Implement #[wasm_bindgen(main)]

.github/workflows/main.yml

@@ -219,7 +219,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - run: rustup update --no-self-update 1.59.0 && rustup default 1.59.0
+    - run: rustup update --no-self-update 1.61.0 && rustup default 1.61.0
     - run: cargo test -p wasm-bindgen-macro
     - run: cargo test -p wasm-bindgen-test-macro
 

crates/macro-support/src/parser.rs

@@ -10,7 +10,7 @@ use proc_macro2::{Delimiter, Ident, Span, TokenStream, TokenTree};
 use quote::ToTokens;
 use syn::parse::{Parse, ParseStream, Result as SynResult};
 use syn::spanned::Spanned;
-use syn::Lit;
+use syn::{ItemFn, Lit, ReturnType};
 
 thread_local!(static ATTRS: AttributeParseState = Default::default());
 
@@ -80,6 +80,7 @@ macro_rules! attrgen {
             (variadic, Variadic(Span)),
             (typescript_custom_section, TypescriptCustomSection(Span)),
             (skip_typescript, SkipTypescript(Span)),
+            (main, Main(Span)),
             (start, Start(Span)),
             (skip, Skip(Span)),
             (typescript_type, TypeScriptType(Span, String, Span)),
@@ -933,6 +934,13 @@ impl<'a> MacroParse<(Option<BindgenAttrs>, &'a mut TokenStream)> for syn::Item {
     ) -> Result<(), Diagnostic> {
         match self {
             syn::Item::Fn(mut f) => {
+                let opts = opts.unwrap_or_default();
+
+                if opts.main().is_some() {
+                    opts.check_used();
+                    return main(f, tokens);
+                }
+
                 let no_mangle = f
                     .attrs
                     .iter()
@@ -951,7 +959,6 @@ impl<'a> MacroParse<(Option<BindgenAttrs>, &'a mut TokenStream)> for syn::Item {
                 // `dead_code` warning. So, add `#[allow(dead_code)]` before it to avoid that.
                 tokens.extend(quote::quote! { #[allow(dead_code)] });
                 f.to_tokens(tokens);
-                let opts = opts.unwrap_or_default();
                 if opts.start().is_some() {
                     if f.sig.generics.params.len() > 0 {
                         bail_span!(&f.sig.generics, "the start function cannot have generics",);
@@ -1689,6 +1696,59 @@ pub fn link_to(opts: BindgenAttrs) -> Result<ast::LinkToModule, Diagnostic> {
     Ok(ast::LinkToModule(program))
 }
 
+fn main(mut f: ItemFn, tokens: &mut TokenStream) -> Result<(), Diagnostic> {
+    if f.sig.ident != "main" {
+        bail_span!(&f.sig.ident, "the main function has to be called main");
+    }
+    if let Some(constness) = f.sig.constness {
+        bail_span!(&constness, "the main function cannot be const");
+    }
+    if !f.sig.generics.params.is_empty() {
+        bail_span!(&f.sig.generics, "the main function cannot have generics");
+    }
+    if !f.sig.inputs.is_empty() {
+        bail_span!(&f.sig.inputs, "the main function cannot have arguments");
+    }
+
+    let r#return = f.sig.output;
+    f.sig.output = ReturnType::Default;
+    let body = f.block;
+
+    if f.sig.asyncness.take().is_some() {
+        f.block = Box::new(
+            syn::parse2(quote::quote! {
+                {
+                    async fn __wasm_bindgen_generated_main() #r#return #body
+                    wasm_bindgen_futures::spawn_local(
+                        async move {
+                            use wasm_bindgen::__rt::Main;
+                            let __ret = __wasm_bindgen_generated_main();
+                            (&mut &mut &mut wasm_bindgen::__rt::MainWrapper(Some(__ret.await))).__wasm_bindgen_main()
+                        },
+                    )
+                }
+            })
+            .unwrap(),
+        );
+    } else {
+        f.block = Box::new(
+            syn::parse2(quote::quote! {
+                {
+                    fn __wasm_bindgen_generated_main() #r#return #body
+                    use wasm_bindgen::__rt::Main;
+                    let __ret = __wasm_bindgen_generated_main();
+                    (&mut &mut &mut wasm_bindgen::__rt::MainWrapper(Some(__ret))).__wasm_bindgen_main()
+                }
+            })
+            .unwrap(),
+        );
+    }
+
+    f.to_tokens(tokens);
+
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     #[test]

crates/macro/ui-tests/main-async.rs

@@ -0,0 +1,7 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+async fn main() {}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-async.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+ --> ui-tests/main-async.rs:7:4
+  |
+7 | fn fail() {}
+  |    ^^^^

crates/macro/ui-tests/main-debug.rs

@@ -0,0 +1,18 @@
+use std::fmt;
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() -> Result<(), Test> {
+    unimplemented!()
+}
+
+struct Test;
+
+impl fmt::Debug for Test {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-debug.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+  --> ui-tests/main-debug.rs:18:4
+   |
+18 | fn fail() {}
+   |    ^^^^

