Experimental cygwin host support

[workingjubilee]

While we do not yet formally support the cygwin target by distributing binaries, that is not because we do not wish for people to be able to build a rustc that uses cygwin as its host if they so please. I am opening this issue so that interested parties can track the progress on enabling a rustc to be built for cygwin. There's a few needed patches:

• compiler-builtins: Add cygwin support compiler-builtins#774
• libc: Add cygwin support libc#4279
• std: Add std support to cygwin target #137621

cc @Berrysoft

[Berrysoft]

The tracking issue for cargo: rust-lang/cargo#15442

[Berrysoft]

• Add cygwin support stacker#122
• feat: add cygwin support nagisa/rust_libloading#173
• Update nix Detegr/rust-ctrlc#131
• Is it possible to continue maintain version 0.2? rust-random/getrandom#653

[jeremyd2019]

In addition to cargo, there seems to be at least one other Cargo.toml that needs a version bump:

rust/src/tools/rust-analyzer/Cargo.toml

Line 119 in 462cc09

libloading = "0.8.6"

[Berrysoft]

Cargo.lock has been updated this week:

rust/src/tools/rust-analyzer/Cargo.lock

Lines 1126 to 1127 in bf64d66

	name = "libloading"
	version = "0.8.7"

[jeremyd2019]

Cool, I was wondering why I wasn't seeing this issue again with the new nightly I got earlier.

[jeremyd2019]

Trying on actual Cygwin instead of MSYS2, I ran into an issue with it trying to use an MSYS2-style path /D/path for D:\path, I changed

rust/src/bootstrap/src/core/build_steps/install.rs

Line 41 in 038d599

Some(format!("/{}/{}", drive, &s[drive.len_utf8() + 2..]))

to

        Some(format!("/proc/cygdrive/{}/{}", drive, &s[drive.len_utf8() + 2..]))

which should work on both Cygwin and MSYS2. I don't know what any other Windows ports of sh would expect (WSL would be /mnt/drive/path, but maybe gnuwin32 or busybox or something also expects /drive/path?)

[Berrysoft]

Now cargo is able to build on cygwin, and I think the patch will be synchronized here next week.
Remain problems:

• miri lacks support for cygwin. It just compiles, but lacks some methods.
• The compiled std lacks debug symbol. The debug symbol is OK if it's cross-compiled, but is missing if it's compiled from cygwin rustc.

[jeremyd2019]

it looks like cargo was updated so 2025-06-01 nightly shouldn't need to be patched for that anymore, but as of yesterday's nightly I get a new error trying to bootstrap from x86_64-pc-windows-gnu:
#141719 (comment)

[jeremyd2019]

rust now requires no patching to build on Cygwin. Curl 8.14.1 is fixed for the crashing on thread exit issue, just need Cygwin to package it (shouldn't take too long).