uefi: Add "unstable" feature

The unstable feature will be used to gate the use of new unstable
features. We hope to make the uefi crate work on the stable channel once
the `abi_efiapi` and `default_alloc_error_handler` features are
stabilized, so we don't want to start relying unconditionally on new
unstable features.

For now the new feature doesn't do anything, but we have a couple
changes landing soon that will be gated by this feature.

Author: nicholasbishop

uefi/Cargo.toml

@@ -19,6 +19,7 @@ logger = []
 # were observed on the VirtualBox UEFI implementation (see uefi-rs#121).
 # In those cases, this feature can be excluded by removing the default features.
 panic-on-logger-errors = []
+unstable = []
 
 [dependencies]
 bitflags = "1.3.1"

uefi/src/lib.rs

@@ -32,6 +32,12 @@
 //!   is not a high-performance logger.
 //! - `panic-on-logger-errors` (enabled by default): Panic if a text
 //!   output error occurs in the logger.
+//! - `unstable`: Enable functionality that depends on [unstable
+//!   features] in the nightly compiler. Note that currently the `uefi`
+//!   crate _always_ requires unstable features even if the `unstable`
+//!   feature is not enabled, but once a couple more required features
+//!   are stabilized we intend to make the `uefi` crate work on the
+//!   stable channel by default.
 //!
 //! The `global_allocator` and `logger` features require special
 //! handling to perform initialization and tear-down. The
@@ -48,6 +54,7 @@
 //!
 //! [`GlobalAlloc`]: alloc::alloc::GlobalAlloc
 //! [`uefi-services`]: https://crates.io/crates/uefi-services
+//! [unstable features]: https://doc.rust-lang.org/unstable-book/
 
 #![feature(abi_efiapi)]
 #![feature(maybe_uninit_slice)]