format: align opening and closing parens in multi-line calls

In a multi-line call, if the opening paren is at the end of a line
but the closing paren is not at the start of a line, or vice versa,
insert a newline so they are placed consistently.

Fixes #74.

Author: mvdan

README.md

@@ -182,6 +182,36 @@ var matrix = [][]int{
 
 </details>
 
+**Multi-line function calls should match opening and closing parentheses**
+
+<details><summary><i>Example</i></summary>
+
+```go
+result := compute(a, b,
+	c,
+)
+
+result := compute(
+	a,
+	b,
+	c)
+```
+
+```go
+result := compute(
+	a, b,
+	c,
+)
+
+result := compute(
+	a,
+	b,
+	c,
+)
+```
+
+</details>
+
 **Empty field lists should use a single line**
 
 <details><summary><i>Example</i></summary>

format/format.go

@@ -961,6 +961,32 @@ func (f *fumpter) applyPost(c *astutil.Cursor) {
 				f.addNewline(elem1.End())
 			}
 		}
+
+	// In a multi-line call, the opening parenthesis at the end of a line
+	// should be matched by a closing parenthesis at the start of a line,
+	// and vice versa. See https://github.com/mvdan/gofumpt/issues/74.
+	case *ast.CallExpr:
+		if len(node.Args) == 0 {
+			break
+		}
+		openLine := f.Line(node.Lparen)
+		closeLine := f.Line(node.Rparen)
+		if openLine == closeLine {
+			break
+		}
+		firstLine := f.Line(node.Args[0].Pos())
+		lastEnd := node.Args[len(node.Args)-1].End()
+		if comment := f.inlineComment(lastEnd); comment != nil {
+			lastEnd = comment.End()
+		}
+		lastLine := f.Line(lastEnd)
+		openAtEOL := openLine != firstLine
+		closeAtBOL := closeLine != lastLine
+		if openAtEOL && !closeAtBOL {
+			f.addNewline(node.Rparen)
+		} else if closeAtBOL && !openAtEOL {
+			f.addNewline(node.Lparen + 1)
+		}
 	}
 }
 

testdata/script/call-multiline.txtar

@@ -0,0 +1,86 @@
+exec gofumpt -w foo.go
+cmp foo.go foo.go.golden
+
+exec gofumpt -d foo.go.golden
+! stdout .
+
+-- foo.go --
+package p
+
+func _() {
+	// already consistent
+	_ = call(a, b, c)
+
+	_ = call(
+		a,
+		b,
+		c,
+	)
+
+	// closing paren at start of line, opening paren not at end of line:
+	// move opening paren so both are consistent.
+	_ = call(a, b,
+		c,
+	)
+
+	// opening paren at end of line, closing paren not at start of line:
+	// move closing paren so both are consistent.
+	_ = call(
+		a,
+		b,
+		c)
+
+	// inline comment after the last argument should still cause the
+	// closing paren to be moved to its own line.
+	_ = call(
+		a,
+		b,
+		c) // trailing
+
+	// nested calls
+	_ = outer(inner(
+		a,
+		b))
+}
+-- foo.go.golden --
+package p
+
+func _() {
+	// already consistent
+	_ = call(a, b, c)
+
+	_ = call(
+		a,
+		b,
+		c,
+	)
+
+	// closing paren at start of line, opening paren not at end of line:
+	// move opening paren so both are consistent.
+	_ = call(
+		a, b,
+		c,
+	)
+
+	// opening paren at end of line, closing paren not at start of line:
+	// move closing paren so both are consistent.
+	_ = call(
+		a,
+		b,
+		c,
+	)
+
+	// inline comment after the last argument should still cause the
+	// closing paren to be moved to its own line.
+	_ = call(
+		a,
+		b,
+		c,
+	) // trailing
+
+	// nested calls
+	_ = outer(inner(
+		a,
+		b,
+	))
+}