Inline most of the code paths for conversions with boxed slices

This helps with the specific problem described in #49541, obviously without
making any large change to how inlining works in the general case.

Everything involved in the conversions is made `#[inline]`, except for the
`<Vec<T>>::into_boxed_slice` entry point which is made `#[inline(always)]`
after checking that duplicating the function mentioned in the issue prevented
its inlining if I only annotate it with `#[inline]`.

For the record, that function was:

```rust
pub fn foo() -> Box<[u8]> {
    vec![0].into_boxed_slice()
}
```

To help the inliner's job, we also hoist a `self.capacity() != self.len` check
in `<Vec<T>>::shrink_to_fit` and mark it as `#[inline]` too.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index aa202e23628..95d5b30b67d 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1587,6 +1587,7 @@ impl String {
     /// let b = s.into_boxed_str();
     /// ```
     #[stable(feature = "box_str", since = "1.4.0")]
+    #[inline]
     pub fn into_boxed_str(self) -> Box<str> {
         let slice = self.vec.into_boxed_slice();
         unsafe { from_boxed_utf8_unchecked(slice) }