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

This commit updates how the linker is invoked on WebAssembly targets
(all of them) to avoid passing the `--allow-undefined` flag to the
linker. Historically, if I remember this correctly, when `wasm-ld` was
first integrated this was practically required because at the time it
was otherwise impossible to import a function from the host into a wasm
binary. Or, at least, I'm pretty sure that was why this was added.

At the time, as the documentation around this option indicates, it was
known that this was going to be a hazard. This doesn't match behavior on
native, for example, and can easily paper over what should be a linker
error with some sort of other obscure runtime error. An example is that
this program currently compiles and links, it just prints null:

    unsafe extern "C" {
        static nonexistent: u8;
    }

    fn main() {
        println!("{:?}", &raw const nonexistent);
    }

This can easily lead to mistakes like rust-lang/libc/4880 and defer what
should be a compile-time link error to weird or unusual behavior at link
time. Additionally, in the intervening time since `wasm-ld` was first
introduced here, lots has changed and notably this program works as
expected:

    #[link(wasm_import_module = "host")]
    unsafe extern "C" {
        fn foo();
    }

    fn main() {
        unsafe {
            foo();
        }
    }

Notably this continues to compile without error and the final wasm
binary indeed has an imported function from the host. What this change
means, however, is that this program:

    unsafe extern "C" {
        fn foo();
    }

    fn main() {
        unsafe {
            foo();
        }
    }

this currently compiles successfully and emits an import from the `env`
module. After this change, however, this will fail to compile with a
link error stating that the `foo` symbol is not defined.

Author: alexcrichton

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