Rustfmt turns significant line into comment for unexpected doc-comment

[SimonSapin]

Describe the bug

For some programs with a doc-comment at an unexpected / invalid location, rustfmt moves the following line of significant code into the comment, effectively removing it and changing the meaning of the program.

To Reproduce

example.rs:

fn foo(
    /// doc
    arg: u32,
) {}

rustfmt example.rs --emit stdout

Expected behavior

Even though this program does not compile because doc-comments are invalid at that location, the source should be printed unchanged.

Actual behavior

In the formatted code, the argument has effectively been commented out

fn foo(/// doc arg: u32
) {
}

Meta

• rustfmt version: rustfmt 1.4.29-nightly (70ce182 2020-12-04)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: rustfmt command line for minimized steps, VSCode + rust-analyzer extension + "format on save" otherwise

[ChinYing-Li]

I can take this one. Test cases added for issue #4245 does not include doc-style and block-style, and indeed rustfmt make the following function signature one-line:

fn test2(
    /* doc */
    arg: u32,
    arg2: u32,
) 

fn foo(
    /// doc
    arg: u32,
)

Unexpectedly, if we put the commend right before the closing brace, the code remain unaffected:

fn test4(
    arg: u32,
    arg2: u32,
    /* doc */
)

[karyon]

This is a duplicate of #4936, which has been fixed by #4985. (I noticed this already works on master and then bisected it).

[karyon]

I couldn't find a corresponding PR on the 2.0 branch, so I'll remove the backport label.