Add `--print=all-target-specs-json` unstable flag

[pietroalbini]

Proposal

This MCP proposes to add the unstable --print=all-target-specs-json compiler flag in addition to the existing unstable --print=target-spec-json flag. The flag will output to the standard output the JSON representation of all builtin target specs, emitting a (sorted) map with the target name as the key and the target spec as the value:

{
  "aarch64-apple-darwin": {
    "abi-return-struct-as-int": true,
    "arch": "aarch64",
    ...
  },
  "aarch64-apple-ios": {
    "abi-return-struct-as-int": true,
    "arch": "aarch64",
    ...
  },
  ...
}

This is needed by rust-lang/rust#108905, as it needs to know the details of all targets in order to validate compiletest directives like only-windows and ignore-x86_64. The alternative of invoking --print=target-list and then iterating each target with --print=target-spec-json takes around 5 seconds on my machine, which is not great.

Mentors or Reviewers

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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

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.

[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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[Mark-Simulacrum]

@rustbot second

This is an unstable flag and we can remove it if we decide there's a better alternative. For now I think this makes sense to add.

[apiraino]

@rustbot label -final-comment-period +major-change-accepted