Doc comments on use declarations are limited to 5 characters less than max_width

[taylordotfish]

If wrap_comments is true, doc comments on use declarations are wrapped to 5 characters less than max_width (or comment_width, if it is less than max_width - 5). This makes it impossible to wrap such comments to the same value as max_width.

Example:

test.rs:

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
pub use u32;

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
pub struct X;

After running rustfmt --config wrap_comments=true,comment_width=79,max_width=79 test.rs:

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a aa a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a a a a a a a a a a a
pub use u32;

/// a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a aa a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
/// a a a a a a a a a a a a a a a
pub struct X;

The comment on the use declaration is wrapped to 74 characters, exactly 5 less than 79. This five-character difference persists across different values of comment_width and max_width, except if comment_width is more than 5 characters less than max_width, both doc comments will be limited to that value.

[taylordotfish]

This issue presents similarly to #5890, but I don't know if the cause is similar—I would expect both doc comments to have the same internal shape.

[ytmimi]

Thanks for reaching out. #5890 only applies to multi-line block comments so it's unrelated.

The issue here is that we're passing the nested_shape to UseTree::rewrite_top_level, and that shape gets used to rewrite both the import and the attributes.

rustfmt/src/reorder.rs

Lines 126 to 141 in da7f678

	// 4 = "use ", 1 = ";"
	let nested_shape = shape.offset_left(4)?.sub_width(1)?;
	let item_vec: Vec<_> = regrouped_items
	.into_iter()
	.filter(|use_group| !use_group.is_empty())
	.map(|use_group| {
	let item_vec: Vec<_> = use_group
	.into_iter()
	.map(|use_tree| ListItem {
	item: use_tree.rewrite_top_level(context, nested_shape),
	..use_tree.list_item.unwrap_or_else(ListItem::empty)
	})
	.collect();
	wrap_reorderable_items(context, &item_vec, nested_shape)
	})
	.collect::<Option<Vec<_>>>()?;

I can think of two possible approaches to fixes this:

1. Move the nested_shape derivation into UseTree::rewrite_top_level (which might also obviate the need for Use the same shape to rewrite import when reordering them or not #5307)
2. Pass both the nested_shape for rewriting imports and the original shape for rewriting attributes to UseTree::rewrite_top_level

[ytmimi]

linking the tracking issue for wrap_comments (#3347)

[taylordotfish]

Is this bug related then?

/* 50 characters:
0        1         2         3         4         5
12345678901234567890123456789012345678901234567890
*/
use aaaaaaaaa::{bbbbbbbbb, ccccccccc, ddddddddd};

After rustfmt --config max_width=50:

/* 50 characters:
0        1         2         3         4         5
12345678901234567890123456789012345678901234567890
*/
use aaaaaaaaa::{
    bbbbbbbbb, ccccccccc, ddddddddd,
};

The use declaration is only 49 characters but gets wrapped anyway. This also happens with the default max_width of 100 (use declarations > 98 characters get wrapped).

[ytmimi]

Is this bug related then?

@taylordotfish It's possible, though my guess is that they're not related since your original issue describes doc comments on imports wrapping too early when using warp_comments=true, while this second case is about rustfmt wrapping nested use declarations too early when close to the max_width boundary.

I very much appreciate you taking the time to check other edge cases 🙏🏼. Probably best to create a new issue so we can track that separately.

[taylordotfish]

@ytmimi Sure, created #5927.