Introduce #[wasm_bindgen(main)]

crates/macro-support/src/parser.rs

@@ -10,7 +10,7 @@ use proc_macro2::{Ident, Span, TokenStream, TokenTree};
 use quote::ToTokens;
 use syn::parse::{Parse, ParseStream, Result as SynResult};
 use syn::spanned::Spanned;
-use syn::{Lit, MacroDelimiter};
+use syn::{ItemFn, Lit, MacroDelimiter, ReturnType};
 
 use crate::ClassMarker;
 
@@ -83,6 +83,7 @@ macro_rules! attrgen {
             (typescript_custom_section, TypescriptCustomSection(Span)),
             (skip_typescript, SkipTypescript(Span)),
             (skip_jsdoc, SkipJsDoc(Span)),
+            (main, Main(Span)),
             (start, Start(Span)),
             (wasm_bindgen, WasmBindgen(Span, syn::Path)),
             (wasm_bindgen_futures, WasmBindgenFutures(Span, syn::Path)),
@@ -949,6 +950,19 @@ 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 let Some(path) = opts.wasm_bindgen() {
+                    program.wasm_bindgen = path.clone();
+                }
+                if let Some(path) = opts.wasm_bindgen_futures() {
+                    program.wasm_bindgen_futures = path.clone();
+                }
+
+                if opts.main().is_some() {
+                    opts.check_used();
+                    return main(program, f, tokens);
+                }
+
                 let no_mangle = f
                     .attrs
                     .iter()
@@ -966,13 +980,6 @@ 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 let Some(path) = opts.wasm_bindgen() {
-                    program.wasm_bindgen = path.clone();
-                }
-                if let Some(path) = opts.wasm_bindgen_futures() {
-                    program.wasm_bindgen_futures = path.clone();
-                }
                 if opts.start().is_some() {
                     if f.sig.generics.params.len() > 0 {
                         bail_span!(&f.sig.generics, "the start function cannot have generics",);
@@ -1740,6 +1747,62 @@ pub fn link_to(opts: BindgenAttrs) -> Result<ast::LinkToModule, Diagnostic> {
     Ok(ast::LinkToModule(program))
 }
 
+fn main(program: &ast::Program, 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;
+
+    let wasm_bindgen = &program.wasm_bindgen;
+    let wasm_bindgen_futures = &program.wasm_bindgen_futures;
+
+    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-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/SUMMARY.md

@@ -85,6 +85,7 @@
       - [`skip`](./reference/attributes/on-rust-exports/skip.md)
       - [`skip_jsdoc`](./reference/attributes/on-rust-exports/skip_jsdoc.md)
       - [`start`](./reference/attributes/on-rust-exports/start.md)
+      - [`main`](./reference/attributes/on-rust-exports/main.md)
       - [`typescript_custom_section`](./reference/attributes/on-rust-exports/typescript_custom_section.md)
       - [`getter` and `setter`](./reference/attributes/on-rust-exports/getter-and-setter.md)
       - [`inspectable`](./reference/attributes/on-rust-exports/inspectable.md)

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

@@ -0,0 +1,33 @@
+# `main`
+
+When attached to the `main` function this attribute will adjust it to properly
+throw errors if they can be.
+
+```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;
+}
+```
+
+This attribute is only intended to be used on the `main` function of binaries or
+examples. Unlike `#[wasm_bindgen(start)]`, it will not cause an arbitrary
+function to be executed on start in a library.
+
+The return type support is modeled after [`Termination`]. `()` and `Infallible`
+are supported, but [`Termination`] itself is not. 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.
+
+[`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 +1729,42 @@ 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_str(&std::format!("{:?}", e));
+            }
+        }
+    }
 }
 
 /// A wrapper type around slices and vectors for binding the `Uint8ClampedArray`