jj_cli/

templater.rs

// Copyright 2020 The Jujutsu Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Tools for lazily evaluating templates that produce text in a fallible
//! manner.

use std::cell::RefCell;
use std::collections::HashMap;
use std::error;
use std::fmt;
use std::io;
use std::io::Write;
use std::iter;
use std::ops::Range;
use std::path::PathBuf;
use std::rc::Rc;

use bstr::BStr;
use bstr::BString;
use jj_lib::backend::Signature;
use jj_lib::backend::Timestamp;
use jj_lib::config::ConfigValue;
use jj_lib::file_util;
use jj_lib::op_store::TimestampRange;

use crate::formatter::FormatRecorder;
use crate::formatter::Formatter;
use crate::formatter::FormatterExt as _;
use crate::formatter::LabeledScope;
use crate::formatter::PlainTextFormatter;
use crate::text_util;
use crate::time_util;

/// Represents a printable type or a compiled template containing a placeholder
/// value.
///
/// This is analogous to [`std::fmt::Display`], but with customized error
/// handling.
pub trait Template {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()>;
}

impl<T: Template + ?Sized> Template for &T {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        <T as Template>::format(self, formatter)
    }
}

impl<T: Template + ?Sized> Template for Box<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        <T as Template>::format(self, formatter)
    }
}

// All optional printable types should be printable, and it's unlikely to
// implement different formatting per type.
impl<T: Template> Template for Option<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        self.as_ref().map_or(Ok(()), |t| t.format(formatter))
    }
}

impl Template for BString {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        formatter.as_mut().write_all(self)
    }
}

impl Template for &BStr {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        formatter.as_mut().write_all(self)
    }
}

impl Template for PathBuf {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        // Render native path bytes directly. Serialization still follows
        // PathBuf's serde behavior, which can reject non-UTF-8 paths.
        let bytes = file_util::path_to_bytes(self).map_err(io::Error::other)?;
        formatter.as_mut().write_all(bytes)
    }
}

impl Template for ConfigValue {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        write!(formatter, "{self}")
    }
}

impl Template for Signature {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        write!(formatter.labeled("name"), "{}", self.name)?;
        if !self.name.is_empty() && !self.email.is_empty() {
            write!(formatter, " ")?;
        }
        if !self.email.is_empty() {
            write!(formatter, "<")?;
            let email = Email(self.email.clone());
            email.format(formatter)?;
            write!(formatter, ">")?;
        }
        Ok(())
    }
}

#[derive(Clone, Debug, Eq, PartialEq, serde::Serialize)]
#[serde(transparent)]
pub struct Email(pub String);

impl Template for Email {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let (local, domain) = text_util::split_email(&self.0);
        write!(formatter.labeled("local"), "{local}")?;
        if let Some(domain) = domain {
            write!(formatter, "@")?;
            write!(formatter.labeled("domain"), "{domain}")?;
        }
        Ok(())
    }
}

// In template language, an integer value is represented as i64. However, we use
// usize here because it's more convenient to guarantee that the lower value is
// bounded to 0.
pub type SizeHint = (usize, Option<usize>);

/// Captures from a regex match, accessible by index or name.
#[derive(Clone, Debug)]
pub struct RegexCaptures {
    /// String that matches were found in.
    haystack: Vec<u8>,
    /// List of byte ranges in `haystack` for capture groups by index (with 0
    /// being the full match).
    capture_ranges: Vec<Range<usize>>,
    /// Mapping from capture group names to their index.
    names: HashMap<String, usize>,
}

impl RegexCaptures {
    pub fn new(
        haystack: Vec<u8>,
        capture_ranges: Vec<Range<usize>>,
        names: HashMap<String, usize>,
    ) -> Self {
        Self {
            haystack,
            capture_ranges,
            names,
        }
    }

