//! Lists in builders

//!

//! Use [`define_list_builder_helper`] and [`define_list_builder_accessors`] together when

//! a configuration (or other struct with a builder)

//! wants to contain a `Vec` of config sub-entries.

//!

//! ### How to use these macros

//!

//!  * For each kind of list, define a `ThingList` type alias for the validated form,

//!    and call [`define_list_builder_helper`] to define a `ThingListBuilder` helper

//!    type.  (Different lists with the same Rust type, but which ought to have a different

//!    default, are different "kinds" and should each have a separately named type alias.)

//!

//!    (Or, alternatively, with a hand-written builder type, make the builder field be

//!    `Option<Vec<ElementBuilder>>`.)

//!

// An alternative design would be declare the field on `Outer` as `Vec<Thing>`, and to provide

// a `VecBuilder`.  But:

//

//  (i) the `.build()` method would have to be from a trait (because it would be `VecBuilder<Item>`

//  which would have to contain some `ItemBuilder`, and for the benefit of `VecBuilder::build()`).

//  Although derive_builder` does not provide that trait now, this problem is not insuperable,

//  but it would mean us inventing a `Buildable` trait and a macro to generate it, or forking

//  derive_builder further.

//

//  (ii) `VecBuilder<Item>::build()` would have to have the same default list for every

//  type Item (an empty list).  So places where the default list is not empty would need special

//  handling.  The special handling would look quite like what we have here.

//

//!  * For each struct field containing a list, in a struct deriving `Builder`,

//!    decorate the field with `#[builder(sub_builder, setter(custom))]`

//!    to (i) get `derive_builder` call the appropriate build method,

//!    (ii) suppress the `derive_builder`-generated setter.

//!

// `ThingListBuilder` exists for two reasons:

//

//  * derive_builder wants to call simply `build` on the builder struct field, and will

//    generate code for attaching the field name to any error which occurs.  We could

//    override the per-field build expression, but it would be quite a lot of typing and

//    would recapitulate the field name three times.

//

//  * The field accessors (which must be generated by a different macro_rules macros, at least

//    unless we soup up derive_builder some more) might need to do defaulting, too.  if

//    the builder field is its own type, that can be a method on that type.

//

//!  * For each struct containing lists, call [`define_list_builder_accessors`]

//!    to define the accessor methods.

//!

//! ### Example - list of structs with builders

//!

//! ```

//! use derive_builder::Builder;

//! use serde::{Deserialize, Serialize};

//! use tor_config::{define_list_builder_helper, define_list_builder_accessors, ConfigBuildError};

//!

//! #[derive(Builder, Debug, Eq, PartialEq)]

//! #[builder(build_fn(error = "ConfigBuildError"))]

//! #[builder(derive(Debug, Serialize, Deserialize))]

//! pub struct Thing { value: i32 }

//!

//! #[derive(Builder, Debug, Eq, PartialEq)]

//! #[builder(build_fn(error = "ConfigBuildError"))]

//! #[builder(derive(Debug, Serialize, Deserialize))]

//! pub struct Outer {

//!     /// List of things, being built as part of the configuration

//!     #[builder(sub_builder, setter(custom))]

//!     things: ThingList,

//! }

//!

//! define_list_builder_accessors! {

//!     struct OuterBuilder {

//!         pub things: [ThingBuilder],

//!     }

//! }

//!

//! /// Type alias for use by list builder macrology

//! type ThingList = Vec<Thing>;

//!

//! define_list_builder_helper! {

//!     pub(crate) struct ThingListBuilder {

//!         pub(crate) things: [ThingBuilder],

//!     }

//!     built: ThingList = things;

//!     default = vec![];

//! }

//!

//! let mut builder = OuterBuilder::default();

//! builder.things().push(ThingBuilder::default().value(42).clone());

//! assert_eq!{ builder.build().unwrap().things, &[Thing { value: 42 }] }

//!

//! builder.set_things(vec![ThingBuilder::default().value(38).clone()]);

//! assert_eq!{ builder.build().unwrap().things, &[Thing { value: 38 }] }

//! ```

//!

//! ### Example - list of trivial values

//!

//! ```

//! use derive_builder::Builder;

//! use serde::{Deserialize, Serialize};

//! use tor_config::{define_list_builder_helper, define_list_builder_accessors, ConfigBuildError};

//!

