Cargo config search meta issue.

[ehuss]

This is a meta issue to coordinate the different requests for changing how cargo searches for config files. Currently there is very little control over this process, and there are several aspects that can be confusing or surprising.

Instead of incrementally making changes to add additional knobs, the Cargo team would like to see an overall strategy to make it flexible enough to accommodate most of the requested issues while still remaining relatively simple to use and understand. A change like this would likely need to go through the RFC process to flesh out the design and make sure any changes consider the big picture.

Linking issues and PRs:

• Issue Cargo does not always search for .cargo/config file in project root #2930 — Cargo does not always search for .cargo/config file in project root
  • Issue Cargo uses .cargo/config of workspace from which it is excluded #7620 -- Cargo loads workspace's .cargo/config when in excluded workspace member
• Issue Want env var specifying overriding config file #6728 — Want env var specifying overriding config file
• Issue Tracking issue for config-include #7723 — Tracking issue for config-include
• PR Implement CARGO_CONFIG_PATH config file search override #7894 — Implement CARGO_CONFIG_PATH config file search override
• Issue Add flag to ignore all parent directory configs #7621 — Add flag to ignore all parent directory configs
• Issue Fine-grain control of Config.toml discovery #7887 — Fine-grain control of Config.toml discovery
• Issue Add a --ignore-local-config flag for ignoring local .cargo/config.toml files #8643 — Add a --ignore-local-config flag for ignoring local .cargo/config.toml files
  • PR Add a unstable --ignore-local-config flag #8652 — Add a unstable --ignore-local-config flag
• Issue env var for original invocation directory #8148 — env var for original invocation directory
• Issue Support multiple cargo configuration files in .cargo #9306 — Support multiple cargo configuration files in .cargo/.
• https://internals.rust-lang.org/t/proposal-move-some-cargo-config-settings-to-cargo-toml/13336
• https://internals.rust-lang.org/t/problems-of-cargo-config-files-and-possible-solutions/12987