xref: /aosp_15_r20/external/pigweed/pw_format/rust/pw_format/macros.rs (revision 61c4878ac05f98d0ceed94b57d316916de578985)

// Copyright 2023 The Pigweed 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.

//! The `macro` module provides helpers that simplify writing proc macros
//! that take format strings and arguments.  This is accomplish with three
//! main constructs:
//! * [`FormatAndArgsFlavor`]: A struct that implements [syn::parse::Parse] to
//!   parse a format string and its following arguments.
//! * [`FormatMacroGenerator`]: A trait used to implement the macro specific
//!   logic to generate code.
//! * [`generate`]: A function to handle the execution of the proc macro by
//!   calling into a [FormatMacroGenerator].
//!
//! Additionally [`PrintfFormatMacroGenerator`] trait and [`generate_printf`]
//! function are provided to help when implementing generators that need to
//! produce `printf` style format strings as part of their code generation.
//!
//! ## Example
//!
//! An example of implementing a proc macro is provided in the
//! [pw_format_example_macro crate](https://pigweed.googlesource.com/pigweed/pigweed/+/refs/heads/main/pw_format/rust/pw_format_example_macro.rs)
//!
//!

use std::collections::VecDeque;
use std::marker::PhantomData;

use proc_macro2::Ident;
use quote::{format_ident, quote, ToTokens};
use syn::{
    parse::{discouraged::Speculative, Parse, ParseStream},
    punctuated::Punctuated,
    spanned::Spanned,
    Expr, ExprCast, LitStr, Token,
};

use crate::{
    ConversionSpec, Flag, FormatFragment, FormatString, Length, MinFieldWidth, Precision,
    Primitive, Style,
};

mod keywords {
    syn::custom_keyword!(PW_FMT_CONCAT);
}

type TokenStream2 = proc_macro2::TokenStream;

/// An error occurring during proc macro evaluation.
///
/// In order to stay as flexible as possible to implementors of
/// [`FormatMacroGenerator`], the error is simply represent by a
/// string.
#[derive(Debug)]
pub struct Error {
    text: String,
}

impl Error {
    /// Create a new proc macro evaluation error.
    pub fn new(text: &str) -> Self {
        Self {
            text: text.to_string(),
        }
    }
}

/// An alias for a Result with an ``Error``
pub type Result<T> = core::result::Result<T, Error>;

/// Formatting parameters passed to an untyped conversion.
#[derive(Clone, Debug, PartialEq)]
pub struct FormatParams {
    /// Style in which to print the corresponding argument.
    pub style: Style,

    /// Minimum field width.  (i.e. the `8` in `{:08x}`).
    pub min_field_width: Option<u32>,

    /// Zero padding.  (i.e. the existence of `0` in `{:08x}`).
    pub zero_padding: bool,
}

impl FormatParams {
    fn printf_format_trait(&self) -> Result<Ident> {
        match self.style {
            Style::None => Ok(format_ident!("PrintfFormatter")),
            Style::Hex => Ok(format_ident!("PrintfHexFormatter")),
            Style::UpperHex => Ok(format_ident!("PrintfUpperHexFormatter")),
            _ => Err(Error::new(&format!(
                "formatting untyped conversions with {:?} style is unsupported",
                self.style
            ))),
        }
    }

    fn field_params(&self) -> String {
        let (zero_pad, min_field_width) = match self.min_field_width {
            None => ("", "".to_string()),
            Some(min_field_width) => (
                if self.zero_padding { "0" } else { "" },
                format!("{min_field_width}"),
            ),
        };

        format!("{zero_pad}{min_field_width}")
    }

    fn core_fmt_specifier(&self) -> Result<String> {
        // If no formatting options are needed, omit the `:`.
        if self.style == Style::None && self.min_field_width.is_none() {
            return Ok("{}".to_string());
        }

        let format = match self.style {
            Style::None => "",
            Style::Octal => "o",
            Style::Hex => "x",
            Style::UpperHex => "X",
            _ => {
                return Err(Error::new(&format!(
                    "formatting untyped conversions with {:?} style is unsupported",
                    self.style
                )))
            }
        };

        let field_params = self.field_params();