//! #[derive(Builder, Debug, Eq, PartialEq)]

//! #[builder(build_fn(error = "ConfigBuildError"))]

//! #[builder(derive(Debug, Serialize, Deserialize))]

//! pub struct Outer {

//!     /// List of values, being built as part of the configuration

//!     #[builder(sub_builder, setter(custom))]

//!     values: ValueList,

//! }

//!

//! define_list_builder_accessors! {

//!    struct OuterBuilder {

//!        pub values: [u32],

//!    }

//! }

//!

//! /// Type alias for use by list builder macrology

//! pub type ValueList = Vec<u32>;

//!

//! define_list_builder_helper! {

//!    pub(crate) struct ValueListBuilder {

//!        pub(crate) values: [u32],

//!    }

//!    built: ValueList = values;

//!    default = vec![27];

//!    item_build: |&value| Ok(value);

//! }

//!

//! let mut builder = OuterBuilder::default();

//! assert_eq!{ builder.build().unwrap().values, &[27] }

//!

//! builder.values().push(12);

//! assert_eq!{ builder.build().unwrap().values, &[27, 12] }

//! ```

use std::fmt;

use std::marker::PhantomData;

use std::str::FromStr;

use educe::Educe;

use itertools::Itertools;

use serde::{Deserialize, Deserializer, Serialize};

use thiserror::Error;

pub use crate::define_list_builder_accessors;

pub use crate::define_list_builder_helper;

/// Define a list builder struct for use with [`define_list_builder_accessors`]

///

/// Generates an builder struct that can be used with derive_builder

/// and [`define_list_builder_accessors`] to configure a list of some kind.

///

/// **See the [`list_builder` module documentation](crate::list_builder) for an overview.**

///

/// ### Generated struct

///

/// This macro-generated builder struct contains `Option<Vec<ThingBuilder>>`, to allow it to

/// distinguish "never set" from "has been adjusted or set, possibly to the empty list".

///

/// This struct is not exposed as part of the API for setting the configuration.

/// Generally the visibility (`$vis`) should be private,

/// but sometimes `pub(crate)` or `pub` is necessary,

/// for example if the list is to be included in a struct in another module or crate.

/// Usually `$field_vis` should be the same as `$vis`.

///

/// `#[derive(Default, Clone, Debug, Serialize, Deserialize)]`

///  will be applied to the generated builder,

/// but you can specify other attributes too.

/// There is no need to supply any documentation; this is an internal struct and

/// the macro will supply a suitable (bland) doc comment.

/// (If you do supply documentation, the autogenerated docs will be appended,

/// so start with a summary line.)

/// Documentation for the semantics and default value should be applied

/// to the field(s) in the containing struct(s).

///

/// `#[serde(transparent)]` will be applied to the generated `ThingBuilder` struct,

/// so that it deserializes just like `Option<Vec<Thing>>`.

///

/// ### Input to the macro

///

/// For the input syntax, refer to the docs autogenerated from the macro's matcher.

///

/// The `built` clause specifies the type of the built value, and how to construct it.

/// In the expression part, `things` (the field name) will be the default-resolved `Vec<Thing>`;

/// it should be consumed by the expression.

/// If the built value is simply a `Vec`, you can just write `built: ThingList = things;`.

///

/// The `default` clause must provide an expression evaluating to a `Vec<ThingBuilder>`.

///

/// The `item_build` clause, if supplied, provides a closure with type

/// `FnMut(&ThingBuilder) -> Result<Thing, ConfigBuildError>`;

/// the default is to call `thing_builder.build()`.

///

/// The `#[ serde $serde_attrs:tt ]`, if supplied, replace the serde attribute

/// `#[serde(transparent)]`.

/// The transparent attribute is applied by default

/// to arrange that the serde view of the list is precisely `Option<Vec>`.

/// If serialisation is done another way, for example with `#[serde(into)]`,

/// that must be specified here.

///

/// `[$generics]` are generics for `$ListBuilder`.

/// Inline bounds (`T: Debug`) are not supported; use a `where` clause instead.

/// Due to limitations of `macro_rules`, the parameters must be within `[ ]` rather than `< >`,

/// and an extraneous pair of `[ ]` must appear around any `$where_clauses`.

//

// This difficulty with macro_rules is not well documented.

// The upstream Rust bug tracker has this issue

//   https://github.com/rust-lang/rust/issues/73174

//   Matching function signature is nearly impossible in declarative macros (mbe)

// which is not precisely this problem but is very nearby.

