Auto merge of #43724 - lukaramu:std-ops-docs, r=QuietMisdreavus

Improve std::ops docs

Fixes #29365. (This fixes all but one point from @steveklabnik's list, but that point was referring to examples of implementing range traits, but there are no range traits in std::ops.)

The main changes are quite a bit of copyediting, adding more "real" examples for some of the traits, incorporating some guidance from the API docs, more linking (cross-docs and to the book & reference), cleaning up examples, moving things around, and so on. Refer to the commit messages for more details.

Note: I decided to link to the second edition of the book since I think it's more appropriate now for the sections I linked, if this is not okay, please say so!

src/libcore/ops/deref.rs

@@ -8,16 +8,44 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-/// The `Deref` trait is used to specify the functionality of dereferencing
-/// operations, like `*v`.
+/// Used for immutable dereferencing operations, like `*v`.
 ///
-/// `Deref` also enables ['`Deref` coercions'][coercions].
+/// In addition to being used for explicit dereferencing operations with the
+/// (unary) `*` operator in immutable contexts, `Deref` is also used implicitly
+/// by the compiler in many circumstances. This mechanism is called
+/// ['`Deref` coercion'][more]. In mutable contexts, [`DerefMut`] is used.
 ///
-/// [coercions]: ../../book/first-edition/deref-coercions.html
+/// Implementing `Deref` for smart pointers makes accessing the data behind them
+/// convenient, which is why they implement `Deref`. On the other hand, the
+/// rules regarding `Deref` and [`DerefMut`] were designed specifically to
+/// accomodate smart pointers. Because of this, **`Deref` should only be
+/// implemented for smart pointers** to avoid confusion.
+///
+/// For similar reasons, **this trait should never fail**. Failure during
+/// dereferencing can be extremely confusing when `Deref` is invoked implicitly.
+///
+/// # More on `Deref` coercion
+///
+/// If `T` implements `Deref<Target = U>`, and `x` is a value of type `T`, then:
+/// * In immutable contexts, `*x` on non-pointer types is equivalent to
+///   `*Deref::deref(&x)`.
+/// * Values of type `&T` are coerced to values of type `&U`
+/// * `T` implicitly implements all the (immutable) methods of the type `U`.
+///
+/// For more details, visit [the chapter in *The Rust Programming Language*]
+/// [book] as well as the reference sections on [the dereference operator]
+/// [ref-deref-op], [the `Deref` trait][ref-deref-trait], and [type coercions].
+///
+/// [book]: ../../book/second-edition/ch15-02-deref.html
+/// [`DerefMut`]: trait.DerefMut.html
+/// [more]: #more-on-deref-coercion
+/// [ref-deref-op]: ../../reference/expressions.html#the-dereference-operator
+/// [ref-deref-trait]: ../../reference/the-deref-trait.html
+/// [type coercions]: ../../reference/type-coercions.html
 ///
 /// # Examples
 ///
-/// A struct with a single field which is accessible via dereferencing the
+/// A struct with a single field which is accessible by dereferencing the
 /// struct.
 ///
 /// ```
@@ -35,19 +63,17 @@
 ///     }
 /// }
 ///
-/// fn main() {
-///     let x = DerefExample { value: 'a' };
-///     assert_eq!('a', *x);
-/// }
+/// let x = DerefExample { value: 'a' };
+/// assert_eq!('a', *x);
 /// ```
 #[lang = "deref"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Deref {
-    /// The resulting type after dereferencing
+    /// The resulting type after dereferencing.
     #[stable(feature = "rust1", since = "1.0.0")]
     type Target: ?Sized;
 
-    /// The method called to dereference a value
+    /// Dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn deref(&self) -> &Self::Target;
 }
@@ -66,16 +92,46 @@ impl<'a, T: ?Sized> Deref for &'a mut T {
     fn deref(&self) -> &T { *self }
 }
 
-/// The `DerefMut` trait is used to specify the functionality of dereferencing
-/// mutably like `*v = 1;`
-///
-/// `DerefMut` also enables ['`Deref` coercions'][coercions].
-///
-/// [coercions]: ../../book/first-edition/deref-coercions.html
+/// Used for mutable dereferencing operations, like in `*v = 1;`.
+///
+/// In addition to being used for explicit dereferencing operations with the
+/// (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly
+/// by the compiler in many circumstances. This mechanism is called
+/// ['`Deref` coercion'][more]. In immutable contexts, [`Deref`] is used.
+///
+/// Implementing `DerefMut` for smart pointers makes mutating the data behind
+/// them convenient, which is why they implement `DerefMut`. On the other hand,
+/// the rules regarding [`Deref`] and `DerefMut` were designed specifically to
+/// accomodate smart pointers. Because of this, **`DerefMut` should only be
+/// implemented for smart pointers** to avoid confusion.
+///
+/// For similar reasons, **this trait should never fail**. Failure during
+/// dereferencing can be extremely confusing when `DerefMut` is invoked
+/// implicitly.
+///
+/// # More on `Deref` coercion
+///
+/// If `T` implements `DerefMut<Target = U>`, and `x` is a value of type `T`,
+/// then:
+/// * In mutable contexts, `*x` on non-pointer types is equivalent to
+///   `*Deref::deref(&x)`.
+/// * Values of type `&mut T` are coerced to values of type `&mut U`
+/// * `T` implicitly implements all the (mutable) methods of the type `U`.
+///
+/// For more details, visit [the chapter in *The Rust Programming Language*]
+/// [book] as well as the reference sections on [the dereference operator]
+/// [ref-deref-op], [the `Deref` trait][ref-deref-trait], and [type coercions].
+///
+/// [book]: ../../book/second-edition/ch15-02-deref.html
+/// [`Deref`]: trait.Deref.html
+/// [more]: #more-on-deref-coercion
+/// [ref-deref-op]: ../../reference/expressions.html#the-dereference-operator
+/// [ref-deref-trait]: ../../reference/the-deref-trait.html
+/// [type coercions]: ../../reference/type-coercions.html
 ///
 /// # Examples
 ///
-/// A struct with a single field which is modifiable via dereferencing the
+/// A struct with a single field which is modifiable by dereferencing the
 /// struct.
 ///
 /// ```
@@ -99,16 +155,14 @@ impl<'a, T: ?Sized> Deref for &'a mut T {
 ///     }
 /// }
 ///
-/// fn main() {
-///     let mut x = DerefMutExample { value: 'a' };
-///     *x = 'b';
-///     assert_eq!('b', *x);
-/// }
+/// let mut x = DerefMutExample { value: 'a' };
+/// *x = 'b';
+/// assert_eq!('b', *x);
 /// ```
 #[lang = "deref_mut"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait DerefMut: Deref {
-    /// The method called to mutably dereference a value
+    /// Mutably dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn deref_mut(&mut self) -> &mut Self::Target;
 }

