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Elaborate some in the documentation and respond to some review comments

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Thom Chiovoloni 2021-10-29 19:35:09 -07:00
parent 06edf082c3
commit f950edbef7
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2 changed files with 19 additions and 8 deletions
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1
library/std/src/io/error.rs
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@ -88,7 +88,6 @@ enum ErrorData<C> {
// requires an alignment >= 4 (note that `#[repr(align)]` will not reduce the
// alignment required by the struct, only increase it).
#[repr(align(4))]
#[doc(hidden)]
pub(crate) struct SimpleMessage {
kind: ErrorKind,
message: &'static str,

26
library/std/src/io/error/repr_bitpacked.rs
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@ -2,17 +2,29 @@
//! 64-bit pointers.
//!
//! (Note that `bitpacked` vs `unpacked` here has no relationship to
//! `#[repr(packed)]`, it just refers to attempting to use any available
//! bits in a more clever manner than `rustc`'s default layout algorithm would).
//! `#[repr(packed)]`, it just refers to attempting to use any available bits in
//! a more clever manner than `rustc`'s default layout algorithm would).
//!
//! Conceptually, it stores the same information as the "unpacked" equivalent we
//! use on other targets: `repr_unpacked::Repr` (see repr_unpacked.rs), however
//! it packs it into a 64bit non-zero value.
//! Conceptually, it stores the same data as the "unpacked" equivalent we use on
//! other targets. Specifically, you can imagine it as an optimized following
//! data (which is equivalent to what's stored by `repr_unpacked::Repr`, e.g.
//! `super::ErrorData<Box<Custom>>`):
//!
//! ```ignore (exposition-only)
//! enum ErrorData {
//! Os(i32),
//! Simple(ErrorKind),
//! SimpleMessage(&'static SimpleMessage),
//! Custom(Box<Custom>),
//! }
//! ```
//!
//! However, it packs this data into a 64bit non-zero value.
//!
//! This optimization not only allows `io::Error` to occupy a single pointer,
//! but improves `io::Result` as well, especially for situations like
//! `Result<()>` (which is now 64 bits) or `Result<u64>` (which i), which are
//! quite common.
//! `io::Result<()>` (which is now 64 bits) or `io::Result<u64>` (which is now
//! 128 bits), which are quite common.
//!
//! # Layout
//! Tagged values are 64 bits, with the 2 least significant bits used for the