Rollup merge of #89835 - jkugelman:must-use-expensive-computations, r=joshtriplett

Add #[must_use] to expensive computations

The unifying theme for this commit is weak, admittedly. I put together a list of "expensive" functions when I originally proposed this whole effort, but nobody's cared about that criterion. Still, it's a decent way to bite off a not-too-big chunk of work.

Given the grab bag nature of this commit, the messages I used vary quite a bit. I'm open to wording changes.

For some reason clippy flagged four `BTreeSet` methods but didn't say boo about equivalent ones on `HashSet`. I stared at them for a while but I can't figure out the difference so I added the `HashSet` ones in.

```rust
// Flagged by clippy.
alloc::collections::btree_set::BTreeSet<T>   fn difference<'a>(&'a self, other: &'a BTreeSet<T>) -> Difference<'a, T>;
alloc::collections::btree_set::BTreeSet<T>   fn symmetric_difference<'a>(&'a self, other: &'a BTreeSet<T>) -> SymmetricDifference<'a, T>
alloc::collections::btree_set::BTreeSet<T>   fn intersection<'a>(&'a self, other: &'a BTreeSet<T>) -> Intersection<'a, T>;
alloc::collections::btree_set::BTreeSet<T>   fn union<'a>(&'a self, other: &'a BTreeSet<T>) -> Union<'a, T>;

// Ignored by clippy, but not by me.
std::collections::HashSet<T, S>              fn difference<'a>(&'a self, other: &'a HashSet<T, S>) -> Difference<'a, T, S>;
std::collections::HashSet<T, S>              fn symmetric_difference<'a>(&'a self, other: &'a HashSet<T, S>) -> SymmetricDifference<'a, T, S>
std::collections::HashSet<T, S>              fn intersection<'a>(&'a self, other: &'a HashSet<T, S>) -> Intersection<'a, T, S>;
std::collections::HashSet<T, S>              fn union<'a>(&'a self, other: &'a HashSet<T, S>) -> Union<'a, T, S>;
```

Parent issue: #89692

r? ```@joshtriplett```

library/core/src/slice/ascii.rs

@@ -11,6 +11,7 @@ use crate::ops;
 impl [u8] {
     /// Checks if all bytes in this slice are within the ASCII range.
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[must_use]
     #[inline]
     pub fn is_ascii(&self) -> bool {
         is_ascii(self)
@@ -21,6 +22,7 @@ impl [u8] {
     /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,
     /// but without allocating and copying temporaries.
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[must_use]
     #[inline]
     pub fn eq_ignore_ascii_case(&self, other: &[u8]) -> bool {
         self.len() == other.len() && iter::zip(self, other).all(|(a, b)| a.eq_ignore_ascii_case(b))

library/core/src/slice/memchr.rs

@@ -37,6 +37,7 @@ fn repeat_byte(b: u8) -> usize {
 }
 
 /// Returns the first index matching the byte `x` in `text`.
+#[must_use]
 #[inline]
 pub fn memchr(x: u8, text: &[u8]) -> Option<usize> {
     // Fast path for small slices
@@ -91,6 +92,7 @@ fn memchr_general_case(x: u8, text: &[u8]) -> Option<usize> {
 }
 
 /// Returns the last index matching the byte `x` in `text`.
+#[must_use]
 pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
     // Scan for a single byte value by reading two `usize` words at a time.
     //