Gate use of unstable try_trait_v2 feature

Add a new enabled-by-default `unstable_try_trait` feature to the `uefi`
library to control whether the unstable `try_trait_v2` rustc feature is
enabled.

It seems like there's still a fair amount of discussion around what the
Try API should look like in the tracking issue for try_trait_v2. This
could lead to the API being changed in the nightly compiler, and
breaking uefi-rs compilation (which is particularly annoying when using
a released version rather than the latest git version). Users who want
to avoid that problem can now disable `unstable_try_trait`.

rust-osdev#452

CHANGELOG.md

@@ -23,6 +23,9 @@
 - Deprecated `BootServices::locate_protocol` and marked it `unsafe`. Use
   `BootServices::get_handle_for_protocol` and
   `BootServices::open_protocol` instead.
+- Use of the unstable `try_trait_v2` rustc feature (allowing `?` to be
+  used with `Status`) has been placed behind an optional
+  `unstable_try_trait` feature. This feature is enabled by default.
 
 ### Fixed
 

Cargo.toml

@@ -23,6 +23,10 @@ default = []
 alloc = []
 exts = []
 logger = []
+# If enabled, the unstable `try_trait_v2` feature
+# (https://github.com/rust-lang/rust/issues/84277) is used to allow
+# using `?` with `Status` values.
+unstable_try_trait = []
 # Ignore text output errors in logger as a workaround for firmware issues that
 # were observed on the VirtualBox UEFI implementation (see uefi-rs#121)
 ignore-logger-errors = []

src/lib.rs

@@ -23,12 +23,12 @@
 //! For example, a PC with no network card might not contain a network driver,
 //! therefore all the network protocols will be unavailable.
 
-#![feature(try_trait_v2)]
 #![feature(abi_efiapi)]
 #![feature(maybe_uninit_slice)]
 #![feature(negative_impls)]
 #![feature(ptr_metadata)]
 #![cfg_attr(feature = "exts", feature(vec_into_raw_parts))]
+#![cfg_attr(feature = "unstable_try_trait", feature(try_trait_v2))]
 #![no_std]
 // Enable some additional warnings and lints.
 #![warn(clippy::ptr_as_ptr, missing_docs, unused)]

src/proto/shim/mod.rs

@@ -108,14 +108,14 @@ impl ShimLock {
             .map_err(|_| Error::from(Status::BAD_BUFFER_SIZE))?;
 
         let mut context = MaybeUninit::<Context>::uninit();
-        (self.context)(ptr, size, context.as_mut_ptr())?;
+        Result::from((self.context)(ptr, size, context.as_mut_ptr()))?;
         (self.hash)(
             ptr,
             size,
             context.as_mut_ptr(),
             &mut hashes.sha256,
             &mut hashes.sha1,
-        )?;
-        Ok(())
+        )
+        .into()
     }
 }

src/result/status.rs

@@ -1,9 +1,11 @@
 use super::{Error, Result};
+use core::fmt::Debug;
+#[cfg(feature = "unstable_try_trait")]
 use core::{
     convert::Infallible,
+    num::NonZeroUsize,
     ops::{ControlFlow, FromResidual, Try},
 };
-use core::{fmt::Debug, num::NonZeroUsize};
 
 /// Bit indicating that an UEFI status code is an error
 const ERROR_BIT: usize = 1 << (core::mem::size_of::<usize>() * 8 - 1);
@@ -172,8 +174,10 @@ impl From<Status> for Result<(), ()> {
     }
 }
 
+#[cfg(feature = "unstable_try_trait")]
 pub struct StatusResidual(NonZeroUsize);
 
+#[cfg(feature = "unstable_try_trait")]
 impl Try for Status {
     type Output = ();
     type Residual = StatusResidual;
@@ -190,18 +194,21 @@ impl Try for Status {
     }
 }
 
+#[cfg(feature = "unstable_try_trait")]
 impl FromResidual for Status {
     fn from_residual(r: StatusResidual) -> Self {
         Status(r.0.into())
     }
 }
 
+#[cfg(feature = "unstable_try_trait")]
 impl<T> FromResidual<StatusResidual> for Result<T, ()> {
     fn from_residual(r: StatusResidual) -> Self {
         Err(Status(r.0.into()).into())
     }
 }
 
+#[cfg(feature = "unstable_try_trait")]
 impl FromResidual<core::result::Result<Infallible, Error>> for Status {
     fn from_residual(r: core::result::Result<Infallible, Error>) -> Self {
         match r {

xtask/src/cargo.rs

@@ -48,6 +48,7 @@ pub enum Feature {
     Alloc,
     Exts,
     Logger,
+    UnstableTryTrait,
 
     Ci,
     Qemu,
@@ -59,6 +60,7 @@ impl Feature {
             Self::Alloc => "alloc",
             Self::Exts => "exts",
             Self::Logger => "logger",
+            Self::UnstableTryTrait => "unstable_try_trait",
 
             Self::Ci => "uefi-test-runner/ci",
             Self::Qemu => "uefi-test-runner/qemu",
@@ -67,7 +69,12 @@ impl Feature {
 
     /// Set of features that enables more code in the root uefi crate.
     pub fn more_code() -> Vec<Self> {
-        vec![Self::Alloc, Self::Exts, Self::Logger]
+        vec![
+            Self::Alloc,
+            Self::Exts,
+            Self::Logger,
+            Self::UnstableTryTrait,
+        ]
     }
 
     fn comma_separated_string(features: &[Feature]) -> String {