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/core/src/task/wake.rs

@@ -159,6 +159,7 @@ pub struct Context<'a> {
 impl<'a> Context<'a> {
     /// Create a new `Context` from a `&Waker`.
     #[stable(feature = "futures_api", since = "1.36.0")]
+    #[must_use]
     #[inline]
     pub fn from_waker(waker: &'a Waker) -> Self {
         Context { waker, _marker: PhantomData }
@@ -252,6 +253,7 @@ impl Waker {
     /// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
     /// Therefore this method is unsafe.
     #[inline]
+    #[must_use]
     #[stable(feature = "futures_api", since = "1.36.0")]
     pub unsafe fn from_raw(waker: RawWaker) -> Waker {
         Waker { waker }