// There's also the vapourware "declarative macros 2.0"

//   https://github.com/rust-lang/rust/issues/39412

#[macro_export]

macro_rules! define_list_builder_helper {

    {

        $(#[ $docs_and_attrs:meta ])*

        $vis:vis

        struct $ListBuilder:ident $( [ $($generics:tt)* ] )?

        $( where [ $($where_clauses:tt)* ] )?

        {

            $field_vis:vis $things:ident : [$EntryBuilder:ty] $(,)?

        }

        built: $Built:ty = $built:expr;

        default = $default:expr;

        $( item_build: $item_build:expr; )?

        $(#[ serde $serde_attrs:tt ] )+

    } => {

        #[educe(Default)]

        $(#[ serde $serde_attrs ])+

        $(#[ $docs_and_attrs ])*

        /// Wrapper struct to help derive_builder find the right types and methods

        ///

        /// This struct is not part of the configuration API.

        /// Refer to the containing structures for information on how to build the config.

        $vis struct $ListBuilder $( < $($generics)* > )?

        $( where $($where_clauses)* )?

        {

            /// The list, as overridden

            $field_vis $things: Option<Vec<$EntryBuilder>>,

        }

        impl $( < $($generics)* > )? $ListBuilder $( < $($generics)* > )?

        $( where $($where_clauses)* )?

        {

            /// Resolve this list to a list of built items.

            ///

            /// If the value is still the [`Default`],

            /// a built-in default list will be built and returned;

            /// otherwise each applicable item will be built,

            /// and the results collected into a single built list.

                let default_buffer;

                    None => {

                    }

                };

            /// Resolve the list to the default if necessary and then return `&mut Vec`

            /// Resolve the list to the default if necessary and then return `&mut Vec`

            /// Resolve the list to the default if necessary and then return `&mut Vec`

        }

    };

    // Expand the version without `#[ serde $serde_attrs ]` into a call

    // which provides `#[serde(transparent)]`.

    //

    // We can't use `macro_first_nonempty!` because macro calls cannot be invoked

    // to generate attributes, only items, expressions, etc.

    {

        $(#[ $docs_and_attrs:meta ])*

        $vis:vis

        struct $ListBuilder:ident $( [ $($generics:tt)* ] )?

        $( where [ $($where_clauses:tt)* ] )?

        {

            $field_vis:vis $things:ident : [$EntryBuilder:ty] $(,)?

        }

        built: $Built:ty = $built:expr;

        default = $default:expr;

        $( item_build: $item_build:expr; )?

    } => {

        define_list_builder_helper! {

            $(#[ $docs_and_attrs ])*

            $vis

            struct $ListBuilder $( [ $($generics)* ] )?

            $( where [ $($where_clauses)* ] )?

            {

                $field_vis $things : [$EntryBuilder],

            }

            built: $Built = $built;

            default = $default;

            $( item_build: $item_build; )?

            #[serde(transparent)]

        }

    };

}

/// Define accessor methods for a configuration item which is a list

///

/// **See the [`list_builder` module documentation](crate::list_builder) for an overview.**

///

/// Generates the following methods for each specified field:

///

/// ```skip

/// impl $OuterBuilder {

///     pub fn $things(&mut self) -> &mut Vec<$EntryBuilder> { .. }

///     pub fn set_$things(&mut self, list: Vec<$EntryBuilder>) { .. }

///     pub fn opt_$things(&self) -> &Option<Vec<$EntryBuilder>> { .. }

///     pub fn opt_$things_mut>](&mut self) -> &mut Option<Vec<$EntryBuilder>> { .. }

/// }

/// ```

///

/// Each `$EntryBuilder` should have been defined by [`define_list_builder_helper`];

/// the method bodies from this macro rely on facilities which will beprovided by that macro.

///

/// You can call `define_list_builder_accessors` once for a particular `$OuterBuilder`,

/// with any number of fields with possibly different entry (`$EntryBuilder`) types.

#[macro_export]

macro_rules! define_list_builder_accessors {

    {

        struct $OuterBuilder:ty {

            $(

                $vis:vis $things:ident: [$EntryBuilder:ty],

            )*

        }

    } => {

        impl $OuterBuilder { $( $crate::deps::paste!{

            /// Access the being-built list (resolving default)

            ///

            /// If the field has not yet been set or accessed, the default list will be

            /// constructed and a mutable reference to the now-defaulted list of builders

            /// will be returned.

