Test and document mut for avoid_slice_indexing

Author: xFrednet

clippy_lints/src/avoidable_slice_indexing.rs

@@ -19,9 +19,14 @@ use std::convert::TryInto;
 
 declare_clippy_lint! {
     /// ### What it does
-    /// The lint checks for slices that come from value deconstruction and are only used to access individual slice values.
+    /// The lint checks for slices that come from value deconstruction and
+    /// are only used to access individual slice values.
     ///
     /// ### Why is this bad?
+    /// Accessing slice values using indices can lead to panics.
+    ///
+    /// ### Limitations
+    /// This lint currently only checks for immutable access.
     ///
     /// ### Example
     /// ```rust

tests/ui-toml/avoidable_slice_indexing_limit/avoidable_slice_indexing_limit.stderr

@@ -11,8 +11,8 @@ LL | #![deny(clippy::avoidable_slice_indexing)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 help: try destructing the slice with a pattern here
    |
-LL |     if let Some(&[_, _, _, _, _, _, _, slice_7, ..]) = slice {
-   |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     if let Some([_, _, _, _, _, _, _, slice_7, ..]) = slice {
+   |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 help: and replacing the index expressions here
    |
 LL |         println!("{}", slice_7);

tests/ui/avoidable_slice_indexing/if_let_slice_destruction.rs

@@ -57,7 +57,7 @@ fn lintable_examples() {
     println!("{:?}", slice);
 }
 
-fn slice_too_long() {
+fn slice_index_above_limit() {
     let slice: Option<&[u32]> = Some(&[1, 2, 3]);
 
     if let Some(slice) = slice {
@@ -78,6 +78,25 @@ fn slice_is_used() {
     }
 }
 
+/// This would be a nice additional feature to have in the future, but adding it
+/// now would make the PR too large. This is therefore only a test that we don't
+/// lint cases we can't make a reasonable suggestion for
+fn mutable_slice_index() {
+    // Mut access
+    let mut slice: Option<[String; 1]> = Some([String::from("Penguin")]);
+    if let Some(ref mut slice) = slice {
+        slice[0] = String::from("Mr. Penguin");
+    }
+    println!("Use after modification: {:?}", slice);
+
+    // Mut access on reference
+    let mut slice: Option<[String; 1]> = Some([String::from("Cat")]);
+    if let Some(slice) = &mut slice {
+        slice[0] = String::from("Lord Meow Meow");
+    }
+    println!("Use after modification: {:?}", slice);
+}
+
 fn complex_binding() {
     let slice: Option<&[u32]> = Some(&[1, 2, 3]);
     if let Some(slice @ [_, _, _]) = slice {