tests: benchmark building ripgrep help text

The ripgrep benchmarks work with lots of options and have really long
help texts. This makes them a good fit for judging the overall time it
takes to construct and format the help text.

benches/05_ripgrep.rs

@@ -14,6 +14,7 @@ use clap::{App, AppSettings, Arg, ArgSettings};
 
 use test::Bencher;
 use std::collections::HashMap;
+use std::io::Cursor;
 
 #[bench]
 fn build_app_short(b: &mut Bencher) { b.iter(|| app_short()); }
@@ -21,6 +22,18 @@ fn build_app_short(b: &mut Bencher) { b.iter(|| app_short()); }
 #[bench]
 fn build_app_long(b: &mut Bencher) { b.iter(|| app_long()); }
 
+#[bench]
+fn build_help_short(b: &mut Bencher) {
+    let app = app_short();
+    b.iter(|| build_help(&app));
+}
+
+#[bench]
+fn build_help_long(b: &mut Bencher) {
+    let app = app_long();
+    b.iter(|| build_help(&app));
+}
+
 #[bench]
 fn parse_clean(b: &mut Bencher) { b.iter(|| app_short().get_matches_from(vec!["rg", "pat"])); }
 
@@ -304,6 +317,16 @@ pub fn app_short() -> App<'static, 'static> { app(false, |k| USAGES[k].short) }
 /// Build a clap application with long help strings.
 pub fn app_long() -> App<'static, 'static> { app(true, |k| USAGES[k].long) }
 
+
+/// Build the help text of an application.
+fn build_help(app: &App) -> String {
+    let mut buf = Cursor::new(Vec::with_capacity(50));
+    app.write_help(&mut buf).unwrap();
+    let content = buf.into_inner();
+    String::from_utf8(content).unwrap()
+}
+
+
 /// Build a clap application parameterized by usage strings.
 ///
 /// The function given should take a clap argument name and return a help