    #[expect(clippy::len_without_is_empty)]
    pub fn len(&self) -> usize {
        self.capture_ranges.len()
    }

    pub fn get(&self, index: usize) -> Option<BString> {
        self.capture_ranges
            .get(index)
            .map(|range| self.haystack[range.start..range.end].into())
    }

    pub fn name(&self, name: &str) -> Option<BString> {
        self.names.get(name).and_then(|&i| self.get(i))
    }
}

impl Template for String {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        write!(formatter, "{self}")
    }
}

impl Template for &str {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        write!(formatter, "{self}")
    }
}

impl Template for Timestamp {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        match time_util::format_absolute_timestamp(self) {
            Ok(formatted) => write!(formatter, "{formatted}"),
            Err(err) => formatter.handle_error(err.into()),
        }
    }
}

impl Template for TimestampRange {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        self.start.format(formatter)?;
        write!(formatter, " - ")?;
        self.end.format(formatter)?;
        Ok(())
    }
}

impl Template for Vec<BString> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        format_joined(formatter, self, " ")
    }
}

impl Template for Vec<String> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        format_joined(formatter, self, " ")
    }
}

impl Template for bool {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let repr = if *self { "true" } else { "false" };
        write!(formatter, "{repr}")
    }
}

impl Template for i64 {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        write!(formatter, "{self}")
    }
}

pub struct LabelTemplate<T, L> {
    content: T,
    labels: L,
}

impl<T, L> LabelTemplate<T, L> {
    pub fn new(content: T, labels: L) -> Self
    where
        T: Template,
        L: TemplateProperty<Output = Vec<String>>,
    {
        Self { content, labels }
    }
}

impl<T, L> Template for LabelTemplate<T, L>
where
    T: Template,
    L: TemplateProperty<Output = Vec<String>>,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        match self.labels.extract() {
            Ok(labels) => format_labeled(formatter, &self.content, &labels),
            Err(err) => formatter.handle_error(err),
        }
    }
}

pub struct RawEscapeSequenceTemplate<T>(pub T);

impl<T: Template> Template for RawEscapeSequenceTemplate<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let rewrap = formatter.rewrap_fn();
        let mut raw_formatter = PlainTextFormatter::new(formatter.raw()?);
        self.0.format(&mut rewrap(&mut raw_formatter))
    }
}

/// Renders a hyperlink using OSC 8 escape sequences when supported, or a
/// fallback otherwise.
pub struct HyperlinkTemplate<U, T, F> {
    url: U,
    text: T,
    fallback: Option<F>,
}

impl<U, T, F> HyperlinkTemplate<U, T, F> {
    pub fn new(url: U, text: T, fallback: Option<F>) -> Self {
        Self {
            url,
            text,
            fallback,
        }
    }
}

impl<U, T, F> Template for HyperlinkTemplate<U, T, F>
where
    U: TemplateProperty<Output = String>,
    T: Template,
    F: Template,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        // Extract URL string
        let url_str = match self.url.extract() {
            Ok(url) => url,
            Err(err) => return formatter.handle_error(err),
        };

        if !formatter.maybe_color() {
            if let Some(fallback) = &self.fallback {
                return fallback.format(formatter);
            }
            return self.text.format(formatter);
        }

        // Write OSC 8 hyperlink via raw()
        write!(formatter.raw()?, "\x1b]8;;{url_str}\x1b\\")?;
        self.text.format(formatter)?;
        write!(formatter.raw()?, "\x1b]8;;\x1b\\")
    }
}

/// Renders contents in order, and returns the first non-empty output.
pub struct CoalesceTemplate<T>(pub Vec<T>);

impl<T: Template> Template for CoalesceTemplate<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let Some((last, contents)) = self.0.split_last() else {
            return Ok(());
        };
        let record_non_empty = record_non_empty_fn(formatter);
        if let Some(recorder) = contents.iter().find_map(record_non_empty) {
            recorder?.replay(formatter.as_mut())
        } else {
            last.format(formatter) // no need to capture the last content
        }
    }
}

pub struct ConcatTemplate<T>(pub Vec<T>);

impl<T: Template> Template for ConcatTemplate<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        for template in &self.0 {
            template.format(formatter)?;
        }
        Ok(())
    }
}

/// Renders the content to buffer, and transforms it without losing labels.
pub struct ReformatTemplate<T, F> {
    content: T,
    reformat: F,
}

impl<T, F> ReformatTemplate<T, F> {
    pub fn new(content: T, reformat: F) -> Self
    where
        T: Template,
        F: Fn(&mut TemplateFormatter, &FormatRecorder) -> io::Result<()>,
    {
        Self { content, reformat }
    }
}

impl<T, F> Template for ReformatTemplate<T, F>
where
    T: Template,
    F: Fn(&mut TemplateFormatter, &FormatRecorder) -> io::Result<()>,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let rewrap = formatter.rewrap_fn();
        let mut recorder = FormatRecorder::new(formatter.maybe_color());
        self.content.format(&mut rewrap(&mut recorder))?;
        (self.reformat)(formatter, &recorder)
    }
}

