From c120199116439827a89a0ac0f24f91f99dbed60f Mon Sep 17 00:00:00 2001
From: folex <0xdxdy@gmail.com>
Date: Sat, 5 Jan 2019 05:56:23 +0300
Subject: [PATCH] Show suggestion to use .char().nth() and link to The Book on
 unimplemented Index trait

---
 src/libcore/ops/index.rs           | 30 ++++++++++++++++++++++++++++++
 src/test/ui/str/str-idx.stderr     |  2 ++
 src/test/ui/str/str-mut-idx.stderr |  2 ++
 3 files changed, 34 insertions(+)

diff --git a/src/libcore/ops/index.rs b/src/libcore/ops/index.rs
index 4f55c68ecd4ae..6cfa36741d0ca 100644
--- a/src/libcore/ops/index.rs
+++ b/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}`",
 )]
diff --git a/src/test/ui/str/str-idx.stderr b/src/test/ui/str/str-idx.stderr
index 108096df9c41c..71b1747492329 100644
--- a/src/test/ui/str/str-idx.stderr
+++ b/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
 
diff --git a/src/test/ui/str/str-mut-idx.stderr b/src/test/ui/str/str-mut-idx.stderr
index a8ab38e5ab6a5..f1e969696b458 100644
--- a/src/test/ui/str/str-mut-idx.stderr
+++ b/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