Emit the emscripten entry point as __main_argc_argv

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1286,9 +1286,24 @@ impl<'a> Linker for EmLinker<'a> {
 
         self.cc_arg("-s");
 
+        // Emscripten exposes the program entry point under the JS name `_main`
+        // regardless of the underlying wasm symbol (which is `__main_argc_argv`
+        // to match Clang's C ABI), bridging the two internally. So the entry
+        // symbol must be requested as `_main` here rather than as a `_`-prefixed
+        // form of its wasm name.
+        let entry_name = self.sess.target.entry_name.as_ref();
         let mut arg = OsString::from("EXPORTED_FUNCTIONS=");
         let encoded = serde_json::to_string(
-            &symbols.iter().map(|sym| "_".to_owned() + &sym.name).collect::<Vec<_>>(),
+            &symbols
+                .iter()
+                .map(|sym| {
+                    if sym.name == entry_name {
+                        "_main".to_owned()
+                    } else {
+                        "_".to_owned() + &sym.name
+                    }
+                })
+                .collect::<Vec<_>>(),
         )
         .unwrap();
         debug!("{encoded}");

compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs

@@ -29,6 +29,13 @@ pub(crate) fn target() -> Target {
         crt_static_default: true,
         crt_static_allows_dylibs: true,
         main_needs_argc_argv: true,
+        // Emit the entry point under the wasm C-ABI name that Clang uses
+        // (`__main_argc_argv` for the argc/argv form) rather than a raw `main`.
+        // This is what emscripten's crt/libc references, and is required for
+        // entry paths such as `-sPROXY_TO_PTHREAD` whose `crt1_proxy_main`
+        // links against `__main_argc_argv`. A raw `main` only resolves on the
+        // JS-driven default entry path and fails to link on the proxied one.
+        entry_name: "__main_argc_argv".into(),
         panic_strategy: PanicStrategy::Unwind,
         no_default_libraries: false,
         families: cvs!["unix", "wasm"],

tests/run-make/wasm-emscripten-main-symbol/main.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/wasm-emscripten-main-symbol/rmake.rs

@@ -0,0 +1,45 @@
+//@ only-wasm32-unknown-emscripten
+
+// On wasm the age-old C trick of a `main` that is either `int main(void)` or
+// `int main(int, char**)` doesn't work, because wasm requires caller and callee
+// signatures to match. The platform ABI (as used by Clang) therefore mangles
+// the entry point by signature; the argc/argv form is emitted as
+// `__main_argc_argv`. Rust's emscripten target uses the argc/argv form, so its
+// entry point must be emitted under that name rather than as a raw `main`.
+//
+// This matters beyond cosmetics: emscripten's own crt references
+// `__main_argc_argv` directly on some entry paths (notably `crt1_proxy_main.o`,
+// used by `-sPROXY_TO_PTHREAD`), which fail to link against a raw `main`.
+//
+// The JS-visible name of the entry point stays `_main`; emscripten bridges that
+// to the wasm `__main_argc_argv` export internally.
+
+use run_make_support::{rfs, rustc, wasmparser};
+use wasmparser::ExternalKind;
+
+fn main() {
+    rustc().input("main.rs").target("wasm32-unknown-emscripten").output("main.js").run();
+
+    let file = rfs::read("main.wasm");
+    let mut entry = None;
+    let mut has_plain_main = false;
+    for payload in wasmparser::Parser::new(0).parse_all(&file) {
+        if let wasmparser::Payload::ExportSection(s) = payload.unwrap() {
+            for export in s {
+                let export = export.unwrap();
+                match export.name {
+                    "__main_argc_argv" => entry = Some(export.kind),
+                    "main" => has_plain_main = true,
+                    _ => {}
+                }
+            }
+        }
+    }
+
+    assert_eq!(
+        entry,
+        Some(ExternalKind::Func),
+        "the emscripten entry point must be exported as the wasm C-ABI symbol `__main_argc_argv`",
+    );
+    assert!(!has_plain_main, "a raw `main` symbol must not be exported on wasm");
+}