Add assume_wasm_js feature

Cargo.toml

@@ -23,6 +23,10 @@ std = []
 # i.e. avoid unconditionally enabling it in library crates.
 wasm_js = ["dep:wasm-bindgen", "dep:js-sys"]
 
+# This flag enables the wasm_js backend and uses it by default on wasm32 where
+# the target_os is unknown. The getrandom_backend cfg may override this.
+assume_wasm_js = ["wasm_js"]
+
 [dependencies]
 cfg-if = "1"
 

README.md

@@ -130,22 +130,26 @@ the `wasm32-unknown-unknown` target (i.e. the target used by `wasm-pack`)
 is not automatically supported since, from the target name alone, we cannot deduce
 which JavaScript interface should be used (or if JavaScript is available at all).
 
+We do not include support for this target in the default configuration because
+our JS backend (supporting web browsers, web workers and Node.js v19 or later)
+requires [`wasm-bindgen`], **bloating `Cargo.lock`** and
+**potentially breaking builds** on non-web WASM platforms.
+
 To enable `getrandom`'s functionality on `wasm32-unknown-unknown` using the Web
 Crypto methods [described above][opt-in] via [`wasm-bindgen`], do
-*both* of the following:
-
--   Use the `wasm_js` feature flag, i.e.
-    `getrandom = { version = "0.3", features = ["wasm_js"] }`.
-    On its own, this only makes the backend available. (As a side effect this
-    will make your `Cargo.lock` significantly larger if you are not already
-    using [`wasm-bindgen`], but otherwise enabling this feature is harmless.)
--   Set `RUSTFLAGS='--cfg getrandom_backend="wasm_js"'` ([see above][opt-in]).
-
-This backend supports both web browsers (main window and Web Workers)
-and Node.js (v19 or later) environments.
-
-WARNING: It is highly recommended to enable the `wasm_js` feature only for
-binary crates and tests, i.e. avoid unconditionally enabling it in library crates.
+one of the following:
+
+-   Use the `wasm_js` feature flag to enable the backend, **and**
+    set `RUSTFLAGS='--cfg getrandom_backend="wasm_js"'` to select this backend
+    ([see above][opt-in]).
+-   Use the `assume_wasm_js` feature flag:
+    `getrandom = { version = "0.4", features = ["assume_wasm_js"] }`.
+    This enables `wasm_js` and uses the backend by default on `wasm32` with
+    unknown OS.
+
+WARNING: enabling the `wasm_js` or `assume_wasm_js` feature will bloat
+`Cargo.lock` on all platforms (where [`wasm-bindgen`] is not an existing
+dependency) and may cause build issues on non-web WASM platforms.
 
 ### Custom backend
 

src/backends.rs

@@ -28,7 +28,7 @@ cfg_if! {
     } else if #[cfg(getrandom_backend = "efi_rng")] {
         mod efi_rng;
         pub use efi_rng::*;
-    } else if #[cfg(all(getrandom_backend = "wasm_js"))] {
+    } else if #[cfg(getrandom_backend = "wasm_js")] {
         cfg_if! {
             if #[cfg(feature = "wasm_js")] {
                 mod wasm_js;
@@ -183,13 +183,20 @@ cfg_if! {
         mod rdrand;
         pub use rdrand::*;
     } else if #[cfg(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")))] {
-        compile_error!(concat!(
-            "The wasm32-unknown-unknown targets are not supported by default; \
-            you may need to enable the \"wasm_js\" configuration flag. Note \
-            that enabling the `wasm_js` feature flag alone is insufficient. \
-            For more information see: \
-            https://docs.rs/getrandom/", env!("CARGO_PKG_VERSION"), "/#webassembly-support"
-        ));
+        cfg_if! {
+            if #[cfg(feature = "assume_wasm_js")] {
+                mod wasm_js;
+                pub use wasm_js::*;
+            } else {
+                compile_error!(concat!(
+                    "The wasm32-unknown-unknown targets are not supported by default; \
+                    you may need to enable the \"wasm_js\" configuration flag. Note \
+                    that enabling the `wasm_js` feature flag alone is insufficient. \
+                    For more information see: \
+                    https://docs.rs/getrandom/", env!("CARGO_PKG_VERSION"), "/#webassembly-support"
+                ));
+            }
+        }
     } else {
         compile_error!(concat!(
             "target is not supported. You may need to define a custom backend see: \