Add StructOpt::from_iter_safe method (#98)

I would like to be able to test argument parsing, which requires an error
value, not a `process::exit()`.

CHANGELOG.md

@@ -1,3 +1,10 @@
+# NEXT
+
+* Add `StructOpt::from_iter_safe()`, which returns an `Error` instead of
+  killing the program when it fails to parse, or parses one of the
+  short-circuiting flags. ([#98](https://github.com/TeXitoi/structopt/pull/98)
+  by [@quodlibetor](https://github.com/quodlibetor))
+
 # v0.2.7 (2018-04-12)
 
 * Add flattening, the insertion of options of another StructOpt struct into another ([#92](https://github.com/TeXitoi/structopt/pull/92)) by [@birkenfeld](https://github.com/birkenfeld)

src/lib.rs

@@ -425,4 +425,18 @@ pub trait StructOpt {
     {
         Self::from_clap(&Self::clap().get_matches_from(iter))
     }
+
+    /// Gets the struct from any iterator such as a `Vec` of your making.
+    ///
+    /// Returns a `clap::Error` in case of failure. This does *not* exit in the
+    /// case of `--help` or `--version`, to achieve the same behavior as
+    /// `from_iter()` you must call `.exit()` on the error value.
+    fn from_iter_safe<I>(iter: I) -> Result<Self, clap::Error>
+    where
+        Self: Sized,
+        I: IntoIterator,
+        I::Item: Into<OsString> + Clone
+    {
+        Ok(Self::from_clap(&Self::clap().get_matches_from_safe(iter)?))
+    }
 }

tests/arguments.rs

@@ -10,6 +10,7 @@
 extern crate structopt;
 
 use structopt::StructOpt;
+use structopt::clap;
 
 #[test]
 fn required_argument() {
@@ -67,3 +68,20 @@ fn arguments() {
     assert_eq!(Opt { arg: vec![] }, Opt::from_iter(&["test"]));
     assert_eq!(Opt { arg: vec![24, 42] }, Opt::from_iter(&["test", "24", "42"]));
 }
+
+
+#[test]
+fn arguments_safe() {
+    #[derive(StructOpt, PartialEq, Debug)]
+    struct Opt {
+        arg: Vec<i32>,
+    }
+    assert_eq!(Opt { arg: vec![24] }, Opt::from_iter_safe(&["test", "24"]).unwrap());
+    assert_eq!(Opt { arg: vec![] }, Opt::from_iter_safe(&["test"]).unwrap());
+    assert_eq!(Opt { arg: vec![24, 42] }, Opt::from_iter_safe(&["test", "24", "42"]).unwrap());
+
+    assert_eq!(
+        clap::ErrorKind::ValueValidation,
+        Opt::from_iter_safe(&["test", "NOPE"]).err().unwrap().kind,
+    );
+}