        Ok(format!("{{:{field_params}{format}}}"))
    }
}

impl TryFrom<&ConversionSpec> for FormatParams {
    type Error = Error;
    fn try_from(spec: &ConversionSpec) -> core::result::Result<Self, Self::Error> {
        let min_field_width = match spec.min_field_width {
            MinFieldWidth::None => None,
            MinFieldWidth::Fixed(len) => Some(len),
            MinFieldWidth::Variable => {
                return Err(Error::new(
                    "Variable width '*' string formats are not supported.",
                ))
            }
        };

        Ok(FormatParams {
            style: spec.style,
            min_field_width,
            zero_padding: spec.flags.contains(&Flag::LeadingZeros),
        })
    }
}

/// Implemented for testing through the pw_format_test_macros crate.
impl ToTokens for FormatParams {
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
        let style = self.style;
        let min_field_width = match self.min_field_width {
            None => quote! {None},
            Some(val) => quote! {Some(#val)},
        };
        let zero_padding = self.zero_padding;

        quote! {
            pw_format::macros::FormatParams {
                style: #style,
                min_field_width: #min_field_width,
                zero_padding: #zero_padding,
            }
        }
        .to_tokens(tokens)
    }
}

/// A code generator for implementing a `pw_format` style macro.
///
/// This trait serves as the primary interface between `pw_format` and a
/// proc macro using it to implement format string and argument parsing.  When
/// evaluating the proc macro and generating code, [`generate`] will make
/// repeated calls to [`string_fragment`](FormatMacroGenerator::string_fragment)
/// and the conversion functions.  These calls will be made in the order they
/// appear in the format string.  After all fragments and conversions are
/// processed, [`generate`] will call
/// [`finalize`](FormatMacroGenerator::finalize).
///
/// For an example of implementing a `FormatMacroGenerator` see the
/// [pw_format_example_macro crate](https://pigweed.googlesource.com/pigweed/pigweed/+/refs/heads/main/pw_format/rust/pw_format_example_macro.rs).
pub trait FormatMacroGenerator {
    /// Called by [`generate`] at the end of code generation.
    ///
    /// Consumes `self` and returns the code to be emitted by the proc macro of
    /// and [`Error`].
    fn finalize(self) -> Result<TokenStream2>;

    /// Process a string fragment.
    ///
    /// A string fragment is a string of characters that appear in a format
    /// string.  This is different than a
    /// [`string_conversion`](FormatMacroGenerator::string_conversion) which is
    /// a string provided through a conversion specifier (i.e. `"%s"`).
    fn string_fragment(&mut self, string: &str) -> Result<()>;

    /// Process an integer conversion.
    fn integer_conversion(
        &mut self,
        params: &FormatParams,
        signed: bool,
        type_width: u8, // This should probably be an enum
        expression: Arg,
    ) -> Result<()>;

    /// Process a string conversion.
    ///
    /// See [`string_fragment`](FormatMacroGenerator::string_fragment) for a
    /// disambiguation between that function and this one.
    fn string_conversion(&mut self, expression: Arg) -> Result<()>;

    /// Process a character conversion.
    fn char_conversion(&mut self, expression: Arg) -> Result<()>;

    /// Process an untyped conversion.
    fn untyped_conversion(&mut self, _expression: Arg, _params: &FormatParams) -> Result<()> {
        Err(Error::new("untyped conversion (%v) not supported"))
    }
}

