An annotation with arguments on a lambda parameter forces a line break after it #101

Formatting a lambda whose parameter carries an annotation with arguments, such as @SuppressWarnings("unused"), breaks the parameter list across lines even when the whole statement fits in 120 columns.

Input

class T {
    BiFunction<Integer, Integer, Integer> g = (final var x, @SuppressWarnings("unused") var y) -> x + y;
}

Output of main (2fbf175), the same on JDK 21, 25 and 27

class T {
    BiFunction<Integer, Integer, Integer> g =
            (final var x,
                    @SuppressWarnings("unused")
                    var y) -> x + y;
}

Expected

The input, unchanged. A lambda parameter is a parameter, and method parameters keep such an annotation on their line: void m(@SuppressWarnings("unused") int y) {} is left alone.

A marker annotation (@Deprecated var x) is already kept inline; it is the arguments that trigger the break. An explicit type instead of var, a block body or a second annotation make no difference.

Cause

JavaInputAstVisitor.visitLambdaExpression declares each parameter with inlineAnnotationDirection(parameter.getModifiers()), which answers VERTICAL for any annotation that has arguments, and a vertical annotation ends in a forced break. That rule is meant for local variable declarations, where

@SuppressWarnings("unchecked")
Function<Object, String> local = o -> (String) o;

is the wanted shape. Method parameters go through visitToDeclare(DeclarationKind.PARAMETER, Direction.HORIZONTAL, ...) instead, so their annotations stay inline.

The same code is in develop of https://github.com/palantir/palantir-java-format and in https://github.com/google/google-java-format, where the method is called fieldAnnotationDirection, so both produce this layout. I found no report of it in either tracker.