setting: IgnoreErrors - Allow parsing despite missing option values

Implemented as AppSetting::Ignore errors as suggested by
@CreepySkeleton in
clap-rs#1880 (comment).

This is not a complete implementation but it works already in
surprisingly many situations.

clap-rs#1880

src/build/app/mod.rs

@@ -2225,7 +2225,13 @@ impl<'help> App<'help> {
 
         // do the real parsing
         let mut parser = Parser::new(self);
-        parser.get_matches_with(&mut matcher, it)?;
+        if let Err(error) = parser.get_matches_with(&mut matcher, it) {
+            if self.is_set(AppSettings::IgnoreErrors) {
+                debug!("ignoring error: {}", error);
+            } else {
+                return Err(error);
+            }
+        }
 
         let global_arg_vec: Vec<Id> = self.get_used_global_args(&matcher);
 
@@ -2376,6 +2382,9 @@ impl<'help> App<'help> {
                         $sc.set(AppSettings::GlobalVersion);
                         $sc.version = Some($_self.version.unwrap());
                     }
+                    if $_self.settings.is_set(AppSettings::IgnoreErrors) {
+                        // $sc.set(AppSettings::PartialParsing);
+                    }
                     $sc.settings = $sc.settings | $_self.g_settings;
                     $sc.g_settings = $sc.g_settings | $_self.g_settings;
                     $sc.term_w = $_self.term_w;

src/build/app/settings.rs

@@ -49,6 +49,7 @@ bitflags! {
         const SUBCOMMAND_PRECEDENCE_OVER_ARG = 1 << 41;
         const DISABLE_HELP_FLAG              = 1 << 42;
         const USE_LONG_FORMAT_FOR_HELP_SC    = 1 << 43;
+        const IGNORE_ERRORS                  = 1 << 44;
     }
 }
 
@@ -135,6 +136,8 @@ impl_settings! { AppSettings, AppFlags,
         => Flags::UNIFIED_HELP,
     NextLineHelp("nextlinehelp")
         => Flags::NEXT_LINE_HELP,
+    IgnoreErrors("ignoreerrors")
+        => Flags::IGNORE_ERRORS,
     DisableVersionForSubcommands("disableversionforsubcommands")
         => Flags::DISABLE_VERSION_FOR_SC,
     WaitOnError("waitonerror")
@@ -841,6 +844,40 @@ pub enum AppSettings {
     /// ```
     NextLineHelp,
 
+    /// Try not to fail on parse errors like missing option values. This is a
+    /// global option that gets propagated sub commands.
+    ///
+    /// Issue: [#1880 Partial / Pre Parsing a
+    /// CLI](https://github.com/clap-rs/clap/issues/1880)
+    ///
+    /// This is the basis for:
+    ///
+    /// * [Changing app settings based on
+    ///   flags](https://github.com/clap-rs/clap/issues/1880#issuecomment-637779787)
+    /// * [#1232  Dynamic completion
+    ///   support](https://github.com/clap-rs/clap/issues/1232)
+    ///
+    /// Support is not complete: Errors are still possible but they can be
+    /// avoided in many cases.
+    ///
+    /// ```rust
+    /// # use clap::{App, AppSettings};
+    /// let app = App::new("app")
+    ///   .setting(AppSettings::IgnoreErrors)
+    ///   .arg("-c, --config=[FILE] 'Sets a custom config file'")
+    ///   .arg("-x, --stuff=[FILE] 'Sets a custom stuff file'")
+    ///   .arg("-f 'Flag'");
+    ///
+    /// let r = app.try_get_matches_from(vec!["app", "-c", "file", "-f", "-x"]);
+    ///
+    /// assert!(r.is_ok(), "unexpected error: {:?}", r);
+    /// let m = r.unwrap();
+    /// assert_eq!(m.value_of("config"), Some("file"));
+    /// assert!(m.is_present("f"));
+    /// assert_eq!(m.value_of("stuff"), None);
+    /// ```
+    IgnoreErrors,
+
     /// Allows [``]s to override all requirements of the parent command.
     /// For example, if you had a subcommand or top level application with a required argument
     /// that is only required as long as there is no subcommand present,

src/parse/parser.rs

@@ -864,6 +864,8 @@ impl<'help, 'app> Parser<'help, 'app> {
             }
         }
 
