add no_std feature and use hashmap_core only on no_std

Cargo.toml

@@ -14,11 +14,14 @@ exclude = [ "/res/*", "/tests/*", "/fuzz/*", "/benches/*" ]
 default = ["std"]
 # Disable for no_std support
 std = ["parity-wasm/std"]
+# Enable for no_std support
+# hashmap_core only works on no_std
+no_std = ["hashmap_core"]
 
 [dependencies]
 parity-wasm = { version = "0.31", default-features = false }
 byteorder = "1.0"
-hashmap_core = "0.1.9"
+hashmap_core = { version = "0.1.9", optional = true }
 memory_units = "0.3.0"
 nan-preserving-float = "0.1.0"
 

README.md

@@ -29,11 +29,14 @@ cargo test
 
 # `no_std` support
 This crate supports `no_std` environments.
-If the `std` feature is disabled, `no_std` is supported.
-Use like that:
+Enable the `no_std` feature and disable default features:
 ```toml
 [dependencies]
-parity-wasm = { version = "0.31", default-features = false }
+parity-wasm = {
+	version = "0.31",
+	default-features = false,
+	features = "no_std"
+}
 ```
 
 `no_std` requires the `core` and `alloc` libraries and a nightly compiler.

src/imports.rs

@@ -1,6 +1,11 @@
 #[allow(unused_imports)]
 use alloc::prelude::*;
+
+#[cfg(feature = "std")]
+use std::collections::HashMap;
+#[cfg(not(feature = "std"))]
 use hashmap_core::HashMap;
+
 use global::GlobalRef;
 use memory::MemoryRef;
 use func::FuncRef;

src/lib.rs

@@ -119,6 +119,7 @@ extern crate assert_matches;
 
 extern crate parity_wasm;
 extern crate byteorder;
+#[cfg(not(feature = "std"))]
 extern crate hashmap_core;
 extern crate memory_units as memory_units_crate;
 

src/module.rs

@@ -5,7 +5,12 @@ use runner::check_function_args;
 use Trap;
 use core::cell::RefCell;
 use core::fmt;
+
+#[cfg(feature = "std")]
+use std::collections::HashMap;
+#[cfg(not(feature = "std"))]
 use hashmap_core::HashMap;
+
 use parity_wasm::elements::{External, InitExpr, Internal, Instruction, ResizableLimits, Type};
 use {Module, Error, Signature, MemoryInstance, RuntimeValue, TableInstance};
 use imports::ImportResolver;

src/validation/mod.rs

@@ -3,7 +3,12 @@ use alloc::prelude::*;
 #[cfg(feature = "std")]
 use std::error;
 use core::fmt;
+
+#[cfg(feature = "std")]
+use std::collections::HashSet;
+#[cfg(not(feature = "std"))]
 use hashmap_core::HashSet;
+
 use parity_wasm::elements::{
 	BlockType, External, GlobalEntry, GlobalType, Internal, MemoryType, Module, Instruction,
 	ResizableLimits, TableType, ValueType, InitExpr, Type,