add dummy and wasm_dummy features

Cargo.toml

@@ -30,3 +30,7 @@ stdweb = { version = "0.4.18", optional = true }
 
 [features]
 std = []
+# enables dummy implementation for wasm32-unknown-unknown
+wasm_dummy = []
+# enables dummy implementation for unsupported targets
+dummy = []

README.md

@@ -45,12 +45,17 @@ This library is `no_std` compatible, but uses `std` on most platforms.
 The `log` library is supported as an optional dependency. If enabled, error
 reporting will be improved on some platforms.
 
-For WebAssembly (`wasm32`), WASI and Emscripten targets are supported directly; otherwise
-one of the following features must be enabled:
+For `wasm32-unknown-unknown` target one of the following features must be
+enabled:
 
 -   [`wasm-bindgen`](https://crates.io/crates/wasm_bindgen)
 -   [`stdweb`](https://crates.io/crates/stdweb)
 
+By default compiling crate for an unsupported platform will result in a
+compilation error. You may either replace this crate with a custom one, which
+will include support for your target, or enable `dummy` feature to use an
+always erroring dummy implementation.
+
 ## Minimum Supported Rust Version
 
 This crate requires Rust 1.32.0 or later.

src/lib.rs

@@ -36,7 +36,7 @@
 //! systems are using the recommended interface and respect maximum buffer
 //! sizes.
 //!
-//! ## Support for WebAssembly and ams.js
+//! ## Support for WebAssembly and asm.js
 //!
 //! The three Emscripten targets `asmjs-unknown-emscripten`,
 //! `wasm32-unknown-emscripten` and `wasm32-experimental-emscripten` use
@@ -46,7 +46,9 @@
 //! methods directly, using either `stdweb` or `wasm-bindgen` depending on what
 //! features are activated for this crate. Note that if both features are
 //! enabled `wasm-bindgen` will be used. If neither feature is enabled,
-//! `getrandom` will always fail.
+//! compiling `getrandom` will result in a compilation error. It can be disabled
+//! by enabling `wasm_dummy` feature, with which `getrandom` will use an always
+//! erroring dummy implementation.
 //!
 //! The WASI target `wasm32-wasi` uses the `__wasi_random_get` function defined
 //! by the WASI standard.
@@ -231,12 +233,24 @@ cfg_if! {
                 #[path = "wasm32_bindgen.rs"] mod imp;
             } else if #[cfg(feature = "stdweb")] {
                 #[path = "wasm32_stdweb.rs"] mod imp;
-            } else {
+            } else if #[cfg(any(feature = "wasm_dummy", feature = "dummy"))] {
                 #[path = "dummy.rs"] mod imp;
+            } else {
+                compile_error!("\
+                    wasm32-unknown-unknown target requires one of the following
+                    features to be enabled: `stdweb`, `wasm-bindgen` or
+                    `dummy`/`wasm_dummy`\
+                ");
             }
         }
-    } else {
+    } else if #[cfg(feature = "dummy")] {
         #[path = "dummy.rs"] mod imp;
+    } else {
+        compile_error!("\
+            target is not supported, you may enable dummy implementation \
+            using `dummy` feature or replace `getrandom` crate with a custom \
+            crate which supports your target\
+        ");
     }
 }