Auto merge of #41424 - frewsxcv:rollup, r=frewsxcv

Rollup of 2 pull requests

- Successful merges: #40812, #40987
- Failed merges:

2 files changed, 142 insertions, 61 deletions

diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index 0b0f831f093..084736685e3 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -17,8 +17,8 @@
 //! Like many traits, these are often used as bounds for generic functions, to
 //! support arguments of multiple types.
 //!
-//! - Impl the `As*` traits for reference-to-reference conversions
-//! - Impl the [`Into`] trait when you want to consume the value in the conversion
+//! - Implement the `As*` traits for reference-to-reference conversions
+//! - Implement the [`Into`] trait when you want to consume the value in the conversion
 //! - The [`From`] trait is the most flexible, useful for value _and_ reference conversions
 //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], but allow for the
 //!   conversion to fail
@@ -26,16 +26,16 @@
 //! As a library author, you should prefer implementing [`From<T>`][`From`] or
 //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
 //! as [`From`] and [`TryFrom`] provide greater flexibility and offer
-//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a blanket implementation
-//! in the standard library.
+//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
+//! blanket implementation in the standard library.
 //!
-//! # Generic impl
+//! # Generic Implementations
 //!
 //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
 //! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
 //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
-//! - [`From`] and [`Into`] are reflexive, which means that all types can `into()`
-//!   themselves and `from()` themselves
+//! - [`From`] and [`Into`] are reflexive, which means that all types can
+//!   `into` themselves and `from` themselves
 //!
 //! See each trait for usage examples.
 //!
@@ -50,20 +50,42 @@
 
 use str::FromStr;
 
-/// A cheap, reference-to-reference conversion.
+/// A cheap reference-to-reference conversion. Used to convert a value to a
+/// reference value within generic code.
 ///
-/// `AsRef` is very similar to, but different than, [`Borrow`]. See
-/// [the book][book] for more.
+/// `AsRef` is very similar to, but serves a slightly different purpose than,
+/// [`Borrow`].
+///
+/// `AsRef` is to be used when wishing to convert to a reference of another
+/// type.
+/// `Borrow` is more related to the notion of taking the reference. It is
+/// useful when wishing to abstract over the type of reference
+/// (`&T`, `&mut T`) or allow both the referenced and owned type to be treated
+/// in the same manner.
+///
+/// The key difference between the two traits is the intention:
+///
+/// - Use `AsRef` when goal is to simply convert into a reference
+/// - Use `Borrow` when goal is related to writing code that is agnostic to the
+///   type of borrow and if is reference or value
+///
+/// See [the book][book] for a more detailed comparison.
 ///
 /// [book]: ../../book/borrow-and-asref.html
 /// [`Borrow`]: ../../std/borrow/trait.Borrow.html
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
-/// returns an [`Option<T>`] or a [`Result<T, E>`].
+/// **Note: this trait must not fail**. If the conversion can fail, use a
+/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
 ///
 /// [`Option<T>`]: ../../std/option/enum.Option.html
 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 ///
+/// # Generic Implementations
+///
+/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
+///   reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
+///   `&mut Foo` or `&&mut Foo`)
+///
 /// # Examples
 ///
 /// Both [`String`] and `&str` implement `AsRef<str>`:
@@ -82,11 +104,6 @@ use str::FromStr;
 /// is_hello(s);
 /// ```
 ///
-/// # Generic Impls
-///
-/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
-/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`)
-///
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait AsRef<T: ?Sized> {
     /// Performs the conversion.
@@ -96,12 +113,21 @@ pub trait AsRef<T: ?Sized> {
 
 /// A cheap, mutable reference-to-mutable reference conversion.
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use a dedicated method which
-/// returns an [`Option<T>`] or a [`Result<T, E>`].
+/// This trait is similar to `AsRef` but used for converting between mutable
+/// references.
+///
+/// **Note: this trait must not fail**. If the conversion can fail, use a
+/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
 ///
 /// [`Option<T>`]: ../../std/option/enum.Option.html
 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 ///
+/// # Generic Implementations
+///
+/// - `AsMut` auto-dereferences if the inner type is a reference or a mutable
+///   reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
+///   `&mut Foo` or `&&mut Foo`)
+///
 /// # Examples
 ///
 /// [`Box<T>`] implements `AsMut<T>`:
@@ -118,10 +144,6 @@ pub trait AsRef<T: ?Sized> {
 /// assert_eq!(*boxed_num, 1);
 /// ```
 ///
-/// # Generic Impls
-///
-/// - `AsMut` auto-dereferences if the inner type is a reference or a mutable
-/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`)
 ///
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait AsMut<T: ?Sized> {
@@ -130,14 +152,22 @@ pub trait AsMut<T: ?Sized> {
     fn as_mut(&mut self) -> &mut T;
 }
 
