[wiggle] Add docs and cargo metadata (#1297)

* test-all: add wiggle & children, plus wasi-common, to cargo test

* wiggle: add licenses, readmes, docs, cargo metadata

crates/wiggle/src/lib.rs

@@ -3,6 +3,84 @@ extern crate proc_macro;
 use proc_macro::TokenStream;
 use syn::parse_macro_input;
 
+/// This macro expands to a set of `pub` Rust modules:
+///
+/// * The `types` module contains definitions for each `typename` declared in
+///   the witx document. Type names are translated to the Rust-idiomatic
+///   CamelCase.
+///
+/// * For each `module` defined in the witx document, a Rust module is defined
+///   containing definitions for that module. Module names are teanslated to the
+///   Rust-idiomatic snake\_case.
+///
+///     * For each `@interface func` defined in a witx module, an abi-level
+///       function is generated which takes ABI-level arguments, along with a
+///       "context" struct (whose type is given by the `ctx` field in the
+///       macro invocation) and a `GuestMemory` implementation.
+///
+///     * A public "module trait" is defined (called the module name, in
+///       SnakeCase) which has a `&self` method for each function in the
+///       module. These methods takes idiomatic Rust types for each argument
+///       and return `Result<($return_types),$error_type>`
+///
+/// Arguments are provided using Rust struct value syntax.
+///
+/// * `witx` takes a list of string literal paths. Paths are relative to the
+///   CARGO_MANIFEST_DIR of the crate where the macro is invoked.
+/// * `ctx` takes a type name. This type must implement all of the module
+///    traits
+///
+/// ## Example
+///
+/// ```
+/// use wiggle_runtime::{GuestPtr, GuestErrorType};
+///
+/// /// The test witx file `arrays.witx` lives in the test directory. For a
+/// /// full-fledged example with runtime tests, see `tests/arrays.rs` and
+/// /// the rest of the files in that directory.
+/// wiggle::from_witx!({
+///     witx: ["tests/arrays.witx"],
+///     ctx: YourCtxType,
+/// });
+///
+/// /// The `ctx` type for this wiggle invocation.
+/// pub struct YourCtxType {}
+///
+/// /// `arrays.witx` contains one module called `arrays`. So, we must
+/// /// implement this one method trait for our ctx type:
+/// impl arrays::Arrays for YourCtxType {
+///     /// The arrays module has two methods, shown here.
+///     /// Note that the `GuestPtr` type comes from `wiggle_runtime`,
+///     /// whereas the witx-defined types like `Excuse` and `Errno` come
+///     /// from the `pub mod types` emitted by the `wiggle::from_witx!`
+///     /// invocation above.
+///     fn reduce_excuses(&self, _a: &GuestPtr<[GuestPtr<types::Excuse>]>)
+///         -> Result<types::Excuse, types::Errno> {
+///         unimplemented!()
+///     }
+///     fn populate_excuses(&self, _a: &GuestPtr<[GuestPtr<types::Excuse>]>)
+///         -> Result<(), types::Errno> {
+///         unimplemented!()
+///     }
+/// }
+///
+/// /// For all types used in the `Error` position of a `Result` in the module
+/// /// traits, you must implement `GuestErrorType` which tells wiggle-generated
+/// /// code how to determine if a method call has been successful, as well as
+/// /// how to translate a wiggle runtime error into an ABI-level error.
+/// impl<'a> GuestErrorType<'a> for types::Errno {
+///     type Context = YourCtxType;
+///     fn success() -> Self {
+///         unimplemented!()
+///     }
+///     fn from_error(_e: wiggle_runtime::GuestError, _c: &Self::Context) -> Self {
+///         unimplemented!()
+///     }
+/// }
+///
+/// # fn main() { println!("this fools doc tests into compiling the above outside a function body")
+/// # }
+/// ```
 #[proc_macro]
 pub fn from_witx(args: TokenStream) -> TokenStream {
     let mut config = parse_macro_input!(args as wiggle_generate::Config);