ci: use pinned nightly rustfmt to make unstable features take effect (#2896)

## What does this PR do?

This PR extends the existing `format` job in
`.github/workflows/build.yml` to
**check Rust formatting with nightly `rustfmt`**:

**Closes #2794**

.github/workflows/build.yml

@@ -13,6 +13,8 @@ env:
 jobs:
   format:
     runs-on: ubuntu-latest
+    env:
+      NIGHTLY_VERSION: nightly-2025-09-01
     steps:
       - name: Checkout
         uses: actions/checkout@v5
@@ -22,10 +24,10 @@ jobs:
           sudo apt update
           sudo apt install gettext yapf3
 
-      - name: Install nightly rustfmt
+      - name: Install pinned nightly for rustfmt
         run: |
-          rustup default nightly
-          rustup component add rustfmt
+          rustup toolchain install --profile minimal "$NIGHTLY_VERSION"
+          rustup component add rustfmt --toolchain "$NIGHTLY_VERSION"
 
       - name: Check formatting
         uses: dprint/check@v2.3

CONTRIBUTING.md

@@ -53,6 +53,16 @@ this:
 
 Run `dprint fmt` to automatically format all files.
 
+**Note:** To make sure you have the correct version of `rustfmt` installed,
+please run:
+
+```bash
+cargo xtask install-tools
+```
+
+This will install the pinned nightly toolchain and add the `rustfmt` component,
+so your local formatting will match the CI.
+
 ### Linux
 
 Install `dprint` using their

dprint.json

@@ -9,7 +9,7 @@
       "command": "yapf3",
       "exts": ["py"]
     }, {
-      "command": "rustup run stable rustfmt --edition 2024",
+      "command": "rustup run nightly-2025-09-01 rustfmt --edition 2024",
       "exts": ["rs"]
     }]
   },

rustfmt.toml

@@ -1,4 +1,5 @@
 # Please use a nightly rustfmt for these settings.
+unstable_features = true
 imports_granularity = "module"
 wrap_comments = true
 

xtask/src/main.rs

@@ -21,9 +21,9 @@
 
 use anyhow::{Ok, Result, anyhow};
 use clap::{Parser, Subcommand};
+use std::env;
 use std::path::{Path, PathBuf};
-use std::process::Stdio;
-use std::{env, process::Command};
+use std::process::{Command, Stdio};
 
 fn main() -> Result<()> {
     if let Err(e) = execute_task() {
@@ -49,7 +49,8 @@ enum Task {
     InstallTools,
     /// Runs the web driver tests in the tests directory.
     WebTests {
-        /// Optional 'book html' directory - if set, will also refresh the list of slides used by slide size test.
+        /// Optional 'book html' directory - if set, will also refresh the list
+        /// of slides used by slide size test.
         #[arg(short, long)]
         dir: Option<PathBuf>,
     },
@@ -61,7 +62,8 @@ enum Task {
         #[arg(short, long)]
         language: Option<String>,
 
-        /// Directory to place the build. If not provided, defaults to the book/ directory (or the book/xx directory if a language is provided).
+        /// Directory to place the build. If not provided, defaults to the book/
+        /// directory (or the book/xx directory if a language is provided).
         #[arg(short, long)]
         output: Option<PathBuf>,
     },
@@ -71,7 +73,8 @@ enum Task {
         #[arg(short, long)]
         language: Option<String>,
 
-        /// Directory to place the build. If not provided, defaults to the book/ directory (or the book/xx directory if a language is provided).
+        /// Directory to place the build. If not provided, defaults to the book/
+        /// directory (or the book/xx directory if a language is provided).
         #[arg(short, long)]
         output: Option<PathBuf>,
     },
@@ -91,6 +94,25 @@ fn execute_task() -> Result<()> {
 
 fn install_tools() -> Result<()> {
     println!("Installing project tools...");
+
+    const PINNED_NIGHTLY: &str = "nightly-2025-09-01";
+
+    let rustup_steps = [
+        ["toolchain", "install", "--profile", "minimal", PINNED_NIGHTLY],
+        ["component", "add", "rustfmt", "--toolchain", PINNED_NIGHTLY],
+    ];
+
+    for args in rustup_steps {
+        let status = std::process::Command::new("rustup").args(args).status()?;
+        if !status.success() {
+            return Err(anyhow!(
+                "Command 'rustup {}' failed with status {:?}",
+                args.join(" "),
+                status.code()
+            ));
+        }
+    }
+
     let path_to_mdbook_exerciser =
         Path::new(env!("CARGO_WORKSPACE_DIR")).join("mdbook-exerciser");
     let path_to_mdbook_course =
@@ -286,8 +308,9 @@ fn build(language: Option<String>, output_arg: Option<PathBuf>) -> Result<()> {
 }
 
 fn get_output_dir(language: Option<String>, output_arg: Option<PathBuf>) -> PathBuf {
-    // If the 'output' arg is specified by the caller, use that, otherwise output to the 'book/' directory
-    // (or the 'book/xx' directory if a language was specified).
+    // If the 'output' arg is specified by the caller, use that, otherwise output to
+    // the 'book/' directory (or the 'book/xx' directory if a language was
+    // specified).
     if let Some(d) = output_arg {
         d
     } else {