crashes: add test for #131451

[matthiaskrgr]

r? @saethlin
How do I find out which mir opts are involved here? 😅

[saethlin]

This should be scriptable:

First off, try with a lower mir-opt-level. This is just a fast way to trim down the pass list.

Build with -Zdump-mir=all. This will produce a mir_dump directory that has the before and after MIR for every pass that was run on any MIR. This produces a directory called mir_dump/ which contains a bunch of files named like <crate>.<function>.<phase_number>-<pass_number>.<pass_name>.{before,after}.mir.

You should be able to manually enable only the passes that produce a diff in the MIR. The before and after files always differ on the first line because that's a comment that repeats the file's name.

Then trim down that list of passes further, if possible.

[matthiaskrgr]

ok ty, I think I got it :D
to get the flags, in the mir-dump dir:

ls -c1 | sort -n | grep after  | grep -o "...-...\..*.after" | cut -c9- | sed 's/\.after$//g'   | sed s/^/\"-Zmir-enable-passes=+/  | sed -e s/$/\",/

Then I throw this into icemaker and let it reduce the flags automatically:
-Zvalidate-mir -Zmir-enable-passes=+DataflowConstProp -Zmir-enable-passes=+Inline
oops these were from a different file that does not require debug assertions
edit: this is

-Clink-dead-code=true -Zmir-enable-passes=+GVN -Zmir-enable-passes=+JumpThreading --crate-type=lib

[saethlin]

Can you drop -Clink-dead-code=true if the fn is pub?