/// An argument to a `pw_format` backed macro.
///
/// `pw_format` backed macros have special case recognition of type casts
/// (`value as ty`) in order to annotate a type for typeless printing w/o
/// relying on experimental features.  If an argument is given in that form,
/// it will be represented as an [`Arg::ExprCast`] here.  Otherwise it will
/// be an [`Arg::Expr`].
#[derive(Clone, Debug)]
pub enum Arg {
    /// An argument that is an type cast expression.
    ExprCast(ExprCast),
    /// An argument that is an expression.
    Expr(Expr),
}

impl Parse for Arg {
    fn parse(input: ParseStream) -> syn::parse::Result<Self> {
        // Try parsing as an explicit cast first.  This lets the user name a
        // type when type_alias_impl_trait is not enabled.
        let fork = input.fork();
        if let Ok(cast) = fork.parse::<ExprCast>() {
            // Speculative parsing and `advance_to` is discouraged due to error
            // presentation.  However, since `ExprCast` is a subset of `Expr`,
            //  any errors in parsing here will be reported when trying to parse
            //  as an `Expr` below.
            input.advance_to(&fork);
            return Ok(Self::ExprCast(cast));
        }

        // Otherwise prase as an expression.
        input.parse::<Expr>().map(Self::Expr)
    }
}

impl ToTokens for Arg {
    fn to_tokens(&self, tokens: &mut TokenStream2) {
        match self {
            Self::Expr(expr) => expr.to_tokens(tokens),
            Self::ExprCast(cast) => cast.to_tokens(tokens),
        }
    }
}

/// A trait for parsing a string into a [`FormatString`].
pub trait FormatStringParser {
    /// Parse `format_string` and return the results as a `[FormatString]`.
    fn parse_format_string(format_string: &str) -> std::result::Result<FormatString, String>;
}

/// An implementation of [`FormatStringParser`] that parsers `printf` style format strings.
#[derive(Debug)]
pub struct PrintfFormatStringParser;
impl FormatStringParser for PrintfFormatStringParser {
    fn parse_format_string(format_string: &str) -> std::result::Result<FormatString, String> {
        FormatString::parse_printf(format_string)
    }
}

/// An implementation of [`FormatStringParser`] that parsers `core::fmt` style format strings.
#[derive(Debug)]
pub struct CoreFmtFormatStringParser;
impl FormatStringParser for CoreFmtFormatStringParser {
    fn parse_format_string(format_string: &str) -> std::result::Result<FormatString, String> {
        FormatString::parse_core_fmt(format_string)
    }
}

/// A parsed format string and it's arguments.
///
/// To parse a `FormatAndArgs`, use the [`FormatAndArgsFlavor`] variant which
/// is generic over [`FormatStringParser`] to allow parsing either `printf` or
/// `core::fmt` style format strings.
///
/// Arguments are parsed according to the pattern:
/// `($format_string:literal, $($args:expr),*)`
///
/// To support uses where format strings need to be built by macros at compile
/// time, the format string can be specified as a set of string literals
/// separated by the custom `PW_FMT_CONCAT` keyword.
#[derive(Debug)]
pub struct FormatAndArgs {
    format_string: LitStr,
    parsed: FormatString,
    args: VecDeque<Arg>,
}

/// A variant of [`FormatAndArgs`] that is generic over format string flavor.
///
/// `FormatAndArgsFlavor` implements [`syn::parse::Parse`] for it's specified
/// format string flavor.  Instantiate `FormatAndArgsFlavor` with either
/// [`PrintfFormatStringParser`] or [`CoreFmtFormatStringParser`] to specify
/// which format string flavor should be used.
///
/// `FormatAndArgsFlavor` trivially converts into [`FormatAndArgs`] with the
/// [`From`] trait.
#[derive(Debug)]
pub struct FormatAndArgsFlavor<T: FormatStringParser> {
    format_and_args: FormatAndArgs,
    phantom: PhantomData<T>,
}

impl<T: FormatStringParser> From<FormatAndArgsFlavor<T>> for FormatAndArgs {
    fn from(val: FormatAndArgsFlavor<T>) -> Self {
        val.format_and_args
    }
}