/// Like `ConcatTemplate`, but inserts a separator between contents.
pub struct JoinTemplate<S, T> {
    separator: S,
    contents: Vec<T>,
}

impl<S, T> JoinTemplate<S, T> {
    pub fn new(separator: S, contents: Vec<T>) -> Self
    where
        S: Template,
        T: Template,
    {
        Self {
            separator,
            contents,
        }
    }
}

impl<S, T> Template for JoinTemplate<S, T>
where
    S: Template,
    T: Template,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        format_joined(formatter, &self.contents, &self.separator)
    }
}

/// Like `JoinTemplate`, but ignores empty contents.
pub struct SeparateTemplate<S, T> {
    separator: S,
    contents: Vec<T>,
}

impl<S, T> SeparateTemplate<S, T> {
    pub fn new(separator: S, contents: Vec<T>) -> Self
    where
        S: Template,
        T: Template,
    {
        Self {
            separator,
            contents,
        }
    }
}

impl<S, T> Template for SeparateTemplate<S, T>
where
    S: Template,
    T: Template,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let record_non_empty = record_non_empty_fn(formatter);
        let content_recorders = self.contents.iter().filter_map(record_non_empty);
        format_joined_with(
            formatter,
            content_recorders,
            &self.separator,
            |formatter, recorder| recorder?.replay(formatter.as_mut()),
        )
    }
}

/// Wrapper around an error occurred during template evaluation.
#[derive(Debug)]
pub struct TemplatePropertyError(pub Box<dyn error::Error + Send + Sync>);

// Implements conversion from any error type to support `expr?` in function
// binding. This type doesn't implement `std::error::Error` instead.
// <https://github.com/dtolnay/anyhow/issues/25#issuecomment-544140480>
impl<E> From<E> for TemplatePropertyError
where
    E: error::Error + Send + Sync + 'static,
{
    fn from(err: E) -> Self {
        Self(err.into())
    }
}

/// Lazily evaluated value which can fail to evaluate.
pub trait TemplateProperty {
    type Output;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError>;
}

impl<P: TemplateProperty + ?Sized> TemplateProperty for Box<P> {
    type Output = <P as TemplateProperty>::Output;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        <P as TemplateProperty>::extract(self)
    }
}

impl<P: TemplateProperty> TemplateProperty for Option<P> {
    type Output = Option<P::Output>;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        self.as_ref().map(|property| property.extract()).transpose()
    }
}

// Implement TemplateProperty for tuples
macro_rules! tuple_impls {
    ($( ( $($n:tt $T:ident),+ ) )+) => {
        $(
            impl<$($T: TemplateProperty,)+> TemplateProperty for ($($T,)+) {
                type Output = ($($T::Output,)+);

                fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
                    Ok(($(self.$n.extract()?,)+))
                }
            }
        )+
    }
}

tuple_impls! {
    (0 T0)
    (0 T0, 1 T1)
    (0 T0, 1 T1, 2 T2)
    (0 T0, 1 T1, 2 T2, 3 T3)
}

/// Type-erased [`TemplateProperty`].
pub type BoxedTemplateProperty<'a, O> = Box<dyn TemplateProperty<Output = O> + 'a>;
pub type BoxedSerializeProperty<'a> =
    BoxedTemplateProperty<'a, Box<dyn erased_serde::Serialize + 'a>>;

/// [`TemplateProperty`] adapters that are useful when implementing methods.
pub trait TemplatePropertyExt: TemplateProperty {
    /// Translates to a property that will apply fallible `function` to an
    /// extracted value.
    fn and_then<O, F>(self, function: F) -> TemplateFunction<Self, F>
    where
        Self: Sized,
        F: Fn(Self::Output) -> Result<O, TemplatePropertyError>,
    {
        TemplateFunction::new(self, function)
    }

    /// Translates to a property that will apply `function` to an extracted
    /// value, leaving `Err` untouched.
    fn map<O, F>(self, function: F) -> impl TemplateProperty<Output = O>
    where
        Self: Sized,
        F: Fn(Self::Output) -> O,
    {
        TemplateFunction::new(self, move |value| Ok(function(value)))
    }

