Vec::try_with_capacity
Related to #91913
Implements try_with_capacity for `Vec`, `VecDeque`, and `String`. I can follow it up with more collections if desired.
`Vec::try_with_capacity()` is functionally equivalent to the current stable:
```rust
let mut v = Vec::new();
v.try_reserve_exact(n)?
```
However, `try_reserve` calls non-inlined `finish_grow`, which requires old and new `Layout`, and is designed to reallocate memory. There is benefit to using `try_with_capacity`, besides syntax convenience, because it generates much smaller code at the call site with a direct call to the allocator. There's codegen test included.
It's also a very desirable functionality for users of `no_global_oom_handling` (Rust-for-Linux), since it makes a very commonly used function available in that environment (`with_capacity` is used much more frequently than all `(try_)reserve(_exact)`).
An example directive is ignore-test. In legacy compiletest style, the header
would be written as
// ignore-test
but in ui_test style, the header would be written as
//@ ignore-test
compiletest is changed to accept only //@ directives for UI tests
(currently), and will reject and report an error if it encounters any
comments // <content> that may be parsed as an legacy compiletest-style
test header. To fix this, you should migrate to the ui_test-style header
//@ <content>.
e3c0158788