Add command to generate shell completions (rust-lang#2507)

Add the `--generate-shell-completions=shell` command which generates completions for the given shell, emits the completion script to `stdout` and exits. The supported shells are: `bash`, `elvish`, `fish`, `powershell` and `zsh`. We depend on the `clap_complete` crate so we can only support the shells they support.

For example, to generate `bash` completions you can run:

```bash
$ bindgen --generate-shell-completions=bash > /usr/share/bash-completion/completions/bindgen.bash
```

CHANGELOG.md

@@ -168,6 +168,8 @@
 
 * Added the `--generate-cstr` CLI flag to generate string constants as `&CStr`
   instead of `&[u8]`. (Requires Rust 1.59 or higher.)
+* Added the `--generate-shell-completions` CLI flag to generate completions for
+  different shells.
 
 ## Changed
 

bindgen-cli/Cargo.toml

@@ -23,6 +23,7 @@ name = "bindgen"
 bindgen = { path = "../bindgen", version = "=0.65.1", features = ["__cli", "experimental"] }
 shlex = "1"
 clap = { version = "4", features = ["derive"] }
+clap_complete = "4"
 env_logger = { version = "0.10.0", optional = true }
 log = { version = "0.4", optional = true }
 

bindgen-cli/options.rs

@@ -4,10 +4,11 @@ use bindgen::{
     FieldVisibilityKind, Formatter, MacroTypeVariation, NonCopyUnionStyle,
     RegexSet, RustTarget, DEFAULT_ANON_FIELDS_PREFIX, RUST_TARGET_STRINGS,
 };
-use clap::Parser;
+use clap::{CommandFactory, Parser};
 use std::fs::File;
 use std::io::{self, Error, ErrorKind};
 use std::path::PathBuf;
+use std::process::exit;
 
 fn rust_target_help() -> String {
     format!(
@@ -376,6 +377,9 @@ struct BindgenCommand {
     /// Whether to emit diagnostics or not.
     #[arg(long, requires = "experimental")]
     emit_diagnostics: bool,
+    /// Generates completions for the specified SHELL, sends them to `stdout` and exits.
+    #[arg(long, value_name = "SHELL")]
+    generate_shell_completions: Option<clap_complete::Shell>,
     /// Enables experimental features.
     #[arg(long)]
     experimental: bool,
@@ -504,11 +508,23 @@ where
         wrap_static_fns_suffix,
         default_visibility,
         emit_diagnostics,
+        generate_shell_completions,
         experimental: _,
         version,
         clang_args,
     } = command;
 
+    if let Some(shell) = generate_shell_completions {
+        clap_complete::generate(
+            shell,
+            &mut BindgenCommand::command(),
+            "bindgen",
+            &mut std::io::stdout(),
+        );
+
+        exit(0);
+    }
+
     if version {
         println!(
             "bindgen {}",

bindgen-tests/Cargo.toml

@@ -9,6 +9,7 @@ bindgen = { path = "../bindgen", features = ["__cli", "experimental"] }
 diff = "0.1"
 shlex = "1"
 clap = { version = "4", features = ["derive"] }
+clap_complete = "4"
 tempfile = "3"
 
 [features]