TypedValueParser cannot return single Vec<String>

[LegionMammal978]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.68.2 (9eb3afe9e 2023-03-27)

Clap Version

4.2.0

Minimal reproducible code

use clap::{builder::ValueParser, ArgAction, Parser};
use std::convert::Infallible;

fn my_parser(arg: &str) -> Result<Vec<String>, Infallible> {
    Ok(arg.split(',').map(str::to_string).collect())
}

#[derive(Parser)]
struct Args {
    #[arg(
        long = "values",
        value_parser = ValueParser::new(my_parser),
        action = ArgAction::Set,
        required = true,
    )]
    values: Vec<String>,
}

fn main() {
    let args = Args::parse_from(["example", "--values=a,b,c"]);
    println!("{:?}", args.values);
}

Steps to reproduce the bug with the above code

cargo run

Actual Behaviour

thread 'main' panicked at 'Mismatch between definition and access of `values`. Could not downcast to alloc::string::String, need to downcast to alloc::vec::Vec<alloc::string::String>
', src/main.rs:16:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Expected Behaviour

["a", "b", "c"]

Additional Context

According to the Derive Reference:

clap assumes some intent based on the type used:

Type	Effect	Implies
Vec<T>	0.. occurrences of argument	.action(ArgAction::Append).required(false)

In addition, .value_parser(value_parser!(T)) is called for each field.

Notes:

• For custom type behavior, you can override the implied attributes/settings and/or set additional ones

This suggests that if I override action and required, then my TypedValueParser can return a Vec<T> just like any other T. However, this always produces an error, since clap_derive unconditionally uses get_many for Vec<T> fields, regardless of what attributes I set. So far, I've been able to work around it by having my TypedValueParser return a Box<[T]>, but this is less than ideal.

Debug Output

[clap_builder::builder::command] 	Command::_do_parse
[clap_builder::builder::command] 	Command::_build: name="example"
[clap_builder::builder::command] 	Command::_propagate:example
[clap_builder::builder::command] 	Command::_check_help_and_version:example expand_help_tree=false
[clap_builder::builder::command] 	Command::long_help_exists
[clap_builder::builder::command] 	Command::_check_help_and_version: Building default --help
[clap_builder::builder::command] 	Command::_propagate_global_args:example
[clap_builder::builder::debug_asserts] 	Command::_debug_asserts
[clap_builder::builder::debug_asserts] 	Arg::_debug_asserts:values
[clap_builder::builder::debug_asserts] 	Arg::_debug_asserts:help
[clap_builder::builder::debug_asserts] 	Command::_verify_positionals
[clap_builder::parser::parser] 	Parser::get_matches_with
[clap_builder::parser::parser] 	Parser::get_matches_with: Begin parsing '"--values=a,b,c"'
[clap_builder::parser::parser] 	Parser::possible_subcommand: arg=Ok("--values=a,b,c")
[clap_builder::parser::parser] 	Parser::get_matches_with: sc=None
[clap_builder::parser::parser] 	Parser::parse_long_arg
[clap_builder::parser::parser] 	Parser::parse_long_arg: Does it contain '='...
[clap_builder::parser::parser] 	Parser::parse_long_arg: Found valid arg or flag '--values <VALUES>'
[clap_builder::parser::parser] 	Parser::parse_long_arg("values"): Found an arg with value 'Some("a,b,c")'
[clap_builder::parser::parser] 	Parser::parse_opt_value; arg=values, val=Some("a,b,c"), has_eq=true
[clap_builder::parser::parser] 	Parser::parse_opt_value; arg.settings=ArgFlags(REQUIRED)
[clap_builder::parser::parser] 	Parser::parse_opt_value; Checking for val...
[clap_builder::parser::parser] 	Parser::react action=Set, identifier=Some(Long), source=CommandLine
[clap_builder::parser::parser] 	Parser::react: cur_idx:=1
[clap_builder::parser::parser] 	Parser::remove_overrides: id="values"
[clap_builder::parser::arg_matcher] 	ArgMatcher::start_custom_arg: id="values", source=CommandLine
[clap_builder::builder::command] 	Command::groups_for_arg: id="values"
[clap_builder::parser::arg_matcher] 	ArgMatcher::start_custom_arg: id="Args", source=CommandLine
[clap_builder::parser::parser] 	Parser::push_arg_values: ["a,b,c"]
[clap_builder::parser::parser] 	Parser::add_single_val_to_arg: cur_idx:=2
[clap_builder::parser::arg_matcher] 	ArgMatcher::needs_more_vals: o=values, pending=0
[clap_builder::parser::arg_matcher] 	ArgMatcher::needs_more_vals: expected=1, actual=0
[clap_builder::parser::parser] 	Parser::react not enough values passed in, leaving it to the validator to complain
[clap_builder::parser::parser] 	Parser::get_matches_with: After parse_long_arg ValuesDone
[clap_builder::parser::parser] 	Parser::add_defaults
[clap_builder::parser::parser] 	Parser::add_defaults:iter:values:
[clap_builder::parser::parser] 	Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser] 	Parser::add_default_value:iter:values: doesn't have default vals
[clap_builder::parser::parser] 	Parser::add_defaults:iter:help:
[clap_builder::parser::parser] 	Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser] 	Parser::add_default_value:iter:help: doesn't have default vals
[clap_builder::parser::validator] 	Validator::validate
[clap_builder::builder::command] 	Command::groups_for_arg: id="values"
[clap_builder::parser::validator] 	Conflicts::gather_direct_conflicts id="values", conflicts=[]
[clap_builder::parser::validator] 	Conflicts::gather_direct_conflicts id="Args", conflicts=[]
[clap_builder::parser::validator] 	Validator::validate_conflicts
[clap_builder::parser::validator] 	Validator::validate_exclusive
[clap_builder::parser::validator] 	Validator::validate_conflicts::iter: id="values"
[clap_builder::parser::validator] 	Conflicts::gather_conflicts: arg="values"
[clap_builder::parser::validator] 	Conflicts::gather_conflicts: conflicts=[]
[clap_builder::parser::validator] 	Validator::validate_required: required=ChildGraph([Child { id: "values", children: [] }])
[clap_builder::parser::validator] 	Validator::gather_requires
[clap_builder::parser::validator] 	Validator::gather_requires:iter:"values"
[clap_builder::parser::validator] 	Validator::gather_requires:iter:"Args"
[clap_builder::parser::validator] 	Validator::gather_requires:iter:"Args":group
[clap_builder::parser::validator] 	Validator::validate_required: is_exclusive_present=false
[clap_builder::parser::arg_matcher] 	ArgMatcher::get_global_values: global_arg_vec=[]
thread 'main' panicked at 'Mismatch between definition and access of `values`. Could not downcast to alloc::string::String, need to downcast to alloc::vec::Vec<alloc::string::String>
', src/main.rs:16:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[epage]

This suggests that if I override action and required, then my TypedValueParser can return a Vec just like any other T. However, this always produces an error, since clap_derive unconditionally uses get_many for Vec fields, regardless of what attributes I set. So far, I've been able to work around it by having my TypedValueParser return a Box<[T]>, but this is less than ideal.

The problem is we do not know what accessor to use, so we do a hard coded lookup based on the type's name. You can work around this by referring to the type by something other than Vec, like std::vec::Vec. We have #4626 for exploring how to improve this.

Closing in favor of #4626

[LegionMammal978]

I wonder if it might be helpful to document this behavior in the meantime, since it took me a fair while to track down the underlying cause of the issue.

[epage]

Huh, I thought we had that somewhere but not seeing it, so added it now.

[LegionMammal978]

There seems to be a typo ("completel").