Use BorrowedFd in arguments

[ids1024]

Implementation of #592.

This is required for IO safety. Or perhaps AsFd generics would be useful.

This will be a breaking API change, and require Rust 1.65.

Currently wayland-client compiles here, but server-side needs the same changes, and things need to be cleaned up a bit.

[ids1024]

Hm, an awkward API stability concern here: adding Request/Event enums are non-exhastive. So adding a variant is considered a non-breaking change. But as this is currently implemented in this PR, adding a variant with an FD adds a type generic, which is breaking.

That is needed because an enum can't have a type generic unless a variant uses that type parameter. Maybe if it's non-exhaustive anyway, it could have a variant that's never constructed with a PhantomData? And an uninhabited type for good measure.

[ids1024]

Currently Request/Event types and methods also accept String and Vec<u8> instead of &str and &[u8]. Not that much of an issue since those types aren't the most common in Wayland, but presumably with a GAT here those could also be changed, avoiding an extra allocation and provide a more idiomatic API.

[codecov]

Codecov Report

Patch coverage: 98.46% and project coverage change: +0.03% 🎉

Comparison is base (799c772) 73.11% compared to head (f9f55d5) 73.15%.

❗ Current head f9f55d5 differs from pull request most recent head 5172cc5. Consider uploading reports for the commit 5172cc5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
+ Coverage   73.11%   73.15%   +0.03%     
==========================================
  Files          47       47              
  Lines        7693     7729      +36     
==========================================
+ Hits         5625     5654      +29     
- Misses       2068     2075       +7     

Flag	Coverage Δ
main	58.57% <86.15%> (-0.03%)	⬇️
test--	78.23% <100.00%> (+0.13%)	⬆️
test--server_system	61.47% <100.00%> (+0.17%)	⬆️
test-client_system-	69.26% <100.00%> (+0.13%)	⬆️
test-client_system-server_system	51.52% <100.00%> (+0.19%)	⬆️

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Files Changed	Coverage Δ
wayland-client/src/lib.rs	2.43% <ø> (ø)
wayland-cursor/src/lib.rs	0.00% <0.00%> (ø)
wayland-server/src/lib.rs	11.62% <ø> (ø)
wayland-client/src/conn.rs	86.29% <100.00%> (+0.45%)	⬆️
wayland-scanner/src/client_gen.rs	99.25% <100.00%> (-0.01%)	⬇️
wayland-scanner/src/common.rs	95.84% <100.00%> (+0.11%)	⬆️
wayland-scanner/src/server_gen.rs	99.11% <100.00%> (-0.01%)	⬇️
wayland-server/src/display.rs	81.92% <100.00%> (+1.15%)	⬆️

... and 7 files with indirect coverage changes

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[ids1024]

Added generation of a #doc(hidden) phantom variant for API stability.

This should be good now.

[elinorbgr]

That's a solid PR, thanks!