Rollup merge of #79997 - coolreader18:wasm-reactor, r=alexcrichton

Emit a reactor for cdylib target on wasi

Fixes #79199, and relevant to #73432

Implements wasi reactors, as described in WebAssembly/WASI#13 and [`design/application-abi.md`](https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md)

Empty `lib.rs`, `lib.crate-type = ["cdylib"]`:

```shell
$ cargo +reactor build --release --target wasm32-wasi
   Compiling wasm-reactor v0.1.0 (/home/coolreader18/wasm-reactor)
    Finished release [optimized] target(s) in 0.08s
$ wasm-dis target/wasm32-wasi/release/wasm_reactor.wasm >reactor.wat
```
`reactor.wat`:
```wat
(module
 (type $none_=>_none (func))
 (type $i32_=>_none (func (param i32)))
 (type $i32_i32_=>_i32 (func (param i32 i32) (result i32)))
 (type $i32_=>_i32 (func (param i32) (result i32)))
 (type $i32_i32_i32_=>_i32 (func (param i32 i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "fd_prestat_get" (func $__wasi_fd_prestat_get (param i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "fd_prestat_dir_name" (func $__wasi_fd_prestat_dir_name (param i32 i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "proc_exit" (func $__wasi_proc_exit (param i32)))
 (import "wasi_snapshot_preview1" "environ_sizes_get" (func $__wasi_environ_sizes_get (param i32 i32) (result i32)))
 (import "wasi_snapshot_preview1" "environ_get" (func $__wasi_environ_get (param i32 i32) (result i32)))
 (memory $0 17)
 (table $0 1 1 funcref)
 (global $global$0 (mut i32) (i32.const 1048576))
 (global $global$1 i32 (i32.const 1049096))
 (global $global$2 i32 (i32.const 1049096))
 (export "memory" (memory $0))
 (export "_initialize" (func $_initialize))
 (export "__data_end" (global $global$1))
 (export "__heap_base" (global $global$2))
 (func $__wasm_call_ctors
  (call $__wasilibc_initialize_environ_eagerly)
  (call $__wasilibc_populate_preopens)
 )
 (func $_initialize
  (call $__wasm_call_ctors)
 )
 (func $malloc (param $0 i32) (result i32)
  (call $dlmalloc
   (local.get $0)
  )
 )
 ;; lots of dlmalloc, memset/memcpy, & libpreopen code
)
```

I went with repurposing cdylib because I figured that it doesn't make much sense to have a wasi shared library that can't be initialized, and even if someone was using it adding an `_initialize` export is a very small change.

2 files changed, 12 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 55fddb38e10..728795cf50b 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1276,6 +1276,7 @@ fn exec_linker(
 
 fn link_output_kind(sess: &Session, crate_type: CrateType) -> LinkOutputKind {
     let kind = match (crate_type, sess.crt_static(Some(crate_type)), sess.relocation_model()) {
+        (CrateType::Executable, _, _) if sess.is_wasi_reactor() => LinkOutputKind::WasiReactorExe,
         (CrateType::Executable, false, RelocModel::Pic) => LinkOutputKind::DynamicPicExe,
         (CrateType::Executable, false, _) => LinkOutputKind::DynamicNoPicExe,
         (CrateType::Executable, true, RelocModel::Pic) => LinkOutputKind::StaticPicExe,
diff --git a/compiler/rustc_codegen_ssa/src/back/linker.rs b/compiler/rustc_codegen_ssa/src/back/linker.rs
index 3df956c465e..bb35e7ec894 100644
--- a/compiler/rustc_codegen_ssa/src/back/linker.rs
+++ b/compiler/rustc_codegen_ssa/src/back/linker.rs
@@ -314,6 +314,10 @@ impl<'a> Linker for GccLinker<'a> {
                 self.cmd.arg("-static");
                 self.build_dylib(out_filename);
             }
+            LinkOutputKind::WasiReactorExe => {
+                self.linker_arg("--entry");
+                self.linker_arg("_initialize");
+            }
         }
         // VxWorks compiler driver introduced `--static-crt` flag specifically for rustc,
         // it switches linking for libc and similar system libraries to static without using
@@ -662,6 +666,9 @@ impl<'a> Linker for MsvcLinker<'a> {
                 arg.push(out_filename.with_extension("dll.lib"));
                 self.cmd.arg(arg);
             }
+            LinkOutputKind::WasiReactorExe => {
+                panic!("can't link as reactor on non-wasi target");
+            }
         }
     }
 
@@ -1085,6 +1092,10 @@ impl<'a> Linker for WasmLd<'a> {
             LinkOutputKind::DynamicDylib | LinkOutputKind::StaticDylib => {
                 self.cmd.arg("--no-entry");
             }
+            LinkOutputKind::WasiReactorExe => {
+                self.cmd.arg("--entry");
+                self.cmd.arg("_initialize");
+            }
         }
     }