Revert #131, renaming `main` back to `run`. (#165)

Changing LLVM and/or Rust to avoid special handling of `main` is a fair
amount of work, and there could be other toolchains with similar special
rules for functions named `main`, so rename the command entrypoint back
to `run`.

We could potentially re-evaluate this in the future, such as in a
preview3 timeframe, but for now, let's go with the simplest thing that
works.

build.rs

@@ -14,12 +14,6 @@ fn main() {
         out_dir.to_str().unwrap()
     );
 
-    // LLVM has special handling for `main` so use a .s file to define the main
-    // function for the adapter. See the comments in src/main.s for details.
-    if env::var("CARGO_FEATURE_COMMAND").is_ok() {
-        println!("cargo:rustc-link-arg=src/main.o");
-    }
-
     // Some specific flags to `wasm-ld` to inform the shape of this adapter.
     // Notably we're importing memory from the main module and additionally our
     // own module has no stack at all since it's specifically allocated at

src/lib.rs

@@ -31,7 +31,7 @@ pub mod bindings {
         raw_strings,
         // The generated definition of command will pull in std, so we are defining it
         // manually below instead
-        skip: ["main", "get-directories", "get-environment"],
+        skip: ["run", "get-directories", "get-environment"],
     });
 
     #[cfg(feature = "reactor")]
@@ -43,6 +43,17 @@ pub mod bindings {
     });
 }
 
+#[no_mangle]
+#[cfg(feature = "command")]
+pub unsafe extern "C" fn run() -> u32 {
+    #[link(wasm_import_module = "__main_module__")]
+    extern "C" {
+        fn _start();
+    }
+    _start();
+    0
+}
+
 // The unwrap/expect methods in std pull panic when they fail, which pulls
 // in unwinding machinery that we can't use in the adapter. Instead, use this
 // extension trait to get postfixed upwrap on Option and Result.
@@ -197,7 +208,7 @@ impl ImportAlloc {
     }
 }
 
-/// This allocator is only used for the `main` entrypoint.
+/// This allocator is only used for the `run` entrypoint.
 ///
 /// The implementation here is a bump allocator into `State::long_lived_arena` which
 /// traps when it runs out of data. This means that the total size of
@@ -2092,7 +2103,7 @@ struct State {
     /// Long-lived bump allocated memory arena.
     ///
     /// This is used for the cabi_export_realloc to allocate data passed to the
-    /// `main` entrypoint. Allocations in this arena are safe to use for
+    /// `run` entrypoint. Allocations in this arena are safe to use for
     /// the lifetime of the State struct. It may also be used for import allocations
     /// which need to be long-lived, by using `import_alloc.with_arena`.
     long_lived_arena: BumpArena,

src/main.s

@@ -1,27 +0,0 @@
-# The component model CLI world exports its entrypoint under the name
-# "main". However, LLVM has special handling for functions named `main`
-# in order to handle the `main(void)` vs `main(int argc, char **argv)`
-# difference on Wasm where the caller needs to know the exact signature.
-# To avoid this, define a function with a different name and export it
-# as `main`.
-#
-# To generate the `main.o` file from this `main.s` file, compile with
-# `clang --target=wasm32-wasi -c main.s`
-
-	.text
-	.functype	main () -> (i32)
-	.export_name	main, main
-	.functype	_start () -> ()
-	.import_name	_start, _start
-	.import_module	_start, __main_module__
-	.section	.text.main,"",@
-	.hidden	main
-	.globl	main
-	.type	main,@function
-main:
-	.functype	main () -> (i32)
-	call	_start
-	i32.const	0
-	return
-	end_function
-	.no_dead_strip	main