Allow phased initializing structs

[dingxiangfei2009]

Partially undoing #54986

This PR introduces an unstable feature partial_init_nodrop_types. This work corresponds to the new action item in rust-lang/rust-project-goals#548.

Action Items

• What should we do with non_exhaustive?

[dingxiangfei2009]

cc @BennoLossin @nxsaken FYI.

[BennoLossin]

Very cool to see that it's less than 200 lines changed!

Just two small things that I noticed. I can't speak on any of the actual modifications :)

View changes since this review

[BennoLossin]

Regarding non-exhaustive: I think it's pretty simple, the reference already covers all of this:

• Within the defining crate, non_exhaustive has no effect.
• Non-exhaustive types cannot be constructed outside of the defining crate

[dingxiangfei2009]

This PR is ready for previewing. It is pending a formal RFC and a tracking issue.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 6742 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [features]: /checkout/compiler/rustc_feature/src/unstable.rs:625: no tracking issue for feature partial_init_locals
tidy [features]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (26.0.1)
linting python files
---
info: ES-Check: there were no ES version matching errors!  🎉
typechecking javascript files
tidy: The following check failed: features
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1388:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:51
  local time: Tue Mar 10 20:44:33 UTC 2026
  network time: Tue, 10 Mar 2026 20:44:33 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[theemathas]

These two code snippets compile with this PR and causes a crash seemingly inside free.

#![feature(partial_init_locals)]

fn main() {
    let x: (String, String);
    x.0 = String::new();
}

#![feature(partial_init_locals)]

struct Thing(String, String);

fn main() {
    let x: Thing;
    x.0 = String::new();
}

[theemathas]

This code compiles with this PR and prints garbage data:

#![feature(partial_init_locals)]

fn main() {
    let x: (i32, i32);
    x.0 = 123;
    let weird = x.1;
    println!("{}", weird);
}