Add a script to help update the changelog

CONTRIBUTING.md

@@ -224,7 +224,8 @@ Instructions for mdBook maintainers to publish a new release:
 1. Create a PR to update the version and update the CHANGELOG:
     1. Update the version in `Cargo.toml`
     2. Run `cargo xtask test-all` to verify that everything is passing, and to update `Cargo.lock`.
-    3. Update `CHANGELOG.md` with any changes that users may be interested in.
+    3. Run `cargo xtask changelog` to add a new entry to the changelog.
+        1. This will add a list of all changes at the top. You will need to move those into the appropriate categories. Most changes that are generally not relevant to a user should be removed. Rewrite the descriptions so that a user can reasonably figure out what it means.
     4. Commit the changes, and open a PR.
 2. After the PR has been merged, create a release in GitHub. This can either be done in the GitHub web UI, or on the command-line:
    ```bash

crates/xtask/src/changelog.rs

@@ -0,0 +1,120 @@
+//! Helper to generate a changelog for a new release.
+
+use super::Result;
+use std::fs;
+use std::process::Command;
+use std::process::exit;
+
+const CHANGELOG_PATH: &str = "CHANGELOG.md";
+
+pub(crate) fn changelog() -> Result<()> {
+    let previous = get_previous()?;
+    let current = get_current()?;
+    if current == previous {
+        eprintln!(
+            "error: Current version is `{current}` which is the same as the \
+             previous version in the changelog. Run `cargo set-version --bump <BUMP> first."
+        );
+        exit(1);
+    }
+    let prs = get_prs(&previous)?;
+    update_changelog(&previous, &current, &prs)?;
+    Ok(())
+}
+
+fn get_previous() -> Result<String> {
+    let contents = fs::read_to_string(CHANGELOG_PATH)?;
+    let version = contents
+        .lines()
+        .filter_map(|line| line.strip_prefix("## mdBook "))
+        .next()
+        .expect("at least one entry")
+        .to_owned();
+    Ok(version)
+}
+
+fn get_current() -> Result<String> {
+    let contents = fs::read_to_string("Cargo.toml")?;
+    let mut lines = contents
+        .lines()
+        .filter_map(|line| line.strip_prefix("version = "))
+        .map(|version| &version[1..version.len() - 1]);
+    let version = lines.next().expect("version should exist").to_owned();
+    assert_eq!(lines.next(), None);
+    Ok(version)
+}
+
+fn get_prs(previous: &str) -> Result<Vec<(String, String)>> {
+    println!("running `git fetch upstream`");
+    let status = Command::new("git").args(["fetch", "upstream"]).status()?;
+    if !status.success() {
+        eprintln!("error: git fetch failed");
+        exit(1);
+    }
+    println!("running `git log`");
+    const SEPARATOR: &str = "---COMMIT_SEPARATOR---";
+    let output = Command::new("git")
+        .args([
+            "log",
+            "--first-parent",
+            &format!("--pretty=format:%B%n{SEPARATOR}"),
+            "upstream/master",
+            &format!("v{previous}...upstream/HEAD"),
+        ])
+        .output()?;
+    if !output.status.success() {
+        eprintln!("error: git log failed");
+        exit(1);
+    }
+    let stdout = std::str::from_utf8(&output.stdout).unwrap();
+    let prs = stdout
+        .split(&format!("{SEPARATOR}\n"))
+        .filter_map(|entry| {
+            let mut lines = entry.lines();
+            let first = match lines.next().unwrap().strip_prefix("Merge pull request #") {
+                Some(f) => f,
+                None => {
+                    println!("warning: merge line not found in {entry}");
+                    return None;
+                }
+            };
+            let number = first.split_whitespace().next().unwrap();
+            assert_eq!(lines.next(), Some(""));
+            let title = lines.next().expect("title is set");
+            assert_eq!(lines.next(), Some(""));
+            Some((number.to_string(), title.to_string()))
+        })
+        .collect();
+    Ok(prs)
+}
+
+fn update_changelog(previous: &str, current: &str, prs: &[(String, String)]) -> Result<()> {
+    let prs: String = prs
+        .iter()
+        .map(|(number, title)| {
+            format!(
+                "- {title}\n  \
+                [#{number}](https://github.com/rust-lang/mdBook/pull/{number})\n"
+            )
+        })
+        .collect();
+    let new = format!(
+        "## mdBook {current}\n\
+        [v{previous}...v{current}](https://github.com/rust-lang/mdBook/compare/v{previous}...v{current})\n\
+        \n\
+        {prs}\
+        \n\
+        ### Added\n\
+        \n\
+        ### Changed\n\
+        \n\
+        ### Fixed\n\
+        \n"
+    );
+
+    let mut contents = fs::read_to_string(CHANGELOG_PATH)?;
+    let insertion_point = contents.find("## ").unwrap();
+    contents.insert_str(insertion_point, &new);
+    fs::write(CHANGELOG_PATH, contents)?;
+    Ok(())
+}

crates/xtask/src/main.rs

@@ -5,11 +5,13 @@ use std::error::Error;
 use std::process::Command;
 use std::process::exit;
 
+mod changelog;
+
 type Result<T> = std::result::Result<T, Box<dyn Error>>;
 
 fn main() -> Result<()> {
     macro_rules! commands {
-        ($($name:literal => $func:ident),* $(,)?) => {
+        ($($name:literal => $func:expr),* $(,)?) => {
             [$(($name, $func as fn() -> Result<()>)),*]
         };
     }
@@ -23,6 +25,7 @@ fn main() -> Result<()> {
         "semver-checks" => semver_checks,
         "eslint" => eslint,
         "gui" => gui,
+        "changelog" => changelog::changelog,
     }
     .into_iter()
     .collect();