    /// Translates to a property that will unwrap an extracted `Option` value
    /// of the specified `type_name`, mapping `None` to `Err`.
    fn try_unwrap<O>(self, type_name: &str) -> impl TemplateProperty<Output = O>
    where
        Self: TemplateProperty<Output = Option<O>> + Sized,
    {
        self.and_then(move |opt| {
            opt.ok_or_else(|| TemplatePropertyError(format!("No {type_name} available").into()))
        })
    }

    /// Converts this property into boxed serialize property.
    fn into_serialize<'a>(self) -> BoxedSerializeProperty<'a>
    where
        Self: Sized + 'a,
        Self::Output: serde::Serialize,
    {
        Box::new(self.map(|value| Box::new(value) as Box<dyn erased_serde::Serialize>))
    }

    /// Converts this property into `Template`.
    fn into_template<'a>(self) -> Box<dyn Template + 'a>
    where
        Self: Sized + 'a,
        Self::Output: Template,
    {
        Box::new(FormattablePropertyTemplate::new(self))
    }

    /// Converts this property into boxed trait object.
    fn into_dyn<'a>(self) -> BoxedTemplateProperty<'a, Self::Output>
    where
        Self: Sized + 'a,
    {
        Box::new(self)
    }

    /// Converts this property into wrapped (or tagged) type.
    ///
    /// Use `W::wrap_property()` if the self type is known to be boxed.
    fn into_dyn_wrapped<'a, W>(self) -> W
    where
        Self: Sized + 'a,
        W: WrapTemplateProperty<'a, Self::Output>,
    {
        W::wrap_property(self.into_dyn())
    }
}

impl<P: TemplateProperty + ?Sized> TemplatePropertyExt for P {}

/// Wraps template property of type `O` in tagged type.
///
/// This is basically [`From<BoxedTemplateProperty<'a, O>>`], but is restricted
/// to property types.
#[diagnostic::on_unimplemented(
    message = "the template property of type `{O}` cannot be wrapped in `{Self}`"
)]
pub trait WrapTemplateProperty<'a, O>: Sized {
    fn wrap_property(property: BoxedTemplateProperty<'a, O>) -> Self;
}

/// Abstraction trait for a type-erased TemplateProperty
pub trait AnyTemplateProperty<'a> {
    fn try_into_serialize(self: Box<Self>) -> Option<BoxedSerializeProperty<'a>>;

    fn try_into_template(self: Box<Self>) -> Option<Box<dyn Template + 'a>>;

    /// If this is a list whose elements convert to `Template`, concatenate with
    /// the given separator.
    fn try_join(
        self: Box<Self>,
        separator: Box<dyn Template + 'a>,
    ) -> Option<Box<dyn Template + 'a>>;
}
pub type BoxedAnyProperty<'a> = Box<dyn AnyTemplateProperty<'a> + 'a>;

/// Adapter that wraps literal value in `TemplateProperty`.
pub struct Literal<O>(pub O);

impl<O: Template> Template for Literal<O> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        self.0.format(formatter)
    }
}

impl<O: Clone> TemplateProperty for Literal<O> {
    type Output = O;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        Ok(self.0.clone())
    }
}

/// Adapter to extract template value from property for displaying.
pub struct FormattablePropertyTemplate<P> {
    property: P,
}

impl<P> FormattablePropertyTemplate<P> {
    pub fn new(property: P) -> Self
    where
        P: TemplateProperty,
        P::Output: Template,
    {
        Self { property }
    }
}

impl<P> Template for FormattablePropertyTemplate<P>
where
    P: TemplateProperty,
    P::Output: Template,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        match self.property.extract() {
            Ok(template) => template.format(formatter),
            Err(err) => formatter.handle_error(err),
        }
    }
}

/// Adapter to turn template back to byte string property.
pub struct PlainTextFormattedProperty<T> {
    template: T,
}

impl<T> PlainTextFormattedProperty<T> {
    pub fn new(template: T) -> Self {
        Self { template }
    }
}

impl<T: Template> TemplateProperty for PlainTextFormattedProperty<T> {
    type Output = BString;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        let mut output = vec![];
        let mut formatter = PlainTextFormatter::new(&mut output);
        let mut wrapper = TemplateFormatter::new(&mut formatter, propagate_property_error);
        self.template.format(&mut wrapper)?;
        Ok(BString::new(output))
    }
}