impl<T: FormatStringParser> Parse for FormatAndArgsFlavor<T> {
    fn parse(input: ParseStream) -> syn::parse::Result<Self> {
        let punctuated =
            Punctuated::<LitStr, keywords::PW_FMT_CONCAT>::parse_separated_nonempty(input)?;
        let span = punctuated.span();
        let format_string = LitStr::new(
            &punctuated.into_iter().fold(String::new(), |mut acc, s| {
                acc.push_str(&s.value());
                acc
            }),
            span,
        );

        let args = if input.is_empty() {
            // If there are no more tokens, no arguments were specified.
            VecDeque::new()
        } else {
            // Eat the `,` following the format string.
            input.parse::<Token![,]>()?;

            let punctuated = Punctuated::<Arg, Token![,]>::parse_terminated(input)?;
            punctuated.into_iter().collect()
        };

        let parsed = T::parse_format_string(&format_string.value()).map_err(|e| {
            syn::Error::new_spanned(
                format_string.to_token_stream(),
                format!("Error parsing format string {e}"),
            )
        })?;

        Ok(FormatAndArgsFlavor {
            format_and_args: FormatAndArgs {
                format_string,
                parsed,
                args,
            },
            phantom: PhantomData,
        })
    }
}

// Grab the next argument returning a descriptive error if no more args are left.
fn next_arg(spec: &ConversionSpec, args: &mut VecDeque<Arg>) -> Result<Arg> {
    args.pop_front()
        .ok_or_else(|| Error::new(&format!("No argument given for {spec:?}")))
}

// Handle a single format conversion specifier (i.e. `%08x`).  Grabs the
// necessary arguments for the specifier from `args` and generates code
// to marshal the arguments into the buffer declared in `_tokenize_to_buffer`.
// Returns an error if args is too short of if a format specifier is unsupported.
fn handle_conversion(
    generator: &mut dyn FormatMacroGenerator,
    spec: &ConversionSpec,
    args: &mut VecDeque<Arg>,
) -> Result<()> {
    match spec.primitive {
        Primitive::Integer | Primitive::Unsigned => {
            // TODO: b/281862660 - Support Width::Variable and Precision::Variable.
            if spec.min_field_width == MinFieldWidth::Variable {
                return Err(Error::new(
                    "Variable width '*' integer formats are not supported.",
                ));
            }

            if spec.precision == Precision::Variable {
                return Err(Error::new(
                    "Variable precision '*' integer formats are not supported.",
                ));
            }

            if spec.style == Style::Binary {
                return Err(Error::new("Binary output style is not supported."));
            }

            let arg = next_arg(spec, args)?;
            let bits = match spec.length.unwrap_or(Length::Long) {
                Length::Char => 8,
                Length::Short => 16,
                Length::Long => 32,
                Length::LongLong => 64,
                Length::IntMax => 64,
                Length::Size => 32,
                Length::PointerDiff => 32,
                Length::LongDouble => {
                    return Err(Error::new(
                        "Long double length parameter invalid for integer formats",
                    ))
                }
            };
            let params = spec.try_into()?;

            generator.integer_conversion(&params, spec.primitive == Primitive::Integer, bits, arg)
        }
        Primitive::String => {
            // TODO: b/281862660 - Support Width::Variable and Precision::Variable.
            if spec.min_field_width == MinFieldWidth::Variable {
                return Err(Error::new(
                    "Variable width '*' string formats are not supported.",
                ));
            }

            if spec.precision == Precision::Variable {
                return Err(Error::new(
                    "Variable precision '*' string formats are not supported.",
                ));
            }

            let arg = next_arg(spec, args)?;
            generator.string_conversion(arg)
        }
        Primitive::Character => {
            let arg = next_arg(spec, args)?;
            generator.char_conversion(arg)
        }

        Primitive::Untyped => {
            let arg = next_arg(spec, args)?;
            let params = spec.try_into()?;
            generator.untyped_conversion(arg, &params)
        }

        Primitive::Float => {
            // TODO: b/281862328 - Support floating point numbers.
            Err(Error::new("Floating point numbers are not supported."))
        }

