rustfmt removes comment between doc comment and generic params

[ytmimi]

This was an issue discovered when implementing a fix for #5122 in PR #5124 (comment). The issue is reproducible on rustfmt 1.4.38-nightly (8572b343 2022-04-24) and uses the rustfmt.toml defined in the rustfmt repo.

# rustfmt.toml
error_on_line_overflow = true
error_on_unformatted = true
version = "Two"

This input:

// Non-doc pre-comment of Foo
/// doc of Foo
// Non-doc post-comment of Foo
struct Foo<
    // Non-doc pre-comment of 'a
    /// doc of 'a
    // Non-doc post-comment of 'a
    'a,
    // Non-doc pre-comment of T
    /// doc of T
    // Non-doc post-comment of T
    T,
    // Non-doc pre-comment of N
    /// doc of N
    // Non-doc post-comment of N
    const N: item,
>;

Gets formatted as follows:

// Non-doc pre-comment of Foo
/// doc of Foo
// Non-doc post-comment of Foo
struct Foo<
    // Non-doc pre-comment of 'a
    /// doc of 'a
    'a,
    // Non-doc pre-comment of T
    /// doc of T
    T,
    // Non-doc pre-comment of N
    /// doc of N
    const N: item,
>;

[ytmimi]

#5215 is somewhat related. It mostly deals with not properly adding a newline between comments and struct fields when there is a doc comment preceding the item instead of dropping the comment altogether, but I imagine the we'll need to take a similar approach when recovering the comments to prevent the issue described in #5215.

[calebcartwright]

Thanks for capturing this so we don't lose track!

[ytmimi]

Just a happy coincidence that I came across this as I was making changes to #5305, and peeking into various issues / PRs to see if I could reword any of the CHANGELOG entries.

[Rudxain]

Related bug (inline block comment):

in

pub struct Foo;
impl /*core*/std::error::Error for Foo {}

out

pub struct Foo;
impl std::error::Error for Foo {}

[ytmimi]

@Rudxain thanks for reaching out! There are several places where rustfmt isn't expected to find comments and doesn't try to recover them when reformatting your code. Between the impl keyword and the trait name is one of them. Although you're observing a similar issue it's happening in a different context so it's unfortunately unrelated to this issue.

Feel free to open up a separate issue to cover the case you're running into.

[Rudxain]

There are several places where rustfmt isn't expected to find comments and doesn't try to recover them when reformatting your code.

Oh. I didn't expect that 😅

Feel free to open up a separate issue to cover the case you're running into.

I'll probably do that. I commented /*core*/ to remind myself to replace std by core when core Errors get stabilized

BTW, thanks for the info!