fix a FP in indexing_slicing

treat refs to arrays the same as arrays
in `indexing_slicing` and `out_of_bounds_indexing`

Author: rail-rain

clippy_lints/src/indexing_slicing.rs

@@ -88,7 +88,7 @@ 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 let ExprKind::Index(ref array, ref index) = &expr.kind {
-            let ty = cx.typeck_results().expr_ty(array);
+            let ty = cx.typeck_results().expr_ty(array).peel_refs();
             if let Some(range) = higher::range(index) {
                 // Ranged indexes, i.e., &x[n..m], &x[n..], &x[..n] and &x[..]
                 if let ty::Array(_, s) = ty.kind() {

tests/ui/indexing_slicing_index.rs

@@ -15,7 +15,8 @@ fn main() {
     x[3]; // Ok, should not produce stderr.
 
     let y = &x;
-    y[0];
+    y[0]; // Ok, referencing shouldn't affect this lint. See the issue 6021
+    y[4]; // Ok, rustc will handle references too.
 
     let v = vec![0; 5];
     v[0];

tests/ui/indexing_slicing_index.stderr

@@ -8,52 +8,44 @@ LL |     x[index];
    = help: Consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic.
-  --> $DIR/indexing_slicing_index.rs:18:5
-   |
-LL |     y[0];
-   |     ^^^^
-   |
-   = help: Consider using `.get(n)` or `.get_mut(n)` instead
-
-error: indexing may panic.
-  --> $DIR/indexing_slicing_index.rs:21:5
+  --> $DIR/indexing_slicing_index.rs:22:5
    |
 LL |     v[0];
    |     ^^^^
    |
    = 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:23:5
    |
 LL |     v[10];
    |     ^^^^^
    |
    = 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:24:5
    |
 LL |     v[1 << 3];
    |     ^^^^^^^^^
    |
    = help: Consider using `.get(n)` or `.get_mut(n)` instead
 
 error: indexing may panic.
-  --> $DIR/indexing_slicing_index.rs:29:5
+  --> $DIR/indexing_slicing_index.rs:30:5
    |
 LL |     v[N];
    |     ^^^^
    |
    = 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:31:5
    |
 LL |     v[M];
    |     ^^^^
    |
    = help: Consider using `.get(n)` or `.get_mut(n)` instead
 
-error: aborting due to 7 previous errors
+error: aborting due to 6 previous errors
 

tests/ui/indexing_slicing_slice.stderr

@@ -71,29 +71,17 @@ LL |     &x[1..][..5];
    |
    = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
 
-error: slicing may panic.
-  --> $DIR/indexing_slicing_slice.rs:24:6
-   |
-LL |     &y[1..2];
-   |      ^^^^^^^
-   |
-   = help: Consider using `.get(n..m)` or `.get_mut(n..m)` instead
-
-error: slicing may panic.
-  --> $DIR/indexing_slicing_slice.rs:25:6
+error: range is out of bounds
+  --> $DIR/indexing_slicing_slice.rs:25:12
    |
 LL |     &y[0..=4];
-   |      ^^^^^^^^
-   |
-   = help: Consider using `.get(n..m)` or `.get_mut(n..m)` instead
+   |            ^
 
-error: slicing may panic.
-  --> $DIR/indexing_slicing_slice.rs:26:6
+error: range is out of bounds
+  --> $DIR/indexing_slicing_slice.rs:26:11
    |
 LL |     &y[..=4];
-   |      ^^^^^^^
-   |
-   = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
+   |           ^
 
 error: slicing may panic.
   --> $DIR/indexing_slicing_slice.rs:31:6
@@ -133,5 +121,5 @@ LL |     &v[..100];
    |
    = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
 
-error: aborting due to 17 previous errors
+error: aborting due to 16 previous errors