Shallow clones can cause `git rev-list --merges` to miss merge
commits. Omit it, because the most recent bors commit is
almost always a merge commit.

Exit with an error if we can't find a commit hash for downloading
LLVM or rustc snapshots.

Rustc was incorrectly reading the value of `RUSTC_LOG` as the
environment vairable with the logging configuration, rather than the
logging configuration itself.

bootstrap: don't use `--merges` to look for commit hashes for downloading artifacts

Shallow clones (and possibly worktrees, though I can't seem to reproduce the problem there) can cause `git rev-list --merges` to falsely return no results, even if a merge commit is present. Stop using the `--merges` option when looking for commit hashes that have build artifacts. `--first-parent` and `[email protected]` should be sufficient.

Also exit with an error if the configuration asks for artifacts to be downloaded and we can't determine an appropriate commit hash to use to download artifacts.

Fixes rust-lang#87890.

r? ``@jyn514``
``@rustbot`` label +A-rustbuild +A-contributor-roadblock

…rs, r=joshtriplett

Add #[must_use] to alloc constructors

Added `#[must_use]`. to the various forms of `new`, `pin`, and `with_capacity` in the `alloc` crate. No extra explanations given as I couldn't think of anything useful to add.

I figure this deserves extra scrutiny compared to the other PRs I've done so far. In particular:

* The 4 `pin`/`pin_in` methods I touched. Are there legitimate use cases for pinning and not using the result? Pinning's a difficult concept I'm not very comfortable with.
* `Box`'s constructors. Do people ever create boxes just for the side effects... allocating or zeroing out memory?

Parent issue: rust-lang#89692

r? ``@joshtriplett``

…ctors, r=joshtriplett

Add #[must_use] to core and std constructors

Parent issue: rust-lang#89692

r? ``@joshtriplett``

Fix RUSTC_LOG handling

Rustc was incorrectly reading the value of `RUSTC_LOG` as the environment vairable with the logging configuration, rather than the logging configuration itself.

…rsions, r=joshtriplett

Add #[must_use] to from_value conversions

I added two methods to the list myself. Clippy did not flag them because they take `mut` args, but neither modifies their argument.

```rust
core::str           const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str;
std::ffi::CString   unsafe fn from_raw(ptr: *mut c_char) -> CString;
```

I put a custom note on `from_raw`:

```rust
#[must_use = "call `drop(from_raw(ptr))` if you intend to drop the `CString`"]
pub unsafe fn from_raw(ptr: *mut c_char) -> CString {
```

Parent issue: rust-lang#89692

r? ``@joshtriplett``

…llaumeGomez

Cleanup .item-table CSS

The main table-like element must be `display: table;`

r? `@GuillaumeGomez`

…=lnicola

⬆️ rust-analyzer

r? ``@ghost``