support local bootstrap profiles

[onur-ozkan]

Copied from the rustc-dev-guide addition:

When working on different tasks, you might need to switch between different bootstrap >configurations.
Sometimes you may want to keep an old configuration for future use. But saving raw config >values in
random files and manually copying and pasting them can quickly become messy, especially if you >have a
long history of different configurations.

To simplify managing multiple configurations, you can create local profiles and switch between >them easily.

For example, you can create cross profile by creating cross.toml file this:

[build]
build = "x86_64-unknown-linux-gnu"
host = ["i686-unknown-linux-gnu"]
target = ["i686-unknown-linux-gnu"]


[llvm]
download-ci-llvm = false

[target.x86_64-unknown-linux-gnu]
llvm-config = "/path/to/llvm-19/bin/llvm-config"

Then, use this profile in your bootstrap.toml:

profile = "cross"

You can also define recursive profiles, meaning that local profiles can inherit other profiles.

For example, you can:

1. Create some-config1 profile in some-config1.toml.
2. Create some-config2 profile in some-config2.toml and inherit some-config1.
3. Create some-config3 profile in some-config3.toml and inherit some-config2.
4. ...

You can chain profiles like this as deep as you need without any limitations.

Note: The outer configuration file always overrides the inner one.

[rustbot]

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[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[TimNN]

Some thoughts (aside from the fact that I like this feature):

1. Consider being explicit about the fact that the additional files must be in the root repository directory, both in the PR description and the docs.

2. Consider changing that, I'd personally prefer having all custom profiles in a single directory (so they are grouped in the file list, and to make it easier to .gitignore them all).

3. As an alternative, switch from custom profiles to "includes", i.e. referencing files instead of profile names.

   • This would clearly differentiate built-in profiles from custom includes.
   • This would make it easy to use a custom directory for custom profiles, possibly even shared between multiple rust checkouts.
   • We could differentiate profiles and includes by checking for the .toml extension, or by having profile and include be separate fields.
   • Relative includes would need somewhat careful handling (should probably be relative to the current file).
   • Includes could open up the possibility of including multiple files (e.g. one to apply my "general" default configuration, and one to switch between different LLVM configurations). Though this may be better left for a follow-up.

[jieyouxu]

Rerolling review due to bandwidth because I'm still slowly testing #119899.
r? bootstrap

[Kobzol]

r? kobzol

Will take a look tomorrow.

[jieyouxu]

I haven't looked too closely, but based on description

Create some-config1 profile in some-config1.toml.
Create some-config2 profile in some-config2.toml and inherit some-config1.
Create some-config3 profile in some-config3.toml and inherit some-config2.

can this infinite loop? I guess even if it does, just don't do that 😆

[onur-ozkan]

I'm planning to rework on this next week.

@rustbot author