                #[allow(unused_imports)]

                use $crate::list_builder::DirectDefaultEmptyListBuilderAccessors as _;

            /// Set the whole list (overriding the default)

                #[allow(unused_imports)]

                use $crate::list_builder::DirectDefaultEmptyListBuilderAccessors as _;

            /// Inspect the being-built list (with default unresolved)

            ///

            /// If the list has not yet been set, or accessed, `&None` is returned.

                #[allow(unused_imports)]

                use $crate::list_builder::DirectDefaultEmptyListBuilderAccessors as _;

            /// Mutably access the being-built list (with default unresolved)

            ///

            /// If the list has not yet been set, or accessed, `&mut None` is returned.

                #[allow(unused_imports)]

                use $crate::list_builder::DirectDefaultEmptyListBuilderAccessors as _;

        } )* }

    }

}

/// Extension trait, an alternative to `define_list_builder_helper`

///

/// Useful for a handwritten `Builder` which wants to contain a list,

/// which is an `Option<Vec<ItemBuilder>>`.

///

/// # Example

///

/// ```

/// use tor_config::define_list_builder_accessors;

///

/// #[derive(Default)]

/// struct WombatBuilder {

///     leg_lengths: Option<Vec<u32>>,

/// }

///

/// define_list_builder_accessors! {

///     struct WombatBuilder {

///         leg_lengths: [u32],

///     }

/// }

///

/// let mut wb = WombatBuilder::default();

/// wb.leg_lengths().push(42);

///

/// assert_eq!(wb.leg_lengths, Some(vec![42]));

/// ```

///

/// It is not necessary to `use` this trait anywhere in your code;

/// the macro `define_list_builder_accessors` arranges to have it in scope where it needs it.

pub trait DirectDefaultEmptyListBuilderAccessors {

    /// Entry type

    type T;

    /// Get access to the `Vec`, defaulting it

    fn access(&mut self) -> &mut Vec<Self::T>;

    /// Get access to the `Option<Vec>`

    fn access_opt(&self) -> &Option<Vec<Self::T>>;

    /// Get mutable access to the `Option<Vec>`

    fn access_opt_mut(&mut self) -> &mut Option<Vec<Self::T>>;

}

impl<T> DirectDefaultEmptyListBuilderAccessors for Option<Vec<T>> {

    type T = T;

}

define_list_builder_helper! {

    /// List of `T`, a straightforward type, being built as part of the configuration

    ///

    /// The default is the empty list.

    ///

    /// ### Example

    ///

    /// ```

    /// use derive_builder::Builder;

    /// use serde::{Deserialize, Serialize};

    /// use tor_config::ConfigBuildError;

    /// use tor_config::{define_list_builder_accessors, list_builder::VecBuilder};

    /// use std::net::SocketAddr;

    ///

    /// #[derive(Debug, Clone, Builder)]

    /// #[builder(build_fn(error = "ConfigBuildError"))]

    /// #[builder(derive(Debug, Serialize, Deserialize))]

    /// pub struct FallbackDir {

    ///     #[builder(sub_builder(fn_name = "build"), setter(custom))]

    ///     orports: Vec<SocketAddr>,

    /// }

    ///

    /// define_list_builder_accessors! {

    ///     struct FallbackDirBuilder {

    ///         pub orports: [SocketAddr],

    ///     }

    /// }

    ///

    /// let mut bld = FallbackDirBuilder::default();

    /// bld.orports().push("[2001:db8:0::42]:12".parse().unwrap());

    /// assert_eq!( bld.build().unwrap().orports[0].to_string(),

    ///             "[2001:db8::42]:12" );

    /// ```

    pub struct VecBuilder[T] where [T: Clone] {

        values: [T],

    }

    built: Vec<T> = values;

    default = vec![];

}

/// Configuration item specifiable as a list, or a single multi-line string

///

/// If a list is supplied, they are deserialized as builders.

/// If a single string is supplied, it is split into lines, and `#`-comments

/// and blank lines and whitespace are stripped, and then each line is parsed

/// as a builder.

/// (Eventually, the builders will be built.)

///

/// For use with `sub_builder` and [`define_list_builder_helper`],

/// with `#[serde(try_from)]` and `#[serde(into)]`.

