Rollup merge of #89753 - jkugelman:must-use-from_value-conversions, 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: #89692

r? ``@joshtriplett``

library/std/src/io/error.rs

@@ -473,6 +473,7 @@ impl Error {
     /// # }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn from_raw_os_error(code: i32) -> Error {
         Error { repr: Repr::Os(code) }