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|
use proc_macro2::{Span, TokenStream};
use quote::quote;
use syn::parse::{Parse, ParseStream, Result};
use syn::punctuated::Punctuated;
use syn::spanned::Spanned;
use syn::{bracketed, parenthesized, token, Error, Expr, Ident, Pat, Token};
#[derive(Debug, Clone)]
struct ChildrenBranch {
pattern_span: Span,
pattern: Punctuated<ChildrenBranchPatternItem, Token![,]>,
body: Expr,
}
#[derive(Debug, Clone)]
enum ChildrenBranchPatternItem {
Single { rule_name: Ident, binder: Pat },
Multiple { rule_name: Ident, binder: Ident },
}
#[derive(Debug, Clone)]
struct ParseChildrenInput {
input_expr: Expr,
branches: Punctuated<ChildrenBranch, Token![,]>,
}
impl Parse for ChildrenBranch {
fn parse(input: ParseStream) -> Result<Self> {
let contents;
let _: token::Bracket = bracketed!(contents in input);
let pattern_unparsed: TokenStream = contents.fork().parse()?;
let pattern_span = pattern_unparsed.span();
let pattern = Punctuated::parse_terminated(&contents)?;
let _: Token![=>] = input.parse()?;
let body = input.parse()?;
Ok(ChildrenBranch {
pattern_span,
pattern,
body,
})
}
}
impl Parse for ChildrenBranchPatternItem {
fn parse(input: ParseStream) -> Result<Self> {
let contents;
let rule_name = input.parse()?;
parenthesized!(contents in input);
if input.peek(Token![..]) {
let binder = contents.parse()?;
let _: Token![..] = input.parse()?;
Ok(ChildrenBranchPatternItem::Multiple { rule_name, binder })
} else if input.is_empty() || input.peek(Token![,]) {
let binder = contents.parse()?;
Ok(ChildrenBranchPatternItem::Single { rule_name, binder })
} else {
Err(input.error("expected `..` or nothing"))
}
}
}
impl Parse for ParseChildrenInput {
fn parse(input: ParseStream) -> Result<Self> {
let input_expr = input.parse()?;
let _: Token![;] = input.parse()?;
let branches = Punctuated::parse_terminated(input)?;
Ok(ParseChildrenInput {
input_expr,
branches,
})
}
}
fn make_parser_branch(
branch: &ChildrenBranch,
i_inputs: &Ident,
) -> Result<TokenStream> {
use ChildrenBranchPatternItem::{Multiple, Single};
let body = &branch.body;
// Convert the input pattern into a pattern-match on the Rules of the children. This uses
// slice_patterns.
// A single pattern just checks that the rule matches; a variable-length pattern binds the
// subslice and checks, in the if-guard, that its elements all match the chosen Rule.
let i_variable_pattern =
Ident::new("___variable_pattern", Span::call_site());
let match_pat = branch.pattern.iter().map(|item| match item {
Single { rule_name, .. } => quote!(stringify!(#rule_name)),
Multiple { .. } => quote!(#i_variable_pattern @ ..),
});
let match_filter = branch.pattern.iter().map(|item| match item {
Single { .. } => quote!(),
Multiple { rule_name, .. } => quote!(
{
// We can't use .all() directly in the pattern guard; see
// https://github.com/rust-lang/rust/issues/59803.
let all_match = |slice: &[_]| {
slice.iter().all(|r|
*r == stringify!(#rule_name)
)
};
all_match(#i_variable_pattern)
} &&
),
});
// Once we have found a branch that matches, we need to parse the children.
let mut singles_before_multiple = Vec::new();
let mut multiple = None;
let mut singles_after_multiple = Vec::new();
for item in &branch.pattern {
match item {
Single {
rule_name, binder, ..
} => {
if multiple.is_none() {
singles_before_multiple.push((rule_name, binder))
} else {
singles_after_multiple.push((rule_name, binder))
}
}
Multiple {
rule_name, binder, ..
} => {
if multiple.is_none() {
multiple = Some((rule_name, binder))
} else {
return Err(Error::new(
branch.pattern_span.clone(),
"multiple variable-length patterns are not allowed",
));
}
}
}
}
let mut parses = Vec::new();
for (rule_name, binder) in singles_before_multiple.into_iter() {
parses.push(quote!(
let #binder = Self::#rule_name(
#i_inputs.next().unwrap()
)?;
))
}
// Note the `rev()`: we are taking inputs from the end of the iterator in reverse order, so that
// only the unmatched inputs are left for the variable-length pattern, if any.
for (rule_name, binder) in singles_after_multiple.into_iter().rev() {
parses.push(quote!(
let #binder = Self::#rule_name(
#i_inputs.next_back().unwrap()
)?;
))
}
if let Some((rule_name, binder)) = multiple {
parses.push(quote!(
let #binder = #i_inputs
.map(|i| Self::#rule_name(i))
.collect::<Result<Vec<_>, _>>()?
.into_iter();
))
}
Ok(quote!(
[#(#match_pat),*] if #(#match_filter)* true => {
#(#parses)*
#body
}
))
}
pub fn match_inputs(
input: proc_macro::TokenStream,
) -> Result<proc_macro2::TokenStream> {
let input: ParseChildrenInput = syn::parse(input)?;
let i_input_rules = Ident::new("___input_rules", Span::call_site());
let i_inputs = Ident::new("___inputs", Span::call_site());
let input_expr = &input.input_expr;
let branches = input
.branches
.iter()
.map(|br| make_parser_branch(br, &i_inputs))
.collect::<Result<Vec<_>>>()?;
Ok(quote!({
#[allow(unused_mut)]
let mut #i_inputs = #input_expr;
let #i_input_rules = #i_inputs.aliased_rules::<Self>();
let #i_input_rules: Vec<&str> = #i_input_rules
.iter()
.map(String::as_str)
.collect();
#[allow(unreachable_code)]
match #i_input_rules.as_slice() {
#(#branches,)*
[..] => return Err(#i_inputs.error(
format!("Unexpected children: {:?}", #i_input_rules)
)),
}
}))
}
|
