feat(Completions): adds the ability to generate completions to io::Write object

src/app/mod.rs

@@ -978,7 +978,7 @@ impl<'a, 'b> App<'a, 'b> {
     /// `get_matches()`, or any of the other normal methods directly after. For example:
     ///
     /// ```ignore
-    /// src/main.rs
+    /// // src/main.rs
     ///
     /// mod cli;
     ///
@@ -1023,6 +1023,44 @@ impl<'a, 'b> App<'a, 'b> {
         self.p.gen_completions(for_shell, out_dir.into());
     }
 
+
+    /// Generate a completions file for a specified shell at runtime.  Until `cargo install` can
+    /// install extra files like a completion script, this may be used e.g. in a command that
+    /// outputs the contents of the completion script, to be redirected into a file by the user.
+    ///
+    /// # Examples
+    ///
+    /// Assuming a separate `cli.rs` like the [example above](./struct.App.html#method.gen_completions),
+    /// we can let users generate a completion script using a command:
+    ///
+    /// ```ignore
+    /// // src/main.rs
+    ///
+    /// mod cli;
+    /// use std::io;
+    ///
+    /// fn main() {
+    ///     let matches = cli::build_cli().get_matches();
+    ///
+    ///     if matches.is_present("generate-bash-completions") {
+    ///         cli::build_cli().gen_completions_to("myapp", Shell::Bash, &mut io::stdout());
+    ///     }
+    ///
+    ///     // normal logic continues...
+    /// }
+    ///
+    /// ```
+    ///
+    /// Usage:
+    ///
+    /// ```shell
+    /// $ myapp generate-bash-completions > /etc/bash_completion.d/myapp
+    /// ```
+    pub fn gen_completions_to<W: Write, S: Into<String>>(&mut self, bin_name: S, for_shell: Shell, buf: &mut W) {
+        self.p.meta.bin_name = Some(bin_name.into());
+        self.p.gen_completions_to(for_shell, buf);
+    }
+
     /// Starts the parsing process, upon a failed parse an error will be displayed to the user and
     /// the process will exit with the appropriate error code. By default this method gets all user
     /// provided arguments from [`env::args_os`] in order to allow for invalid UTF-8 code points,

src/app/parser.rs

@@ -26,6 +26,8 @@ use app::meta::AppMeta;
 use args::MatchedArg;
 use shell::Shell;
 use completions::ComplGen;
+use std::fs::File;
+use std::path::PathBuf;
 
 #[allow(missing_debug_implementations)]
 #[doc(hidden)]
@@ -99,10 +101,25 @@ impl<'a, 'b> Parser<'a, 'b>
                               .nth(0);
     }
 
-    pub fn gen_completions(&mut self, for_shell: Shell, od: OsString) {
+    pub fn gen_completions_to<W: Write>(&mut self, for_shell: Shell, buf: &mut W) {
+
         self.propogate_help_version();
         self.build_bin_names();
-        ComplGen::new(self, od).generate(for_shell)
+
+        ComplGen::new(self).generate(for_shell, buf)
+    }
+
+    pub fn gen_completions(&mut self, for_shell: Shell, od: OsString) {
+        use std::error::Error;
+
+        let out_dir = PathBuf::from(od);
+
+        let mut file = match File::create(out_dir.join(format!("{}_bash.sh", &*self.meta.bin_name.as_ref().unwrap()))) {
+            Err(why) => panic!("couldn't create bash completion file: {}",
+                why.description()),
+            Ok(file) => file,
+        };
+        self.gen_completions_to(for_shell, &mut file)
     }
 
     // actually adds the arguments

src/completions.rs

@@ -1,6 +1,3 @@
-use std::path::PathBuf;
-use std::fs::File;
-use std::ffi::OsString;
 use std::io::Write;
 
 use app::parser::Parser;
@@ -8,8 +5,8 @@ use shell::Shell;
 use args::{ArgSettings, OptBuilder};
 
 macro_rules! w {
-    ($_self:ident, $f:ident, $to_w:expr) => {
-        match $f.write_all($to_w) {
+    ($buf:expr, $to_w:expr) => {
+        match $buf.write_all($to_w) {
             Ok(..) => (),
             Err(..) => panic!(format!("Failed to write to file completions file")),
         }
@@ -18,33 +15,23 @@ macro_rules! w {
 
 pub struct ComplGen<'a, 'b> where 'a: 'b {
     p: &'b Parser<'a, 'b>,
-    out_dir: OsString,
 }
 
 impl<'a, 'b> ComplGen<'a, 'b> {
-    pub fn new(p: &'b Parser<'a, 'b>, od: OsString) -> Self {
+    pub fn new(p: &'b Parser<'a, 'b>) -> Self {
         ComplGen {
             p: p,
-            out_dir: od,
         }
     }
 
-    pub fn generate(&self, for_shell: Shell) {
+    pub fn generate<W: Write>(&self, for_shell: Shell, buf: &mut W) {
         match for_shell {
-            Shell::Bash => self.gen_bash(),
+            Shell::Bash => self.gen_bash(buf),
         }
     }
 
-    fn gen_bash(&self) {
-        use std::error::Error;
-        let out_dir = PathBuf::from(&self.out_dir);
-
-        let mut file = match File::create(out_dir.join(format!("{}_bash.sh", &*self.p.meta.bin_name.as_ref().unwrap()))) {
-            Err(why) => panic!("couldn't create bash completion file: {}",
-                why.description()),
-            Ok(file) => file,
-        };
-        w!(self, file, format!(
+    fn gen_bash<W: Write>(&self, buf: &mut W) {
+        w!(buf, format!(
 "_{name}() {{
     local i cur prev opts cmds
     COMPREPLY=()

tests/completions.rs

@@ -0,0 +1,25 @@
+extern crate clap;
+
+use clap::{App, Arg, SubCommand, Shell};
+
+#[test]
+fn test_generation() {
+   let mut app = App::new("myapp")
+        .about("Tests completions")
+        .arg(Arg::with_name("file")
+            .help("some input file"))
+        .subcommand(SubCommand::with_name("test")
+            .about("tests things")
+            .arg(Arg::with_name("case")
+                .long("case")
+                .takes_value(true)
+                .help("the case to test")));
+    let mut buf = vec![];
+    app.gen_completions_to("myapp", Shell::Bash, &mut buf);
+    let string = String::from_utf8(buf).unwrap();
+    let first_line = string.lines().nth(0).unwrap();
+    let last_line = string.lines().rev().nth(0).unwrap();
+
+    assert_eq!(first_line, "_myapp() {");
+    assert_eq!(last_line, "complete -F _myapp myapp");
+}