        // TODO: b/281862333 - Support pointers.
        Primitive::Pointer => Err(Error::new("Pointer types are not supported.")),
    }
}

/// Generate code for a `pw_format` style proc macro.
///
/// `generate` takes a [`FormatMacroGenerator`] and a [`FormatAndArgs`] struct
/// and uses them to produce the code output for a proc macro.
pub fn generate(
    mut generator: impl FormatMacroGenerator,
    format_and_args: FormatAndArgs,
) -> core::result::Result<TokenStream2, syn::Error> {
    let mut args = format_and_args.args;
    let mut errors = Vec::new();

    for fragment in format_and_args.parsed.fragments {
        let result = match fragment {
            FormatFragment::Conversion(spec) => handle_conversion(&mut generator, &spec, &mut args),
            FormatFragment::Literal(string) => generator.string_fragment(&string),
        };
        if let Err(e) = result {
            errors.push(syn::Error::new_spanned(
                format_and_args.format_string.to_token_stream(),
                e.text,
            ));
        }
    }

    if !errors.is_empty() {
        return Err(errors
            .into_iter()
            .reduce(|mut accumulated_errors, error| {
                accumulated_errors.combine(error);
                accumulated_errors
            })
            .expect("errors should not be empty"));
    }

    generator.finalize().map_err(|e| {
        syn::Error::new_spanned(format_and_args.format_string.to_token_stream(), e.text)
    })
}

/// A specialized generator for proc macros that produce `printf` style format strings.
///
/// For proc macros that need to translate a `pw_format` invocation into a
/// `printf` style format string, `PrintfFormatMacroGenerator` offer a
/// specialized form of [`FormatMacroGenerator`] that builds the format string
/// and provides it as an argument to
/// [`finalize`](PrintfFormatMacroGenerator::finalize).
///
/// In cases where a generator needs to override the conversion specifier it
/// can return it from its appropriate conversion method.  An example of using
/// this would be wanting to pass a Rust string directly to a `printf` call
/// over FFI.  In that case,
/// [`string_conversion`](PrintfFormatMacroGenerator::string_conversion) could
/// return `Ok(Some("%.*s".to_string()))` to allow both the length and string
/// pointer to be passed to `printf`.
pub trait PrintfFormatMacroGenerator {
    /// Called by [`generate_printf`] at the end of code generation.
    ///
    /// Works like [`FormatMacroGenerator::finalize`] with the addition of
    /// being provided a `printf_style` format string.
    fn finalize(
        self,
        format_string_fragments: &[PrintfFormatStringFragment],
    ) -> Result<TokenStream2>;

    /// Process a string fragment.
    ///
    /// **NOTE**: This string may contain unescaped `%` characters.
    /// However, most implementations of this train can simply ignore string
    /// fragments as they will be included (with properly escaped `%`
    /// characters) as part of the format string passed to
    /// [`PrintfFormatMacroGenerator::finalize`].
    ///
    /// See [`FormatMacroGenerator::string_fragment`] for a disambiguation
    /// between a string fragment and string conversion.
    fn string_fragment(&mut self, string: &str) -> Result<()>;

    /// Process an integer conversion.
    ///
    /// May optionally return a printf format string (i.e. "%d") to override the
    /// default.
    fn integer_conversion(&mut self, ty: Ident, expression: Arg) -> Result<Option<String>>;

    /// Process a string conversion.
    ///
    /// May optionally return a printf format string (i.e. "%s") to override the
    /// default.
    ///
    /// See [`FormatMacroGenerator::string_fragment`] for a disambiguation
    /// between a string fragment and string conversion.
    fn string_conversion(&mut self, expression: Arg) -> Result<Option<String>>;

    /// Process a character conversion.
    ///
    /// May optionally return a printf format string (i.e. "%c") to override the
    /// default.
    fn char_conversion(&mut self, expression: Arg) -> Result<Option<String>>;

