Implement --drop-message for jj squash

If `-d/--drop-message` is passed to `jj squash`, the descriptions of the
commits being squashed are dropped and the message of the destination commit is
used.

CHANGELOG.md

@@ -40,6 +40,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   It can thereby be for all use cases where `jj move` can be used. The `--from`
   argument accepts a revset that resolves to more than one revision.
 
+* `jj squash` now accepts a `--drop-message/-d` option that discards the
+  descriptions of the commits being squashed.
+
 * Commit templates now support `immutable` keyword.
 
 * New template function `coalesce(content, ..)` is added.

cli/src/commands/move.rs

@@ -16,7 +16,7 @@ use clap::ArgGroup;
 use jj_lib::object_id::ObjectId;
 use tracing::instrument;
 
-use super::squash::move_diff;
+use super::squash::{move_diff, SquashedDescription};
 use crate::cli_util::{CommandHelper, RevisionArg};
 use crate::command_error::{user_error, CommandError};
 use crate::ui::Ui;
@@ -93,7 +93,7 @@ pub(crate) fn cmd_move(
         &destination,
         matcher.as_ref(),
         &diff_selector,
-        None,
+        SquashedDescription::Combine,
         false,
         &args.paths,
     )?;

cli/src/commands/squash.rs

@@ -62,6 +62,9 @@ pub(crate) struct SquashArgs {
     /// The description to use for squashed revision (don't open editor)
     #[arg(long = "message", short, value_name = "MESSAGE")]
     message_paragraphs: Vec<String>,
+    /// Discard the descriptions of the squashed revisions.
+    #[arg(long, short)]
+    drop_message: bool,
     /// Interactively choose which parts to squash
     #[arg(long, short)]
     interactive: bool,
@@ -112,8 +115,6 @@ pub(crate) fn cmd_squash(
         workspace_command.diff_selector(ui, args.tool.as_deref(), args.interactive)?;
     let mut tx = workspace_command.start_transaction();
     let tx_description = format!("squash commits into {}", destination.id().hex());
-    let description = (!args.message_paragraphs.is_empty())
-        .then(|| join_message_paragraphs(&args.message_paragraphs));
     move_diff(
         ui,
         &mut tx,
@@ -122,14 +123,38 @@ pub(crate) fn cmd_squash(
         &destination,
         matcher.as_ref(),
         &diff_selector,
-        description,
+        SquashedDescription::from_args(args),
         args.revision.is_none() && args.from.is_none() && args.into.is_none(),
         &args.paths,
     )?;
     tx.finish(ui, tx_description)?;
     Ok(())
 }
 
+// TODO(#2882): Remove public visibility once `jj move` is deleted.
+pub(crate) enum SquashedDescription {
+    // Use this exact description.
+    Exact(String),
+    // Use the destination's description and discard the descriptions of the
+    // source revisions.
+    UseDestination,
+    // Combine the descriptions of the source and destination revisions.
+    Combine,
+}
+
+// TODO(#2882): Remove public visibility once `jj move` is deleted.
+impl SquashedDescription {
+    pub(crate) fn from_args(args: &SquashArgs) -> Self {
+        if !args.message_paragraphs.is_empty() {
+            SquashedDescription::Exact(join_message_paragraphs(&args.message_paragraphs))
+        } else if args.drop_message {
+            SquashedDescription::UseDestination
+        } else {
+            SquashedDescription::Combine
+        }
+    }
+}
+
 #[allow(clippy::too_many_arguments)]
 pub fn move_diff(
     ui: &mut Ui,
@@ -139,7 +164,7 @@ pub fn move_diff(
     destination: &Commit,
     matcher: &dyn Matcher,
     diff_selector: &DiffSelector,
-    description: Option<String>,
+    description: SquashedDescription,
     no_rev_arg: bool,
     path_arg: &[String],
 ) -> Result<(), CommandError> {
@@ -229,8 +254,11 @@ from the source will be moved into the destination.
         destination_tree = destination_tree.merge(&tree1, &tree2)?;
     }
     let description = match description {
-        Some(description) => description,
-        None => combine_messages(tx.base_repo(), &abandoned_commits, destination, settings)?,
+        SquashedDescription::Exact(description) => description,
+        SquashedDescription::UseDestination => destination.description().to_owned(),
+        SquashedDescription::Combine => {
+            combine_messages(tx.base_repo(), &abandoned_commits, destination, settings)?
+        }
     };
     let mut predecessors = vec![destination.id().clone()];
     predecessors.extend(sources.iter().map(|source| source.id().clone()));

cli/tests/cli-reference@.md.snap

@@ -1723,6 +1723,10 @@ If a working-copy commit gets abandoned, it will be given a new, empty commit. T
 * `--from <FROM>` — Revision to squash from (default: @)
 * `--into <INTO>` — Revision to squash into (default: @)
 * `-m`, `--message <MESSAGE>` — The description to use for squashed revision (don't open editor)
+* `-d`, `--discard-message` — Discard the descriptions of the squashed revisions
+
+  Possible values: `true`, `false`
+
 * `-i`, `--interactive` — Interactively choose which parts to squash
 
   Possible values: `true`, `false`

cli/tests/test_squash_command.rs

@@ -968,9 +968,53 @@ fn test_squash_empty() {
     "###);
 }
 
+#[test]
+fn test_squash_drop_messages() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    test_env.jj_cmd_ok(&repo_path, &["commit", "-m=a"]);
+    test_env.jj_cmd_ok(&repo_path, &["commit", "-m=b"]);
+    test_env.jj_cmd_ok(&repo_path, &["describe", "-m=c"]);
+    // Test the setup
+    insta::assert_snapshot!(get_log_output_with_description(&test_env, &repo_path), @r###"
+    @  71f7c810d8ed c
+    ◉  10dd87c3b4e2 b
+    ◉  4c5b3042d9e0 a
+    ◉  000000000000
+    "###);
+
+    // Squash the current revision using the short name for the option.
+    test_env.jj_cmd_ok(&repo_path, &["squash", "-d"]);
+    insta::assert_snapshot!(get_log_output_with_description(&test_env, &repo_path), @r###"
+    @  10e30ce4a910
+    ◉  1c21278b775f b
+    ◉  4c5b3042d9e0 a
+    ◉  000000000000
+    "###);
+
+    // Undo and squash again, but this time squash both "b" and "c" into "a".
+    test_env.jj_cmd_ok(&repo_path, &["undo"]);
+    test_env.jj_cmd_ok(
+        &repo_path,
+        &["squash", "-d", "--from", "10d::", "--into", "4c5"],
+    );
+    insta::assert_snapshot!(get_log_output_with_description(&test_env, &repo_path), @r###"
+    @  da1507508bdf
+    ◉  f1387f804776 a
+    ◉  000000000000
+    "###);
+}
+
 fn get_description(test_env: &TestEnvironment, repo_path: &Path, rev: &str) -> String {
     test_env.jj_cmd_success(
         repo_path,
         &["log", "--no-graph", "-T", "description", "-r", rev],
     )
 }
+
+fn get_log_output_with_description(test_env: &TestEnvironment, repo_path: &Path) -> String {
+    let template = r#"separate(" ", commit_id.short(), description)"#;
+    test_env.jj_cmd_success(repo_path, &["log", "-T", template])
+}