-/// A conversion that consumes `self`, which may or may not be expensive.
+/// A conversion that consumes `self`, which may or may not be expensive. The
+/// reciprocal of [`From`][From].
+///
+/// **Note: this trait must not fail**. If the conversion can fail, use
+/// [`TryInto`] or a dedicated method which returns an [`Option<T>`] or a
+/// [`Result<T, E>`].
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use [`TryInto`] or a dedicated
-/// method which returns an [`Option<T>`] or a [`Result<T, E>`].
+/// Library authors should not directly implement this trait, but should prefer
+/// implementing the [`From`][From] trait, which offers greater flexibility and
+/// provides an equivalent `Into` implementation for free, thanks to a blanket
+/// implementation in the standard library.
 ///
-/// Library authors should not directly implement this trait, but should prefer implementing
-/// the [`From`][From] trait, which offers greater flexibility and provides an equivalent `Into`
-/// implementation for free, thanks to a blanket implementation in the standard library.
+/// # Generic Implementations
+///
+/// - [`From<T>`][From]` for U` implies `Into<U> for T`
+/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
 ///
 /// # Examples
 ///
@@ -153,11 +183,6 @@ pub trait AsMut<T: ?Sized> {
 /// is_hello(s);
 /// ```
 ///
-/// # Generic Impls
-///
-/// - [`From<T>`][From]` for U` implies `Into<U> for T`
-/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
-///
 /// [`TryInto`]: trait.TryInto.html
 /// [`Option<T>`]: ../../std/option/enum.Option.html
 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
@@ -171,10 +196,31 @@ pub trait Into<T>: Sized {
     fn into(self) -> T;
 }
 
-/// Construct `Self` via a conversion.
+/// Simple and safe type conversions in to `Self`. It is the reciprocal of
+/// `Into`.
+///
+/// This trait is useful when performing error handling as described by
+/// [the book][book] and is closely related to the `?` operator.
+///
+/// When constructing a function that is capable of failing the return type
+/// will generally be of the form `Result<T, E>`.
+///
+/// The `From` trait allows for simplification of error handling by providing a
+/// means of returning a single error type that encapsulates numerous possible
+/// erroneous situations.
+///
+/// This trait is not limited to error handling, rather the general case for
+/// this trait would be in any type conversions to have an explicit definition
+/// of how they are performed.
+///
+/// **Note: this trait must not fail**. If the conversion can fail, use
+/// [`TryFrom`] or a dedicated method which returns an [`Option<T>`] or a
+/// [`Result<T, E>`].
 ///
-/// **Note: this trait must not fail**. If the conversion can fail, use [`TryFrom`] or a dedicated
-/// method which returns an [`Option<T>`] or a [`Result<T, E>`].
+/// # Generic Implementations
+///
+/// - `From<T> for U` implies [`Into<U>`]` for T`
+/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
 ///
 /// # Examples
 ///
@@ -186,10 +232,38 @@ pub trait Into<T>: Sized {
 ///
 /// assert_eq!(string, other_string);
 /// ```
-/// # Generic impls
 ///
-/// - `From<T> for U` implies [`Into<U>`]` for T`
-/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
+/// An example usage for error handling:
+///
+/// ```
+/// use std::io::{self, Read};
+/// use std::num;
+///
+/// enum CliError {
+///     IoError(io::Error),
+///     ParseError(num::ParseIntError),
+/// }
+///
+/// impl From<io::Error> for CliError {
+///     fn from(error: io::Error) -> Self {
+///         CliError::IoError(error)
+///     }
+/// }
+///
+/// impl From<num::ParseIntError> for CliError {
+///     fn from(error: num::ParseIntError) -> Self {
+///         CliError::ParseError(error)
+///     }
+/// }
+///
+/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
+///     let mut file = std::fs::File::open("test")?;
+///     let mut contents = String::new();
+///     file.read_to_string(&mut contents)?;
+///     let num: i32 = contents.trim().parse()?;
+///     Ok(num)
+/// }
+/// ```
 ///
 /// [`TryFrom`]: trait.TryFrom.html
 /// [`Option<T>`]: ../../std/option/enum.Option.html
@@ -197,6 +271,7 @@ pub trait Into<T>: Sized {
 /// [`String`]: ../../std/string/struct.String.html
 /// [`Into<U>`]: trait.Into.html
 /// [`from`]: trait.From.html#tymethod.from
+/// [book]: ../../book/error-handling.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait From<T>: Sized {
     /// Performs the conversion.
@@ -204,11 +279,13 @@ pub trait From<T>: Sized {
     fn from(T) -> Self;
 }
 
-/// An attempted conversion that consumes `self`, which may or may not be expensive.
+/// An attempted conversion that consumes `self`, which may or may not be
+/// expensive.
 ///
-/// Library authors should not directly implement this trait, but should prefer implementing
-/// the [`TryFrom`] trait, which offers greater flexibility and provides an equivalent `TryInto`
-/// implementation for free, thanks to a blanket implementation in the standard library.
+/// Library authors should not directly implement this trait, but should prefer
+/// implementing the [`TryFrom`] trait, which offers greater flexibility and
+/// provides an equivalent `TryInto` implementation for free, thanks to a
+/// blanket implementation in the standard library.
 ///
 /// [`TryFrom`]: trait.TryFrom.html
 #[unstable(feature = "try_from", issue = "33417")]
@@ -236,7 +313,8 @@ pub trait TryFrom<T>: Sized {
 
 // As lifts over &
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U> {
+impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U>
+{
     fn as_ref(&self) -> &U {
         <T as AsRef<U>>::as_ref(*self)
     }
@@ -244,7 +322,8 @@ impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U> {
 
 // As lifts over &mut
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> {
+impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U>
+{
     fn as_ref(&self) -> &U {
         <T as AsRef<U>>::as_ref(*self)
     }
@@ -260,7 +339,8 @@ impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U> {
 
 // AsMut lifts over &mut
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> {
+impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U>
+{
     fn as_mut(&mut self) -> &mut U {
         (*self).as_mut()
     }
@@ -276,7 +356,8 @@ impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U> {
 
 // From implies Into
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, U> Into<U> for T where U: From<T> {
+impl<T, U> Into<U> for T where U: From<T>
+{
     fn into(self) -> U {
         U::from(self)
     }
@@ -291,7 +372,8 @@ impl<T> From<T> for T {
 
 // TryFrom implies TryInto
 #[unstable(feature = "try_from", issue = "33417")]
-impl<T, U> TryInto<U> for T where U: TryFrom<T> {
+impl<T, U> TryInto<U> for T where U: TryFrom<T>
+{
     type Error = U::Error;
 
     fn try_into(self) -> Result<U, U::Error> {
@@ -327,7 +409,8 @@ impl AsRef<str> for str {
 
 // FromStr implies TryFrom<&str>
 #[unstable(feature = "try_from", issue = "33417")]
-impl<'a, T> TryFrom<&'a str> for T where T: FromStr {
+impl<'a, T> TryFrom<&'a str> for T where T: FromStr
+{
     type Error = <T as FromStr>::Err;
 
     fn try_from(s: &'a str) -> Result<T, Self::Error> {
diff --git a/src/libstd/process.rs b/src/libstd/process.rs
index 3795c42f5ef..4ec9076de61 100644
--- a/src/libstd/process.rs
+++ b/src/libstd/process.rs
@@ -73,6 +73,15 @@ use sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 /// spawning process and can itself be constructed using a builder-style
 /// interface.
 ///
+/// There is no implementation of [`Drop`] for child processes,
+/// so if you do not ensure the `Child` has exited then it will continue to
+/// run, even after the `Child` handle to the child process has gone out of
+/// scope.
+///
+/// Calling [`wait`](#method.wait) (or other functions that wrap around it) will make
+/// the parent process wait until the child has actually exited before
+/// continuing.
+///
 /// # Examples
 ///
 /// ```should_panic
@@ -89,17 +98,6 @@ use sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 /// assert!(ecode.success());
 /// ```
 ///
-/// # Note
-///
-/// Take note that there is no implementation of [`Drop`] for child processes,
-/// so if you do not ensure the `Child` has exited then it will continue to
-/// run, even after the `Child` handle to the child process has gone out of
-/// scope.
-///
-/// Calling [`wait`][`wait`] (or other functions that wrap around it) will make
-/// the parent process wait until the child has actually exited before
-/// continuing.
-///
 /// [`Command`]: struct.Command.html
 /// [`Drop`]: ../../core/ops/trait.Drop.html
 /// [`wait`]: #method.wait