    /// Process and untyped conversion.
    fn untyped_conversion(&mut self, _expression: Arg) -> Result<()> {
        Err(Error::new("untyped conversion not supported"))
    }
}

/// A fragment of a printf format string.
///
/// Printf format strings are built of multiple fragments.  These fragments can
/// be either a string ([`PrintfFormatStringFragment::String`]) or an expression
/// that evaluates to a `const &str` ([`PrintfFormatStringFragment::Expr`]).
/// These fragments can then be used to create a single `const &str` for use by
/// code generation.
///
/// # Example
/// ```
/// use pw_bytes::concat_static_strs;
/// use pw_format::macros::{PrintfFormatStringFragment, Result};
/// use quote::quote;
///
/// fn handle_fragments(format_string_fragments: &[PrintfFormatStringFragment]) -> Result<()> {
///   let format_string_pieces: Vec<_> = format_string_fragments
///     .iter()
///     .map(|fragment| fragment.as_token_stream("__pw_log_backend_crate"))
///     .collect::<Result<Vec<_>>>()?;
///
///   quote! {
///     let format_string = concat_static_strs!("prefix: ", #(#format_string_pieces),*, "\n");
///   };
///   Ok(())
/// }
/// ```
pub enum PrintfFormatStringFragment {
    /// A fragment that is a string.
    String(String),

    /// An expressions that can be converted to a `const &str`.
    Expr {
        /// Argument to convert.
        arg: Arg,
        /// Trait to used for getting the format specifier for the argument.
        ///
        /// One of `PrintfFormatter`, `PrintfHexFormatter`, `PrintfUpperHexFormatter
        format_trait: Ident,
    },
}

impl PrintfFormatStringFragment {
    /// Returns a [`proc_macro2::TokenStream`] representing this fragment.
    pub fn as_token_stream(&self, printf_formatter_trait_location: &str) -> Result<TokenStream2> {
        let crate_name = format_ident!("{}", printf_formatter_trait_location);
        match self {
            Self::String(s) => Ok(quote! {#s}),
            #[cfg(not(feature = "nightly_tait"))]
            Self::Expr { arg, format_trait } => {
                let Arg::ExprCast(cast) = arg else {
                    return Err(Error::new(&format!(
                      "Expected argument to untyped format (%v/{{}}) to be a cast expression (e.g. x as i32), but found {}.",
                      arg.to_token_stream()
                    )));
                };
                let ty = &cast.ty;
                Ok(quote! {
                  {
                    use #crate_name::#format_trait;
                    <#ty as #format_trait>::FORMAT_ARG
                  }
                })
            }
            #[cfg(feature = "nightly_tait")]
            Self::Expr { expr, format_trait } => Ok(quote! {
              {
                use #crate_name::#format_trait;
                type T = impl #format_trait;
                let _: &T = &(#expr);
                let arg = <T as #format_trait>::FORMAT_ARG;
                arg
              }
            }),
        }
    }
}

// Wraps a `PrintfFormatMacroGenerator` in a `FormatMacroGenerator` that
// generates the format string as it goes.
struct PrintfGenerator<GENERATOR: PrintfFormatMacroGenerator> {
    inner: GENERATOR,
    format_string_fragments: Vec<PrintfFormatStringFragment>,
}

impl<GENERATOR: PrintfFormatMacroGenerator> PrintfGenerator<GENERATOR> {
    // Append `format_string` to the current set of format string fragments.
    fn append_format_string(&mut self, format_string: &str) {
        // If the last fragment is a string, append to that.
        if let PrintfFormatStringFragment::String(s) = self
            .format_string_fragments
            .last_mut()
            .expect("format_string_fragments always has at least one entry")
        {
            s.push_str(format_string)
        } else {
            // If the last fragment is not a string, add a new string fragment.
            self.format_string_fragments
                .push(PrintfFormatStringFragment::String(
                    format_string.to_string(),
                ));
        }
    }
}

