Rollup merge of #89251 - estebank:negative-index-literals, r=davidtwco

Detect when negative literal indices are used and suggest appropriate code

src/test/ui/suggestions/negative-literal-index.fixed

@@ -0,0 +1,22 @@
+// run-rustfix
+
+use std::ops::Index;
+struct X;
+impl Index<i32> for X {
+    type Output = ();
+
+    fn index(&self, _: i32) -> &() {
+        &()
+    }
+}
+
+fn main() {
+    let x = vec![1, 2, 3];
+    x[x.len() -1]; //~ ERROR negative integers cannot be used to index on a
+    let x = [1, 2, 3];
+    x[x.len() -1]; //~ ERROR negative integers cannot be used to index on a
+    let x = &[1, 2, 3];
+    x[x.len() -1]; //~ ERROR negative integers cannot be used to index on a
+    let _ = x;
+    X[-1];
+}

src/test/ui/suggestions/negative-literal-index.rs

@@ -0,0 +1,22 @@
+// run-rustfix
+
+use std::ops::Index;
+struct X;
+impl Index<i32> for X {
+    type Output = ();
+
+    fn index(&self, _: i32) -> &() {
+        &()
+    }
+}
+
+fn main() {
+    let x = vec![1, 2, 3];
+    x[-1]; //~ ERROR negative integers cannot be used to index on a
+    let x = [1, 2, 3];
+    x[-1]; //~ ERROR negative integers cannot be used to index on a
+    let x = &[1, 2, 3];
+    x[-1]; //~ ERROR negative integers cannot be used to index on a
+    let _ = x;
+    X[-1];
+}

src/test/ui/suggestions/negative-literal-index.stderr

@@ -0,0 +1,35 @@
+error: negative integers cannot be used to index on a `Vec<{integer}>`
+  --> $DIR/negative-literal-index.rs:15:7
+   |
+LL |     x[-1];
+   |       ^^ cannot use a negative integer for indexing on `Vec<{integer}>`
+   |
+help: to access an element starting from the end of the `Vec<{integer}>`, compute the index
+   |
+LL |     x[x.len() -1];
+   |       +++++++
+
+error: negative integers cannot be used to index on a `[{integer}; 3]`
+  --> $DIR/negative-literal-index.rs:17:7
+   |
+LL |     x[-1];
+   |       ^^ cannot use a negative integer for indexing on `[{integer}; 3]`
+   |
+help: to access an element starting from the end of the `[{integer}; 3]`, compute the index
+   |
+LL |     x[x.len() -1];
+   |       +++++++
+
+error: negative integers cannot be used to index on a `[{integer}; 3]`
+  --> $DIR/negative-literal-index.rs:19:7
+   |
+LL |     x[-1];
+   |       ^^ cannot use a negative integer for indexing on `[{integer}; 3]`
+   |
+help: to access an element starting from the end of the `[{integer}; 3]`, compute the index
+   |
+LL |     x[x.len() -1];
+   |       +++++++
+
+error: aborting due to 3 previous errors
+