derive_syn_parse/

fields.rs

//! Handling for generating a `Parse` implementation using fields

use proc_macro2::{Span, TokenStream};
use quote::{format_ident, quote, quote_spanned, ToTokens};
use std::convert::{TryFrom, TryInto};
use syn::parse::{Parse, ParseStream};
use syn::spanned::Spanned;
use syn::{AttrStyle, Attribute, Expr, Fields, Ident, Path, Result, Token, Type};

pub(crate) fn generate_fn_body(
    base_tyname: &impl ToTokens,
    fields: Fields,
    with_return: bool,
) -> Result<TokenStream> {
    let initialize_self = initialize_type_or_variant(base_tyname, &fields);
    let parse_fields = fields
        .into_iter()
        .enumerate()
        .map(parse_field)
        .collect::<Result<Vec<_>>>()?;

    let maybe_return = match with_return {
        true => Token![return](Span::call_site()).into_token_stream(),
        false => TokenStream::new(),
    };

    Ok(quote! {
        #( #parse_fields )*

        #maybe_return Ok(#initialize_self)
    })
}

enum FieldAttr {
    Inside(Ident),
    Tree(TreeKind),
    Call(Expr),
    ParseTerminated(Path),
    Peek(PeekAttr),
    Prefix(NeighborAttr),
    Postfix(NeighborAttr),
}

enum TreeKind {
    Paren,
    Bracket,
    Brace,
}

enum ParseMethod {
    Tree(TreeKind, Span),
    Call(Expr),
    ParseTerminated(Path),
    Default,
}

enum PeekAttr {
    Peek(Expr),
    PeekWith(Expr),
    ParseIf(Expr),
}

// An attribute that's either a #[prefix] or #[postfix] directive. These can either be
// free-standing (i.e. discarded immediately after parsing) or named - saved for later parsing.
//
// They can additionally be specified as inside a particular token stream, so the full syntax is:
//
//   "#[prefix(" <Type> [ "as" <Ident> ] [ "in" <Ident> ] ")]"
struct NeighborAttr {
    parse_ty: Type,
    maybe_named: Option<Ident>,
    maybe_inside: Option<Ident>,
}

impl Parse for NeighborAttr {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let parse_ty = input.parse()?;
        let as_token: Option<Token![as]> = input.parse()?;
        let maybe_named = match as_token.is_some() {
            true => Some(input.parse()?),
            false => None,
        };

        let in_token: Option<Token![in]> = input.parse()?;
        let maybe_inside = match in_token.is_some() {
            true => Some(input.parse()?),
            false => None,
        };

        Ok(NeighborAttr {
            parse_ty,
            maybe_named,
            maybe_inside,
        })
    }
}

impl ParseMethod {
    fn is_default(&self) -> bool {
        matches!(self, Self::Default)
    }
}

struct FieldAttrs {
    prefix: Vec<NeighborAttr>,
    postfix: Vec<NeighborAttr>,
    inside: Option<Ident>,
    parse_method: ParseMethod,
    peek: Option<PeekAttr>,
}

struct ParseField {
    required_var_defs: Option<Ident>,
    parse_expr: TokenStream,
    pre_parse: TokenStream,
    post_parse: TokenStream,
}

// This needs to return tokenstreams because tuple structs use integer indices as field names
//
// For example: We'd end up writing the following
//   Ok(Self {
//       0: _field_0,
//       1: _field_1,
//       ...
//   })
//
// Otherwise, we would totally just return a list of identifiers
fn initialize_type_or_variant(name: &impl ToTokens, fields: &syn::Fields) -> TokenStream {
    use syn::Fields::{Named, Unit, Unnamed};

    match fields {
        Unit => name.to_token_stream(),
        Named(fields) => {
            let iter = fields.named.iter().map(|f| {
                f.ident
                    .as_ref()
                    .expect("named field was unnamed! the impossible is now possible!")
            });
            quote! {
                #name { #( #iter, )* }
            }
        }
        Unnamed(fields) => {
            let iter = fields
                .unnamed
                .iter()
                .enumerate()
                .map(|(i, f)| field_name_for_idx(i, f.span()));
            quote! {
                #name( #( #iter, )* )
            }
        }
    }
}

fn field_name_for_idx(idx: usize, span: Span) -> Ident {
    format_ident!("_field_{}", idx, span = span)
}

fn parse_field((idx, field): (usize, syn::Field)) -> Result<TokenStream> {
    let span = field.span();

    let assigned_name = field.ident.unwrap_or_else(|| field_name_for_idx(idx, span));

    let attrs = (field.attrs)
        .into_iter()
        .filter_map(try_as_field_attr)
        .collect::<Result<Vec<_>>>()?
        .try_into()?;

    let ParseField {
        required_var_defs,
        parse_expr,
        pre_parse,
        post_parse,
    } = handle_field_attrs(&assigned_name, field.ty.span(), attrs);