+        let partial_parsing_enabled = self.is_set(AS::IgnoreErrors);
+
         if let Some(sc) = self.app.subcommands.iter_mut().find(|s| s.name == sc_name) {
             let mut sc_matcher = ArgMatcher::default();
             // Display subcommand name, short and long in usage
@@ -912,7 +914,13 @@ impl<'help, 'app> Parser<'help, 'app> {
                     p.cur_idx.set(self.cur_idx.get());
                     p.skip_idxs = self.skip_idxs;
                 }
-                p.get_matches_with(&mut sc_matcher, it)?;
+                if let Err(error) = p.get_matches_with(&mut sc_matcher, it) {
+                    if partial_parsing_enabled {
+                        debug!("ignored error in subcommand {}: {:?}", sc_name, error);
+                    } else {
+                        return Err(error);
+                    }
+                }
             }
             let name = sc.name.clone();
             matcher.subcommand(SubCommand {

tests/partial_parsing.rs

@@ -0,0 +1,107 @@
+use clap::{App, AppSettings, Arg};
+
+#[test]
+fn single_short_arg_without_value() {
+    let app = App::new("app")
+        .setting(AppSettings::IgnoreErrors)
+        .arg("-c, --config=[FILE] 'Sets a custom config file'");
+
+    let r = app.try_get_matches_from(vec!["app", "-c" /* missing: , "config file" */]);
+
+    assert!(r.is_ok(), "unexpected error: {:?}", r);
+    let m = r.unwrap();
+    assert!(m.is_present("config"));
+}
+
+#[test]
+fn single_long_arg_without_value() {
+    let app = App::new("app")
+        .setting(AppSettings::IgnoreErrors)
+        .arg("-c, --config=[FILE] 'Sets a custom config file'");
+
+    let r = app.try_get_matches_from(vec!["app", "--config" /* missing: , "config file" */]);
+
+    assert!(r.is_ok(), "unexpected error: {:?}", r);
+    let m = r.unwrap();
+    assert!(m.is_present("config"));
+}
+
+#[test]
+fn multiple_args_and_final_arg_without_value() {
+    let app = App::new("app")
+        .setting(AppSettings::IgnoreErrors)
+        .arg("-c, --config=[FILE] 'Sets a custom config file'")
+        .arg("-x, --stuff=[FILE] 'Sets a custom stuff file'")
+        .arg("-f 'Flag'");
+
+    let r = app.try_get_matches_from(vec![
+        "app", "-c", "file", "-f", "-x", /* missing: , "some stuff" */
+    ]);
+
+    assert!(r.is_ok(), "unexpected error: {:?}", r);
+    let m = r.unwrap();
+    assert_eq!(m.value_of("config"), Some("file"));
+    assert!(m.is_present("f"));
+    assert_eq!(m.value_of("stuff"), None);
+}
+
+#[test]
+fn multiple_args_and_intermittent_arg_without_value() {
+    let app = App::new("app")
+        .setting(AppSettings::IgnoreErrors)
+        .arg("-c, --config=[FILE] 'Sets a custom config file'")
+        .arg("-x, --stuff=[FILE] 'Sets a custom stuff file'")
+        .arg("-f 'Flag'");
+
+    let r = app.try_get_matches_from(vec![
+        "app", "-x", /* missing: ,"some stuff" */
+        "-c", "file", "-f",
+    ]);
+
+    assert!(r.is_ok(), "unexpected error: {:?}", r);
+    let m = r.unwrap();
+    assert_eq!(m.value_of("config"), Some("file"));
+    assert!(m.is_present("f"));
+    assert_eq!(m.value_of("stuff"), None);
+}
+
+#[test]
+fn subcommand() {
+    let app = App::new("test")
+        .setting(AppSettings::IgnoreErrors)
+        .subcommand(
+            App::new("some")
+                .arg(
+                    Arg::new("test")
+                        .short('t')
+                        .long("test")
+                        .takes_value(true)
+                        .about("testing testing"),
+                )
+                .arg(
+                    Arg::new("stuff")
+                        .short('x')
+                        .long("stuff")
+                        .takes_value(true)
+                        .about("stuf value"),
+                ),
+        )
+        .arg(Arg::new("other").long("other"));
+
+    let m = app.get_matches_from(vec![
+        "myprog",
+        "some",
+        "--test", /* missing: ,"some val" */
+        "-x",
+        "some other val",
+    ]);
+
+    assert_eq!(m.subcommand_name().unwrap(), "some");
+    let sub_m = m.subcommand_matches("some").unwrap();
+    assert!(
+        sub_m.is_present("test"),
+        "expected subcommand to be present due to partial parsing"
+    );
+    assert_eq!(sub_m.value_of("test"), None);
+    assert_eq!(sub_m.value_of("stuff"), Some("some other val"));
+}