reserved width for unsafe in match arm pattern

[AsthaMishra]

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

Issue : unsafe is a block with label None and when patterns code executes and label: None is found, execution goes to fallback arm where only 5 column spaces are reserved (which is correct for async, const, gen and try blocks) but unsafe needs 12 column spaces, this is what causing max_width violation and when run with --config error_on_line_overflow=true it does give line overflow error for max-width 80

 // Patterns
    let pat_shape = match &arm.body.as_ref().unknown_error()?.kind {
        ast::ExprKind::Block(_, Some(label)) => {
            // Some block with a label ` => 'label: {`
            // 7 = ` => : {`
            let label_len = label.ident.as_str().len();
            shape
                .sub_width(7 + label_len, arm.span)?
                .offset_left(pipe_offset, arm.span)?
        }
        _ => {
            // 5 = ` => {`
            shape
                .sub_width(5, arm.span)?
                .offset_left(pipe_offset, arm.span)?
        }
    };

Fix : added an arm for unsafe block to reserve required space for unsafe

 ast::ExprKind::Block(block, None) if is_unsafe_block(block) => {
            // 12 = ` => unsafe {`
            shape
                .sub_width(12, arm.span)?
                .offset_left(pipe_offset, arm.span)?
        }

Fixes : #6848

[AsthaMishra]

one question? should this change be gated?

[ytmimi]

@AsthaMishra Thank you for working on this. Yeah, let's gate this fix on style_edition=2027

[ytmimi]

Also are we sure that things are correct for other types of blocks? Let's add test cases for them just to be sure.

[ytmimi]

Looks like we're forcing const, async, gen, and try blocks to be wrapped in an outer block. Does that always happen or just when we can't fit everything onto one line?

unsafe 6 chars
const 5 chars
async 5 chars
gen 3 chars
try 3 chars

All of these are getting tested against the same ( ExampleTypeX::VariantAlphaSampleXYZ, ExampleTypeX::VariantBetaXYZ) tuple. To make sure we're exhaustively testing this I would like to test the following cases for each type of block:

1. 1 char below the max_width limit when accounting for the pattern, =>, keyword, {, and any whitespace in between..
2. everything properly fits on 1 line at exactly the max_width limit when accounting for the pattern, =>, keyword, {, and any whitespace in between..
3. 1 char over the max_width limit when accounting for the pattern, =>, keyword, {, and any whitespace in between.

View changes since the review

[AsthaMishra]

Looks like we're forcing const, async, gen, and try blocks to be wrapped in an outer block. Does that always happen or just when we can't fit everything onto one line?

outer block only added when we can't fit everything in one line

[AsthaMishra]

Test cases added for above mentioned pointers. There was one more bug , when empty blocks exceeds max-width, we get line overflow error.

reason -

rustfmt/src/matches.rs

Lines 530 to 537 in 2d897e2

	Ok(ref body_str)
	if is_block
	|| (!body_str.contains('\n')
	&& unicode_str_width(body_str) <= body_shape.width) =>
	{
	return combine_orig_body(body_str);
	}
	_ => rewrite,

if is_block || (!body_str.contains('\n') && unicode_str_width(body_str) <= body_shape.width)

code sees if is_block is true and 'OR' condition is bypassed i.e. if there is no new line in body_str and if it fits in the available column space. this is only for what comes after => in same line.

Fix: add a way to enforce execution of (!body_str.contains('\n') && unicode_str_width(body_str) <= body_shape.width for empty blocks

  let enforce_empty_block_width =
            is_empty_block && context.config.style_edition() >= StyleEdition::Edition2027;
        match rewrite {
            Ok(ref body_str)
                if (is_block && !enforce_empty_block_width)
                    || (!body_str.contains('\n')
                        && unicode_str_width(body_str) <= body_shape.width) =>
            {
                return combine_orig_body(body_str);
            }
            _ => rewrite,
        }