Rollup merge of #121533 - ratmice:wasm_init_fini_array, r=nnethercote

Handle .init_array link_section specially on wasm

Given that wasm-ld now has support for [.init_array](https://github.com/llvm/llvm-project/blob/8f2bd8ae68883592a333f4bdbed9798d66e68630/llvm/lib/MC/WasmObjectWriter.cpp#L1852), it appears we can easily implement that section by falling through to the normal path rather than taking the typical custom_section path for wasm.

The wasm-ld appears to have a bunch of limitations. Only one static with the `link_section` in a crate or else you hit the fatal error in the link above "only one .init_array section fragment supported". They do not get merged.

You can still call multiple constructors by setting it to an array.

```
unsafe extern "C" fn ctor() {
    println!("foo");
}
#[used]
#[link_section = ".init_array"]
static FOO: [unsafe extern "C" fn(); 2] = [ctor, ctor];
```

Another issue appears to be that if crate *A* depends on crate *B*, but *A* doesn't call any symbols from *B* and *B* doesn't `#[export_name = ...]` any symbols, then crate *B*'s constructor will not be called.  The workaround to this is to provide an exported symbol in crate *B*.

1 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index a2314f4850c..164d1681a36 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -495,8 +495,14 @@ impl<'ll> CodegenCx<'ll, '_> {
             }
 
             // Wasm statics with custom link sections get special treatment as they
-            // go into custom sections of the wasm executable.
-            if self.tcx.sess.target.is_like_wasm {
+            // go into custom sections of the wasm executable. The exception to this
+            // is the `.init_array` section which are treated specially by the wasm linker.
+            if self.tcx.sess.target.is_like_wasm
+                && attrs
+                    .link_section
+                    .map(|link_section| !link_section.as_str().starts_with(".init_array"))
+                    .unwrap_or(true)
+            {
                 if let Some(section) = attrs.link_section {
                     let section = llvm::LLVMMDStringInContext2(
                         self.llcx,