rename `rust-toolchain` to `rust-toolchain.toml` (#14442)

The configuration of the toolchain using `rust-toolchain` is only
retained for backward compatibility, and it is recommended to explicitly
specify the TOML format, as in `rust-toolchain.toml`.

r? flip1995

(This is because the change affects the sync process, and you are the
one responsible for sync (I believe).)

changelog: none

6 files changed, 5 insertions, 5 deletions

diff --git a/book/src/development/basics.md b/book/src/development/basics.md
index 931e5c3a294..4219724ed5d 100644
--- a/book/src/development/basics.md
+++ b/book/src/development/basics.md
@@ -147,7 +147,7 @@ following:
 
 First, take note of the toolchain
 [override](https://rust-lang.github.io/rustup/overrides.html) in
-`/rust-toolchain`. We will use this override to install Clippy into the right
+`/rust-toolchain.toml`. We will use this override to install Clippy into the right
 toolchain.
 
 > Tip: You can view the active toolchain for the current directory with `rustup
diff --git a/book/src/development/infrastructure/sync.md b/book/src/development/infrastructure/sync.md
index da1ad586607..2bbdf47a835 100644
--- a/book/src/development/infrastructure/sync.md
+++ b/book/src/development/infrastructure/sync.md
@@ -86,7 +86,7 @@ to be run inside the `rust` directory):
 4. Bump the nightly version in the Clippy repository by running these commands:
    ```bash
    cargo dev sync update_nightly
-   git commit -m "Bump nightly version -> YYYY-MM-DD" rust-toolchain clippy_utils/README.md
+   git commit -m "Bump nightly version -> YYYY-MM-DD" rust-toolchain.toml clippy_utils/README.md
    ```
 5. Open a PR to `rust-lang/rust-clippy` and wait for it to get merged (to
    accelerate the process ping the `@rust-lang/clippy` team in your PR and/or
diff --git a/clippy_dev/src/main.rs b/clippy_dev/src/main.rs
index 074dea4ab77..0380b7e6a6d 100644
--- a/clippy_dev/src/main.rs
+++ b/clippy_dev/src/main.rs
@@ -334,7 +334,7 @@ struct SyncCommand {
 #[derive(Subcommand)]
 enum SyncSubcommand {
     #[command(name = "update_nightly")]
-    /// Update nightly version in rust-toolchain and `clippy_utils`
+    /// Update nightly version in `rust-toolchain.toml` and `clippy_utils`
     UpdateNightly,
 }
 
diff --git a/clippy_dev/src/setup/toolchain.rs b/clippy_dev/src/setup/toolchain.rs
index 2966629cf70..ecd80215f7e 100644
--- a/clippy_dev/src/setup/toolchain.rs
+++ b/clippy_dev/src/setup/toolchain.rs
@@ -62,7 +62,7 @@ pub fn create(standalone: bool, force: bool, release: bool, name: &str) {
 
     println!("Created toolchain {name}, use it in other projects with e.g. `cargo +{name} clippy`");
     if !standalone {
-        println!("Note: This will need to be re-run whenever the Clippy `rust-toolchain` changes");
+        println!("Note: This will need to be re-run whenever the Clippy `rust-toolchain.toml` changes");
     }
 }
 
diff --git a/clippy_dev/src/sync.rs b/clippy_dev/src/sync.rs
index 3522d182e90..a6b65e561c2 100644
--- a/clippy_dev/src/sync.rs
+++ b/clippy_dev/src/sync.rs
@@ -10,7 +10,7 @@ pub fn update_nightly() {
     let date = Utc::now().format("%Y-%m-%d").to_string();
     replace_region_in_file(
         UpdateMode::Change,
-        Path::new("rust-toolchain"),
+        Path::new("rust-toolchain.toml"),
         "# begin autogenerated nightly\n",
         "# end autogenerated nightly",
         |res| {
diff --git a/rust-toolchain b/rust-toolchain.toml
index fcaeedc9a66..fcaeedc9a66 100644
--- a/rust-toolchain
+++ b/rust-toolchain.toml