feat(revset): Add exactly function

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - ReleaseDate
 
+### Added
+
+- (#508) Added `exactly(<revset>, n)` revset function to allow assertions on the number of commits within a set.
+
 ### Changed
 
 - (#512) Fixed so that the setting for `--color` is now respected.

git-branchless/src/revset/builtins.rs

@@ -48,6 +48,7 @@ lazy_static! {
             ("committer.name", &fn_committer_name),
             ("committer.email", &fn_committer_email),
             ("committer.date", &fn_committer_date),
+            ("exactly", &fn_exactly),
         ];
         functions.iter().cloned().collect()
     };
@@ -377,3 +378,21 @@ fn fn_committer_date(ctx: &mut Context, name: &str, args: &[Expr]) -> EvalResult
         }),
     )
 }
+
+fn fn_exactly(ctx: &mut Context, name: &str, args: &[Expr]) -> EvalResult {
+    let (lhs, expected_len) = eval_number_rhs(ctx, name, args)?;
+    let actual_len: usize = lhs
+        .count()
+        .wrap_err("Counting commit set")
+        .map_err(EvalError::OtherError)?;
+
+    if actual_len == expected_len {
+        Ok(lhs)
+    } else {
+        Err(EvalError::UnexpectedSetLength {
+            expr: format!("{}", args[0]),
+            expected_len,
+            actual_len,
+        })
+    }
+}

git-branchless/src/revset/eval.rs

@@ -10,6 +10,7 @@ use once_cell::unsync::OnceCell;
 use thiserror::Error;
 
 use lib::core::dag::{CommitSet, Dag};
+use lib::core::formatting::Pluralize;
 use lib::git::{Repo, ResolvedReferenceInfo};
 use tracing::instrument;
 
@@ -104,8 +105,19 @@ pub enum EvalError {
         actual_arity: usize,
     },
 
-    #[error("expected {expr} to evaluate to 1 element, but got {actual_len}")]
-    ExpectedSingletonSet { expr: String, actual_len: usize },
+    #[error(
+        "expected '{expr}' to evaluate to {}, but got {actual_len}",
+        Pluralize {
+            determiner: None,
+            amount: *expected_len,
+            unit: ("element", "elements"),
+        }
+    )]
+    UnexpectedSetLength {
+        expr: String,
+        expected_len: usize,
+        actual_len: usize,
+    },
 
     #[error("not an integer: {from}")]
     ParseInt {
@@ -708,6 +720,59 @@ mod tests {
             "###);
         }
 
+        {
+            let expr = Expr::FunctionCall(
+                Cow::Borrowed("exactly"),
+                vec![
+                    Expr::FunctionCall(Cow::Borrowed("stack"), vec![]),
+                    Expr::Name(Cow::Borrowed("3")),
+                ],
+            );
+            insta::assert_debug_snapshot!(eval_and_sort(&effects, &repo, &mut dag, &expr), @r###"
+            Ok(
+                [
+                    Commit {
+                        inner: Commit {
+                            id: 848121cb21bf9af8b064c91bc8930bd16d624a22,
+                            summary: "create test5.txt",
+                        },
+                    },
+                    Commit {
+                        inner: Commit {
+                            id: f0abf649939928fe5475179fd84e738d3d3725dc,
+                            summary: "create test6.txt",
+                        },
+                    },
+                    Commit {
+                        inner: Commit {
+                            id: ba07500a4adc661dc06a748d200ef92120e1b355,
+                            summary: "create test7.txt",
+                        },
+                    },
+                ],
+            )
+            "###);
+        }
+
+        {
+            let expr = Expr::FunctionCall(
+                Cow::Borrowed("exactly"),
+                vec![
+                    Expr::FunctionCall(Cow::Borrowed("stack"), vec![]),
+                    Expr::Name(Cow::Borrowed("2")),
+                ],
+            );
+            insta::assert_debug_snapshot!(eval_and_sort(&effects, &repo, &mut dag, &expr), @r###"
+            Err(
+                UnexpectedSetLength {
+                    expr: "stack()",
+                    expected_len: 2,
+                    actual_len: 3,
+                },
+            )
+            "###);
+        }
+
         Ok(())
     }
 

git-branchless/tests/command/test_query.rs

@@ -84,7 +84,7 @@ fn test_query_eval_error() -> eyre::Result<()> {
             },
         )?;
         insta::assert_snapshot!(stderr, @r###"
-        Evaluation error for expression 'foo()': no function with the name 'foo' could be found; these functions are available: all, ancestors, ancestors.nth, author.date, author.email, author.name, branches, children, committer.date, committer.email, committer.name, descendants, difference, draft, heads, intersection, message, none, not, only, parents, parents.nth, paths.changed, range, roots, stack, union
+        Evaluation error for expression 'foo()': no function with the name 'foo' could be found; these functions are available: all, ancestors, ancestors.nth, author.date, author.email, author.name, branches, children, committer.date, committer.email, committer.name, descendants, difference, draft, exactly, heads, intersection, message, none, not, only, parents, parents.nth, paths.changed, range, roots, stack, union
         "###);
         insta::assert_snapshot!(stdout, @"");
     }