Implement exporting interfaces from adapter modules (#421)

* start of support for adapter exports. doesnt actually work

we're stuck on wit-bindgen-guest-rust performing allocations when
it creates all the glue for the default export. we need a mode in
the guest-rust crate for putting the type information about the
default export into the object file, but not generating the actual
functions, because in the case of this adapter, we want to create
those functions manually so that they dont pull in std & therefore
an allocator.

* testwasi: export a "command" via an off-by-default feature

* encode_exports: optional instance & realloc index in case the adapter isnt required

* Remove command support from dummy preview1

This is only used for tests and none of the tests are exercising
commands.

* Disable default features for workspace deps

Gets `wasi_snapshot_preview1` compiling correctly by actually turning
off default features.

* Finish up exported interfaces from adapters

* Add missing default feature

Co-authored-by: Pat Hickey <phickey@fastly.com>

Author: alexcrichton

Cargo.toml

@@ -41,7 +41,7 @@ wit-bindgen-gen-host-wasmtime-py = { path = 'crates/gen-host-wasmtime-py', versi
 wit-bindgen-gen-host-wasmtime-rust = { path = 'crates/gen-host-wasmtime-rust', version = '0.3.0' }
 wit-bindgen-gen-markdown = { path = 'crates/gen-markdown', version = '0.3.0' }
 wit-bindgen-gen-rust-lib = { path = 'crates/gen-rust-lib', version = '0.3.0' }
-wit-bindgen-guest-rust = { path = 'crates/guest-rust', version = '0.3.0' }
+wit-bindgen-guest-rust = { path = 'crates/guest-rust', version = '0.3.0', default-features = false }
 wit-bindgen-host-wasmtime-rust = { path = 'crates/host-wasmtime-rust', version = '0.3.0' }
 wit-parser = { path = 'crates/wit-parser', version = '0.3.0' }
 wit-component = { path = 'crates/wit-component', version = '0.3.0', default-features = false }

crates/guest-rust/Cargo.toml

@@ -9,5 +9,6 @@ wit-bindgen-guest-rust-macro = { path = "../guest-rust-macro", optional = true }
 bitflags = { workspace = true }
 
 [features]
-default = ["macros"]
+default = ["macros", "realloc"]
 macros = ["wit-bindgen-guest-rust-macro"]
+realloc = []

crates/guest-rust/src/lib.rs

@@ -16,6 +16,7 @@ pub mod rt {
     // Re-export things from liballoc for convenient use.
     pub use super::alloc::{alloc, string, vec};
 
+    #[cfg(feature = "realloc")]
     #[no_mangle]
     unsafe extern "C" fn cabi_realloc(
         old_ptr: *mut u8,

crates/wasi_snapshot_preview1/Cargo.toml

@@ -5,7 +5,7 @@ edition.workspace = true
 
 [dependencies]
 wasi = "0.11.0"
-wit-bindgen-guest-rust = { workspace = true }
+wit-bindgen-guest-rust = { workspace = true, default-features = false, features = ["macros"] }
 
 [lib]
 crate-type = ["cdylib"]

crates/wasi_snapshot_preview1/src/lib.rs

@@ -20,25 +20,9 @@ use wasi::*;
 
 wit_bindgen_guest_rust::generate!({
     import: "testwasi.wit",
-    name: "testwasi",
+    name: "test_wasi",
 });
 
-// Nothing in this wasm module should end up needing cabi_realloc. However, if
-// we don't define this trapping implementation of the export, we'll pull in
-// the one from wit_bindgen_guest_rust, which will pull in the libc allocator
-// and a bunch of panic related machinery from std, which will use vtables
-// and therefore create a Wasm ElementSection, which will make the resulting
-// wasm unusable as an adapter module.
-#[no_mangle]
-unsafe extern "C" fn cabi_realloc(
-    old_ptr: *mut u8,
-    old_len: usize,
-    align: usize,
-    new_len: usize,
-) -> *mut u8 {
-    unreachable()
-}
-
 #[no_mangle]
 pub extern "C" fn environ_get(environ: *mut *mut u8, environ_buf: *mut u8) -> Errno {
     ERRNO_SUCCESS

crates/wit-bindgen-demo/Cargo.toml

@@ -20,7 +20,7 @@ wit-bindgen-gen-host-js = { workspace = true }
 wit-bindgen-gen-guest-c = { workspace = true }
 wit-bindgen-gen-guest-teavm-java = { workspace = true }
 wit-bindgen-gen-markdown = { workspace = true }
-wit-bindgen-guest-rust = { workspace = true }
+wit-bindgen-guest-rust = { workspace = true, features = ['default'] }
 wasmprinter = { workspace = true }
 wit-component = { workspace = true }
 test-helpers = { path = '../test-helpers', default-features = false }

crates/wit-component/src/decoding.rs

@@ -99,6 +99,18 @@ pub struct ComponentInterfaces {
     pub exports: IndexMap<String, Interface>,
 }
 
+impl ComponentInterfaces {
+    /// Returns an iterator which visits all the exported interfaces, both named
+    /// and default. The second entry in each pair is whether the interface is
+    /// the default interface.
+    pub fn exports(&self) -> impl Iterator<Item = (&Interface, bool)> + '_ {
+        self.exports
+            .values()
+            .map(|i| (i, false))
+            .chain(self.default.iter().map(|i| (i, true)))
+    }
+}
+
 /// Decode the interfaces imported and exported by a component.
 ///
 /// This function takes a binary component as input and will infer the