rustc: Stop passing --allow-undefined on wasm targets

compiler/rustc_target/src/spec/base/wasm.rs

@@ -28,16 +28,6 @@ pub(crate) fn options() -> TargetOptions {
                 // stack overflow will be guaranteed to trap as it underflows instead of
                 // corrupting static data.
                 concat!($prefix, "--stack-first"),
-                // FIXME we probably shouldn't pass this but instead pass an explicit list
-                // of symbols we'll allow to be undefined. We don't currently have a
-                // mechanism of knowing, however, which symbols are intended to be imported
-                // from the environment and which are intended to be imported from other
-                // objects linked elsewhere. This is a coarse approximation but is sure to
-                // hide some bugs and frustrate someone at some point, so we should ideally
-                // work towards a world where we can explicitly list symbols that are
-                // supposed to be imported and have all other symbols generate errors if
-                // they remain undefined.
-                concat!($prefix, "--allow-undefined"),
                 // LLD only implements C++-like demangling, which doesn't match our own
                 // mangling scheme. Tell LLD to not demangle anything and leave it up to
                 // us to demangle these symbols later. Currently rustc does not perform

tests/run-make/wasm-stringify-ints-small/foo.rs

@@ -1,12 +1,14 @@
 #![crate_type = "cdylib"]
 
+#[link(wasm_import_module = "the-world")]
 extern "C" {
     fn observe(ptr: *const u8, len: usize);
 }
 
 macro_rules! s {
     ( $( $f:ident -> $t:ty );* $(;)* ) => {
         $(
+            #[link(wasm_import_module = "the-world")]
             extern "C" {
                 fn $f() -> $t;
             }

tests/run-make/wasm-unexpected-features/foo.rs

@@ -14,6 +14,11 @@ unsafe extern "Rust" {
     fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
 }
 
+#[rustc_std_internal_symbol]
+pub unsafe fn __rdl_alloc_error_handler(_size: usize, _align: usize) -> ! {
+    loop {}
+}
+
 #[used]
 static mut BUF: [u8; 1024] = [0; 1024];
 

tests/ui/linking/executable-no-mangle-strip.rs

@@ -1,5 +1,6 @@
 //@ run-pass
 //@ ignore-windows-gnu: only statics marked with used can be GC-ed on windows-gnu
+//@ ignore-wasm: wasm, for better or worse, exports all #[no_mangle]
 
 // Regression test for <https://github.com/rust-lang/rust/issues/139744>.
 // Functions in the binary marked with no_mangle should be GC-ed if they