Enable build with global-context-less-secure

Currently this command fails:

 cargo build --no-default-features --features=global-context-less-secure

All features should be able to be built individually, this is currently
not the case with `global-context-less-secure`.

Enable `std` if `global-context-less-secure` is enabled (because `Once`
only comes from `std`, not available in `core`).

Add `global-context-less-secure` to the features test array in
`contrib/test.sh`.

With this applied the build command above runs successfully.

Author: tcharding

Cargo.toml

@@ -27,7 +27,7 @@ rand-std = ["rand/std"]
 recovery = ["secp256k1-sys/recovery"]
 lowmemory = ["secp256k1-sys/lowmemory"]
 global-context = ["std", "rand-std", "global-context-less-secure"]
-global-context-less-secure = []
+global-context-less-secure = ["std"]
 
 [dependencies]
 secp256k1-sys = { version = "0.4.2", default-features = false, path = "./secp256k1-sys" }

contrib/test.sh

@@ -1,6 +1,6 @@
 #!/bin/sh -ex
 
-FEATURES="bitcoin_hashes global-context lowmemory rand rand-std recovery serde"
+FEATURES="bitcoin_hashes global-context-less-secure global-context lowmemory rand rand-std recovery serde"
 
 # Use toolchain if explicitly specified
 if [ -n "$TOOLCHAIN" ]

src/context.rs

@@ -35,6 +35,7 @@ pub mod global {
     impl Deref for GlobalContext {
         type Target = Secp256k1<All>;
 
+        #[allow(unused_mut)]    // Unused when "global-context" is not enabled.
         fn deref(&self) -> &Self::Target {
             static ONCE: Once = Once::new();
             static mut CONTEXT: Option<Secp256k1<All>> = None;