From ab2187089a8dea0d7f1ea611fe8f62db874498b7 Mon Sep 17 00:00:00 2001
From: Kunal Behbudzade <121988281+Kunalbehbud@users.noreply.github.com>
Date: Tue, 28 Jul 2026 19:47:31 +0300
Subject: [PATCH] format: Keep rule body inline when the head spans multiple
 lines (#8904)

### Why the changes in this PR are needed?

Fixes #8894.

`opa fmt` expands a one-line `if` condition into a block whenever the
rule head's *value* expression spans multiple lines, even though the
condition itself is a single simple term. For example:

```rego
foo := sprintf(
	"%d",
	[1],
) if allow
```

was reformatted to:

```rego
foo := sprintf(
	"%d",
	[1],
) if {
	allow
}
```

### What are the changes in this PR?

The inline-`if` path in `writeRule` decides whether to keep `if <term>`
on one line by comparing the body term's row to the rule head's row:

```go
if rule.Body[0].Location.Row == rule.Head.Location.Row {
```

`rule.Head.Location.Row` is the head's **start** row. Once the head
value wraps onto later lines, the single body term sits on a later row
than the head start, the equality fails, and formatting falls through to
the block form.

The fix compares against the head's **end** row instead (start row plus
the number of newlines in the head's location text), so a single body
term on the same line as `if` stays inline regardless of how many lines
the head value occupies. Single-line heads are unaffected (end row ==
start row), and genuinely multi-statement bodies still expand as before
(they don't hit the `len(rule.Body) == 1` branch).

### Notes to assist PR review:

Added `v1/format/testfiles/v1/test_issue_8894.rego` (+`.formatted`) with
the exact repro from the issue; it fails on `master` (expands to a
block) and passes with this change. The rest of the format golden suite
is unchanged.

Signed-off-by: Kunalbehbud <b.kunal2002@gmail.com>
---
 v1/format/format.go                                   | 7 ++++++-
 v1/format/testfiles/v1/test_issue_8894.rego           | 6 ++++++
 v1/format/testfiles/v1/test_issue_8894.rego.formatted | 6 ++++++
 3 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 v1/format/testfiles/v1/test_issue_8894.rego
 create mode 100644 v1/format/testfiles/v1/test_issue_8894.rego.formatted

diff --git a/v1/format/format.go b/v1/format/format.go
index dfb4746334..1f25938bae 100644
--- a/v1/format/format.go
+++ b/v1/format/format.go
@@ -640,7 +640,12 @@ func (w *writer) writeRule(rule *ast.Rule, isElse bool, comments []*ast.Comment)
 	if (w.fmtOpts.regoV1 || w.fmtOpts.ifs) && partialSetException {
 		w.write(" if")
 		if len(rule.Body) == 1 {
-			if rule.Body[0].Location.Row == rule.Head.Location.Row {
+			// Keep `if <term>` on one line when the single body term sits on the
+			// same line as the end of the head. Comparing against the head's
+			// start row would wrongly expand the condition into a block whenever
+			// the head value spans multiple lines (e.g. a multi-line call).
+			headEndRow := rule.Head.Location.Row + strings.Count(string(rule.Head.Location.Text), "\n")
+			if rule.Body[0].Location.Row == headEndRow {
 				w.write(" ")
 				var err error
 				comments, err = w.writeExpr(rule.Body[0], comments)
diff --git a/v1/format/testfiles/v1/test_issue_8894.rego b/v1/format/testfiles/v1/test_issue_8894.rego
new file mode 100644
index 0000000000..b524d9ade4
--- /dev/null
+++ b/v1/format/testfiles/v1/test_issue_8894.rego
@@ -0,0 +1,6 @@
+package test
+
+foo := sprintf(
+	"%d",
+	[1],
+) if allow
diff --git a/v1/format/testfiles/v1/test_issue_8894.rego.formatted b/v1/format/testfiles/v1/test_issue_8894.rego.formatted
new file mode 100644
index 0000000000..b524d9ade4
--- /dev/null
+++ b/v1/format/testfiles/v1/test_issue_8894.rego.formatted
@@ -0,0 +1,6 @@
+package test
+
+foo := sprintf(
+	"%d",
+	[1],
+) if allow