impl<GENERATOR: PrintfFormatMacroGenerator> FormatMacroGenerator for PrintfGenerator<GENERATOR> {
    fn finalize(self) -> Result<TokenStream2> {
        self.inner.finalize(&self.format_string_fragments)
    }

    fn string_fragment(&mut self, string: &str) -> Result<()> {
        // Escape '%' characters.
        let format_string = string.replace('%', "%%");

        self.append_format_string(&format_string);
        self.inner.string_fragment(string)
    }

    fn integer_conversion(
        &mut self,
        params: &FormatParams,
        signed: bool,
        type_width: u8, // in bits
        expression: Arg,
    ) -> Result<()> {
        let length_modifier = match type_width {
            8 => "hh",
            16 => "h",
            32 => "",
            64 => "ll",
            _ => {
                return Err(Error::new(&format!(
                    "printf backend does not support {} bit field width",
                    type_width
                )))
            }
        };

        let (conversion, ty) = match params.style {
            Style::None => {
                if signed {
                    ("d", format_ident!("i{type_width}"))
                } else {
                    ("u", format_ident!("u{type_width}"))
                }
            }
            Style::Octal => ("o", format_ident!("u{type_width}")),
            Style::Hex => ("x", format_ident!("u{type_width}")),
            Style::UpperHex => ("X", format_ident!("u{type_width}")),
            _ => {
                return Err(Error::new(&format!(
                    "printf backend does not support formatting integers with {:?} style",
                    params.style
                )))
            }
        };

        match self.inner.integer_conversion(ty, expression)? {
            Some(s) => self.append_format_string(&s),
            None => self.append_format_string(&format!(
                "%{}{}{}",
                params.field_params(),
                length_modifier,
                conversion
            )),
        }

        Ok(())
    }

    fn string_conversion(&mut self, expression: Arg) -> Result<()> {
        match self.inner.string_conversion(expression)? {
            Some(s) => self.append_format_string(&s),
            None => self.append_format_string("%s"),
        }
        Ok(())
    }

    fn char_conversion(&mut self, expression: Arg) -> Result<()> {
        match self.inner.char_conversion(expression)? {
            Some(s) => self.append_format_string(&s),
            None => self.append_format_string("%c"),
        }
        Ok(())
    }

    fn untyped_conversion(&mut self, expression: Arg, params: &FormatParams) -> Result<()> {
        self.inner.untyped_conversion(expression.clone())?;

        self.append_format_string(&format!("%{}", params.field_params()));
        self.format_string_fragments
            .push(PrintfFormatStringFragment::Expr {
                arg: expression,
                format_trait: params.printf_format_trait()?,
            });
        Ok(())
    }
}

/// Generate code for a `pw_format` style proc macro that needs a `printf` format string.
///
/// `generate_printf` is a specialized version of [`generate`] which works with
/// [`PrintfFormatMacroGenerator`]
pub fn generate_printf(
    generator: impl PrintfFormatMacroGenerator,
    format_and_args: FormatAndArgs,
) -> core::result::Result<TokenStream2, syn::Error> {
    let generator = PrintfGenerator {
        inner: generator,
        format_string_fragments: vec![PrintfFormatStringFragment::String("".into())],
    };
    generate(generator, format_and_args)
}

/// A specialized generator for proc macros that produce [`core::fmt`] style format strings.
///
/// For proc macros that need to translate a `pw_format` invocation into a
/// [`core::fmt`] style format string, `CoreFmtFormatMacroGenerator` offer a
/// specialized form of [`FormatMacroGenerator`] that builds the format string
/// and provides it as an argument to
/// [`finalize`](CoreFmtFormatMacroGenerator::finalize).
///
/// In cases where a generator needs to override the conversion specifier (i.e.
/// `{}`, it can return it from its appropriate conversion method.
pub trait CoreFmtFormatMacroGenerator {
    /// Called by [`generate_core_fmt`] at the end of code generation.
    ///
    /// Works like [`FormatMacroGenerator::finalize`] with the addition of
    /// being provided a [`core::fmt`] format string.
    fn finalize(self, format_string: String) -> Result<TokenStream2>;

