Support custom containers for many functions

[epage]

Please complete the following tasks

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

winnow version

0.2.0

Describe your use case

I wanted to experiment with porting toml_edit to nom, one thing I especially found missing was custom collection support. If I use many0, it only supports Vec. The naive approach in code is to do

fn ml_body(input: &str) -> IResult<&str, String> {
    many(substr).map(|s| s.join(""))
}

Leading me to allocate a Vec<String> and then merge them. combine defaults to Vec but allows any Extend type, allowing

fn ml_body(input: &str) -> IResult<&str, String> {
    many(substr)
}

which will collect directly into the String, saving on a lot of allocations

I'm also looking at optimizing a section of my parser where collect into a Vec and then turn it into a HashMap with errors by creating a custom type that wraps HashMap with my error type and stores off errors on Extend, allowing me to bypass the Vec allocation.

I could do all of this with fold_ variants but it will be a bit messier.

Describe the solution you'd like

Minimum

• Supports common containers (Vec, String, etc)

Ideal

• Supports all containers
• Pre-allocates

One option I've been considering is

trait Foo {
    type Item;
    type Err;

    fn initial(capacity: Option<usize>) -> Result<Self, Self::Err>;

    fn push(&mut self, item: Self::Item) -> Result<(), Self::Err>;
}

(Err was added last minute to support #99)

We could implement this for

• ()
• usize
• String
• Vec
• HashMap
• BTreeMap
• winnow::*::Extendable which wraps any Default + Extend

Questions

• Name of trait
• Where trait and Extendable live

Alternatives, if applicable

rust-bakery/nom#1621 solved this by supporting any Default + Extends container, losing pre-allocations

Additional Context

No response

[epage]

I wonder if this can replace the existing ExtendInto trait used for escapes. That instead is determined based on the input type and we'd be switching it to be based on the output type.