Use getrandom directly instead of rand

This works around rust-lang/cargo#5730
for no_std crates that try to use rand.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "const-random"
-version = "0.1.6"
+version = "0.1.7"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/tkaitchuck/constrandom"
 documentation = "https://docs.rs/const-random"
@@ -11,5 +11,5 @@ readme = "README.md"
 edition = "2018"
 
 [dependencies]
-const-random-macro = { path = "macro", version = "0.1.6"}
+const-random-macro = { path = "macro", version = "0.1.7"}
 proc-macro-hack = { version = "0.5" }

macro/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "const-random-macro"
-version = "0.1.6"
+version = "0.1.7"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/tkaitchuck/constrandom"
 documentation = "https://docs.rs/const-random"
@@ -14,4 +14,4 @@ proc-macro = true
 
 [dependencies]
 proc-macro-hack = { version = "0.5" }
-rand = { version = "0.7", default-features = false, features = ["getrandom"] }
+getrandom = "0.1"

macro/src/lib.rs

@@ -1,24 +1,33 @@
 extern crate proc_macro;
 
+use getrandom;
 use proc_macro::TokenStream;
 use proc_macro_hack::proc_macro_hack;
-use rand::rngs::OsRng;
-use rand::Rng;
+use std::mem;
+
+// Ideally we would use the proper interface for this through the rand crate,
+// but due to https://github.com/rust-lang/cargo/issues/5730 this leads to
+// issues for no_std crates that try to use rand themselves. So instead we skip
+// rand and generate random bytes straight from the OS.
+fn gen_random<T>() -> T {
+    let mut out = [0u8; 16];
+    getrandom::getrandom(&mut out).unwrap();
+    unsafe { mem::transmute_copy(&out) }
+}
 
 #[proc_macro_hack]
 pub fn const_random(input: TokenStream) -> TokenStream {
     match &input.to_string()[..] {
-        "u8" => format!("0x{:x}", OsRng.gen::<u8>()).parse().unwrap(),
-        "u16" => format!("0x{:x}", OsRng.gen::<u16>()).parse().unwrap(),
-        "u32" => format!("0x{:x}", OsRng.gen::<u32>()).parse().unwrap(),
-        "u64" => format!("0x{:x}", OsRng.gen::<u64>()).parse().unwrap(),
-        "u128" => format!("0x{:x}", OsRng.gen::<u128>()).parse().unwrap(),
-        "i8" => format!("0x{:x}", OsRng.gen::<i8>()).parse().unwrap(),
-        "i16" => format!("0x{:x}", OsRng.gen::<i16>()).parse().unwrap(),
-        "i32" => format!("0x{:x}", OsRng.gen::<i32>()).parse().unwrap(),
-        "i64" => format!("0x{:x}", OsRng.gen::<i64>()).parse().unwrap(),
-        "i128" => format!("0x{:x}", OsRng.gen::<i128>()).parse().unwrap(),
+        "u8" => format!("0x{:x}", gen_random::<u8>()).parse().unwrap(),
+        "u16" => format!("0x{:x}", gen_random::<u16>()).parse().unwrap(),
+        "u32" => format!("0x{:x}", gen_random::<u32>()).parse().unwrap(),
+        "u64" => format!("0x{:x}", gen_random::<u64>()).parse().unwrap(),
+        "u128" => format!("0x{:x}", gen_random::<u128>()).parse().unwrap(),
+        "i8" => format!("0x{:x}", gen_random::<i8>()).parse().unwrap(),
+        "i16" => format!("0x{:x}", gen_random::<i16>()).parse().unwrap(),
+        "i32" => format!("0x{:x}", gen_random::<i32>()).parse().unwrap(),
+        "i64" => format!("0x{:x}", gen_random::<i64>()).parse().unwrap(),
+        "i128" => format!("0x{:x}", gen_random::<i128>()).parse().unwrap(),
         _ => panic!("Invalid integer type"),
     }
-
 }