revset: add empty() predicate to find commits with no file change

The expression 'x ~ empty()' is identical to 'x & file(".")', but more
intuitive.

Note that 'x ~ empty()' is slower than 'x & file(".")' since the negative
intersection isn't optimized right now. I think that can be handled as
follows: 'x ~ filter(f)' -> 'x & filter(!f)' -> 'filter(!f, x)'

CHANGELOG.md

@@ -40,6 +40,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The new revset function `file(pattern..)` finds commits modifying the
   paths specified by the `pattern..`.
 
+* The new revset function `empty()` finds commits modifying no files.
+
 * It is now possible to specify configuration options on the command line
   with the new `--config-toml` global option.
 

docs/revsets.md

@@ -108,6 +108,8 @@ revsets (expressions) as arguments.
   email.
 * `committer(needle)`: Commits with the given string in the committer's
   name or email.
+* `empty()`: Commits modifying no files. This also includes `merges()` without
+  user modifications and `root`.
 * `file(pattern..)`: Commits modifying the paths specified by the `pattern..`.
 * `present(x)`: Same as `x`, but evaluated to `none()` if any of the commits
   in `x` doesn't exist (e.g. is an unknown branch name.)

lib/src/revset.rs

@@ -31,7 +31,7 @@ use thiserror::Error;
 use crate::backend::{BackendError, BackendResult, CommitId};
 use crate::commit::Commit;
 use crate::index::{HexPrefix, IndexEntry, IndexPosition, PrefixResolution, RevWalk};
-use crate::matchers::{Matcher, PrefixMatcher};
+use crate::matchers::{EverythingMatcher, Matcher, PrefixMatcher};
 use crate::op_store::WorkspaceId;
 use crate::repo::RepoRef;
 use crate::repo_path::{FsPathParseError, RepoPath};
@@ -280,6 +280,8 @@ pub enum RevsetFilterPredicate {
     Author(String),
     /// Commits with committer's name or email containing the needle.
     Committer(String),
+    /// Commits modifying no files. Equivalent to `Not(File(["."]))`.
+    Empty,
     /// Commits modifying the paths specified by the pattern.
     File(Vec<RepoPath>),
 }
@@ -727,6 +729,10 @@ fn parse_function_expression(
                 needle,
             )))
         }
+        "empty" => {
+            expect_no_arguments(name, arguments_pair)?;
+            Ok(RevsetExpression::filter(RevsetFilterPredicate::Empty))
+        }
         "file" => {
             if let Some(ctx) = workspace_ctx {
                 let arguments_span = arguments_pair.as_span();
@@ -1589,6 +1595,12 @@ pub fn evaluate_expression<'repo>(
                         }),
                     }))
                 }
+                RevsetFilterPredicate::Empty => Ok(Box::new(FilterRevset {
+                    candidates,
+                    predicate: Box::new(move |entry| {
+                        !has_diff_from_parent(repo, entry, &EverythingMatcher)
+                    }),
+                })),
                 RevsetFilterPredicate::File(paths) => {
                     // TODO: Add support for globs and other formats
                     let matcher: Box<dyn Matcher> = Box::new(PrefixMatcher::new(paths));
@@ -1911,6 +1923,11 @@ mod tests {
                 RevsetFilterPredicate::Description("(foo)".to_string())
             ))
         );
+        assert_eq!(
+            parse("empty()"),
+            Ok(RevsetExpression::filter(RevsetFilterPredicate::Empty))
+        );
+        assert!(parse("empty(foo)").is_err());
         assert!(parse("file()").is_err());
         assert_eq!(
             parse("file(foo)"),

lib/tests/test_revset.rs

@@ -1816,6 +1816,9 @@ fn test_filter_by_diff(use_git: bool) {
     let commit3 =
         CommitBuilder::for_new_commit(&settings, vec![commit2.id().clone()], tree3.id().clone())
             .write_to_repo(mut_repo);
+    let commit4 =
+        CommitBuilder::for_new_commit(&settings, vec![commit3.id().clone()], tree3.id().clone())
+            .write_to_repo(mut_repo);
 
     // matcher API:
     let resolve = |file_path: &RepoPath| -> Vec<CommitId> {
@@ -1862,4 +1865,13 @@ fn test_filter_by_diff(use_git: bool) {
         ),
         vec![commit2.id().clone()]
     );
+
+    // empty() revset, which is identical to ~file(".")
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo.as_repo_ref(),
+            &format!("{}: & empty()", commit1.id().hex())
+        ),
+        vec![commit4.id().clone()]
+    );
 }