/// Renders template property of list type with the given separator.
///
/// Each list item will be formatted by the given `format_item()` function.
pub struct ListPropertyTemplate<P, S, F> {
    property: P,
    separator: S,
    format_item: F,
}

impl<P, S, F> ListPropertyTemplate<P, S, F> {
    pub fn new<O>(property: P, separator: S, format_item: F) -> Self
    where
        P: TemplateProperty,
        P::Output: IntoIterator<Item = O>,
        S: Template,
        F: Fn(&mut TemplateFormatter, O) -> io::Result<()>,
    {
        Self {
            property,
            separator,
            format_item,
        }
    }
}

impl<O, P, S, F> Template for ListPropertyTemplate<P, S, F>
where
    P: TemplateProperty,
    P::Output: IntoIterator<Item = O>,
    S: Template,
    F: Fn(&mut TemplateFormatter, O) -> io::Result<()>,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let contents = match self.property.extract() {
            Ok(contents) => contents,
            Err(err) => return formatter.handle_error(err),
        };
        format_joined_with(formatter, contents, &self.separator, &self.format_item)
    }
}

/// Result of a `map()` operation.
///
/// The operations supported on MappedProperty are dependent on the "true" type
/// of `mapped`.
pub struct ListMapProperty<'a, P, O> {
    property: P,
    placeholder: PropertyPlaceholder<O>,
    mapped: BoxedAnyProperty<'a>,
}

impl<'a, P, O> ListMapProperty<'a, P, O> {
    pub fn new(
        property: P,
        placeholder: PropertyPlaceholder<O>,
        mapped: BoxedAnyProperty<'a>,
    ) -> Self {
        Self {
            property,
            placeholder,
            mapped,
        }
    }
}

impl<'a, P, O> AnyTemplateProperty<'a> for ListMapProperty<'a, P, O>
where
    P: TemplateProperty + 'a,
    P::Output: IntoIterator<Item = O>,
    O: Clone + 'a,
{
    fn try_into_serialize(self: Box<Self>) -> Option<BoxedSerializeProperty<'a>> {
        let placeholder = self.placeholder;
        let mapped = self.mapped.try_into_serialize()?;
        Some(
            self.property
                .and_then(move |property| {
                    property
                        .into_iter()
                        .map(|i| placeholder.with_value(i, || mapped.extract()))
                        .collect::<Result<Vec<_>, _>>()
                })
                .into_serialize(),
        )
    }

    fn try_into_template(self: Box<Self>) -> Option<Box<dyn Template + 'a>> {
        self.try_join(Box::new(Literal(" ")))
    }

    fn try_join(
        self: Box<Self>,
        separator: Box<dyn Template + 'a>,
    ) -> Option<Box<dyn Template + 'a>> {
        let placeholder = self.placeholder;
        let mapped = self.mapped.try_into_template()?;
        Some(Box::new(ListPropertyTemplate::new(
            self.property,
            separator,
            move |formatter, value| placeholder.with_value(value, || mapped.format(formatter)),
        )))
    }
}

/// AnyTemplateProperty which selects an output based on a boolean condition.
pub struct ConditionalProperty<'a, P> {
    pub condition: P,
    pub on_true: BoxedAnyProperty<'a>,
    pub on_false: Option<BoxedAnyProperty<'a>>,
}

impl<'a, P> ConditionalProperty<'a, P> {
    pub fn new(
        condition: P,
        on_true: BoxedAnyProperty<'a>,
        on_false: Option<BoxedAnyProperty<'a>>,
    ) -> Self
    where
        P: TemplateProperty<Output = bool> + 'a,
    {
        Self {
            condition,
            on_true,
            on_false,
        }
    }
}

impl<'a, P> AnyTemplateProperty<'a> for ConditionalProperty<'a, P>
where
    P: TemplateProperty<Output = bool> + 'a,
{
    fn try_into_serialize(self: Box<Self>) -> Option<BoxedSerializeProperty<'a>> {
        Some(
            (
                self.condition,
                self.on_true.try_into_serialize()?,
                self.on_false?.try_into_serialize()?,
            )
                .map(
                    move |(condition, on_true, on_false)| {
                        if condition { on_true } else { on_false }
                    },
                )
                .into_dyn(),
        )
    }

