Rust 1.75.0

[rubdos]

Based on #21, but doesn't compile at all yet.

Plan:

• Make 1.73.0 compile (Rust 1.73.0 #21)
• Rebase this on the 1.73 fix
• Make 1.75 compile

It would be highly beneficial for Whisperfish to have 1.75, because I expect the Rust ecosystem to adapt the return position impl Trait in trait feature very rapidly.

Part of The Rust Plan

[rubdos]

To cite @direc85 in rubdos#1, the remaining issue can be resolved in the spec file:

As it turned out, the bug is fixable in environment (Whisperfish .spec in my use case) instead:

export TMPDIR=$(realpath "%{_sourcedir}/.tmp")
mkdir -p $TMPDIR

So, it's a bug somewhere in Scratchbox instead -- closing this PR as redundant (and potentially harmful).

[direc85]

I'm afraid my patch has a few extra lines that should be removed. I'm afk currently, but I'll check and fix that late this evening.

[rainemak]

Minor remarks. Looks good and we got Gecko esr78 building against newer Rust as well. We'll do final verification still.