Accept TOML format in rust-toolchain

README.md

@@ -404,9 +404,21 @@ source repository. This is most often the case for nightly-only
 software that pins to a revision from the release archives.
 
 In these cases the toolchain can be named in the project's directory
-in a file called `rust-toolchain`, the content of which is the name of
-a single `rustup` toolchain, and which is suitable to check in to
-source control. This file has to be encoded in US-ASCII (if you are on
+in a file called `rust-toolchain`, the content of which is either the name of
+a single `rustup` toolchain, or a TOML file with the following layout:
+
+``` toml
+[toolchain]
+channel = "nightly-2020-07-10"
+components = [ "rustfmt", "rustc-dev" ]
+targets = [ "wasm32-unknown-unknown", "thumbv2-none-eabi" ]
+```
+
+If the TOML format is used, the `[toolchain]` section is mandatory,
+and at least one property must be specified.
+
+The `rust-toolchain` file is suitable to check in to source control. 
+This file has to be encoded in US-ASCII (if you are on
 Windows, check the encoding and that it does not starts with a BOM).
 
 The toolchains named in this file have a more restricted form than

src/cli/common.rs

@@ -480,9 +480,8 @@ pub fn list_toolchains(cfg: &Cfg, verbose: bool) -> Result<utils::ExitCode> {
             String::new()
         };
         let cwd = utils::current_dir()?;
-        let ovr_toolchain_name = if let Ok(Some((ovr_toolchain, _reason))) = cfg.find_override(&cwd)
-        {
-            ovr_toolchain.name().to_string()
+        let ovr_toolchain_name = if let Ok(Some((toolchain, _reason))) = cfg.find_override(&cwd) {
+            toolchain.name().to_string()
         } else {
             String::new()
         };