Rollup merge of #89796 - jkugelman:must-use-non-mutating-verb-methods, r=joshtriplett

Add #[must_use] to non-mutating verb methods

These are methods that could be misconstrued to mutate their input, similar to #89694. I gave each one a different custom message.

I wrote that `upgrade` and `downgrade` don't modify the input pointers. Logically they don't, but technically they do...

Parent issue: #89692

r? ```@joshtriplett```

Author: the8472

library/alloc/src/rc.rs

@@ -2230,6 +2230,8 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// assert!(weak_five.upgrade().is_none());
     /// ```
+    #[must_use = "this returns a new `Rc`, \
+                  without modifying the original weak pointer"]
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Rc<T>> {
         let inner = self.inner()?;

library/alloc/src/sync.rs

@@ -146,7 +146,7 @@ macro_rules! acquire {
 /// use std::sync::Arc;
 ///
 /// let my_arc = Arc::new(());
-/// Arc::downgrade(&my_arc);
+/// let my_weak = Arc::downgrade(&my_arc);
 /// ```
 ///
 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
@@ -898,6 +898,8 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// let weak_five = Arc::downgrade(&five);
     /// ```
+    #[must_use = "this returns a new `Weak` pointer, \
+                  without modifying the original `Arc`"]
     #[stable(feature = "arc_weak", since = "1.4.0")]
     pub fn downgrade(this: &Self) -> Weak<T> {
         // This Relaxed is OK because we're checking the value in the CAS
@@ -1863,6 +1865,8 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// assert!(weak_five.upgrade().is_none());
     /// ```
+    #[must_use = "this returns a new `Arc`, \
+                  without modifying the original weak pointer"]
     #[stable(feature = "arc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Arc<T>> {
         // We use a CAS loop to increment the strong count instead of a

library/core/src/alloc/layout.rs

@@ -112,6 +112,8 @@ impl Layout {
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
+    #[must_use = "this returns the minimum alignment, \
+                  without modifying the layout"]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()
@@ -229,6 +231,8 @@ impl Layout {
     /// satisfy this constraint is to ensure `align <= self.align()`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[must_use = "this returns the padding needed, \
+                  without modifying the `Layout`"]
     #[inline]
     pub const fn padding_needed_for(&self, align: usize) -> usize {
         let len = self.size();
@@ -262,6 +266,8 @@ impl Layout {
     /// This is equivalent to adding the result of `padding_needed_for`
     /// to the layout's current size.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
+    #[must_use = "this returns a new `Layout`, \
+                  without modifying the original"]
     #[inline]
     pub fn pad_to_align(&self) -> Layout {
         let pad = self.padding_needed_for(self.align());

library/std/src/path.rs

@@ -2516,6 +2516,8 @@ impl Path {
     /// println!("{}", path.display());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this does not display the path, \
+                  it returns an object that can be displayed"]
     #[inline]
     pub fn display(&self) -> Display<'_> {
         Display { path: self }