src/libcore/ops/drop.rs

@@ -8,20 +8,27 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-/// The `Drop` trait is used to run some code when a value goes out of scope.
+/// Used to run some code when a value goes out of scope.
 /// This is sometimes called a 'destructor'.
 ///
-/// When a value goes out of scope, if it implements this trait, it will have
-/// its `drop` method called. Then any fields the value contains will also
+/// When a value goes out of scope, it will have its `drop` method called if
+/// its type implements `Drop`. Then, any fields the value contains will also
 /// be dropped recursively.
 ///
-/// Because of the recursive dropping, you do not need to implement this trait
+/// Because of this recursive dropping, you do not need to implement this trait
 /// unless your type needs its own destructor logic.
 ///
+/// Refer to [the chapter on `Drop` in *The Rust Programming Language*][book]
+/// for some more elaboration.
+///
+/// [book]: ../../book/second-edition/ch15-03-drop.html
+///
 /// # Examples
 ///
-/// A trivial implementation of `Drop`. The `drop` method is called when `_x`
-/// goes out of scope, and therefore `main` prints `Dropping!`.
+/// ## Implementing `Drop`
+///
+/// The `drop` method is called when `_x` goes out of scope, and therefore
+/// `main` prints `Dropping!`.
 ///
 /// ```
 /// struct HasDrop;
@@ -37,9 +44,11 @@
 /// }
 /// ```
 ///
-/// Showing the recursive nature of `Drop`. When `outer` goes out of scope, the
-/// `drop` method will be called first for `Outer`, then for `Inner`. Therefore
-/// `main` prints `Dropping Outer!` and then `Dropping Inner!`.
+/// ## Dropping is done recursively
+///
+/// When `outer` goes out of scope, the `drop` method will be called first for
+/// `Outer`, then for `Inner`. Therefore, `main` prints `Dropping Outer!` and
+/// then `Dropping Inner!`.
 ///
 /// ```
 /// struct Inner;
@@ -62,12 +71,20 @@
 /// }
 /// ```
 ///
-/// Because variables are dropped in the reverse order they are declared,
-/// `main` will print `Declared second!` and then `Declared first!`.
+/// ## Variables are dropped in reverse order of declaration
+///
+/// `_first` is declared first and `_second` is declared second, so `main` will
+/// print `Declared second!` and then `Declared first!`.
 ///
 /// ```
 /// struct PrintOnDrop(&'static str);
 ///
+/// impl Drop for PrintOnDrop {
+///     fn drop(&mut self) {
+///         println!("{}", self.0);
+///     }
+/// }
+///
 /// fn main() {
 ///     let _first = PrintOnDrop("Declared first!");
 ///     let _second = PrintOnDrop("Declared second!");
@@ -76,24 +93,25 @@
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Drop {
-    /// A method called when the value goes out of scope.
+    /// Executes the destructor for this type.
+    ///
+    /// This method is called implilcitly when the value goes out of scope,
+    /// and cannot be called explicitly (this is compiler error [E0040]).
+    /// However, the [`std::mem::drop`] function in the prelude can be
+    /// used to call the argument's `Drop` implementation.
     ///
     /// When this method has been called, `self` has not yet been deallocated.
-    /// If it were, `self` would be a dangling reference.
+    /// That only happens after the method is over.
+    /// If this wasn't the case, `self` would be a dangling reference.
     ///
-    /// After this function is over, the memory of `self` will be deallocated.
+    /// # Panics
     ///
-    /// This function cannot be called explicitly. This is compiler error
-    /// [E0040]. However, the [`std::mem::drop`] function in the prelude can be
-    /// used to call the argument's `Drop` implementation.
+    /// Given that a [`panic!`] will call `drop` as it unwinds, any [`panic!`]
+    /// in a `drop` implementation will likely abort.
     ///
     /// [E0040]: ../../error-index.html#E0040
+    /// [`panic!`]: ../macro.panic.html
     /// [`std::mem::drop`]: ../../std/mem/fn.drop.html
-    ///
-    /// # Panics
-    ///
-    /// Given that a `panic!` will call `drop()` as it unwinds, any `panic!` in
-    /// a `drop()` implementation will likely abort.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn drop(&mut self);
 }