    // convert the Option to an iterator, so we can declare variables conditionally:
    let required_var_defs = required_var_defs.into_iter();
    let field_ty = field.ty;
    Ok(quote_spanned! {
        span=>
        #( let #required_var_defs; )*
        #pre_parse
        let #assigned_name: #field_ty = #parse_expr;
        #post_parse
    })
}

fn try_as_field_attr(attr: Attribute) -> Option<Result<(FieldAttr, Span)>> {
    let name = attr.path().get_ident()?.to_string();
    let span = attr.span();

    macro_rules! expect_outer_attr {
        ($name:literal) => {{
            if let AttrStyle::Inner(_) = attr.style {
                return Some(Err(syn::Error::new(
                    span,
                    concat!(
                        "the `#[",
                        $name,
                        "]` parsing attribute can only be used as an outer attribute"
                    ),
                )));
            }
        }};
    }

    #[rustfmt::skip]
    macro_rules! expect_no_attr_args {
        ($name:expr) => {{
            use syn::Meta::*;
            match attr.meta {
                List(args)      =>  return Some(Err(syn::Error::new(
                    span,
                    format!("the `#[{}]` parsing attribute does not expect arguments {:?}", $name, args.tokens.to_string()),
                ))),
                NameValue(val)  =>  return Some(Err(syn::Error::new(
                    span,
                    format!("the `#[{}]` parsing attribute does not expect value {:?}", $name, val.value.to_token_stream().to_string()),
                ))),
                _               =>  {}
            }
        }};
    }

    macro_rules! expect_parenthesis {
        ($attr_lit:literal) => {{
            use syn::{MacroDelimiter::*, Meta};
            match attr.meta {
                Meta::List(ref ml) => match ml.delimiter {
                    Paren(_) => {}
                    Brace(_) => {
                        return Some(Err(syn::Error::new(
                            span,
                            concat!(
                                "expected parenthesis in `#[",
                                $attr_lit,
                                "(...)]`, found braces"
                            ),
                        )))
                    }
                    Bracket(_) => {
                        return Some(Err(syn::Error::new(
                            span,
                            concat!(
                                "expected parenthesis in `#[",
                                $attr_lit,
                                "(...)]`, found brackets"
                            ),
                        )))
                    }
                },
                _ => {
                    return Some(Err(syn::Error::new(
                        span,
                        concat!("expected parenthesis in `#[", $attr_lit, "(...)]`"),
                    )))
                }
            }
        }};
    }
    match name.as_str() {
        "inside" => {
            expect_outer_attr!("inside(...)");
            expect_parenthesis!("inside");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Inside(id), span)),
            )
        }
        "call" => {
            expect_outer_attr!("call(...)");
            expect_parenthesis!("call");
            Some(attr.parse_args().map(move |id| (FieldAttr::Call(id), span)))
        }
        "parse_terminated" => {
            expect_outer_attr!("parse_terminated(...)");
            expect_parenthesis!("parse_terminated");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::ParseTerminated(id), span)),
            )
        }
        "paren" => {
            expect_outer_attr!("paren");
            expect_no_attr_args!("paren");
            Some(Ok((FieldAttr::Tree(TreeKind::Paren), span)))
        }
        "bracket" => {
            expect_outer_attr!("bracket");
            expect_no_attr_args!("bracket");
            Some(Ok((FieldAttr::Tree(TreeKind::Bracket), span)))
        }
        "brace" => {
            expect_outer_attr!("brace");
            expect_no_attr_args!("brace");
            Some(Ok((FieldAttr::Tree(TreeKind::Brace), span)))
        }
        "peek" => {
            expect_outer_attr!("peek(...)");
            expect_parenthesis!("peek");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Peek(PeekAttr::Peek(id)), span)),
            )
        }
        "peek_with" => {
            expect_outer_attr!("peek_with(...)");
            expect_parenthesis!("peek_with");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Peek(PeekAttr::PeekWith(id)), span)),
            )
        }
        "parse_if" => {
            expect_outer_attr!("parse_if(...)");
            expect_parenthesis!("parse_if");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Peek(PeekAttr::ParseIf(id)), span)),
            )
        }
        "prefix" => {
            expect_outer_attr!("prefix(...)");
            expect_parenthesis!("prefix");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Prefix(id), span)),
            )
        }
        "postfix" => {
            expect_outer_attr!("postifx(...)");
            expect_parenthesis!("postfix");
            Some(
                attr.parse_args()
                    .map(move |id| (FieldAttr::Postfix(id), span)),
            )
        }
        _ => None,
    }
}

impl TryFrom<Vec<(FieldAttr, Span)>> for FieldAttrs {
    type Error = syn::Error;

    fn try_from(vec: Vec<(FieldAttr, Span)>) -> Result<Self> {
        use FieldAttr::{Call, Inside, ParseTerminated, Peek, Postfix, Prefix, Tree};

        let mut inside = None;
        let mut peek = None;
        let mut parse_method = ParseMethod::Default;
        let mut prefix = Vec::new();
        let mut postfix = Vec::new();

        for (attr, span) in vec {
            match attr {
                Tree(_) | Call(_) | ParseTerminated(_) if !parse_method.is_default() => {
                    return Err(syn::Error::new(span, "parsing method specified twice"));
                }
                Inside(_) if inside.is_some() => {
                    return Err(syn::Error::new(
                        span,
                        "containing parse stream is specified twice",
                    ));
                }
                Peek(_) if peek.is_some() => {
                    return Err(syn::Error::new(span, "peeking can only be specified once"));
                }

