Auto merge of rust-lang#12915 - lnicola:promote-subtree, r=lnicola

internal: Update `xtask promote` and release instructions

Update `xtask` for the subtree workflow. This doesn't explain how to do a `rust -> RA`  sync, since that's definitely more involved, but will probably only happen rarely.

Author: bors

docs/dev/README.md

@@ -210,7 +210,8 @@ Release process is handled by `release`, `dist` and `promote` xtasks, `release`
 ./rust-rust-analyzer  # Note the name!
 ```
 
-Additionally, it assumes that the remote for `rust-analyzer` is called `upstream` (I use `origin` to point to my fork).
+The remote for `rust-analyzer` must be called `upstream` (I use `origin` to point to my fork).
+In addition, for `xtask promote` (see below), `rust-rust-analyzer` must have a `rust-analyzer` remote pointing to this repository on GitHub.
 
 `release` calls the GitHub API calls to scrape pull request comments and categorize them in the changelog.
 This step uses the `curl` and `jq` applications, which need to be available in `PATH`.
@@ -225,13 +226,13 @@ Release steps:
    * push it to `upstream`. This triggers GitHub Actions which:
      * runs `cargo xtask dist` to package binaries and VS Code extension
      * makes a GitHub release
-     * pushes VS Code extension to the marketplace
+     * publishes the VS Code extension to the marketplace
    * call the GitHub API for PR details
    * create a new changelog in `rust-analyzer.github.io`
 3. While the release is in progress, fill in the changelog
 4. Commit & push the changelog
 5. Tweet
-6. Inside `rust-analyzer`, run `cargo xtask promote` -- this will create a PR to rust-lang/rust updating rust-analyzer's submodule.
+6. Inside `rust-analyzer`, run `cargo xtask promote` -- this will create a PR to rust-lang/rust updating rust-analyzer's subtree.
    Self-approve the PR.
 
 If the GitHub Actions release fails because of a transient problem like a timeout, you can re-run the job from the Actions console.

xtask/src/release.rs

@@ -77,18 +77,12 @@ impl flags::Promote {
         cmd!(sh, "git switch master").run()?;
         cmd!(sh, "git fetch upstream").run()?;
         cmd!(sh, "git reset --hard upstream/master").run()?;
-        cmd!(sh, "git submodule update --recursive").run()?;
 
         let date = date_iso(sh)?;
         let branch = format!("rust-analyzer-{date}");
         cmd!(sh, "git switch -c {branch}").run()?;
-        {
-            let _dir = sh.push_dir("src/tools/rust-analyzer");
-            cmd!(sh, "git fetch origin").run()?;
-            cmd!(sh, "git reset --hard origin/release").run()?;
-        }
-        cmd!(sh, "git add src/tools/rust-analyzer").run()?;
-        cmd!(sh, "git commit -m':arrow_up: rust-analyzer'").run()?;
+        cmd!(sh, "git subtree pull -P src/tools/rust-analyzer rust-analyzer master").run()?;
+
         if !self.dry_run {
             cmd!(sh, "git push -u origin {branch}").run()?;
             cmd!(