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//! Common parsing utilities for derive macros.
//!
//! Fair parsing of [`syn::Expr`] requires [`syn`]'s `full` feature to be enabled, which unnecessary
//! increases compile times. As we don't have complex AST manipulation, usually requiring only
//! understanding where syntax item begins and ends, simpler manual parsing is implemented.
use proc_macro2::{Spacing, TokenStream};
use quote::ToTokens;
use syn::{
buffer::Cursor,
parse::{Parse, ParseStream},
};
/// [`syn::Expr`] [`Parse`]ing polyfill.
#[derive(Clone, Debug)]
pub(crate) enum Expr {
/// [`syn::Expr::Path`] of length 1 [`Parse`]ing polyfill.
Ident(syn::Ident),
/// Every other [`syn::Expr`] variant.
Other(TokenStream),
}
impl Expr {
/// Returns a [`syn::Ident`] in case this [`Expr`] is represented only by it.
///
/// [`syn::Ident`]: struct@syn::Ident
pub(crate) fn ident(&self) -> Option<&syn::Ident> {
match self {
Self::Ident(ident) => Some(ident),
Self::Other(_) => None,
}
}
}
impl From<syn::Ident> for Expr {
fn from(ident: syn::Ident) -> Self {
Self::Ident(ident)
}
}
impl Parse for Expr {
fn parse(input: ParseStream) -> syn::Result<Self> {
if let Ok(ident) = input.step(|c| {
c.ident()
.filter(|(_, c)| c.eof() || punct(',')(*c).is_some())
.ok_or_else(|| syn::Error::new(c.span(), "expected `ident(,|eof)`"))
}) {
Ok(Self::Ident(ident))
} else {
input.step(|c| {
take_until1(
alt([
&mut seq([
&mut path_sep,
&mut balanced_pair(punct('<'), punct('>')),
]),
&mut seq([
&mut balanced_pair(punct('<'), punct('>')),
&mut path_sep,
]),
&mut balanced_pair(punct('|'), punct('|')),
&mut token_tree,
]),
punct(','),
)(*c)
.map(|(stream, cursor)| (Self::Other(stream), cursor))
.ok_or_else(|| syn::Error::new(c.span(), "failed to parse expression"))
})
}
}
}
impl PartialEq<syn::Ident> for Expr {
fn eq(&self, other: &syn::Ident) -> bool {
self.ident().is_some_and(|i| i == other)
}
}
impl ToTokens for Expr {
fn to_tokens(&self, tokens: &mut TokenStream) {
match self {
Self::Ident(ident) => ident.to_tokens(tokens),
Self::Other(other) => other.to_tokens(tokens),
}
}
}
/// Result of parsing.
type ParsingResult<'a> = Option<(TokenStream, Cursor<'a>)>;
/// Tries to parse a [`token::PathSep`].
///
/// [`token::PathSep`]: struct@syn::token::PathSep
pub fn path_sep(c: Cursor<'_>) -> ParsingResult<'_> {
seq([
&mut punct_with_spacing(':', Spacing::Joint),
&mut punct(':'),
])(c)
}
/// Tries to parse a [`punct`] with [`Spacing`].
pub fn punct_with_spacing(
p: char,
spacing: Spacing,
) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_> {
move |c| {
c.punct().and_then(|(punct, c)| {
(punct.as_char() == p && punct.spacing() == spacing)
.then(|| (punct.into_token_stream(), c))
})
}
}
/// Tries to parse a [`Punct`].
///
/// [`Punct`]: proc_macro2::Punct
pub fn punct(p: char) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_> {
move |c| {
c.punct().and_then(|(punct, c)| {
(punct.as_char() == p).then(|| (punct.into_token_stream(), c))
})
}
}
/// Tries to parse any [`TokenTree`].
///
/// [`TokenTree`]: proc_macro2::TokenTree
pub fn token_tree(c: Cursor<'_>) -> ParsingResult<'_> {
c.token_tree().map(|(tt, c)| (tt.into_token_stream(), c))
}
/// Parses until balanced amount of `open` and `close` or eof.
