Add parse-bridges CLI subcommand (#274)

## What's this?

A new `parse-bridges` subcommand.
Also, an update to the documentation in the example showing this feature's preview.

This is handy for usage in higher-level build systems.
For example, in my case, I build both Rust and Swift with CMake, and having this CLI command implemented would allow generating the glue code from CMake as well, as a dependency step before building Rust/Swift targets (via `add_custom_target` API).

## Notes

One caveat of the current implementation is that one would have to duplicate the crate name in the invocation line (the first argument).

This is fine for cases like mine, where this would be taken from the build system anyway, but it may not be handy for other cases.

The package name can be automatically deduced if one's running in the correct directory (the package root).
In this case, we can parse the `cargo read-manifest` output and get the name from there.

This would require a new dependency, though (`serde_json`), so I decided not to do this just yet, but if this sounds okay to you, I'll go ahead and implement this as well.

## Examples

Single file:
```shell
swift-bridge-cli parse-bridges --crate-name my-awesome-crate -f src/lib.rs -o generated
```

Multiple files:
```shell
swift-bridge-cli parse-bridges --crate-name my-superb-crate \
-f src/lib.rs \
-f src/some_other_file.rs \
-o generated
```

book/src/building/swiftc-and-cargo/README.md

@@ -70,7 +70,7 @@ func run() {
 ```
 
 ```sh
-# briding-header.h
+# bridging-header.h
 
 #ifndef BridgingHeader_h
 #define BridgingHeader_h
@@ -134,25 +134,13 @@ chmod +x build-swiftc-links-rust.sh
 
 ## Rust links to a Swift native library
 
-Unlike the when we had `swiftc` in the Rust code, you do not need to set the `crate-type`
+Unlike when we had `swiftc` linking in the Rust code, you do not need to set the `crate-type`
 when you have `Cargo` linking in the Swift code.
 
 ```rust
 // build.rs
-use std::path::PathBuf;
-
 fn main() {
-    let out_dir = PathBuf::from("./generated");
-
-    let bridges = vec!["src/lib.rs"];
-    for path in &bridges {
-        println!("cargo:rerun-if-changed={}", path);
-    }
-
-    swift_bridge_build::parse_bridges(bridges)
-        .write_all_concatenated(out_dir, env!("CARGO_PKG_NAME"));
-
-    println!("cargo:rustc-link-lib=static=swiftc_link_rust");
+    println!("cargo:rustc-link-lib=static=my_swift");
     println!("cargo:rustc-link-search=./");
 }
 ```
@@ -163,9 +151,10 @@ fn main() {
 #!/bin/bash
 set -e
 
-# The swift-bridge CLI does not exist yet. Open an issue if you need to use
-# this approach and I'll happily whip up the CLI.
-swift-bridge -f src/lib.rs > generated
+# Generate the bridging headers.
+# Multiple files can be supplied with "-f" flag, e.g. "-f file1 -f file2".
+# Substitute <package name> for the crate name in your Cargo.toml file.
+swift-bridge-cli parse-bridges --crate-name <package name> -f src/lib.rs -o generated
 
 swiftc -emit-library -static -module-name my_swift -import-objc-header bridging-header.h \
   lib.swift ./generated/swift-and-rust/swift-and-rust.swift

crates/swift-bridge-cli/src/clap_app.rs

@@ -1,4 +1,4 @@
-use clap::{Arg, Command};
+use clap::{Arg, ArgAction, Command};
 
 /// The CLI application
 pub fn cli() -> Command<'static> {
@@ -7,6 +7,7 @@ pub fn cli() -> Command<'static> {
         .version(env!("CARGO_PKG_VERSION"))
         .subcommand_required(true)
         .subcommand(create_package_command())
+        .subcommand(create_bridges_command())
 }
 
 /// The command for creating a Swift Package
@@ -101,3 +102,35 @@ fn create_package_command() -> Command<'static> {
                 .help("The name for the Swift Package"),
         )
 }
+
+fn create_bridges_command() -> Command<'static> {
+    Command::new("parse-bridges")
+        .about("Parse bridge library files and output generated headers")
+        .arg(
+            Arg::new("crate-name")
+                .action(ArgAction::Set)
+                .help(
+                    "Crate name for which the bridging headers are generated; \
+                          used as a part of header names",
+                )
+                .long("--crate-name")
+                .required(true),
+        )
+        .arg(
+            Arg::new("source-file")
+                .action(ArgAction::Append)
+                .help("source file(s) containing #[swift_bridge::bridge] macro")
+                .long("file")
+                .short('f')
+                .required(true),
+        )
+        .arg(
+            Arg::new("output")
+                .action(ArgAction::Set)
+                .help("Output destination folder")
+                .long("output")
+                .short('o')
+                .value_name("PATH")
+                .required(true),
+        )
+}

crates/swift-bridge-cli/src/clap_exec.rs

@@ -1,13 +1,16 @@
 use clap::ArgMatches;
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
-use swift_bridge_build::{create_package, ApplePlatform, CreatePackageConfig};
+use swift_bridge_build::{create_package, parse_bridges, ApplePlatform, CreatePackageConfig};
 
 /// Executes the correct function depending on the cli input
 pub fn handle_matches(matches: ArgMatches) {
     match matches.subcommand_name() {
-        Some("create-package") => {
-            handle_create_package(matches.subcommand_matches("create-package").unwrap())
+        Some(cmd @ "create-package") => {
+            handle_create_package(matches.subcommand_matches(cmd).unwrap())
+        }
+        Some(cmd @ "parse-bridges") => {
+            handle_parse_bridges(matches.subcommand_matches(cmd).unwrap())
         }
         _ => unreachable!("No subcommand or unknown subcommand given"), // Shouldn't happen
     }
@@ -16,7 +19,7 @@ pub fn handle_matches(matches: ArgMatches) {
 /// Executes the `create-package` command
 fn handle_create_package(matches: &ArgMatches) {
     let bridges_dir = matches.value_of("bridges-dir").unwrap(); // required
-    let out_dir = matches.value_of("out-dir").map(|p| Path::new(p)).unwrap(); // required
+    let out_dir = matches.value_of("out-dir").map(Path::new).unwrap(); // required
     let name = matches.value_of("name").unwrap(); // required
 
     let mut config = CreatePackageConfig {
@@ -34,3 +37,12 @@ fn handle_create_package(matches: &ArgMatches) {
 
     create_package(config);
 }
+
+/// Executes the `parse-bridges` command
+fn handle_parse_bridges(matches: &ArgMatches) {
+    let crate_name = matches.get_one::<String>("crate-name").unwrap(); // required
+    let source_files: Vec<&String> = matches.get_many("source-file").unwrap().collect(); // required
+    let output = matches.get_one::<String>("output").map(Path::new).unwrap(); // required
+
+    parse_bridges(source_files.iter().map(Path::new)).write_all_concatenated(output, crate_name);
+}

crates/swift-bridge-macro/tests/ui/args-into-argument-not-found.rs

@@ -15,12 +15,12 @@ mod ffi {
     }
 }
 
-fn some_function(arg: u8) {}
+fn some_function(_arg: u8) {}
 
 struct SomeType;
 
 impl SomeType {
-    fn some_method(&self, foo: u8) {}
+    fn some_method(&self, _foo: u8) {}
 }
 
 fn main() {}