Rollup merge of #50511 - Manishearth:must-use, r=QuietMisdreavus

Add some explanations for #[must_use]

`#[must_use]` can be given a string argument which is shown whilst warning for things.

We should add a string argument to most of the user-exposed ones.

I added these for everything but the operators, mostly because I'm not sure what to write there or if we need anything there.

6 files changed, 14 insertions, 12 deletions

diff --git a/src/liballoc/str.rs b/src/liballoc/str.rs
index 0cbe65db53c..9e693c89be9 100644
--- a/src/liballoc/str.rs
+++ b/src/liballoc/str.rs
@@ -207,7 +207,8 @@ impl str {
     /// let s = "this is old";
     /// assert_eq!(s, s.replace("cookie monster", "little lamb"));
     /// ```
-    #[must_use]
+    #[must_use = "this returns the replaced string as a new allocation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn replace<'a, P: Pattern<'a>>(&'a self, from: P, to: &str) -> String {
@@ -247,7 +248,8 @@ impl str {
     /// let s = "this is old";
     /// assert_eq!(s, s.replacen("cookie monster", "little lamb", 10));
     /// ```
-    #[must_use]
+    #[must_use = "this returns the replaced string as a new allocation, \
+                  without modifying the original"]
     #[stable(feature = "str_replacen", since = "1.16.0")]
     pub fn replacen<'a, P: Pattern<'a>>(&'a self, pat: P, to: &str, count: usize) -> String {
         // Hope to reduce the times of re-allocation
diff --git a/src/libcore/fmt/builders.rs b/src/libcore/fmt/builders.rs
index a1f4c6995da..e8ea2e743da 100644
--- a/src/libcore/fmt/builders.rs
+++ b/src/libcore/fmt/builders.rs
@@ -84,7 +84,7 @@ impl<'a> fmt::Write for PadAdapter<'a> {
 /// // prints "Foo { bar: 10, baz: "Hello World" }"
 /// println!("{:?}", Foo { bar: 10, baz: "Hello World".to_string() });
 /// ```
-#[must_use]
+#[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub struct DebugStruct<'a, 'b: 'a> {
@@ -181,7 +181,7 @@ impl<'a, 'b: 'a> DebugStruct<'a, 'b> {
 /// // prints "Foo(10, "Hello World")"
 /// println!("{:?}", Foo(10, "Hello World".to_string()));
 /// ```
-#[must_use]
+#[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub struct DebugTuple<'a, 'b: 'a> {
@@ -319,7 +319,7 @@ impl<'a, 'b: 'a> DebugInner<'a, 'b> {
 /// // prints "{10, 11}"
 /// println!("{:?}", Foo(vec![10, 11]));
 /// ```
-#[must_use]
+#[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub struct DebugSet<'a, 'b: 'a> {
@@ -390,7 +390,7 @@ impl<'a, 'b: 'a> DebugSet<'a, 'b> {
 /// // prints "[10, 11]"
 /// println!("{:?}", Foo(vec![10, 11]));
 /// ```
-#[must_use]
+#[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub struct DebugList<'a, 'b: 'a> {
@@ -461,7 +461,7 @@ impl<'a, 'b: 'a> DebugList<'a, 'b> {
 /// // prints "{"A": 10, "B": 11}"
 /// println!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)]));
 /// ```
-#[must_use]
+#[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
 pub struct DebugMap<'a, 'b: 'a> {
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index c152d4979b9..e0cc669d035 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -251,7 +251,7 @@ use ops;
 /// [`Ok`]: enum.Result.html#variant.Ok
 /// [`Err`]: enum.Result.html#variant.Err
 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
-#[must_use]
+#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub enum Result<T, E> {
     /// Contains the success value
diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index 3b4904c98e8..f3503b0b3a6 100644
--- a/src/libstd/sync/mutex.rs
+++ b/src/libstd/sync/mutex.rs
@@ -150,7 +150,7 @@ unsafe impl<T: ?Sized + Send> Sync for Mutex<T> { }
 /// [`lock`]: struct.Mutex.html#method.lock
 /// [`try_lock`]: struct.Mutex.html#method.try_lock
 /// [`Mutex`]: struct.Mutex.html
-#[must_use]
+#[must_use = "if unused the Mutex will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct MutexGuard<'a, T: ?Sized + 'a> {
     // funny underscores due to how Deref/DerefMut currently work (they
diff --git a/src/libstd/sync/rwlock.rs b/src/libstd/sync/rwlock.rs
index f7fdedc0d21..e3db60cff84 100644
--- a/src/libstd/sync/rwlock.rs
+++ b/src/libstd/sync/rwlock.rs
@@ -94,7 +94,7 @@ unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
 /// [`read`]: struct.RwLock.html#method.read
 /// [`try_read`]: struct.RwLock.html#method.try_read
 /// [`RwLock`]: struct.RwLock.html
-#[must_use]
+#[must_use = "if unused the RwLock will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
     __lock: &'a RwLock<T>,
@@ -115,7 +115,7 @@ unsafe impl<'a, T: ?Sized + Sync> Sync for RwLockReadGuard<'a, T> {}
 /// [`write`]: struct.RwLock.html#method.write
 /// [`try_write`]: struct.RwLock.html#method.try_write
 /// [`RwLock`]: struct.RwLock.html
-#[must_use]
+#[must_use = "if unused the RwLock will immediately unlock"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
     __lock: &'a RwLock<T>,
diff --git a/src/libstd/sys_common/remutex.rs b/src/libstd/sys_common/remutex.rs
index ce43ec6d9ab..022056f8a8a 100644
--- a/src/libstd/sys_common/remutex.rs
+++ b/src/libstd/sys_common/remutex.rs
@@ -41,7 +41,7 @@ unsafe impl<T: Send> Sync for ReentrantMutex<T> {}
 /// because implementation of the trait would violate Rust’s reference aliasing
 /// rules. Use interior mutability (usually `RefCell`) in order to mutate the
 /// guarded data.
-#[must_use]
+#[must_use = "if unused the ReentrantMutex will immediately unlock"]
 pub struct ReentrantMutexGuard<'a, T: 'a> {
     // funny underscores due to how Deref currently works (it disregards field
     // privacy).