Merge pull request #902 from alexcrichton/target-docs

Add documentation about supported targets

guide/src/SUMMARY.md

@@ -37,6 +37,7 @@
   - [Working with Duck-Typed Interfaces](./reference/working-with-duck-typed-interfaces.md)
   - [Command Line Interface](./reference/cli.md)
   - [Optimizing for Size](./reference/optimize-size.md)
+  - [Supported Rust Targets](./reference/rust-targets.md)
   - [Supported Types](./reference/types.md)
     - [Imported JavaScript Types](./reference/types/imported-js-types.md)
     - [Exported Rust Types](./reference/types/exported-rust-types.md)

guide/src/reference/rust-targets.md

@@ -0,0 +1,72 @@
+# Supported Rust Targets
+
+> **Note**: This section is about Rust target triples, not targets like node/web
+> workers/browsers. More information on that coming soon!
+
+The `wasm-bindgen` project is designed to target the `wasm32-unknown-unknown`
+target in Rust. This target is a "bare bones" target for Rust which emits
+WebAssembly as output. The standard library is largely inert as modules like
+`std::fs` and `std::net` will simply return errors.
+
+## Non-wasm targets
+
+Note that `wasm-bindgen` also aims to compile on all targets. This means that it
+should be safe, if you like, to use `#[wasm_bindgen]` even when compiling for
+Windows (for example). For example:
+
+```rust
+#[wasm_bindgen]
+pub fn add(a: u32, b: u32) -> u32 {
+    a + b
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+fn main() {
+    println!("1 + 2 = {}", add(1, 2));
+}
+```
+
+This program will compile and work on all platforms, not just
+`wasm32-unknown-unknown`. Note that imported functions with `#[wasm_bindgen]`
+will unconditionally panic on non-wasm targets. For example:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(js_namespace = console)]
+    fn log(s: &str);
+}
+
+fn main() {
+    log("hello!");
+}
+```
+
+This program will unconditionally panic on all platforms other than
+`wasm32-unknown-unknown`.
+
+For better compile times, however, you likely want to only use `#[wasm_bindgen]`
+on the `wasm32-unknown-unknown` target. You can have a target-specific
+dependency like so:
+
+```toml
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+wasm-bindgen = "0.2"
+```
+
+And in your code you can use:
+
+```rust
+#[cfg(target_arch = "wasm32")]
+#[wasm_bindgen]
+pub fn only_on_the_wasm_target() {
+    // ...
+}
+```
+
+## Other Web Targets
+
+The `wasm-bindgen` target does not support the `wasm32-unknown-emscripten` nor
+the `asmjs-unknown-emscripten` targets. There are currently no plans to support
+these targets either. All annotations work like other platforms on the targets,
+retaining exported functions and causing all imports to panic.