The diagnostic emitter doesn't deal with non-ASCII characters correctly when eliding parts of overly long source lines

Found while reviewing #126597 for the nth and final time. Many parts of the emitter look fishy to me wrt. Unicode handling and to be honest it was quite frustrating to review code related to it because the emitter doesn't make any attempt to newtype the different units used, namely byte lengths/offsets, char / Unicode scalar lengths/offsets and Unicode widths (I hope rust-lang/annotate-snippets-rs will remedy that). That's just an aside.

The string offset acrobatics performed in HumanEmitter::render_source_line and HumanEmitter::draw_line look incredibly suspicious to me. Let me just link some parts where we likely incorrectly reinterpret different units (byte len, char count, Unicode width):

https://github.com/rust-lang/rust/blob/c22887b4d97400e8e024e19fb5f724eda65ad58d/compiler/rustc_errors/src/emitter.rs#L733-L736

https://github.com/rust-lang/rust/blob/c22887b4d97400e8e024e19fb5f724eda65ad58d/compiler/rustc_errors/src/emitter.rs#L663-L681

I gave up trying to make sense of this – having to look at all the weakly typed variables and fields of type usize. However, based on these functions I crafted a pathological input file where it's clear something is amiss.

Example Reproducer

/*这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。这是宽的。*/?

Compiler Output

Clearly butchered:

error: expected item, found `?`
 --> col.rs:1:170
  |
1.|....
  |^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

error: aborting due to 1 previous error

Counterexample

Compare this to ASCII-only input:

/*aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa*/?

Compiler Output

Perfectly sensible:

error: expected item, found `?`
 --> col_ascii.rs:1:335
  |
1 | ...aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa*/?
  |                                                                                       ^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

error: aborting due to 1 previous error

rust-lang / rust