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, 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, } impl Parse for ChildrenBranch { fn parse(input: ParseStream) -> Result { 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 { 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 { 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 { 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::, _>>()? .into_iter(); )) } Ok(quote!( [#(#match_pat),*] if #(#match_filter)* true => { #(#parses)* #body } )) } pub fn match_inputs( input: proc_macro::TokenStream, ) -> Result { 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::>>()?; Ok(quote!({ #[allow(unused_mut)] let mut #i_inputs = #input_expr; let #i_input_rules = #i_inputs.aliased_rules::(); 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) )), } })) }