Show suggestion to use .char().nth() and link to The Book on unimplem…

…ented Index trait

src/libcore/ops/index.rs

@@ -51,6 +51,21 @@
 /// ```
 #[lang = "index"]
 #[rustc_on_unimplemented(
+    on(
+        _Self="&str",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
+    on(
+        _Self="str",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
+    on(
+        _Self="std::string::String",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
     message="the type `{Self}` cannot be indexed by `{Idx}`",
     label="`{Self}` cannot be indexed by `{Idx}`",
 )]
@@ -141,6 +156,21 @@ pub trait Index<Idx: ?Sized> {
 /// ```
 #[lang = "index_mut"]
 #[rustc_on_unimplemented(
+    on(
+        _Self="&str",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
+    on(
+        _Self="str",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
+    on(
+        _Self="std::string::String",
+        note="you can use `.chars().nth()` or `.bytes().nth()`
+see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>"
+    ),
     message="the type `{Self}` cannot be mutably indexed by `{Idx}`",
     label="`{Self}` cannot be mutably indexed by `{Idx}`",
 )]

src/test/ui/str/str-idx.stderr

@@ -5,6 +5,8 @@ LL |     let c: u8 = s[4]; //~ ERROR the type `str` cannot be indexed by `{integ
    |                 ^^^^ `str` cannot be indexed by `{integer}`
    |
    = help: the trait `std::ops::Index<{integer}>` is not implemented for `str`
+   = note: you can use `.chars().nth()` or `.bytes().nth()`
+           see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
 
 error: aborting due to previous error
 

src/test/ui/str/str-mut-idx.stderr

@@ -29,6 +29,8 @@ LL |     s[1usize] = bot();
    |     ^^^^^^^^^ `str` cannot be mutably indexed by `usize`
    |
    = help: the trait `std::ops::IndexMut<usize>` is not implemented for `str`
+   = note: you can use `.chars().nth()` or `.bytes().nth()`
+           see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings>
 
 error: aborting due to 3 previous errors