Removing lint for constant usize array indexing

This commit removes the logic in this PR that linted out-of-bounds constant `usize` indexing on arrays. That case is already handled by rustc's `const_err` lint. Beyond removing the linting logic, the test file and its associated stderr were updated to verify that const `usize` indexing operations on arrays are no longer handled by this `indexing_slicing` lint.

Author: shnewto

clippy_lints/src/indexing_slicing.rs

@@ -34,9 +34,9 @@ declare_clippy_lint! {
     "out of bounds constant indexing"
 }
 
-/// **What it does:** Checks for usage of indexing or slicing. Does not report
-/// on arrays if we can tell that the indexing or slicing operations are in
-/// bounds.
+/// **What it does:** Checks for usage of indexing or slicing. Arrays are special cased, this lint
+/// does report on arrays if we can tell that slicing operations are in bounds and does not
+/// lint on constant `usize` indexing on arrays because that is handled by rustc's `const_err` lint.
 ///
 /// **Why is this bad?** Indexing and slicing can panic at runtime and there are
 /// safe alternatives.
@@ -64,13 +64,11 @@ declare_clippy_lint! {
 /// let y = [0, 1, 2, 3];
 ///
 /// // Bad
-/// y[10];
 /// &y[10..100];
 /// &y[10..];
 /// &y[..100];
 ///
 /// // Good
-/// y[2];
 /// &y[2..];
 /// &y[..2];
 /// &y[0..3];
@@ -132,20 +130,10 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IndexingSlicing {
                 );
             } else {
                 // Catchall non-range index, i.e. [n] or [n << m]
-                if let ty::TyArray(_, s) = ty.sty {
-                    let size: u128 = s.assert_usize(cx.tcx).unwrap().into();
+                if let ty::TyArray(..) = ty.sty {
                     // Index is a constant uint.
-                    if let Some((Constant::Int(const_index), _)) = constant(cx, cx.tables, index) {
-                        if size <= const_index {
-                            utils::span_lint(
-                                cx,
-                                OUT_OF_BOUNDS_INDEXING,
-                                expr.span,
-                                "const index is out of bounds",
-                            );
-                        }
-                        // Else index is in bounds, ok.
-
+                    if let Some(..) = constant(cx, cx.tables, index) {
+                        // Let rustc's `const_err` lint handle constant `usize` indexing on arrays.
                         return;
                     }
                 }

tests/ui/indexing_slicing.rs

@@ -13,8 +13,8 @@ fn main() {
     &x[..index];
     &x[index_from..index_to];
     &x[index_from..][..index_to]; // Two lint reports, one for [index_from..] and another for [..index_to].
-    x[4];
-    x[1 << 3];
+    x[4]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
+    x[1 << 3]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
     &x[..=4];
     &x[1..5];
     &x[5..][..10]; // Two lint reports, one for [5..] and another for [..10].
@@ -44,7 +44,7 @@ fn main() {
     &y[..]; // Ok, should not produce stderr.
 
     let empty: [i8; 0] = [];
-    empty[0];
+    empty[0]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
     &empty[1..5];
     &empty[0..=4];
     &empty[..=4];
@@ -75,7 +75,7 @@ fn main() {
 
     const N: usize = 15; // Out of bounds
     const M: usize = 3; // In bounds
-    x[N];
+    x[N]; // Ok, let rustc's `const_err` lint handle `usize` indexing on arrays.
     x[M]; // Ok, should not produce stderr.
     v[N];
     v[M];

tests/ui/indexing_slicing.stderr

@@ -47,25 +47,13 @@ error: slicing may panic.
    |
    = help: Consider using `.get(n..)` or .get_mut(n..)` instead
 
-error: const index is out of bounds
-  --> $DIR/indexing_slicing.rs:16:5
-   |
-16 |     x[4];
-   |     ^^^^
-   |
-   = note: `-D out-of-bounds-indexing` implied by `-D warnings`
-
-error: const index is out of bounds
-  --> $DIR/indexing_slicing.rs:17:5
-   |
-17 |     x[1 << 3];
-   |     ^^^^^^^^^
-
 error: range is out of bounds
   --> $DIR/indexing_slicing.rs:18:6
    |
 18 |     &x[..=4];
    |      ^^^^^^^
+   |
+   = note: `-D out-of-bounds-indexing` implied by `-D warnings`
 
 error: range is out of bounds
   --> $DIR/indexing_slicing.rs:19:6
@@ -159,12 +147,6 @@ error: slicing may panic.
    |
    = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
 
-error: const index is out of bounds
-  --> $DIR/indexing_slicing.rs:47:5
-   |
-47 |     empty[0];
-   |     ^^^^^^^^
-
 error: range is out of bounds
   --> $DIR/indexing_slicing.rs:48:6
    |
@@ -269,12 +251,6 @@ error: slicing may panic.
    |
    = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
 
-error: const index is out of bounds
-  --> $DIR/indexing_slicing.rs:78:5
-   |
-78 |     x[N];
-   |     ^^^^
-
 error: indexing may panic.
   --> $DIR/indexing_slicing.rs:80:5
    |
@@ -291,5 +267,5 @@ error: indexing may panic.
    |
    = help: Consider using `.get(n)` or `.get_mut(n)` instead
 
-error: aborting due to 41 previous errors
+error: aborting due to 37 previous errors