update docs for the "std" feature

Previously "std" enabled runtime CPU feature detection on x86, but as of
https://github.com/BLAKE3-team/BLAKE3/pull/469 that's always on.

2 files changed, 10 insertions, 12 deletions

diff --git a/Cargo.toml b/Cargo.toml
index bdd719f..61ca47f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,11 +24,11 @@ neon = []
 # This may become the default in the future.
 wasm32_simd = []
 
-# This crate uses libstd for std::io trait implementations, and also for
-# runtime CPU feature detection. This feature is enabled by default. If you use
-# --no-default-features, the only way to use the SIMD implementations in this
-# crate is to enable the corresponding instruction sets statically for the
-# entire build, with e.g. RUSTFLAGS="-C target-cpu=native".
+# Enables std::io traits, including impl Write for Hasher, impl Read and Seek
+# for OutputReader, and the Hasher::update_reader method. This feature is
+# enabled by default. (Previously this also controlled runtime CPU feature
+# detection on x86, but now we use the no-std-compatible `cpufeatures` crate
+# for that.)
 std = []
 
 # The `rayon` feature (disabled by default, but enabled for docs.rs) adds the
diff --git a/src/lib.rs b/src/lib.rs
index d777896..280964b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -32,13 +32,10 @@
 //!
 //! # Cargo Features
 //!
-//! The `std` feature (the only feature enabled by default) is required for
-//! implementations of the [`Write`] and [`Seek`] traits, the
-//! [`update_reader`](Hasher::update_reader) helper method, and runtime CPU
-//! feature detection on x86. If this feature is disabled, the only way to use
-//! the x86 SIMD implementations is to enable the corresponding instruction sets
-//! globally, with e.g. `RUSTFLAGS="-C target-cpu=native"`. The resulting binary
-//! will not be portable to other machines.
+//! The `std` feature (the only feature enabled by default) enables the
+//! [`Write`] implementation and the [`update_reader`](Hasher::update_reader)
+//! method for [`Hasher`], and also the [`Read`] and [`Seek`] implementations
+//! for [`OutputReader`].
 //!
 //! The `rayon` feature (disabled by default, but enabled for [docs.rs]) adds
 //! the [`update_rayon`](Hasher::update_rayon) and (in combination with `mmap`
@@ -82,6 +79,7 @@
 //! [BLAKE3]: https://blake3.io
 //! [Rayon]: https://github.com/rayon-rs/rayon
 //! [docs.rs]: https://docs.rs/
+//! [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html
 //! [`Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
 //! [`Seek`]: https://doc.rust-lang.org/std/io/trait.Seek.html
 //! [`digest`]: https://crates.io/crates/digest