Add JSON schema for target spec JSON

[Noratrieb]

Proposal

rustc has the ability to use a custom target spec via JSON (--target ./uwu.json). The properties for this target are undocumented and unstable (even though it can be used on stable, lol). The way people use this in practice is by dumping the JSON for the target closest to their desired target and then modifying the JSON. This is hard because of the lack of documentation.

With a JSON schema, it would be significantly easier to write such a target spec JSON. It would give autocomplete in the editor, suggesting fields and their types. It can even contain some documentation generated from doc comments in rustc (of which a few exist already, and more could be written (or not)).

I want to add the flag --print target-spec-json-schema (unstable, with no interest in stabilizing it) which prints the JSON schema file, and then also add the schema as a file into the sysroot (./etc/target-spec-json-schema.json). None of this would be guaranteed stable, it would just exist (and be documented in the rustc book as existing and unstable). People can then configure their editors to point to this schema file in the sysroot.

draft implementation: rust-lang/rust#144498

resulting schema: https://gist.github.com/Noratrieb/e6f51b13d59b097d647f637314de37c5#file-schema-json

Mentors or Reviewers

anyone lol

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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  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[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

[WaffleLapkin]

@rustbot second

[rustbot]

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

As the automated representative, I would like to thank 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.