Provide a way to 'anonymize' line numbers when building a Snippet

[phansch]

In the rust test suite there's the option to replace line numbers in the diagnostics output with letters, so that changes in the output produce less noisy diffs.

With that flag enabled, the following

error: cannot concatenate a byte string literal
  --> $DIR/concat.rs:2:13
   |
 2 |     concat!(b'f');
   |             ^^^^

is turned into

error: cannot concatenate a byte string literal
  --> $DIR/concat.rs:2:13
   |
LL |     concat!(b'f');
   |             ^^^^

Any line number is replaced by LL, no matter the digits of that number.

AFAICT, rustc would currently have to implement its own Snippet -> DisplayList conversion, which would duplicate a lot of the conversion code just to anonymize the line numbers.

I think this would be nice to have inside annotate-snippets too, maybe as an option when constructing a Snippet? I believe other users of this library could also benefit from this option.

[zbraniecki]

What happens if there are multiple lines? Are all of them LL or is it LM and so on? If it's all LL then it should be trivial to implement.

First, you need to adjust the lineno_width algo to recognize this option and hardcode it to 2 here - https://github.com/rust-lang/annotate-snippets-rs/blob/master/src/formatter/mod.rs#L74

Second, you need to branch in format_line to use LL here: https://github.com/rust-lang/annotate-snippets-rs/blob/master/src/formatter/mod.rs#L288

Done! :)

[Manishearth]

All of them are LL.

The idea is that the line number shows up on the top with the filename and nowhere else