    fn try_into_template(self: Box<Self>) -> Option<Box<dyn Template + 'a>> {
        Some(Box::new(ConditionalTemplate::new(
            self.condition,
            self.on_true.try_into_template()?,
            // NOTE: We need to propagate out the inner `None` (or else we would
            // allow a non-template `else` property).
            match self.on_false {
                Some(on_false) => on_false.try_into_template()?,
                None => Box::new(Literal("")),
            },
        )))
    }

    fn try_join(
        self: Box<Self>,
        _separator: Box<dyn Template + 'a>,
    ) -> Option<Box<dyn Template + 'a>> {
        // NOTE: This is implementable, but currently cannot be called.
        None
    }
}

/// Template which selects an output based on a boolean condition.
pub struct ConditionalTemplate<P, T, U> {
    pub condition: P,
    pub true_template: T,
    pub false_template: U,
}

impl<P, T, U> ConditionalTemplate<P, T, U> {
    pub fn new(condition: P, true_template: T, false_template: U) -> Self
    where
        P: TemplateProperty<Output = bool>,
        T: Template,
        U: Template,
    {
        Self {
            condition,
            true_template,
            false_template,
        }
    }
}

impl<P, T, U> Template for ConditionalTemplate<P, T, U>
where
    P: TemplateProperty<Output = bool>,
    T: Template,
    U: Template,
{
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let condition = match self.condition.extract() {
            Ok(condition) => condition,
            Err(err) => return formatter.handle_error(err),
        };
        match condition {
            true => self.true_template.format(formatter),
            false => self.false_template.format(formatter),
        }
    }
}

/// Attempts to render or extract contents in order, returns the first
/// successful output.
pub struct TryList<T>(Vec<T>);

impl<T> TryList<T> {
    pub fn new(contents: Vec<T>) -> Self {
        assert!(!contents.is_empty());
        Self(contents)
    }

    fn try_into_inner<U>(self, mut f: impl FnMut(T) -> Option<U>) -> Option<Vec<U>> {
        self.0.into_iter().map(&mut f).collect()
    }
}

/// Converts type-erased properties if all items support the operation.
impl<'a> AnyTemplateProperty<'a> for TryList<BoxedAnyProperty<'a>> {
    fn try_into_serialize(self: Box<Self>) -> Option<BoxedSerializeProperty<'a>> {
        let properties = self.try_into_inner(|p| p.try_into_serialize())?;
        Some(Box::new(TryList(properties)))
    }

    fn try_into_template(self: Box<Self>) -> Option<Box<dyn Template + 'a>> {
        let templates = self.try_into_inner(|p| p.try_into_template())?;
        Some(Box::new(TryList(templates)))
    }

    fn try_join(
        self: Box<Self>,
        _separator: Box<dyn Template + 'a>,
    ) -> Option<Box<dyn Template + 'a>> {
        // NOTE: This is implementable, but currently cannot be called.
        None
    }
}

impl<T: Template> Template for TryList<T> {
    fn format(&self, formatter: &mut TemplateFormatter) -> io::Result<()> {
        let (last, contents) = self.0.split_last().unwrap();
        if let Some(recorder) = contents.iter().find_map(|content| {
            let mut recorder = FormatRecorder::new(formatter.maybe_color());
            let mut wrapper = TemplateFormatter::new(&mut recorder, propagate_property_error);
            content.format(&mut wrapper).is_ok().then_some(recorder)
        }) {
            recorder.replay(formatter.as_mut())
        } else {
            last.format(formatter) // render the last error normally
        }
    }
}

impl<T: TemplateProperty> TemplateProperty for TryList<T> {
    type Output = T::Output;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        let (last, contents) = self.0.split_last().unwrap();
        if let Some(value) = contents.iter().find_map(|p| p.extract().ok()) {
            Ok(value)
        } else {
            last.extract() // propagate the last error
        }
    }
}

/// Adapter to apply fallible `function` to the `property`.
///
/// This is usually created by `TemplatePropertyExt::and_then()`/`map()`.
pub struct TemplateFunction<P, F> {
    pub property: P,
    pub function: F,
}

impl<P, F> TemplateFunction<P, F> {
    pub fn new<O>(property: P, function: F) -> Self
    where
        P: TemplateProperty,
        F: Fn(P::Output) -> Result<O, TemplatePropertyError>,
    {
        Self { property, function }
    }
}

impl<O, P, F> TemplateProperty for TemplateFunction<P, F>
where
    P: TemplateProperty,
    F: Fn(P::Output) -> Result<O, TemplatePropertyError>,
{
    type Output = O;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        (self.function)(self.property.extract()?)
    }
}