///
/// [`Cursor`] should be pointing **right after** the first `open`ing.
pub fn balanced_pair(
mut open: impl FnMut(Cursor<'_>) -> ParsingResult<'_>,
mut close: impl FnMut(Cursor<'_>) -> ParsingResult<'_>,
) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_> {
move |c| {
let (mut out, mut c) = open(c)?;
let mut count = 1;
while count != 0 {
let (stream, cursor) = if let Some(closing) = close(c) {
count -= 1;
closing
} else if let Some(opening) = open(c) {
count += 1;
opening
} else {
let (tt, c) = c.token_tree()?;
(tt.into_token_stream(), c)
};
out.extend(stream);
c = cursor;
}
Some((out, c))
}
}
/// Tries to execute the provided sequence of `parsers`.
pub fn seq<const N: usize>(
mut parsers: [&mut dyn FnMut(Cursor<'_>) -> ParsingResult<'_>; N],
) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_> + '_ {
move |c| {
parsers.iter_mut().try_fold(
(TokenStream::new(), c),
|(mut out, mut c), parser| {
let (stream, cursor) = parser(c)?;
out.extend(stream);
c = cursor;
Some((out, c))
},
)
}
}
/// Tries to execute the first successful parser.
pub fn alt<const N: usize>(
mut parsers: [&mut dyn FnMut(Cursor<'_>) -> ParsingResult<'_>; N],
) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_> + '_ {
move |c| parsers.iter_mut().find_map(|parser| parser(c))
}
/// Parses with `basic` while `until` fails. Returns [`None`] in case
/// `until` succeeded initially or `basic` never succeeded. Doesn't consume
/// tokens parsed by `until`.
pub fn take_until1<P, U>(
mut parser: P,
mut until: U,
) -> impl FnMut(Cursor<'_>) -> ParsingResult<'_>
where
P: FnMut(Cursor<'_>) -> ParsingResult<'_>,
U: FnMut(Cursor<'_>) -> ParsingResult<'_>,
{
move |mut cursor| {
let mut out = TokenStream::new();
let mut parsed = false;
loop {
if cursor.eof() || until(cursor).is_some() {
return parsed.then_some((out, cursor));
}
let (stream, c) = parser(cursor)?;
out.extend(stream);
cursor = c;
parsed = true;
}
}
}
#[cfg(test)]
mod spec {
use std::{fmt::Debug, str::FromStr};
use itertools::Itertools as _;
use proc_macro2::TokenStream;
use quote::ToTokens;
use syn::{
parse::{Parse, Parser as _},
punctuated::Punctuated,
token::Comma,
};
use super::Expr;
fn assert<'a, T: Debug + Parse + ToTokens>(
input: &'a str,
parsed: impl AsRef<[&'a str]>,
) {
let parsed = parsed.as_ref();
let punctuated = Punctuated::<T, Comma>::parse_terminated
.parse2(TokenStream::from_str(input).unwrap())
.unwrap();
assert_eq!(
parsed.len(),
punctuated.len(),
"Wrong length\n\
Expected: {parsed:?}\n\
Found: {punctuated:?}",
);
punctuated
.iter()
.map(|ty| ty.to_token_stream().to_string())
.zip(parsed)
.enumerate()
.for_each(|(i, (found, expected))| {
assert_eq!(
*expected, &found,
"Mismatch at index {i}\n\
Expected: {parsed:?}\n\
Found: {punctuated:?}",
);
});
}
mod expr {
use super::*;
#[test]
fn cases() {
let cases = [
"ident",
"[a , b , c , d]",
"counter += 1",
"async { fut . await }",
"a < b",
"a > b",
"{ let x = (a , b) ; }",
"invoke (a , b)",
"foo as f64",
"| a , b | a + b",
"obj . k",
"for pat in expr { break pat ; }",
"if expr { true } else { false }",
"vector [2]",
"1",
"\"foo\"",
"loop { break i ; }",
"format ! (\"{}\" , q)",
"match n { Some (n) => { } , None => { } }",
"x . foo ::< T > (a , b)",
"x . foo ::< T < [T < T >; if a < b { 1 } else { 2 }] >, { a < b } > (a , b)",
"(a + b)",
"i32 :: MAX",
"1 .. 2",
"& a",
"[0u8 ; N]",
"(a , b , c , d)",
"< Ty as Trait > :: T",
"< Ty < Ty < T >, { a < b } > as Trait < T > > :: T",
];
assert::<Expr>("", []);
for i in 1..4 {
for permutations in cases.into_iter().permutations(i) {
let mut input = permutations.clone().join(",");
assert::<Expr>(&input, &permutations);
input.push(',');
assert::<Expr>(&input, &permutations);
}
}
}
}
}