Vec::push in consts MVP

[fee1-dead]

Example:

const X: &'static [u32] = {
    let mut v = Vec::with_capacity(6);
    let mut x = 1;
    while x < 42 {
        v.push(x);
        x *= 2;
    }
    assert!(v.len() == 6);
    v.const_leak()
};

assert_eq!([1, 2, 4, 8, 16, 32], X);

Oh this is fun...

• We split out the implementation of Global such that it calls intrinsics::const_allocate and intrinsics::const_deallocate during compile time. This is achieved using const_eval_select
• This allows us to impl const Allocator for Global
• We then constify everything necessary for Vec::with_capacity and Vec::push.
• Added Vec::const_leak to leak the final value via intrinsics::const_make_global. If we see any pointer in the final value of a const that did not call const_make_global, we error as implemented in add const_make_global; err for const_allocate ptrs if didn't call #143595. (should we name the Vec method const_make_global as well?)

r? @rust-lang/wg-const-eval

To-do for me:

• Assess the rustdoc impact of additional bounds in the method
• Increase test coverage

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [mir-opt] tests/mir-opt/pre-codegen/drop_boxed_slice.rs stdout ----
25                         }
26                     }
27                     scope 18 (inlined <std::alloc::Global as Allocator>::deallocate) {
-                         let mut _9: *mut u8;
-                         scope 19 (inlined Layout::size) {
-                         }
-                         scope 20 (inlined NonNull::<u8>::as_ptr) {
-                         }
-                         scope 21 (inlined std::alloc::dealloc) {
-                             let mut _10: usize;
---
40                             }
41                         }


thread '[mir-opt] tests/mir-opt/pre-codegen/drop_boxed_slice.rs' panicked at src/tools/compiletest/src/runtest/mir_opt.rs:84:21:
Actual MIR output differs from expected MIR output /checkout/tests/mir-opt/pre-codegen/drop_boxed_slice.generic_in_place.PreCodegen.after.64bit.panic-unwind.mir
stack backtrace:
   5: __rustc::rust_begin_unwind
             at /rustc/bb624dcb4c8ab987e10c0808d92d76f3b84dd117/library/std/src/panicking.rs:698:5
   6: core::panicking::panic_fmt
             at /rustc/bb624dcb4c8ab987e10c0808d92d76f3b84dd117/library/core/src/panicking.rs:75:14

[RalfJung]

We used to have the plan to have a dedicated compile-time allocator type... this is an interesting alternative. Given that it also involves the library surface, we should probably involve t-libs-api.

Why have you picked Vec as the first type for this? I think it'd make more sense for Box to go first since that is the most primitive type for heap allocations.

View changes since this review

[RalfJung]

It would be much more efficient to use write_bytes here.

[RalfJung]

This is conceptually quite different from leak which IMO should be reflected in the name, so I would indeed prefer const_make_global.

[RalfJung]

This needs a where T: Freeze for soundness.

[RalfJung]

It doesn't need to be done for all Vec created during compile-time -- only for those that you want to carry over to runtime. You can use intermediate Vec as scratch space or so during compile time that you never call this method on.