Tracking Issue for target-spec-json (flexible target specifications)

This is a tracking issue for the RFC "Flexible target specification" (rust-lang/rfcs#131).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Overview

This is the tracking issue for the ability to specify custom targets using a file that defines the target (currently JSON).
See also #38338 for the related --print=target-spec-json.

Documentation: https://doc.rust-lang.org/nightly/rustc/targets/custom.html
Issues: https://github.com/rust-lang/rust/labels/A-target-specs

Unresolved Questions

• Should the file format be JSON or something else like TOML?
  • See https://github.com/davidtwco/rfcs/pull/5 for a draft RFC proposal that also includes stability information.
• The format is somewhat tied to LLVM. There are some concerns that may make using other backends more difficult. Here is a suggestion for making the backend explicit.
• How will stability be handled?
• Validate that the standard library can be reliably built (no TARGET detection, see https://github.com/rust-lang/wg-cargo-std-aware/issues/6#issuecomment-679237914).

Implementation history

• Original tracking issue: https://github.com/rust-lang/rust/issues/16093
• Initial implementation: https://github.com/rust-lang/rust/pull/16156
• Added --print target-spec-json: https://github.com/rust-lang/rust/pull/38061
• Support for absolute paths: https://github.com/rust-lang/rust/pull/49019
• Added sysroot path search: https://github.com/rust-lang/rust/pull/83800
• Target search cleanup: https://github.com/rust-lang/rust/pull/85152
• Added --print target-spec-json-schema: https://github.com/rust-lang/rust/pull/144498
• Destabilization:
  • https://github.com/rust-lang/rust/pull/150151
  • https://github.com/rust-lang/rust/pull/151534
  • https://github.com/rust-lang/rust/pull/152236
  • https://github.com/rust-lang/rust/pull/152677
  • https://github.com/rust-lang/rust/pull/152721
  • https://github.com/rust-lang/rust/pull/154428
  • https://github.com/rust-lang/cargo/pull/16557

Start with RFC 131 and the custom-target documentation, then review the unresolved questions and linked implementation history. This tracking issue does not define a self-contained change or a concrete done condition; pick a dedicated issue under A-target-specs before coding.