Add a wasmtime::component::bindgen! macro (#5317)

* Import Wasmtime support from the `wit-bindgen` repo

This commit imports the `wit-bindgen-gen-host-wasmtime-rust` crate from
the `wit-bindgen` repository into the upstream Wasmtime repository. I've
chosen to not import the full history here since the crate is relatively
small and doesn't have a ton of complexity. While the history of the
crate is quite long the current iteration of the crate's history is
relatively short so there's not a ton of import there anyway. The
thinking is that this can now continue to evolve in-tree.

* Refactor `wasmtime-component-macro` a bit

Make room for a `wit_bindgen` macro to slot in.

* Add initial support for a `bindgen` macro

* Add tests for `wasmtime::component::bindgen!`

* Improve error forgetting `async` feature

* Add end-to-end tests for bindgen

* Add an audit of `unicase`

* Add a license to the test-helpers crate

* Add vet entry for `pulldown-cmark`

* Update publish script with new crate

* Try to fix publish script

* Update audits

* Update lock file

crates/component-macro/src/bindgen.rs

@@ -0,0 +1,137 @@
+use proc_macro2::{Span, TokenStream};
+use std::path::{Path, PathBuf};
+use syn::parse::{Error, Parse, ParseStream, Result};
+use syn::punctuated::Punctuated;
+use syn::token;
+use syn::Token;
+use wasmtime_wit_bindgen::Opts;
+use wit_parser::World;
+
+#[derive(Default)]
+pub struct Config {
+    opts: Opts, // ...
+    world: World,
+    files: Vec<String>,
+}
+
+pub fn expand(input: &Config) -> Result<TokenStream> {
+    if !cfg!(feature = "async") && input.opts.async_ {
+        return Err(Error::new(
+            Span::call_site(),
+            "cannot enable async bindings unless `async` crate feature is active",
+        ));
+    }
+
+    let src = input.opts.generate(&input.world);
+    let mut contents = src.parse::<TokenStream>().unwrap();
+
+    // Include a dummy `include_str!` for any files we read so rustc knows that
+    // we depend on the contents of those files.
+    let cwd = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+    for file in input.files.iter() {
+        contents.extend(
+            format!(
+                "const _: &str = include_str!(r#\"{}\"#);\n",
+                Path::new(&cwd).join(file).display()
+            )
+            .parse::<TokenStream>()
+            .unwrap(),
+        );
+    }
+
+    Ok(contents)
+}
+
+impl Parse for Config {
+    fn parse(input: ParseStream<'_>) -> Result<Self> {
+        let call_site = Span::call_site();
+        let mut world = None;
+        let mut ret = Config::default();
+
+        if input.peek(token::Brace) {
+            let content;
+            syn::braced!(content in input);
+            let fields = Punctuated::<Opt, Token![,]>::parse_terminated(&content)?;
+            for field in fields.into_pairs() {
+                match field.into_value() {
+                    Opt::Path(path) => {
+                        if world.is_some() {
+                            return Err(Error::new(path.span(), "cannot specify second world"));
+                        }
+                        world = Some(ret.parse(path)?);
+                    }
+                    Opt::Inline(span, w) => {
+                        if world.is_some() {
+                            return Err(Error::new(span, "cannot specify second world"));
+                        }
+                        world = Some(w);
+                    }
+                    Opt::Tracing(val) => ret.opts.tracing = val,
+                    Opt::Async(val) => ret.opts.async_ = val,
+                }
+            }
+        } else {
+            let s = input.parse::<syn::LitStr>()?;
+            world = Some(ret.parse(s)?);
+        }
+        ret.world = world.ok_or_else(|| {
+            Error::new(
+                call_site,
+                "must specify a `*.wit` file to generate bindings for",
+            )
+        })?;
+        Ok(ret)
+    }
+}
+
+impl Config {
+    fn parse(&mut self, path: syn::LitStr) -> Result<World> {
+        let span = path.span();
+        let path = path.value();
+        let manifest_dir = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR").unwrap());
+        let path = manifest_dir.join(path);
+        self.files.push(path.to_str().unwrap().to_string());
+        World::parse_file(path).map_err(|e| Error::new(span, e))
+    }
+}
+
+mod kw {
+    syn::custom_keyword!(path);
+    syn::custom_keyword!(inline);
+    syn::custom_keyword!(tracing);
+}
+
+enum Opt {
+    Path(syn::LitStr),
+    Inline(Span, World),
+    Tracing(bool),
+    Async(bool),
+}
+
+impl Parse for Opt {
+    fn parse(input: ParseStream<'_>) -> Result<Self> {
+        let l = input.lookahead1();
+        if l.peek(kw::path) {
+            input.parse::<kw::path>()?;
+            input.parse::<Token![:]>()?;
+            Ok(Opt::Path(input.parse()?))
+        } else if l.peek(kw::inline) {
+            let span = input.parse::<kw::inline>()?.span;
+            input.parse::<Token![:]>()?;
+            let s = input.parse::<syn::LitStr>()?;
+            let world =
+                World::parse("<macro-input>", &s.value()).map_err(|e| Error::new(s.span(), e))?;
+            Ok(Opt::Inline(span, world))
+        } else if l.peek(kw::tracing) {
+            input.parse::<kw::tracing>()?;
+            input.parse::<Token![:]>()?;
+            Ok(Opt::Tracing(input.parse::<syn::LitBool>()?.value))
+        } else if l.peek(Token![async]) {
+            input.parse::<Token![async]>()?;
+            input.parse::<Token![:]>()?;
+            Ok(Opt::Async(input.parse::<syn::LitBool>()?.value))
+        } else {
+            Err(l.error())
+        }
+    }
+}