/// Property which will be compiled into template once, and substituted later.
#[derive(Clone, Debug)]
pub struct PropertyPlaceholder<O> {
    value: Rc<RefCell<Option<O>>>,
}

impl<O> PropertyPlaceholder<O> {
    pub fn new() -> Self {
        Self {
            value: Rc::new(RefCell::new(None)),
        }
    }

    pub fn set(&self, value: O) {
        *self.value.borrow_mut() = Some(value);
    }

    pub fn take(&self) -> Option<O> {
        self.value.borrow_mut().take()
    }

    pub fn with_value<R>(&self, value: O, f: impl FnOnce() -> R) -> R {
        self.set(value);
        let result = f();
        self.take();
        result
    }
}

impl<O> Default for PropertyPlaceholder<O> {
    fn default() -> Self {
        Self::new()
    }
}

impl<O: Clone> TemplateProperty for PropertyPlaceholder<O> {
    type Output = O;

    fn extract(&self) -> Result<Self::Output, TemplatePropertyError> {
        if let Some(value) = self.value.borrow().as_ref() {
            Ok(value.clone())
        } else {
            Err(TemplatePropertyError("Placeholder value is not set".into()))
        }
    }
}

/// Adapter that renders compiled `template` with the `placeholder` value set.
pub struct TemplateRenderer<'a, C> {
    template: Box<dyn Template + 'a>,
    placeholder: PropertyPlaceholder<C>,
    labels: Vec<String>,
}

impl<'a, C: Clone> TemplateRenderer<'a, C> {
    pub fn new(template: Box<dyn Template + 'a>, placeholder: PropertyPlaceholder<C>) -> Self {
        Self {
            template,
            placeholder,
            labels: Vec::new(),
        }
    }

    /// Returns renderer that will format template with the given `labels`.
    ///
    /// This is equivalent to wrapping the content template with `label()`
    /// function. For example,
    /// `content.labeled(["foo", "bar"]).labeled(["baz"])` can be expressed as
    /// `label("baz", label("foo bar", content))` in template.
    pub fn labeled<S: Into<String>>(mut self, labels: impl IntoIterator<Item = S>) -> Self {
        self.labels.splice(0..0, labels.into_iter().map(Into::into));
        self
    }

    pub fn format(&self, context: &C, formatter: &mut dyn Formatter) -> io::Result<()> {
        let mut wrapper = TemplateFormatter::new(formatter, format_property_error_inline);
        self.placeholder.with_value(context.clone(), || {
            format_labeled(&mut wrapper, &self.template, &self.labels)
        })
    }

    /// Renders template into buffer ignoring any color labels.
    ///
    /// The output is usually UTF-8, but it can contain arbitrary bytes such as
    /// file content.
    pub fn format_plain_text(&self, context: &C) -> Vec<u8> {
        let mut output = Vec::new();
        self.format(context, &mut PlainTextFormatter::new(&mut output))
            .expect("write() to vec backed formatter should never fail");
        output
    }
}

/// Wrapper to pass around `Formatter` and error handler.
pub struct TemplateFormatter<'a> {
    formatter: &'a mut dyn Formatter,
    error_handler: PropertyErrorHandler,
}

impl<'a> TemplateFormatter<'a> {
    fn new(formatter: &'a mut dyn Formatter, error_handler: PropertyErrorHandler) -> Self {
        Self {
            formatter,
            error_handler,
        }
    }

    /// Returns function that wraps another `Formatter` with the current error
    /// handling strategy.
    ///
    /// This does not borrow `self` so the underlying formatter can be mutably
    /// borrowed.
    pub fn rewrap_fn(&self) -> impl Fn(&mut dyn Formatter) -> TemplateFormatter<'_> + use<> {
        let error_handler = self.error_handler;
        move |formatter| TemplateFormatter::new(formatter, error_handler)
    }

    pub fn raw(&mut self) -> io::Result<Box<dyn Write + '_>> {
        self.formatter.raw()
    }

    pub fn labeled(&mut self, label: &str) -> LabeledScope<&mut (dyn Formatter + 'a)> {
        self.formatter.labeled(label)
    }

    pub fn push_label(&mut self, label: &str) {
        self.formatter.push_label(label);
    }

    pub fn pop_label(&mut self) {
        self.formatter.pop_label();
    }

    pub fn maybe_color(&self) -> bool {
        self.formatter.maybe_color()
    }

    pub fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
        self.formatter.write_fmt(args)
    }