///

/// # Example

///

/// ```

/// use derive_builder::Builder;

/// use serde::{Deserialize, Serialize};

/// use tor_config::{ConfigBuildError, MultilineListBuilder};

/// use tor_config::convert_helper_via_multi_line_list_builder;

/// use tor_config::{define_list_builder_accessors, define_list_builder_helper};

/// use tor_config::impl_standard_builder;

///

/// # fn generate_random<T: Default>() -> T { Default::default() }

///

/// #[derive(Debug, Clone, Builder, Eq, PartialEq)]

/// #[builder(build_fn(error = "ConfigBuildError"))]

/// #[builder(derive(Debug, Serialize, Deserialize))]

/// #[non_exhaustive]

/// pub struct LotteryConfig {

///     /// What numbers should win the lottery?  Setting this is lottery fraud.

///     #[builder(sub_builder, setter(custom))]

///     #[builder_field_attr(serde(default))]

///     winners: LotteryNumberList,

/// }

/// impl_standard_builder! { LotteryConfig }

///

/// /// List of lottery winners

/// //

/// // This type alias arranges that we can put `LotteryNumberList` in `LotteryConfig`

/// // and have derive_builder put a `LotteryNumberListBuilder` in `LotteryConfigBuilder`.

/// pub type LotteryNumberList = Vec<u16>;

///

/// define_list_builder_helper! {

///     struct LotteryNumberListBuilder {

///         numbers: [u16],

///     }

///     built: LotteryNumberList = numbers;

///     default = generate_random();

///     item_build: |number| Ok(*number);

///     #[serde(try_from="MultilineListBuilder<u16>")]

///     #[serde(into="MultilineListBuilder<u16>")]

/// }

///

/// convert_helper_via_multi_line_list_builder! {

///     struct LotteryNumberListBuilder {

///         numbers: [u16],

///     }

/// }

///

/// define_list_builder_accessors! {

///     struct LotteryConfigBuilder {

///         pub winners: [u16],

///     }

/// }

///

/// let lc: LotteryConfigBuilder = toml::from_str(r#"winners = [1,2,3]"#).unwrap();

/// let lc = lc.build().unwrap();

/// assert_eq!{ lc.winners, [1,2,3] }

///

/// let lc = r#"

/// winners = '''

///   ## Enny tells us this is the ticket they bought:

///

///   4

///   5

///   6

/// '''

/// "#;

/// let lc: LotteryConfigBuilder = toml::from_str(lc).unwrap();

/// let lc = lc.build().unwrap();

/// assert_eq!{ lc.winners, [4,5,6] }

/// ```

#[derive(Clone, Debug, Default, Serialize)]

#[serde(untagged)]

#[non_exhaustive]

pub enum MultilineListBuilder<EB> {

    /// Config key not present

    #[default]

    Unspecified,

    /// Config key was a string which is to be parsed line-by-line

    String(String),

    /// Config key was a list of the individual entry builders

    List(Vec<EB>),

}

/// Error from trying to parse a MultilineListBuilder as a list of particular items

///

/// Usually, this error is generated during deserialization.

#[derive(Error, Debug, Clone)]

#[error("multi-line string, line/item {item_number}: could not parse {line:?}: {error}")]

#[non_exhaustive]

pub struct MultilineListBuilderError<E: std::error::Error + Clone + Send + Sync> {

    /// The line number (in the multi-line text string) that could not be parsed

    ///

    /// Starting at 1.

    item_number: usize,

    /// The line that could not be parsed

    line: String,

    /// The parse error from `FromStr`

    ///

    /// This is not a `source` because we want to include it in the `Display`

    /// implementation so that serde errors are useful.

    error: E,

}

// We could derive this with `#[serde(untagged)]` but that produces quite terrible error

// messages, which do not reproduce the error messages from any of the variants.

//

// Instead, have a manual implementation, which can see whether the input is a list or a string.

impl<'de, EB: Deserialize<'de>> Deserialize<'de> for MultilineListBuilder<EB> {

}

/// Visitor for deserialize_any for [`MultilineListBuilder`]

#[educe(Default)]

struct MllbVisitor<EB> {

    /// Variance: this visitor constructs `EB`s

    ret: PhantomData<fn() -> EB>,

}

impl<'de, EB: Deserialize<'de>> serde::de::Visitor<'de> for MllbVisitor<EB> {

    type Value = MultilineListBuilder<EB>;

}

impl<EB> From<Option<Vec<EB>>> for MultilineListBuilder<EB> {

        use MultilineListBuilder as MlLB;

        }

}

impl<EB> TryInto<Option<Vec<EB>>> for MultilineListBuilder<EB>

where

    EB: FromStr,

    EB::Err: std::error::Error + Clone + Send + Sync,

{

    type Error = MultilineListBuilderError<EB::Err>;

        use MultilineListBuilder as MlLB;

        /// Helper for parsing each line of `iter` and collecting the results

            ))

        })

}

/// Implement `TryFrom<MultilineListBuilder>` and `Into<MultilineListBuilder>` for $Builder.

