add ./miri squash

src/tools/miri/CONTRIBUTING.md

@@ -19,12 +19,10 @@ When you get a review, please take care of the requested changes in new commits.
 existing commits. Generally avoid force-pushing. The only time you should force push is when there
 is a conflict with the master branch (in that case you should rebase across master, not merge), and
 all the way at the end of the review process when the reviewer tells you that the PR is done and you
-should squash the commits. For the latter case, use `git rebase --keep-base ...` to squash without
-changing the base commit your PR branches off of. Use your own judgment and the reviewer's guidance
-to decide whether the PR should be squashed into a single commit or multiple logically separate
-commits. (All this is to work around the fact that Github is quite bad at dealing with force pushes
-and does not support `git range-diff`. Maybe one day Github will be good at git and then life can
-become easier.)
+should squash the commits. If you are unsure how to use `git rebase` to squash commits, use `./miri
+squash` which automates the process but leaves little room for customization. (All this is to work
+around the fact that Github is quite bad at dealing with force pushes and does not support `git
+range-diff`. Maybe one day Github will be good at git and then life can become easier.)
 
 Most PRs bounce back and forth between the reviewer and the author several times, so it is good to
 keep track of who is expected to take the next step. We are using the `S-waiting-for-review` and

src/tools/miri/miri-script/src/commands.rs

@@ -1,7 +1,8 @@
 use std::collections::HashMap;
 use std::ffi::{OsStr, OsString};
-use std::fs::File;
-use std::io::{BufReader, BufWriter, Write};
+use std::fmt::Write as _;
+use std::fs::{self, File};
+use std::io::{self, BufRead, BufReader, BufWriter, Write as _};
 use std::ops::Not;
 use std::path::PathBuf;
 use std::time::Duration;
@@ -169,7 +170,8 @@ impl Command {
             | Command::Toolchain { .. }
             | Command::Bench { .. }
             | Command::RustcPull { .. }
-            | Command::RustcPush { .. } => {}
+            | Command::RustcPush { .. }
+            | Command::Squash => {}
         }
         // Then run the actual command.
         match self {
@@ -188,6 +190,7 @@ impl Command {
             Command::Toolchain { flags } => Self::toolchain(flags),
             Command::RustcPull { commit } => Self::rustc_pull(commit.clone()),
             Command::RustcPush { github_user, branch } => Self::rustc_push(github_user, branch),
+            Command::Squash => Self::squash(),
         }
     }
 
@@ -383,6 +386,72 @@ impl Command {
         Ok(())
     }
 
+    fn squash() -> Result<()> {
+        let sh = Shell::new()?;
+        sh.change_dir(miri_dir()?);
+        // Figure out base wrt latest upstream master.
+        // (We can't trust any of the local ones, they can all be outdated.)
+        let origin_master = {
+            cmd!(sh, "git fetch https://github.com/rust-lang/miri/")
+                .quiet()
+                .ignore_stdout()
+                .ignore_stderr()
+                .run()?;
+            cmd!(sh, "git rev-parse FETCH_HEAD").read()?
+        };
+        let base = cmd!(sh, "git merge-base HEAD {origin_master}").read()?;
+        // Rebase onto that, setting ourselves as the sequence editor so that we can edit the sequence programmatically.
+        // We want to forward the host stdin so apparently we cannot use `cmd!`.
+        let mut cmd = process::Command::new("git");
+        cmd.arg("rebase").arg(&base).arg("--interactive");
+        cmd.env("GIT_SEQUENCE_EDITOR", env::current_exe()?);
+        cmd.env("MIRI_SCRIPT_IS_GIT_SEQUENCE_EDITOR", "1");
+        cmd.current_dir(sh.current_dir());
+        let result = cmd.status()?;
+        if !result.success() {
+            bail!("`git rebase` failed");
+        }
+        Ok(())
+    }
+
+    pub fn squash_sequence_editor() -> Result<()> {
+        let sequence_file = env::args().nth(1).expect("git should pass us a filename");
+        if sequence_file == "fmt" {
+            // This is probably us being called as a git hook as part of the rebase. Let's just
+            // ignore this. Sadly `git rebase` does not have a flag to skip running hooks.
+            return Ok(());
+        }
+        // Read the provided sequence and adjust it.
+        let rebase_sequence = {
+            let mut rebase_sequence = String::new();
+            let file = fs::File::open(&sequence_file).with_context(|| {
+                format!("failed to read rebase sequence from {sequence_file:?}")
+            })?;
+            let file = io::BufReader::new(file);
+            for line in file.lines() {
+                let line = line?;
+                // The first line is left unchanged.
+                if rebase_sequence.is_empty() {
+                    writeln!(rebase_sequence, "{line}").unwrap();
+                    continue;
+                }
+                // If this is a "pick" like, make it "squash".
+                if let Some(rest) = line.strip_prefix("pick ") {
+                    writeln!(rebase_sequence, "squash {rest}").unwrap();
+                    continue;
+                }
+                // We've reached the end of the relevant part of the sequence, and we can stop.
+                break;
+            }
+            rebase_sequence
+        };
+        // Write out the adjusted sequence.
+        fs::write(&sequence_file, rebase_sequence).with_context(|| {
+            format!("failed to write adjusted rebase sequence to {sequence_file:?}")
+        })?;
+        Ok(())
+    }
+
     fn bench(
         target: Option<String>,
         no_install: bool,

src/tools/miri/miri-script/src/main.rs

@@ -133,6 +133,8 @@ pub enum Command {
         #[arg(default_value = "miri-sync")]
         branch: String,
     },
+    /// Squash the commits of the current feature branch into one.
+    Squash,
 }
 
 impl Command {
@@ -154,7 +156,7 @@ impl Command {
                 flags.extend(remainder);
                 Ok(())
             }
-            Self::Bench { .. } | Self::RustcPull { .. } | Self::RustcPush { .. } =>
+            Self::Bench { .. } | Self::RustcPull { .. } | Self::RustcPush { .. } | Self::Squash =>
                 bail!("unexpected \"--\" found in arguments"),
         }
     }
@@ -170,6 +172,11 @@ pub struct Cli {
 }
 
 fn main() -> Result<()> {
+    // If we are invoked as the git sequence editor, jump to that logic.
+    if !std::env::var_os("MIRI_SCRIPT_IS_GIT_SEQUENCE_EDITOR").unwrap_or_default().is_empty() {
+        return Command::squash_sequence_editor();
+    }
+
     // Split the arguments into the part before the `--` and the part after.
     // The `--` itself ends up in the second part.
     let miri_args: Vec<_> = std::env::args().take_while(|x| *x != "--").collect();