updated

src/app/mod.rs

@@ -1525,6 +1525,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for App<'n, 'e> {
     fn num_vals(&self) -> Option<u64> { None }
     fn possible_vals(&self) -> Option<&[&'e str]> { None }
     fn validator(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> { None }
+    fn validator_os(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> { None }
     fn min_vals(&self) -> Option<u64> { None }
     fn short(&self) -> Option<char> { None }
     fn long(&self) -> Option<&'e str> { None }

src/app/parser.rs

@@ -1533,6 +1533,11 @@ impl<'a, 'b> Parser<'a, 'b>
                 return Err(Error::value_validation(Some(arg), e, self.color()));
             }
         }
+        if let Some(vtor) = arg.validator_os() {
+            if let Err(e) = vtor(val.to_string_lossy().into_owned()) {
+                return Err(Error::value_validation(Some(arg), e, self.color()));
+            }
+        }
         if matcher.needs_more_vals(arg) {
             return Ok(Some(arg.name()));
         }

src/args/any_arg.rs

@@ -26,6 +26,7 @@ pub trait AnyArg<'n, 'e>: std_fmt::Display {
     fn num_vals(&self) -> Option<u64>;
     fn possible_vals(&self) -> Option<&[&'e str]>;
     fn validator(&self) -> Option<&Rc<Fn(String) -> Result<(), String>>>;
+    fn validator_os(&self) -> Option<&Rc<Fn(String) -> Result<(), String>>>;
     fn short(&self) -> Option<char>;
     fn long(&self) -> Option<&'e str>;
     fn val_delim(&self) -> Option<char>;

src/args/arg.rs

@@ -66,6 +66,8 @@ pub struct Arg<'a, 'b>
     #[doc(hidden)]
     pub validator: Option<Rc<Fn(String) -> Result<(), String>>>,
     #[doc(hidden)]
+    pub validator_os: Option<Rc<Fn(String) -> Result<(), String>>>,
+    #[doc(hidden)]
     pub overrides: Option<Vec<&'a str>>,
     #[doc(hidden)]
     pub settings: ArgFlags,
@@ -97,6 +99,7 @@ impl<'a, 'b> Default for Arg<'a, 'b> {
             max_vals: None,
             min_vals: None,
             validator: None,
+            validator_os: None,
             overrides: None,
             settings: ArgFlags::new(),
             val_delim: None,
@@ -1908,6 +1911,35 @@ impl<'a, 'b> Arg<'a, 'b> {
         self
     }
 
+    ///Works identically to Validator but is intended to be used with non UTF-8 formatted strings.
+    /// # Examples
+    /// ```rust
+    /// # use clap::{App, Arg};
+    ///fn has_ampersand(v: String) -> Result<(), String> {
+    ///     if v.contains("&") { return Ok(()); }
+    ///     Err(String::from("The value did not contain the required & sigil"))
+    /// }
+    /// let res = App::new("validators")
+    ///     .arg(Arg::with_name("file")
+    ///         .index(1)
+    ///         .validator(has_ampersand))
+    ///     .get_matches_from_safe(vec![
+    ///         "validators", "Fish & chips"
+    ///     ]);
+    /// assert!(res.is_ok());
+    /// assert_eq!(res.unwrap().value_of("file"), Some("Fish & chips"));
+    /// ```
+    /// [`String`]: https://doc.rust-lang.org/std/string/struct.String.html
+    /// [`Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
+    /// [`Err(String)`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
+    /// [`Rc`]: https://doc.rust-lang.org/std/rc/struct.Rc.html
+    pub fn validator_os<F>(mut self, f: F) -> Self
+        where F: Fn(String) -> Result<(), String> + 'static
+    {
+        self.validator = Some(Rc::new(f));
+        self
+    }
+
     /// Specifies the *maximum* number of values are for this argument. For example, if you had a
     /// `-f <file>` argument where you wanted up to 3 'files' you would set `.max_values(3)`, and
     /// this argument would be satisfied if the user provided, 1, 2, or 3 values.
@@ -2563,6 +2595,7 @@ impl<'a, 'b, 'z> From<&'z Arg<'a, 'b>> for Arg<'a, 'b> {
             val_names: a.val_names.clone(),
             groups: a.groups.clone(),
             validator: a.validator.clone(),
+            validator_os: a.validator_os.clone(),
             overrides: a.overrides.clone(),
             settings: a.settings,
             val_delim: a.val_delim,
@@ -2591,6 +2624,7 @@ impl<'a, 'b> Clone for Arg<'a, 'b> {
             val_names: self.val_names.clone(),
             groups: self.groups.clone(),
             validator: self.validator.clone(),
+            validator_os: self.validator_os.clone(),
             overrides: self.overrides.clone(),
             settings: self.settings,
             val_delim: self.val_delim,

src/args/arg_builder/flag.rs

@@ -64,6 +64,7 @@ impl<'n, 'e> AnyArg<'n, 'e> for FlagBuilder<'n, 'e> {
     fn num_vals(&self) -> Option<u64> { None }
     fn possible_vals(&self) -> Option<&[&'e str]> { None }
     fn validator(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> { None }
+    fn validator_os(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> { None }
     fn min_vals(&self) -> Option<u64> { None }
     fn short(&self) -> Option<char> { self.s.short }
     fn long(&self) -> Option<&'e str> { self.s.long }

src/args/arg_builder/option.rs

@@ -107,6 +107,9 @@ impl<'n, 'e> AnyArg<'n, 'e> for OptBuilder<'n, 'e> {
     fn validator(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> {
         self.v.validator.as_ref()
     }
+    fn validator_os(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> {
+        self.v.validator_os.as_ref()
+    }
     fn min_vals(&self) -> Option<u64> { self.v.min_vals }
     fn short(&self) -> Option<char> { self.s.short }
     fn long(&self) -> Option<&'e str> { self.s.long }

src/args/arg_builder/positional.rs

@@ -111,6 +111,9 @@ impl<'n, 'e> AnyArg<'n, 'e> for PosBuilder<'n, 'e> {
     fn validator(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> {
         self.v.validator.as_ref()
     }
+    fn validator_os(&self) -> Option<&Rc<Fn(String) -> StdResult<(), String>>> {
+        self.v.validator_os.as_ref()
+    }
     fn min_vals(&self) -> Option<u64> { self.v.min_vals }
     fn short(&self) -> Option<char> { None }
     fn long(&self) -> Option<&'e str> { None }

src/args/arg_builder/valued.rs

@@ -15,6 +15,7 @@ pub struct Valued<'a, 'b>
     pub max_vals: Option<u64>,
     pub min_vals: Option<u64>,
     pub validator: Option<Rc<Fn(String) -> Result<(), String>>>,
+    pub validator_os: Option<Rc<Fn(String) -> Result<(), String>>>,
     pub val_delim: Option<char>,
     pub default_val: Option<&'a str>,
 }
@@ -28,6 +29,7 @@ impl<'n, 'e> Default for Valued<'n, 'e> {
             max_vals: None,
             val_names: None,
             validator: None,
+            validator_os: None,
             val_delim: Some(','),
             default_val: None,
         }
@@ -43,6 +45,7 @@ impl<'n, 'e, 'z> From<&'z Arg<'n, 'e>> for Valued<'n, 'e> {
             max_vals: a.max_vals,
             val_names: a.val_names.clone(),
             validator: a.validator.clone(),
+            validator_os: a.validator_os.clone(),
             val_delim: a.val_delim,
             default_val: a.default_val,
         };