Add new targets for Cygwin (and MSYS2)

[Berrysoft]

Proposal

Cygwin is a POSIX-compatible layer on Windows. MSYS2 is a fork of Cygwin. I want to add new targets for Cygwin. The main motivation is to compile fish-shell to Windows.

My proposed targets are x86_64-pc-cygwin and x86_64-pc-cygwin-msys2. The PR is here: rust-lang/rust#134999 . They are tier 3 targets, but we faced some problems: should this target in windows family or in unix family? I would suggest unix family, because Cygwin is such an layer to provide POSIX apis and make porting unix programs easy.

Mentors or Reviewers

@jieyouxu suggested me to open this issue.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

I have succeeded building my own rustc targeting cygwin and compiled fish on Windows.

• rust-lang/rust@master...Berrysoft:rust:dev/cygwin
• https://github.com/Berrysoft/fish-shell/releases

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rfcbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rfcbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler