Rollup merge of #93162 - camsteffen:std-prim-docs, r=Mark-Simulacrum

Std module docs improvements

My primary goal is to create a cleaner separation between primitive types and primitive type helper modules (fixes #92777). I also changed a few header lines in other top-level std modules (seen at https://doc.rust-lang.org/std/) for consistency.

Some conventions used/established:

 * "The \`Box\<T>` type for heap allocation." - if a module mainly provides a single type, name it and summarize its purpose in the module header
 * "Utilities for the _ primitive type." - this wording is used for the header of helper modules
 * Documentation for primitive types themselves are removed from helper modules
 * provided-by-core functionality of primitive types is documented in the primitive type instead of the helper module (such as the "Iteration" section in the slice docs)

I wonder if some content in `std::ptr` should be in `pointer` but I did not address this.

4 files changed, 56 insertions, 11 deletions

diff --git a/library/std/src/error.rs b/library/std/src/error.rs
index df7a49d2582..4fbcfd85d7c 100644
--- a/library/std/src/error.rs
+++ b/library/std/src/error.rs
@@ -1,4 +1,4 @@
-//! Interfaces for working with Errors.
+//! The `Error` trait provides common functionality for errors.
 //!
 //! # Error Handling In Rust
 //!
diff --git a/library/std/src/f32.rs b/library/std/src/f32.rs
index 933b52b4dcc..3dd5b12507f 100644
--- a/library/std/src/f32.rs
+++ b/library/std/src/f32.rs
@@ -1,4 +1,4 @@
-//! Constants specific to the `f32` single-precision floating point type.
+//! Constants for the `f32` single-precision floating point type.
 //!
 //! *[See also the `f32` primitive type](primitive@f32).*
 //!
diff --git a/library/std/src/f64.rs b/library/std/src/f64.rs
index a9aa84f70d1..31351a87978 100644
--- a/library/std/src/f64.rs
+++ b/library/std/src/f64.rs
@@ -1,4 +1,4 @@
-//! Constants specific to the `f64` double-precision floating point type.
+//! Constants for the `f64` double-precision floating point type.
 //!
 //! *[See also the `f64` primitive type](primitive@f64).*
 //!
diff --git a/library/std/src/primitive_docs.rs b/library/std/src/primitive_docs.rs
index 1c14b9341ca..2b2ef64fdb1 100644
--- a/library/std/src/primitive_docs.rs
+++ b/library/std/src/primitive_docs.rs
@@ -801,11 +801,53 @@ mod prim_array {}
 /// assert_eq!(2 * pointer_size, std::mem::size_of::<Box<[u8]>>());
 /// assert_eq!(2 * pointer_size, std::mem::size_of::<Rc<[u8]>>());
 /// ```
+///
+/// ## Trait Implementations
+///
+/// Some traits are implemented for slices if the element type implements
+/// that trait. This includes [`Eq`], [`Hash`] and [`Ord`].
+///
+/// ## Iteration
+///
+/// The slices implement `IntoIterator`. The iterator yields references to the
+/// slice elements.
+///
+/// ```
+/// let numbers: &[i32] = &[0, 1, 2];
+/// for n in numbers {
+///     println!("{n} is a number!");
+/// }
+/// ```
+///
+/// The mutable slice yields mutable references to the elements:
+///
+/// ```
+/// let mut scores: &mut [i32] = &mut [7, 8, 9];
+/// for score in scores {
+///     *score += 1;
+/// }
+/// ```
+///
+/// This iterator yields mutable references to the slice's elements, so while
+/// the element type of the slice is `i32`, the element type of the iterator is
+/// `&mut i32`.
+///
+/// * [`.iter`] and [`.iter_mut`] are the explicit methods to return the default
+///   iterators.
+/// * Further methods that return iterators are [`.split`], [`.splitn`],
+///   [`.chunks`], [`.windows`] and more.
+///
+/// [`Hash`]: core::hash::Hash
+/// [`.iter`]: slice::iter
+/// [`.iter_mut`]: slice::iter_mut
+/// [`.split`]: slice::split
+/// [`.splitn`]: slice::splitn
+/// [`.chunks`]: slice::chunks
+/// [`.windows`]: slice::windows
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_slice {}
 
 #[doc(primitive = "str")]
-//
 /// String slices.
 ///
 /// *[See also the `std::str` module](crate::str).*
@@ -816,19 +858,22 @@ mod prim_slice {}
 ///
 /// String slices are always valid UTF-8.
 ///
-/// # Examples
+/// # Basic Usage
 ///
 /// String literals are string slices:
 ///
 /// ```
-/// let hello = "Hello, world!";
-///
-/// // with an explicit type annotation
-/// let hello: &'static str = "Hello, world!";
+/// let hello_world = "Hello, World!";
 /// ```
 ///
-/// They are `'static` because they're stored directly in the final binary, and
-/// so will be valid for the `'static` duration.
+/// Here we have declared a string slice initialized with a string literal.
+/// String literals have a static lifetime, which means the string `hello_world`
+/// is guaranteed to be valid for the duration of the entire program.
+/// We can explicitly specify `hello_world`'s lifetime as well:
+///
+/// ```
+/// let hello_world: &'static str = "Hello, world!";
+/// ```
 ///
 /// # Representation
 ///