crates/macro/ui-tests/main-infallible.rs

@@ -0,0 +1,10 @@
+use std::convert::Infallible;
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() -> Infallible {
+    unimplemented!()
+}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-infallible.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+  --> ui-tests/main-infallible.rs:10:4
+   |
+10 | fn fail() {}
+   |    ^^^^

crates/macro/ui-tests/main-jsvalue.rs

@@ -0,0 +1,9 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() -> Result<(), JsValue> {
+    unimplemented!()
+}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-jsvalue.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+ --> ui-tests/main-jsvalue.rs:9:4
+  |
+9 | fn fail() {}
+  |    ^^^^

crates/macro/ui-tests/main-termination.rs

@@ -0,0 +1,18 @@
+use std::process;
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() -> Test {
+    unimplemented!()
+}
+
+struct Test;
+
+impl process::Termination for Test {
+    fn report(self) -> process::ExitCode {
+        unimplemented!()
+    }
+}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-termination.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+  --> ui-tests/main-termination.rs:18:4
+   |
+18 | fn fail() {}
+   |    ^^^^

crates/macro/ui-tests/main-unit.rs

@@ -0,0 +1,7 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() -> () {}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main-unit.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+ --> ui-tests/main-unit.rs:7:4
+  |
+7 | fn fail() {}
+  |    ^^^^

crates/macro/ui-tests/main.rs

@@ -0,0 +1,7 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(main)]
+fn main() {}
+
+#[wasm_bindgen(main)]
+fn fail() {}

crates/macro/ui-tests/main.stderr

@@ -0,0 +1,5 @@
+error: the main function has to be called main
+ --> ui-tests/main.rs:7:4
+  |
+7 | fn fail() {}
+  |    ^^^^

guide/src/reference/attributes/on-rust-exports/main.md

@@ -0,0 +1,37 @@
+# `main`
+
+When attached to the `fn main()` function this attribute will adjust it to
+properly throw errors if they can be. This is only intended to be used for
+binaries.
+
+```rust
+#[wasm_bindgen(main)]
+fn main() -> Result<(), JsValue> {
+    Err(JsValue::from("this error message will be thrown"))
+}
+```
+
+The attribute also allows using `async fn main()` in Cargo binaries.
+
+```rust
+#[wasm_bindgen(main)]
+async fn main() {
+    // ...
+    future.await;
+}
+```
+
+Unlike `#[wasm_bindgen(start)]` this will not export a function to be executed
+on startup, it should only be used in Cargo binaries or examples for the `main`
+function. `#[wasm_bindgen(start)]` will prevent the `main` function to start and
+should not be used in conjunction.
+
+Any return value that is supported by Rust is supported here, see
+[`Termination`]. In order, wasm-bindgen will first detect a
+`Result<(), impl Into<JsValue>>` and will throw proper `JsValue`s,
+`Result<(), impl Debug>` will convert an error to a string and throw that.
+Lastly anything implementing [`Termination`] will throw it's reported
+[`ExitCode`](https://doc.rust-lang.org/std/process/struct.ExitCode.html) by
+using it's `Debug` representation.
+
+[`termination`]: https://doc.rust-lang.org/std/process/trait.Termination.html

src/lib.rs

@@ -1374,6 +1374,7 @@ pub mod __rt {
     use crate::JsValue;
     use core::borrow::{Borrow, BorrowMut};
     use core::cell::{Cell, UnsafeCell};
+    use core::convert::Infallible;
     use core::ops::{Deref, DerefMut};
 
     pub extern crate core;
@@ -1728,6 +1729,49 @@ pub mod __rt {
             }
         }
     }
+
+    /// An internal helper struct for usage in `#[wasm_bindgen(main)]`
+    /// functions to throw the error (if it is `Err`).
+    pub struct MainWrapper<T>(pub Option<T>);
+
+    pub trait Main {
+        fn __wasm_bindgen_main(&mut self);
+    }
+
+    impl Main for &mut &mut MainWrapper<()> {
+        #[inline]
+        fn __wasm_bindgen_main(&mut self) {}
+    }
+
+    impl Main for &mut &mut MainWrapper<Infallible> {
+        #[inline]
+        fn __wasm_bindgen_main(&mut self) {}
+    }
+
+    impl<E: Into<JsValue>> Main for &mut &mut MainWrapper<Result<(), E>> {
+        #[inline]
+        fn __wasm_bindgen_main(&mut self) {
+            if let Err(e) = self.0.take().unwrap() {
+                crate::throw_val(e.into());
+            }
+        }
+    }
+
+    impl<E: std::fmt::Debug> Main for &mut MainWrapper<Result<(), E>> {
+        #[inline]
+        fn __wasm_bindgen_main(&mut self) {
+            if let Err(e) = self.0.take().unwrap() {
+                crate::throw_val(std::format!("{:?}", e).into());
+            }
+        }
+    }
+
+    impl<T: std::process::Termination> Main for MainWrapper<T> {
+        #[inline]
+        fn __wasm_bindgen_main(&mut self) {
+            crate::throw_val(std::format!("{:?}", self.0.take().unwrap().report()).into());
+        }
+    }
 }
 
 /// A wrapper type around slices and vectors for binding the `Uint8ClampedArray`