Pass --no-entry to emcc when linking emscripten cdylib/dylib

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1233,6 +1233,9 @@ impl<'a> Linker for EmLinker<'a> {
             }
             LinkOutputKind::DynamicDylib | LinkOutputKind::StaticDylib => {
                 self.cmd.arg("-sSIDE_MODULE=2");
+                // Shared libraries have no `main`, so emcc must not link in the
+                // standalone-wasm entry shim that expects one. Mirrors `WasmLd`.
+                self.link_arg("--no-entry");
             }
             // -fno-pie is the default on Emscripten.
             LinkOutputKind::StaticNoPicExe | LinkOutputKind::StaticPicExe => {}

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

@@ -0,0 +1,3 @@
+fn main() {
+    println!("{}", 42);
+}

tests/run-make/wasm-emscripten-cdylib/rmake.rs

@@ -1,11 +1,17 @@
 //! Check that cdylib crate type is supported for the wasm32-unknown-emscripten
 //! target and produces a valid Emscripten dynamic library.
+//!
+//! A cdylib has no `main`, so rustc must link it as entryless (`--no-entry`),
+//! otherwise emcc pulls in the standalone-wasm entry shim and `wasm-ld` errors
+//! with `undefined symbol: main`. Executables must keep their entry, so this
+//! also checks that a `bin` crate with `main` still links.
 
 //@ only-wasm32-unknown-emscripten
 
 use run_make_support::{bare_rustc, rfs, wasmparser};
 
 fn main() {
+    // A cdylib must link without `--no-entry` being supplied by the user.
     bare_rustc().input("foo.rs").target("wasm32-unknown-emscripten").crate_type("cdylib").run();
 
     // Verify the output is a valid wasm file with a dylink.0 section
@@ -22,4 +28,8 @@ fn main() {
     }
 
     assert!(has_dylink, "expected dylink.0 section in emscripten cdylib output");
+
+    // An executable has a `main`, so `--no-entry` must NOT be applied: it must
+    // still link with its entry preserved.
+    bare_rustc().input("main.rs").target("wasm32-unknown-emscripten").crate_type("bin").run();
 }