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I feel like this is very arbitrary. The 9999 limit is just an internal implementation detail (an internal "this is obviously wrong" check and would be bumped if we ever get to that many error codes) and shouldn't be exposed to the user. I would've just expected

$ rustc --explain E12345
error: E12345 is not a valid error code

to behave the same as

$ rustc --explain E4444
error: E4444 is not a valid error code

and

$ rustc --explain E12345678901234567890 # exceeds u32::MAX
error: E12345678901234567890 is not a valid error code

to gracefully fail with the same error message.

@jieyouxu any actual idea why tidy start fails here?

tidy check
tidy error: Error code `E9999` is used in the compiler but not defined and documented in `compiler/rustc_error_codes/src/lib.rs`.

like... should I just add E9999 there..?
and about your suggestion, you asking to merge both errors into one error_code is not a valid error code right?

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

oh no, looks like adding E9999 was a miskate here
but it actually makes tidy pass, so idk, correct me here pls

You may want to add a regression test based on tests/ui/explain.rs

@ShE3py oh, yes i want to add tests for it, didnt knew that there is some for --explain, should i create it separate tests or can do it in one file?

yeah, i guess i cant do this in one file
error: Option 'explain' given more than once

Yup, that's unfortunate but AFAIK you can only run rustc once per file.

Btw, //~? may be interesting to use:
https://rustc-dev-guide.rust-lang.org/tests/ui.html#error-without-line-information

yep, //~? is a nice one

@fmease seems done, but check pls if i got you and made all right
i also may should add some tests for other cases like valid error code for error that not exists or just something weird like word instead of number etc

i also may should add some tests for other cases like valid error code for error that not exists or just something weird like word instead of number etc

If you'd like to, go right ahead, sure sounds like a good idea.

@fmease should be good to go, added few more test cases

@bors r=davidtwco,fmease rollup

📌 Commit 3c1c072 has been approved by davidtwco,fmease

It is now in the queue for this repository.