Loosely integrate implements into bindgen! (#13497)

* Loosely integrate `implements` into `bindgen!`

This commit is the next step in fleshing out #12698 and following in the
footsteps of #13361 to integrate the `(implements ...)` directive into
`bindgen!`. Notably this world:

    world foo {
        import a: thing;
        import b: thing;
    }

will only generate a single trait for `thing::Host` as opposed to the
previous 2 traits that were generated. This enables hosts to use the
same trait implementation for both, or customize as appropriate.

Additionally the `add_to_linker`-generated methods for interfaces are
now refactored to additionally have an `add_to_linker_instance`
entrypoint. This new entrypoint takes a `LinkerInstance` instead of a
`Linker` and fills in the provided instance directly. This is in-turn
used to fill out a linker for the `foo` world above.

Notably, however, embedders can call `add_to_linker_instance` with
distinct closures accessing different parts of `T`, the store's state,
meaning that it's possible to have two distinct implementations for `a`
and `b` above.

Many golden tests were updated with this new `add_to_linker_instance`
method, and codegen/runtime tests were added for a simple world as well
showcasing the bindings modes that are possible. The (rarely used)
top-level world `add_to_linker` only uses a single trait implementation,
but invoking `add_to_linker_instance` enables passing in two separate
implementations.

* Hide generated files in diffs by default

* Add some runtime tests

Author: alexcrichton

.gitattributes

@@ -8,3 +8,6 @@
 
 # ISLE should use lisp syntax highlighting.
 *.isle linguist-language=lisp
+
+# Generated files don't need diffs rendered by default
+crates/component-macro/tests/expanded/** linguist-generated=true

crates/component-macro/src/bindgen.rs

@@ -20,8 +20,8 @@ pub struct Config {
     include_generated_code_from_file: bool,
 }
 
-pub fn expand(input: &Config) -> Result<TokenStream> {
-    let mut src = match input.opts.generate(&input.resolve, input.world) {
+pub fn expand(input: &mut Config) -> Result<TokenStream> {
+    let mut src = match input.opts.generate(&mut input.resolve, input.world) {
         Ok(s) => s,
         Err(e) => return Err(Error::new(Span::call_site(), e.to_string())),
     };

crates/component-macro/src/lib.rs

@@ -49,7 +49,7 @@ pub fn flags(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 
 #[proc_macro]
 pub fn bindgen(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
-    bindgen::expand(&parse_macro_input!(input as bindgen::Config))
+    bindgen::expand(&mut parse_macro_input!(input as bindgen::Config))
         .unwrap_or_else(Error::into_compile_error)
         .into()
 }

crates/component-macro/tests/codegen/implements.wit

@@ -0,0 +1,11 @@
+package foo:foo;
+
+interface store {
+  get: func(key: string) -> option<string>;
+  set: func(key: string, value: string);
+}
+
+world cache {
+  import hot-cache: store;
+  import durable: store;
+}

crates/component-macro/tests/expanded.rs

@@ -43,6 +43,11 @@ fn process_expanded(path: &str, suffix: &str, src: &str) -> Result<()> {
         Path::new("tests/expanded").join(stem).with_extension("rs")
     };
     if std::env::var("BINDGEN_TEST_BLESS").is_ok_and(|val| !val.is_empty()) {
+        if let Ok(prev) = std::fs::read(&expanded_path)
+            && prev == formatted_src.as_bytes()
+        {
+            return Ok(());
+        }
         std::fs::write(expanded_path, formatted_src)?;
     } else {
         match std::fs::read_to_string(&expanded_path) {