Tracking Issue for `partial_init_locals`

This is a tracking issue for the RFC "XXX" (rust-lang/rfcs#NNN).
The feature gate for the issue is #![feature(partial_init_locals)].

This is a small subgoal for the experiment rust-lang/lang-team#336.

This feature is to partially undo #54986 on a sane set-up. Types with no Drop can be sanely initialised gradually one field at a time.

struct A {
    a: u8,
    b: (u64, u64),
}
let data: A;
data.b.0 = 0;
//~ `data` not initialised
data.b.1 = 1;
//~ `data` still not initialised, but `data.b` is
data.a = 0;
// `data` is fully initialised.
data.a = 1;
//~ ERROR `data` is not mutable.

Steps

• Implement the feature, see #152774
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

• Initial implementation in #152774