fix indexing_slicing with const

- should not fire if indexing with a constant block
- should not fire if indexing within a constant context
(const statement or const block)

clippy_lints/src/indexing_slicing.rs

@@ -96,6 +96,10 @@ declare_lint_pass!(IndexingSlicing => [INDEXING_SLICING, OUT_OF_BOUNDS_INDEXING]
 
 impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if cx.tcx.hir().is_inside_const_context(expr.hir_id) {
+            return;
+        }
+
         if let ExprKind::Index(array, index) = &expr.kind {
             let ty = cx.typeck_results().expr_ty(array).peel_refs();
             if let Some(range) = higher::Range::hir(index) {
@@ -151,6 +155,10 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
             } else {
                 // Catchall non-range index, i.e., [n] or [n << m]
                 if let ty::Array(..) = ty.kind() {
+                    // Index is a const block.
+                    if let ExprKind::ConstBlock(..) = index.kind {
+                        return;
+                    }
                     // Index is a constant uint.
                     if let Some(..) = constant(cx, cx.typeck_results(), index) {
                         // Let rustc's `const_err` lint handle constant `usize` indexing on arrays.

tests/ui/indexing_slicing_index.rs

@@ -1,18 +1,34 @@
+#![feature(inline_const)]
 #![warn(clippy::indexing_slicing)]
 // We also check the out_of_bounds_indexing lint here, because it lints similar things and
 // we want to avoid false positives.
 #![warn(clippy::out_of_bounds_indexing)]
-#![allow(clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(const_err, clippy::no_effect, clippy::unnecessary_operation)]
+
+const ARR: [i32; 2] = [1, 2];
+const REF: &i32 = &ARR[idx()]; // Ok, should not produce stderr.
+const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
+
+const fn idx() -> usize {
+    1
+}
+const fn idx4() -> usize {
+    4
+}
 
 fn main() {
     let x = [1, 2, 3, 4];
     let index: usize = 1;
     x[index];
-    x[4]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
+    x[4]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
-    x[1 << 3]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
+    x[1 << 3]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
 
     x[0]; // Ok, should not produce stderr.
     x[3]; // Ok, should not produce stderr.
+    x[const { idx() }]; // Ok, should not produce stderr.
+    x[const { idx4() }]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
+    const { &ARR[idx()] }; // Ok, should not produce stderr.
+    const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
 
     let y = &x;
     y[0]; // Ok, referencing shouldn't affect this lint. See the issue 6021
@@ -25,7 +41,7 @@ fn main() {
 
     const N: usize = 15; // Out of bounds
     const M: usize = 3; // In bounds
-    x[N]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
+    x[N]; // Ok, let rustc's `unconditional_panic` lint handle `usize` indexing on arrays.
     x[M]; // Ok, should not produce stderr.
     v[N];
     v[M];

tests/ui/indexing_slicing_index.stderr

@@ -1,5 +1,17 @@
+error[E0080]: evaluation of `main::{constant#3}::<&i32>` failed
+  --> $DIR/indexing_slicing_index.rs:31:14
+   |
+LL |     const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
+   |              ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
+
+error[E0080]: erroneous constant used
+  --> $DIR/indexing_slicing_index.rs:31:5
+   |
+LL |     const { &ARR[idx4()] }; // Ok, let rustc handle const contexts.
+   |     ^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:10:5
+  --> $DIR/indexing_slicing_index.rs:22:5
    |
 LL |     x[index];
    |     ^^^^^^^^
@@ -8,7 +20,7 @@ LL |     x[index];
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:22:5
+  --> $DIR/indexing_slicing_index.rs:38:5
    |
 LL |     v[0];
    |     ^^^^
@@ -16,7 +28,7 @@ LL |     v[0];
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:23:5
+  --> $DIR/indexing_slicing_index.rs:39:5
    |
 LL |     v[10];
    |     ^^^^^
@@ -24,7 +36,7 @@ LL |     v[10];
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:24:5
+  --> $DIR/indexing_slicing_index.rs:40:5
    |
 LL |     v[1 << 3];
    |     ^^^^^^^^^
@@ -32,7 +44,7 @@ LL |     v[1 << 3];
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:30:5
+  --> $DIR/indexing_slicing_index.rs:46:5
    |
 LL |     v[N];
    |     ^^^^
@@ -40,12 +52,13 @@ LL |     v[N];
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic
-  --> $DIR/indexing_slicing_index.rs:31:5
+  --> $DIR/indexing_slicing_index.rs:47:5
    |
 LL |     v[M];
    |     ^^^^
    |
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
-error: aborting due to 6 previous errors
+error: aborting due to 8 previous errors
 
+For more information about this error, try `rustc --explain E0080`.