aggregate(..)
             | Rvalue::Use(..)
             | Rvalue::CopyForDeref(..)
             | Rvalue::Repeat(..)
-            | Rvalue::Len(..)
             | Rvalue::Cast(..)
             | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)

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
+}

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
+

src/test/ui/consts/issue-65348.rs

@@ -1,4 +1,5 @@
 // check-pass
+#![allow(unconditional_panic)]
 
 struct Generic<T>(T);
 

src/tools/clippy/tests/ui/indexing_slicing_index.stderr

@@ -1,3 +1,23 @@
+error: this operation will panic at runtime
+  --> $DIR/indexing_slicing_index.rs:23:5
+   |
+LL |     x[4]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+   |     ^^^^ index out of bounds: the length is 4 but the index is 4
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: this operation will panic at runtime
+  --> $DIR/indexing_slicing_index.rs:24:5
+   |
+LL |     x[1 << 3]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+   |     ^^^^^^^^^ index out of bounds: the length is 4 but the index is 8
+
+error: this operation will panic at runtime
+  --> $DIR/indexing_slicing_index.rs:29:5
+   |
+LL |     x[const { idx4() }]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+   |     ^^^^^^^^^^^^^^^^^^^ index out of bounds: the length is 4 but the index is 4
+
 error[E0080]: evaluation of `main::{constant#3}` failed
   --> $DIR/indexing_slicing_index.rs:31:14
    |
@@ -10,6 +30,18 @@ error[E0080]: erroneous constant used
 LL |     const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
    |     ^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
 
+error: this operation will panic at runtime
+  --> $DIR/indexing_slicing_index.rs:35:5
+   |
+LL |     y[4]; // Ok, rustc will handle references too.
+   |     ^^^^ index out of bounds: the length is 4 but the index is 4
+
+error: this operation will panic at runtime
+  --> $DIR/indexing_slicing_index.rs:44:5
+   |
+LL |     x[N]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+   |     ^^^^ index out of bounds: the length is 4 but the index is 15
+
 error: indexing may panic
   --> $DIR/indexing_slicing_index.rs:22:5
    |
@@ -59,6 +91,6 @@ LL |     v[M];
    |
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
-error: aborting due to 8 previous errors
+error: aborting due to 13 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

src/tools/clippy/tests/ui/self_assignment.stderr

@@ -1,3 +1,17 @@
+error: this operation will panic at runtime
+  --> $DIR/self_assignment.rs:16:15
+   |
+LL |     s.b[10] = s.b[5 + 5];
+   |               ^^^^^^^^^^ index out of bounds: the length is 10 but the index is 10
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: this operation will panic at runtime
+  --> $DIR/self_assignment.rs:16:5
+   |
+LL |     s.b[10] = s.b[5 + 5];
+   |     ^^^^^^^ index out of bounds: the length is 10 but the index is 10
+
 error: self-assignment of `a` to `a`
   --> $DIR/self_assignment.rs:12:5
    |
@@ -66,5 +80,5 @@ error: self-assignment of `(t.0)` to `t.0`
 LL |     t.0 = (t.0);
    |     ^^^^^^^^^^^
 
-error: aborting due to 11 previous errors
+error: aborting due to 13 previous errors