    /// Handles the given template property evaluation error.
    ///
    /// This usually prints the given error inline, and returns `Ok`. It's up to
    /// caller to decide whether or not to continue template processing on `Ok`.
    /// For example, `if(cond, ..)` expression will terminate if the `cond`
    /// failed to evaluate, whereas `concat(x, y, ..)` will continue processing.
    ///
    /// If `Err` is returned, the error should be propagated.
    pub fn handle_error(&mut self, err: TemplatePropertyError) -> io::Result<()> {
        (self.error_handler)(self.formatter, err)
    }
}

impl<'a> AsMut<dyn Formatter + 'a> for TemplateFormatter<'a> {
    fn as_mut(&mut self) -> &mut (dyn Formatter + 'a) {
        self.formatter
    }
}

pub fn format_joined<I, S>(
    formatter: &mut TemplateFormatter,
    contents: I,
    separator: S,
) -> io::Result<()>
where
    I: IntoIterator,
    I::Item: Template,
    S: Template,
{
    format_joined_with(formatter, contents, separator, |formatter, item| {
        item.format(formatter)
    })
}

fn format_joined_with<I, S, F>(
    formatter: &mut TemplateFormatter,
    contents: I,
    separator: S,
    mut format_item: F,
) -> io::Result<()>
where
    I: IntoIterator,
    S: Template,
    F: FnMut(&mut TemplateFormatter, I::Item) -> io::Result<()>,
{
    let mut contents_iter = contents.into_iter().fuse();
    if let Some(item) = contents_iter.next() {
        format_item(formatter, item)?;
    }
    for item in contents_iter {
        separator.format(formatter)?;
        format_item(formatter, item)?;
    }
    Ok(())
}

fn format_labeled<T: Template + ?Sized>(
    formatter: &mut TemplateFormatter,
    content: &T,
    labels: &[String],
) -> io::Result<()> {
    for label in labels {
        formatter.push_label(label);
    }
    content.format(formatter)?;
    for _label in labels {
        formatter.pop_label();
    }
    Ok(())
}

type PropertyErrorHandler = fn(&mut dyn Formatter, TemplatePropertyError) -> io::Result<()>;

/// Prints property evaluation error as inline template output.
fn format_property_error_inline(
    formatter: &mut dyn Formatter,
    err: TemplatePropertyError,
) -> io::Result<()> {
    let TemplatePropertyError(err) = &err;
    let mut formatter = formatter.labeled("error");
    write!(formatter, "<")?;
    write!(formatter.labeled("heading"), "Error: ")?;
    write!(formatter, "{err}")?;
    for err in iter::successors(err.source(), |err| err.source()) {
        write!(formatter, ": {err}")?;
    }
    write!(formatter, ">")?;
    Ok(())
}

fn propagate_property_error(
    _formatter: &mut dyn Formatter,
    err: TemplatePropertyError,
) -> io::Result<()> {
    Err(io::Error::other(err.0))
}

/// Creates function that renders a template to buffer and returns the buffer
/// only if it isn't empty.
///
/// This inherits the error handling strategy from the given `formatter`.
fn record_non_empty_fn<T: Template + ?Sized>(
    formatter: &TemplateFormatter,
    // TODO: T doesn't have to be captured, but "currently, all type parameters
    // are required to be mentioned in the precise captures list" as of rustc
    // 1.85.0.
) -> impl Fn(&T) -> Option<io::Result<FormatRecorder>> + use<T> {
    let rewrap = formatter.rewrap_fn();
    let maybe_color = formatter.maybe_color();
    move |template| {
        let mut recorder = FormatRecorder::new(maybe_color);
        match template.format(&mut rewrap(&mut recorder)) {
            Ok(()) if recorder.data().is_empty() => None, // omit empty content
            Ok(()) => Some(Ok(recorder)),
            Err(e) => Some(Err(e)),
        }
    }
}