    /// Process a string fragment.
    ///
    /// **NOTE**: This string may contain unescaped `{` and `}` characters.
    /// However, most implementations of this train can simply ignore string
    /// fragments as they will be included (with properly escaped `{` and `}`
    /// characters) as part of the format string passed to
    /// [`CoreFmtFormatMacroGenerator::finalize`].
    ///
    ///
    /// See [`FormatMacroGenerator::string_fragment`] for a disambiguation
    /// between a string fragment and string conversion.
    fn string_fragment(&mut self, string: &str) -> Result<()>;

    /// Process an integer conversion.
    fn integer_conversion(&mut self, ty: Ident, expression: Arg) -> Result<Option<String>>;

    /// Process a string conversion.
    fn string_conversion(&mut self, expression: Arg) -> Result<Option<String>>;

    /// Process a character conversion.
    fn char_conversion(&mut self, expression: Arg) -> Result<Option<String>>;

    /// Process an untyped conversion.
    fn untyped_conversion(&mut self, _expression: Arg) -> Result<()> {
        Err(Error::new("untyped conversion ({}) not supported"))
    }
}

// Wraps a `CoreFmtFormatMacroGenerator` in a `FormatMacroGenerator` that
// generates the format string as it goes.
struct CoreFmtGenerator<GENERATOR: CoreFmtFormatMacroGenerator> {
    inner: GENERATOR,
    format_string: String,
}

impl<GENERATOR: CoreFmtFormatMacroGenerator> FormatMacroGenerator for CoreFmtGenerator<GENERATOR> {
    fn finalize(self) -> Result<TokenStream2> {
        self.inner.finalize(self.format_string)
    }

    fn string_fragment(&mut self, string: &str) -> Result<()> {
        // Escape '{' and '} characters.
        let format_string = string.replace('{', "{{").replace('}', "}}");

        self.format_string.push_str(&format_string);
        self.inner.string_fragment(string)
    }

    fn integer_conversion(
        &mut self,
        params: &FormatParams,
        signed: bool,
        type_width: u8, // in bits
        expression: Arg,
    ) -> Result<()> {
        let ty = if signed {
            format_ident!("i{type_width}")
        } else {
            format_ident!("u{type_width}")
        };

        let conversion = params.core_fmt_specifier()?;

        match self.inner.integer_conversion(ty, expression)? {
            Some(s) => self.format_string.push_str(&s),
            None => self.format_string.push_str(&conversion),
        }

        Ok(())
    }

    fn string_conversion(&mut self, expression: Arg) -> Result<()> {
        match self.inner.string_conversion(expression)? {
            Some(s) => self.format_string.push_str(&s),
            None => self.format_string.push_str("{}"),
        }
        Ok(())
    }

    fn char_conversion(&mut self, expression: Arg) -> Result<()> {
        match self.inner.char_conversion(expression)? {
            Some(s) => self.format_string.push_str(&s),
            None => self.format_string.push_str("{}"),
        }
        Ok(())
    }

    fn untyped_conversion(&mut self, expression: Arg, params: &FormatParams) -> Result<()> {
        self.inner.untyped_conversion(expression)?;
        self.format_string.push_str(&params.core_fmt_specifier()?);
        Ok(())
    }
}

/// Generate code for a `pw_format` style proc macro that needs a [`core::fmt`] format string.
///
/// `generate_core_fmt` is a specialized version of [`generate`] which works with
/// [`CoreFmtFormatMacroGenerator`]
pub fn generate_core_fmt(
    generator: impl CoreFmtFormatMacroGenerator,
    format_and_args: FormatAndArgs,
) -> core::result::Result<TokenStream2, syn::Error> {
    let generator = CoreFmtGenerator {
        inner: generator,
        format_string: "".into(),
    };
    generate(generator, format_and_args)
}