Create configuration conditional bench

Currently we are unable to build with all features enabled with a
non-nightly toolchain, this is because of the use of

    `#![cfg_attr(all(test, feature = "unstable"), feature(test))]`

which causes the following error when building:

 error[E0554]: `#![feature]` may not be used on the stable release
 channel

The "unstable" feature is used to guard bench mark modules, this is
widely suggested online but there is a better way.

When running the bench marks use the following incantation:

    `RUSTFLAGS='--cfg=bench' cargo bench`

This creates a configuration conditional "bench" that can be used to
guard the bench mark modules.

    #[cfg(bench)]
    mod benches {
        ...
    }

Cargo.toml

@@ -15,7 +15,6 @@ edition = "2018"
 # Please don't forget to add relevant features to docs.rs below
 [features]
 default = [ "std", "secp-recovery" ]
-unstable = []
 rand = ["secp256k1/rand-std"]
 serde = ["actual-serde", "bitcoin_hashes/serde", "secp256k1/serde"]
 secp-lowmemory = ["secp256k1/lowmemory"]

README.md

@@ -110,6 +110,11 @@ shell alias to check your documentation changes build correctly.
 alias build-docs='RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links'
 ```
 
+### Running benchmarks
+
+We use a custom Rust compiler configuration conditional to guard the bench mark code. To run the
+bench marks use: `RUSTFLAGS='--cfg=bench' cargo +nightly bench`.
+
 ## Pull Requests
 
 Every PR needs at least two reviews to get merged. During the review phase

contrib/test.sh

@@ -77,7 +77,7 @@ then
     )
 fi
 
-# Bench if told to
+# Bench if told to, only works with non-stable toolchain (nightly, beta).
 if [ "$DO_BENCH" = true ]
 then
     if [ "NIGHTLY" = false ]
@@ -90,7 +90,7 @@ then
         fi
         exit 1
     fi
-    cargo bench --features unstable
+    RUSTFLAGS='--cfg=bench' cargo bench
 fi
 
 # Use as dependency if told to

src/blockdata/block.rs

@@ -528,7 +528,7 @@ mod tests {
     }
 }
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(bench)]
 mod benches {
     use super::Block;
     use crate::EmptyWrite;

src/blockdata/transaction.rs

@@ -1724,7 +1724,7 @@ mod tests {
     }
 }
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(bench)]
 mod benches {
     use super::Transaction;
     use crate::EmptyWrite;

src/blockdata/witness.rs

@@ -441,7 +441,7 @@ mod test {
 }
 
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(bench)]
 mod benches {
     use test::{Bencher, black_box};
     use super::Witness;

src/lib.rs

@@ -19,7 +19,6 @@
 //! * `std` - the usual dependency on `std` (default).
 //! * `secp-recovery` - enables calculating public key from a signature and message.
 //! * `base64` - (dependency), enables encoding of PSBTs and message signatures.
-//! * `unstable` - enables unstable features for testing.
 //! * `rand` - (dependency), makes it more convenient to generate random values.
 //! * `serde` - (dependency), implements `serde`-based serialization and
 //!                 deserialization.
@@ -31,9 +30,8 @@
 
 #![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
 
-// Experimental features we need
+// Experimental features we need.
-#![cfg_attr(all(test, feature = "unstable"), feature(test))]
+#![cfg_attr(bench, feature(test))]
-
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 // Coding conventions
@@ -56,6 +54,8 @@ compile_error!("rust-bitcoin currently only supports architectures with pointers
                 than 16 bits, let us know if you want 16-bit support. Note that we do
                 NOT guarantee that we will implement it!");
 
+#[cfg(bench)] extern crate test;
+
 #[cfg(feature = "no-std")]
 #[macro_use]
 extern crate alloc;
@@ -184,10 +184,10 @@ mod prelude {
     pub use std::collections::HashSet;
 }
 
-#[cfg(all(test, feature = "unstable"))] use tests::EmptyWrite;
+#[cfg(bench)] use bench::EmptyWrite;
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(bench)]
-mod tests {
+mod bench {
     use core::fmt::Arguments;
     use crate::io::{IoSlice, Result, Write};