                Call(expr) => parse_method = ParseMethod::Call(expr),
                ParseTerminated(path) => parse_method = ParseMethod::ParseTerminated(path),
                Tree(kind) => parse_method = ParseMethod::Tree(kind, span),
                Inside(name) => inside = Some(name),
                Peek(p) => peek = Some(p),

                Prefix(_) if inside.is_some() => {
                    return Err(syn::Error::new(
                        span,
                        "`#[prefix]` cannot be used after `#[inside]`. Perhaps try `#[prefix(<Type> in <token>)]`?",
                    ));
                }

                Postfix(_) if inside.is_some() => {
                    return Err(syn::Error::new(
                        span,
                        "`#[postfix]` cannot be used after `#[inside]`. Perhaps try `#[prefix(<Type> in <token>)]`?",
                    ));
                }

                Prefix(attr) => prefix.push(attr),
                Postfix(attr) => postfix.push(attr),
            }
        }

        Ok(FieldAttrs {
            prefix,
            postfix,
            inside,
            parse_method,
            peek,
        })
    }
}

fn handle_field_attrs(field_name: &Ident, ty_span: Span, attrs: FieldAttrs) -> ParseField {
    use ParseMethod::{Call, Default, ParseTerminated, Tree};

    let input_source = attrs
        .inside
        .as_ref()
        .map(tree_name)
        .unwrap_or_else(crate::parse_input);

    let required_var_defs;
    let mut parse_expr;

    match attrs.parse_method {
        Default => {
            required_var_defs = None;
            parse_expr = quote_spanned! { ty_span=> #input_source.parse()? };
        }
        Call(expr) => {
            required_var_defs = None;
            parse_expr = quote_spanned!(expr.span()=> {
                let result: ::syn::Result<_> = (#expr)(&#input_source);
                result?
            });
        }
        ParseTerminated(path) => {
            required_var_defs = None;
            parse_expr = quote_spanned! { path.span()=> #input_source.parse_terminated(#path)? };
        }
        Tree(tree_kind, span) => {
            required_var_defs = Some(tree_name(field_name));

            let macro_name = tree_kind.macro_name();
            let tree_name = tree_name(field_name);
            parse_expr = quote_spanned! { span=> ::syn::#macro_name!(#tree_name in #input_source) };
        }
    }

    if let Some(p) = attrs.peek {
        parse_expr = match p {
            PeekAttr::Peek(expr) => quote_spanned!(expr.span()=> match #input_source.peek(#expr) {
                true => Some(#parse_expr),
                false => None,
            }),
            PeekAttr::PeekWith(expr) => quote_spanned!(expr.span()=> match (#expr)(#input_source) {
                true => Some(#parse_expr),
                false => None,
            }),
            PeekAttr::ParseIf(expr) => quote_spanned!(expr.span()=> match #expr {
                true => Some(#parse_expr),
                false => None,
            }),
        };
    }

    let neighbor_map = |na: NeighborAttr| -> TokenStream {
        let assigned_name = na
            .maybe_named
            .unwrap_or_else(|| Ident::new("_", Span::call_site()));
        let source = na
            .maybe_inside
            .as_ref()
            .map(tree_name)
            .unwrap_or_else(crate::parse_input);
        let parse_ty = na.parse_ty;
        quote! {
            let #assigned_name: #parse_ty = #source.parse()?;
        }
    };

    let pre_parse = attrs.prefix.into_iter().map(neighbor_map).collect();
    let post_parse = attrs.postfix.into_iter().map(neighbor_map).collect();

    ParseField {
        required_var_defs,
        parse_expr,
        pre_parse,
        post_parse,
    }
}

fn tree_name(field_name: &Ident) -> Ident {
    format_ident!("__{}_backing_token_stream", field_name)
}

impl TreeKind {
    // Gives the name of the syn macro that corresponds to attempting parse the next token as a
    // certain group
    fn macro_name(&self) -> Ident {
        let span = Span::call_site();
        match self {
            TreeKind::Paren => Ident::new("parenthesized", span),
            TreeKind::Bracket => Ident::new("bracketed", span),
            TreeKind::Brace => Ident::new("braced", span),
        }
    }
}