Merge pull request #2697 from jstasiak/document-rust-toolchain-error

Document "invalid channel name '[toolchain]'" error

Author: kinnison

doc/src/overrides.md

@@ -1,4 +1,4 @@
-# Overrides
+[[#]] Overrides
 
 `rustup` automatically determines which [toolchain] to use when one of the
 installed commands like `rustc` is executed. There are several ways to control
@@ -97,6 +97,16 @@ just `nightly-2021-01-21`. The file has to be encoded in US-ASCII this case
 (if you are on Windows, check the encoding and that it does not start with a
 BOM). The legacy format is not available in `rust-toolchain.toml` files.
 
+If you see the following error (when running `rustc`, `cargo` or other command)
+
+```
+error: invalid channel name '[toolchain]' in '/PATH/TO/DIRECTORY/rust-toolchain'
+```
+
+it means you're running `rustup` pre-1.23.0 and trying to interact with a project
+that uses the new TOML encoding in the `rust-toolchain` file. You need to upgrade
+`rustup` to 1.23.0+.
+
 The `rust-toolchain.toml`/`rust-toolchain` files are suitable to check in to
 source control.