Make the std feature inert (#250)

In more recent versions of Rust the `Error` trait no longer needs to be
gated, so use it through `core::error::Error` to remove the only usage
of the `feature = "std"` directive. The `std` feature is left for now to
avoid breaking downstream users (if any applicable) but should be
removable later.

---------

Co-authored-by: Chris Fallin <chris@cfallin.org>

Author: alexcrichton

.github/workflows/rust.yml

@@ -56,11 +56,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         submodules: true
-    - run: |
-        set -e
-        curl -L https://github.com/EmbarkStudios/cargo-deny/releases/download/0.14.22/cargo-deny-0.14.22-x86_64-unknown-linux-musl.tar.gz | tar xzf -
-        mv cargo-deny-*-x86_64-unknown-linux-musl/cargo-deny cargo-deny
-        echo `pwd` >> $GITHUB_PATH
+    - run: cargo install cargo-deny
     - run: cargo deny check
 
   # Builds the fuzz targets.

Cargo.toml

@@ -40,7 +40,8 @@ overflow-checks = true
 [features]
 default = ["std"]
 
-# Enables std-specific features such as the Error trait for RegAllocError.
+# Currently does nothing, but historically enabled some `std` features such as
+# the `Error` trait.
 std = []
 
 # Enables generation of DefAlloc edits for the checker.

src/lib.rs

@@ -14,7 +14,7 @@
 #![allow(clippy::all)]
 #![no_std]
 
-#[cfg(feature = "std")]
+#[cfg(feature = "fuzzing")]
 extern crate std;
 
 extern crate alloc;
@@ -1627,8 +1627,7 @@ impl core::fmt::Display for RegAllocError {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for RegAllocError {}
+impl core::error::Error for RegAllocError {}
 
 /// Run the allocator.
 pub fn run<F: Function>(