feat(mcp): generate a Rust MCP server

src/bin/forge/main.rs

@@ -2,6 +2,7 @@
 //!
 //! ```text
 //! forge openapi.json --target ts --out sdk
+//! forge openapi.json --target mcp --out server
 //! forge openapi.json --target wasm://plugin.wasm --out sdk
 //! ```
 
@@ -13,6 +14,7 @@ use std::process::ExitCode;
 use clap::Parser;
 use forge::{Err, Forge};
 
+mod mcp;
 mod ts;
 
 /// Generates an SDK from an OpenAPI document.
@@ -21,8 +23,9 @@ mod ts;
 struct Args {
     /// The OpenAPI JSON document, or - to read it from standard input.
     spec: PathBuf,
-    /// The language of the SDK: `ts` for TypeScript, or `wasm://PATH` for
-    /// the plugin compiled to a WebAssembly component at PATH.
+    /// The language of the SDK: `ts` for TypeScript, `mcp` for an MCP server
+    /// in Rust, or `wasm://PATH` for the plugin compiled to a WebAssembly
+    /// component at PATH.
     #[arg(long, value_parser = Target::parse)]
     target: Target,
     /// The directory to write the SDK to.
@@ -35,6 +38,8 @@ struct Args {
 enum Target {
     /// The TypeScript generator.
     Ts,
+    /// The MCP server generator.
+    Mcp,
     /// A plugin compiled to a WebAssembly component, at the path.
     #[cfg(feature = "wasm")]
     Wasm(PathBuf),
@@ -45,8 +50,11 @@ impl Target {
         if s == "ts" {
             return Ok(Target::Ts);
         }
+        if s == "mcp" {
+            return Ok(Target::Mcp);
+        }
         let Some(path) = s.strip_prefix("wasm://") else {
-            return Err("possible values: ts, wasm://PATH".to_owned());
+            return Err("possible values: ts, mcp, wasm://PATH".to_owned());
         };
         if path.is_empty() {
             return Err("wasm:// needs the path of a plugin, as in wasm://plugin.wasm".to_owned());
@@ -75,6 +83,10 @@ fn run(args: Args) -> Result<(), Err> {
             let forge = read(&args.spec)?;
             forge::write(&args.out, forge::run(&forge, &ts::Generator)?)
         }
+        Target::Mcp => {
+            let forge = read(&args.spec)?;
+            forge::write(&args.out, forge::run(&forge, &mcp::Generator)?)
+        }
         #[cfg(feature = "wasm")]
         Target::Wasm(path) => {
             // Compiling the plugin and reading the document are both slow,