Add first-class support for binary crates

crates/cli-support/src/webidl/mod.rs

@@ -512,6 +512,10 @@ pub fn process(
         cx.program(program)?;
     }
 
+    if !cx.start_found {
+        cx.discover_main()?;
+    }
+
     if let Some(standard) = cx.module.customs.delete_typed::<ast::WebidlBindings>() {
         cx.standard(&standard)?;
     }
@@ -623,6 +627,63 @@ impl<'a> Context<'a> {
         Ok(())
     }
 
+    fn discover_main(&mut self) -> Result<(), Error> {
+        let main_info = self
+            .module
+            .functions()
+            .find(|x| x.name.as_ref().map_or(false, |x| x == "main"))
+            .map(|x| (x.ty(), x.id()));
+        if let Some((main_type, main_id)) = main_info {
+            use walrus::ValType::I32;
+            // an entry point `main` has signature (i32, i32) -> i32
+            let type_matches = {
+                let ty = self.module.types.get(main_type);
+                ty.params() == [I32, I32] && ty.results() == [I32]
+            };
+            if !type_matches {
+                return Ok(());
+            }
+            let mut wrapper = walrus::FunctionBuilder::new(&mut self.module.types, &[], &[]);
+            const NAME: &str = "__wasm_bindgen_autodiscoveredmain_wrapper";
+            wrapper
+                .func_body()
+                .i32_const(0)
+                .i32_const(0)
+                .call(main_id)
+                .drop()
+                .return_();
+            let wrapper = wrapper.finish(vec![], &mut self.module.funcs);
+            let export = self.module.exports.add(NAME, wrapper);
+            self.function_exports
+                .insert(NAME.to_string(), (export, wrapper));
+            let descriptor = Descriptor::Function(Box::new(Function {
+                arguments: vec![],
+                shim_idx: 0, // TODO is this bad
+                ret: Descriptor::Unit,
+            }));
+            self.descriptors.insert(NAME.to_string(), descriptor);
+
+            let export = decode::Export {
+                class: None,
+                comments: vec![],
+                consumed: false,
+                function: decode::Function {
+                    arg_names: vec![],
+                    name: NAME,
+                },
+                method_kind: decode::MethodKind::Operation(decode::Operation {
+                    is_static: true,
+                    kind: decode::OperationKind::Regular,
+                }),
+                start: true,
+            };
+
+            self.export(export)?;
+        }
+
+        Ok(())
+    }
+
     // Ensure that the `start` function for this module calls the
     // `__wbindgen_init_anyref_table` function. This'll ensure that all
     // instances of this module have the initial slots of the anyref table

crates/cli/tests/wasm-bindgen/main.rs

@@ -151,3 +151,49 @@ fn one_export_works() {
         .wasm_bindgen("");
     cmd.assert().success();
 }
+
+#[test]
+fn bin_crate_works() {
+    let (mut cmd, out_dir) = Project::new("bin_crate_works")
+        .file(
+            "src/main.rs",
+            r#"
+                use wasm_bindgen::prelude::*;
+                #[wasm_bindgen]
+                extern "C" {
+                    #[wasm_bindgen(js_namespace = console)]
+                    fn log(data: &str);
+                }
+
+                fn main() {
+                    log("hello, world");
+                }
+            "#,
+        )
+        .file(
+            "Cargo.toml",
+            &format!(
+                "
+                    [package]
+                    name = \"bin_crate_works\"
+                    authors = []
+                    version = \"1.0.0\"
+                    edition = '2018'
+
+                    [dependencies]
+                    wasm-bindgen = {{ path = '{}' }}
+
+                    [workspace]
+                ",
+                repo_root().display(),
+            ),
+        )
+        .wasm_bindgen("--target nodejs");
+    cmd.assert().success();
+    Command::new("node")
+        .arg("bin_crate_works.js")
+        .current_dir(out_dir)
+        .assert()
+        .success()
+        .stdout("hello, world\n");
+}