Incorrect span used for doctests defined using #[doc(include)] or #[doc = include_str!("...")]

[lambda-fairy]

Given these two files:

ducks.rs

#![feature(extended_key_value_attributes)]
#[doc = include_str!("ducks.md")]
fn ducks() {}

ducks.md

test
test
test
test
test
test
test
test
test
test

```rust
invalid syntax
```

When I run rustdoc --test ducks.rs, I get an error:

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `syntax`
 --> ducks.rs:14:9
  |
3 | invalid syntax
  |         ^^^^^^ expected one of 8 possible tokens

error: aborting due to previous error

The error message is correct. However, the position (ducks.rs:14:9) is invalid. I would expect it to reference either the original include_str! call, or (preferred) the snippet in the Markdown file.

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (bc39d4d9c 2021-01-15)
binary: rustc
commit-hash: bc39d4d9c514e5fdb40a5782e6ca08924f979c35
commit-date: 2021-01-15
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0

cc #78835

[camelid]

Hmm, this is especially interesting given that the error message has 3 | for the line number gutter but 14:9 for the location...

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `syntax`
 --> ducks.rs:14:9
  |
3 | invalid syntax
  |         ^^^^^^ expected one of 8 possible tokens

error: aborting due to previous error

However, this bug also occurs with old-school doc(include):

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `syntax`
 --> ducks.rs:13:9
  |
3 | invalid syntax
  |         ^^^^^^ expected one of 8 possible tokens

error: aborting due to previous error

I wonder if the reason the doc(include) one shows 13:9 as the location while the include_str! shows 14:9 is because include_str! uses macro expansion whereas doc(include) is dealt with in rustdoc itself? Not sure though.

[camelid]

Ok, I've looked into this and found the underlying issue. Rustdoc actually uses environment variables as a side channel to tell rustc, but the code is really messy and the filename and line number seem to be obtained separately:

rust/src/librustdoc/doctest.rs

Lines 327 to 328 in 7d6f948

	compiler.env("UNSTABLE_RUSTDOC_TEST_PATH", path);
	compiler.env("UNSTABLE_RUSTDOC_TEST_LINE", format!("{}", line as isize - line_offset as isize));

I'm still looking into ways to get the filename based on the span of the docs themselves rather than the attribute they're in.