Replace `rustc_target::specTarget::arch` string with enum

[tamird]

Proposal

I propose to replace the stringly-typed Target::arch: Cow<'static, str> with a new Architecture enum. The work is already done.

The change is mechanical but large, and this MCP is filed per @nnethercote's request.

Mentors or Reviewers

• @nnethercote (reviewer).

Process

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

• File an issue describing the proposal.
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• Once an MCP is seconded, the Final Comment Period begins.
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  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

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Note

Concerns (0 active)

• rust-gpu-use-case resolved in this comment

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[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
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cc @rust-lang/compiler

[davidtwco]

@rustbot second

[workingjubilee]

@rustbot concern rust-gpu-use-case

The reason for the current design is partially that it allows a codegen backend to support an architecture that we do not explicitly enumerate amongst our supported architectures, without having to completely fork the target handling code. In other words, the stringly typing is on-purpose. This needs to be examined/discussed before we move forward with this. I name the concern for Rust GPU as a practical example, but the more general principle was articulated in https://rust-lang.github.io/rfcs/0131-target-specification.html

[tamird]

We could add an escape hatch of "other" but I'd like to understand how that would work. What is the interface between the compiler and the backend? It'd obviously be nice to have flexibility here but as you can see from the PR - a bunch of decisions are made using exact matches on the target string.

[workingjubilee]

discussion is meant to take place on the Zulip topic, I can't answer your questions. an escape hatch is sufficient if that is what we prefer.

@rustbot resolve rust-gpu-use-case

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#148352.