Add #![no_std] support

.github/ci.rs

@@ -23,6 +23,11 @@ fn try_main() -> Result<()> {
         cwd.display()
     );
 
+    {
+        let _s = Section::new("BUILD_NO_DEFAULT_FEATURES");
+        shell("cargo test --all-features --workspace --no-run --no-default-features")?;
+    }
+
     {
         let _s = Section::new("BUILD");
         shell("cargo test --all-features --workspace --no-run")?;

Cargo.toml

@@ -8,10 +8,14 @@ authors = ["Aleksey Kladov <aleksey.kladov@gmail.com>"]
 edition = "2018"
 
 [dependencies]
-serde = { version = "1", optional = true, default_features = false, features = [ "std" ] }
+serde = { version = "1", optional = true, default_features = false }
 arbitrary = { version = "1", optional = true }
 
 [dev-dependencies]
 proptest = "0.10"
 serde_json = "1"
 serde =  { version = "1", features = [ "derive" ] }
+
+[features]
+default = ["std"]
+std = ["serde/std"]

src/lib.rs

@@ -1,11 +1,27 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(feature = "std"))]
+extern crate core as std;
+
+#[cfg(not(feature = "std"))]
+extern crate alloc;
+
 use std::{
     borrow::Borrow,
     cmp::{self, Ordering},
     fmt, hash, iter,
     ops::Deref,
+};
+
+#[cfg(not(feature = "std"))]
+use alloc::{
+    string::{String, ToString},
     sync::Arc,
 };
 
+#[cfg(feature = "std")]
+use std::sync::Arc;
+
 /// A `SmolStr` is a string type that has the following properties:
 ///
 /// * `size_of::<SmolStr>() == size_of::<String>()`