Return slices with 'data lifetime in BorrowedBuf

[SUPERCILEX]

Tracking issue: #117693

Seems like an oversight? I don't think there's any reason to limit the lifetime of the filled buffer... fixing this enables the following style of use case:

pub fn direct_file_name(
    buf: &mut [MaybeUninit<u8>; DIRECT_FILE_NAME_LEN + 1],
    to: RingKind,
    index: u32,
) -> DirectFileNameToken<()> {
    let mut buf = BorrowedBuf::from(buf.as_mut_slice());
    write!(buf.unfilled(), "{:0>13}\0", composite_id(to, index)).unwrap();
    DirectFileNameToken(buf.filled_mut(), PhantomData) // Doesn't compile!
}

[rustbot]

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

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#16 3.093 Building wheels for collected packages: reuse
#16 3.094   Building wheel for reuse (pyproject.toml): started
#16 3.341   Building wheel for reuse (pyproject.toml): finished with status 'done'
#16 3.342   Created wheel for reuse: filename=reuse-4.0.3-cp310-cp310-manylinux_2_35_x86_64.whl size=132720 sha256=026f3bb0f1aa8090b861fd0a0939cb1a782396d84c8aab7875096557d637a0f6
#16 3.342   Stored in directory: /tmp/pip-ephem-wheel-cache-yp186m7o/wheels/3d/8d/0a/e0fc6aba4494b28a967ab5eaf951c121d9c677958714e34532
#16 3.345 Installing collected packages: boolean-py, binaryornot, tomlkit, reuse, python-debian, markupsafe, license-expression, jinja2, chardet, attrs
#16 3.730 Successfully installed attrs-23.2.0 binaryornot-0.4.4 boolean-py-4.0 chardet-5.2.0 jinja2-3.1.4 license-expression-30.3.0 markupsafe-2.1.5 python-debian-0.1.49 reuse-4.0.3 tomlkit-0.13.0
#16 3.731 WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
#16 DONE 3.8s
---
   Compiling std v0.0.0 (/checkout/library/std)
error: lifetime may not live long enough
  --> core/src/io/borrowed_buf.rs:97:13
   |
72 | impl<'data> BorrowedBuf<'data> {
   |      ----- lifetime `'data` defined here
...
93 |     pub fn filled(&self) -> &'data [u8] {
   |                   - let's call the lifetime of this reference `'1`
97 |             MaybeUninit::slice_assume_init_ref(buf)
97 |             MaybeUninit::slice_assume_init_ref(buf)
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method was supposed to return data with lifetime `'data` but it is returning data with lifetime `'1`
error: lifetime may not live long enough
   --> core/src/io/borrowed_buf.rs:105:9
    |
    |
72  |   impl<'data> BorrowedBuf<'data> {
    |        ----- lifetime `'data` defined here
103 |       pub fn filled_mut(&mut self) -> &'data mut [u8] {
    |                         - let's call the lifetime of this reference `'1`
    |                         - let's call the lifetime of this reference `'1`
104 |           // SAFETY: We only slice the filled part of the buffer, which is always valid
105 | /         unsafe {
106 | |             let buf = self.buf.get_unchecked_mut(..self.filled);
107 | |             MaybeUninit::slice_assume_init_mut(buf)
108 | |         }
    | |_________^ method was supposed to return data with lifetime `'data` but it is returning data with lifetime `'1`
error: could not compile `core` (lib) due to 2 previous errors
Build completed unsuccessfully in 0:00:43
  local time: Sat Nov  2 17:55:57 UTC 2024
  network time: Sat, 02 Nov 2024 17:55:57 GMT

[SUPERCILEX]

Ah right this doesn't work because you could alias. Will make an ACP.

[SUPERCILEX]

ACP: rust-lang/libs-team#473