///

/// The input syntax is the `struct` part of that for `define_list_builder_helper`.

/// `$EntryBuilder` must implement `FromStr`.

//

// This is a macro because a helper trait to enable blanket impl would have to provide

// access to `$things`, defeating much of the point.

#[macro_export]

macro_rules! convert_helper_via_multi_line_list_builder { {

    struct $ListBuilder:ident { $things:ident: [$EntryBuilder:ty] $(,)? }

} => {

    impl std::convert::TryFrom<$crate::MultilineListBuilder<$EntryBuilder>> for $ListBuilder {

        type Error = $crate::MultilineListBuilderError<<$EntryBuilder as std::str::FromStr>::Err>;

    }

} }

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_duration_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    use derive_builder::Builder;

    #[derive(Eq, PartialEq, Builder)]

    #[builder(derive(Deserialize))]

    struct Outer {

        #[builder(sub_builder, setter(custom))]

        list: List,

    }

    define_list_builder_accessors! {

        struct OuterBuilder {

            list: [char],

        }

    }

    type List = Vec<char>;

    define_list_builder_helper! {

        struct ListBuilder {

            list: [char],

        }

        built: List = list;

        default = vec!['a'];

        item_build: |&c| Ok(c);

    }

    #[test]

    fn nonempty_default() {

        let mut b = OuterBuilder::default();

        assert!(b.opt_list().is_none());

        assert_eq! { b.build().expect("build failed").list, ['a'] };

        b.list().push('b');

        assert!(b.opt_list().is_some());

        assert_eq! { b.build().expect("build failed").list, ['a', 'b'] };

        for mut b in [b.clone(), OuterBuilder::default()] {

            b.set_list(vec!['x', 'y']);

            assert!(b.opt_list().is_some());

            assert_eq! { b.build().expect("build failed").list, ['x', 'y'] };

        }

        *b.opt_list_mut() = None;

        assert_eq! { b.build().expect("build failed").list, ['a'] };

    }

    #[test]

    fn vecbuilder() {

        // Minimal test, since rustdoc tests seem not to be finding the documentation inside

        // the declaration of VecBuilder.  (Or at least that's what the coverage says.)

        let mut b = VecBuilder::<u32>::default();

        b.access().push(1);

        b.access().push(2);

        b.access().push(3);

        assert_eq!(b.build().unwrap(), vec![1, 2, 3]);

    }

    #[test]

    fn deser() {

        let o: OuterBuilder = toml::from_str("list = ['x','y']").unwrap();

        let o = o.build().unwrap();

        assert_eq!(o.list, ['x', 'y']);

        #[derive(Deserialize, Debug)]

        struct OuterWithMllb {

            #[serde(default)]

            list: MultilineListBuilder<u32>,

        }

        let parse_ok = |s: &str| {

            let o: OuterWithMllb = toml::from_str(s).unwrap();

            let l: Option<Vec<_>> = o.list.try_into().unwrap();

            l

        };

        let l = parse_ok("");

        assert!(l.is_none());

        let l = parse_ok("list = []");

        assert!(l.unwrap().is_empty());

        let l = parse_ok("list = [12,42]");

        assert_eq!(l.unwrap(), [12, 42]);

        let l = parse_ok(r#"list = """#);

        assert!(l.unwrap().is_empty());

        let l = parse_ok("list = \"\"\"\n12\n42\n\"\"\"\n");

        assert_eq!(l.unwrap(), [12, 42]);

        let e = toml::from_str::<OuterWithMllb>("list = [\"fail\"]")

            .unwrap_err()

            .to_string();

        assert!(dbg!(e).contains(r#"invalid type: string "fail", expected u32"#));

        let o = toml::from_str::<OuterWithMllb>("list = \"\"\"\nfail\n\"\"\"").unwrap();

        let l: Result<Option<Vec<_>>, _> = o.list.try_into();

        let e = l.unwrap_err().to_string();

        assert_eq!(e, "multi-line string, line/item 1: could not parse \"fail\": invalid digit found in string");

    }

}