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/alloc/src/collections/btree/set.rs

@@ -137,6 +137,8 @@ pub struct Range<'a, T: 'a> {
 /// See its documentation for more.
 ///
 /// [`difference`]: BTreeSet::difference
+#[must_use = "this returns the difference as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Difference<'a, T: 'a> {
     inner: DifferenceInner<'a, T>,
@@ -169,6 +171,8 @@ impl<T: fmt::Debug> fmt::Debug for Difference<'_, T> {
 /// [`BTreeSet`]. See its documentation for more.
 ///
 /// [`symmetric_difference`]: BTreeSet::symmetric_difference
+#[must_use = "this returns the difference as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct SymmetricDifference<'a, T: 'a>(MergeIterInner<Iter<'a, T>>);
 
@@ -185,6 +189,8 @@ impl<T: fmt::Debug> fmt::Debug for SymmetricDifference<'_, T> {
 /// See its documentation for more.
 ///
 /// [`intersection`]: BTreeSet::intersection
+#[must_use = "this returns the intersection as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Intersection<'a, T: 'a> {
     inner: IntersectionInner<'a, T>,
@@ -217,6 +223,8 @@ impl<T: fmt::Debug> fmt::Debug for Intersection<'_, T> {
 /// See its documentation for more.
 ///
 /// [`union`]: BTreeSet::union
+#[must_use = "this returns the union as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Union<'a, T: 'a>(MergeIterInner<Iter<'a, T>>);
 

library/alloc/src/string.rs

@@ -552,6 +552,7 @@ impl String {
     ///
     /// assert_eq!("Hello <20>World", output);
     /// ```
+    #[must_use]
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
@@ -646,6 +647,7 @@ impl String {
     ///            String::from_utf16_lossy(v));
     /// ```
     #[cfg(not(no_global_oom_handling))]
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16_lossy(v: &[u16]) -> String {