rustdoc: intra-doc link errors give incorrect spans on certain block doc comments

[QuietMisdreavus]

Given the following source:

/** # for example,
 *
 * time to introduce a link [error]*/
pub struct SomeStruct;

The span conversion calculation in the error reporting will give the wrong offset when reporting the resolution failure, leading to a confusing report:

$ cargo +nightly doc
 Documenting asdf v0.1.0 (/home/misdreavus/git/asdf)
warning: `[error]` cannot be resolved, ignoring it...
 --> src/lib.rs:3:33
  |
3 |   time to introduce a link [error]*/
  |  _________________________________^
4 | | pub struct SomeStruct;
  | |__^ cannot be resolved, ignoring
  |
  = note: #[warn(intra_doc_link_resolution_failure)] on by default
  = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

    Finished dev [unoptimized + debuginfo] target(s) in 2.12s

Strangely enough, putting either the leading /** or the closing */ on its own line will cause the report to change to something that looks correct, if a little too broad:

/**
 * # for example,
 *
 * time to introduce a link [error]
 */
pub struct SomeStruct;

$ cargo +nightly doc
 Documenting asdf v0.1.0 (/home/misdreavus/git/asdf)
warning: `[error]` cannot be resolved, ignoring it...
 --> src/lib.rs:1:1
  |
1 | / /**
2 | |  * # for example,
3 | |  *
4 | |  * time to introduce a link [error]
5 | |  */
  | |___^
  |
  = note: #[warn(intra_doc_link_resolution_failure)] on by default
  = note: the link appears in this line:

           time to introduce a link [error]
                                     ^^^^^
  = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

    Finished dev [unoptimized + debuginfo] target(s) in 2.12s

I believe the code at fault is this section here:

rust/src/librustdoc/passes/collect_intra_doc_links.rs

Lines 521 to 538 in 6f244c9

	let line_offset = dox[..link_range.start].lines().count();
	// The span starts in the `///`, so we don't have to account for the leading whitespace
	let code_dox_len = if line_offset <= 1 {
	doc_comment_padding
	} else {
	// The first `///`
	doc_comment_padding +
	// Each subsequent leading whitespace and `///`
	code_dox.lines().skip(1).take(line_offset - 1).fold(0, |sum, line| {
	sum + doc_comment_padding + line.len() - line.trim().len()
	})
	};
	// Extract the specific span
	let sp = sp.from_inner_byte_pos(
	link_range.start + code_dox_len,
	link_range.end + code_dox_len,
	);

Because it assumes that every doc comment will have a padding of 3 characters (the /// or //!), it includes that in its calculation of each line's offset. However, for block-style comments, this assumption is false.

[euclio]

I'm working on this.

[euclio]

You can also reproduce this with unsugared docs:

warning: `[error]` cannot be resolved, ignoring it...
  --> src/test/rustdoc-ui/intra-links-warning.rs:71:17
   |
71 | #[doc = "single line [error]"]
   |                 ^^^^^ cannot be resolved, ignoring
   |
   = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`