Rollup merge of #49555 - nox:inline-into-boxed, r=alexcrichton

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, 3 insertions, 1 deletions

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 8ab25dfe7d8..7d1b2ed85c7 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -583,7 +583,9 @@ impl<T> Vec<T> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn shrink_to_fit(&mut self) {
-        self.buf.shrink_to_fit(self.len);
+        if self.capacity() != self.len {
+            self.buf.shrink_to_fit(self.len);
+        }
     }
 
     /// Shrinks the capacity of the vector with a lower bound.