compiler/rustc_expand/src/mbe/metavar_expr.rs

   1 use rustc_ast::token::{self, Delimiter};
   2 use rustc_ast::tokenstream::{Cursor, TokenStream, TokenTree};
   3 use rustc_ast::{LitIntType, LitKind};
   4 use rustc_ast_pretty::pprust;
   5 use rustc_errors::{Applicability, PResult};
   6 use rustc_session::parse::ParseSess;
   7 use rustc_span::symbol::Ident;
   8 use rustc_span::Span;
   9
  10 /// A meta-variable expression, for expansions based on properties of meta-variables.
  11 #[derive(Debug, Clone, PartialEq, Encodable, Decodable)]
  12 crate enum MetaVarExpr {
  13     /// The number of repetitions of an identifier, optionally limited to a number
  14     /// of outer-most repetition depths. If the depth limit is `None` then the depth is unlimited.
  15     Count(Ident, Option<usize>),
  16
  17     /// Ignore a meta-variable for repetition without expansion.
  18     Ignore(Ident),
  19
  20     /// The index of the repetition at a particular depth, where 0 is the inner-most
  21     /// repetition. The `usize` is the depth.
  22     Index(usize),
  23
  24     /// The length of the repetition at a particular depth, where 0 is the inner-most
  25     /// repetition. The `usize` is the depth.
  26     Length(usize),
  27 }
  28
  29 impl MetaVarExpr {
  30     /// Attempt to parse a meta-variable expression from a token stream.
  31     crate fn parse<'sess>(
  32         input: &TokenStream,
  33         outer_span: Span,
  34         sess: &'sess ParseSess,
  35     ) -> PResult<'sess, MetaVarExpr> {
  36         let mut tts = input.trees();
  37         let ident = parse_ident(&mut tts, sess, outer_span)?;
  38         let Some(TokenTree::Delimited(_, Delimiter::Parenthesis, args)) = tts.next() else {
  39             let msg = "meta-variable expression parameter must be wrapped in parentheses";
  40             return Err(sess.span_diagnostic.struct_span_err(ident.span, msg));
  41         };
  42         check_trailing_token(&mut tts, sess)?;
  43         let mut iter = args.trees();
  44         let rslt = match &*ident.as_str() {
  45             "count" => parse_count(&mut iter, sess, ident.span)?,
  46             "ignore" => MetaVarExpr::Ignore(parse_ident(&mut iter, sess, ident.span)?),
  47             "index" => MetaVarExpr::Index(parse_depth(&mut iter, sess, ident.span)?),
  48             "length" => MetaVarExpr::Length(parse_depth(&mut iter, sess, ident.span)?),
  49             _ => {
  50                 let err_msg = "unrecognized meta-variable expression";
  51                 let mut err = sess.span_diagnostic.struct_span_err(ident.span, err_msg);
  52                 err.span_suggestion(
  53                     ident.span,
  54                     "supported expressions are count, ignore, index and length",
  55                     String::new(),
  56                     Applicability::MachineApplicable,
  57                 );
  58                 return Err(err);
  59             }
  60         };
  61         check_trailing_token(&mut iter, sess)?;
  62         Ok(rslt)
  63     }
  64
  65     crate fn ident(&self) -> Option<Ident> {
  66         match *self {
  67             MetaVarExpr::Count(ident, _) | MetaVarExpr::Ignore(ident) => Some(ident),
  68             MetaVarExpr::Index(..) | MetaVarExpr::Length(..) => None,
  69         }
  70     }
  71 }
  72
  73 // Checks if there are any remaining tokens. For example, `${ignore(ident ... a b c ...)}`
  74 fn check_trailing_token<'sess>(iter: &mut Cursor, sess: &'sess ParseSess) -> PResult<'sess, ()> {
  75     if let Some(tt) = iter.next() {
  76         let mut diag = sess.span_diagnostic.struct_span_err(
  77             tt.span(),
  78             &format!("unexpected token: {}", pprust::tt_to_string(&tt)),
  79         );
  80         diag.span_note(tt.span(), "meta-variable expression must not have trailing tokens");
  81         Err(diag)
  82     } else {
  83         Ok(())
  84     }
  85 }
  86
  87 /// Parse a meta-variable `count` expression: `count(ident[, depth])`
  88 fn parse_count<'sess>(
  89     iter: &mut Cursor,
  90     sess: &'sess ParseSess,
  91     span: Span,
  92 ) -> PResult<'sess, MetaVarExpr> {
  93     let ident = parse_ident(iter, sess, span)?;
  94     let depth = if try_eat_comma(iter) { Some(parse_depth(iter, sess, span)?) } else { None };
  95     Ok(MetaVarExpr::Count(ident, depth))
  96 }
  97
  98 /// Parses the depth used by index(depth) and length(depth).
  99 fn parse_depth<'sess>(
 100     iter: &mut Cursor,
 101     sess: &'sess ParseSess,
 102     span: Span,
 103 ) -> PResult<'sess, usize> {
 104     let Some(tt) = iter.next() else { return Ok(0) };
 105     let TokenTree::Token(token::Token {
 106         kind: token::TokenKind::Literal(lit), ..
 107     }) = tt else {
 108         return Err(sess.span_diagnostic.struct_span_err(
 109             span,
 110             "meta-variable expression depth must be a literal"
 111         ));
 112     };
 113     if let Ok(lit_kind) = LitKind::from_lit_token(lit)
 114         && let LitKind::Int(n_u128, LitIntType::Unsuffixed) = lit_kind
 115         && let Ok(n_usize) = usize::try_from(n_u128)
 116     {
 117         Ok(n_usize)
 118     }
 119     else {
 120         let msg = "only unsuffixes integer literals are supported in meta-variable expressions";
 121         Err(sess.span_diagnostic.struct_span_err(span, msg))
 122     }
 123 }
 124
 125 /// Parses an generic ident
 126 fn parse_ident<'sess>(
 127     iter: &mut Cursor,
 128     sess: &'sess ParseSess,
 129     span: Span,
 130 ) -> PResult<'sess, Ident> {
 131     if let Some(tt) = iter.next() && let TokenTree::Token(token) = tt {
 132         if let Some((elem, false)) = token.ident() {
 133             return Ok(elem);
 134         }
 135         let token_str = pprust::token_to_string(&token);
 136         let mut err = sess.span_diagnostic.struct_span_err(
 137             span,
 138             &format!("expected identifier, found `{}`", &token_str)
 139         );
 140         err.span_suggestion(
 141             token.span,
 142             &format!("try removing `{}`", &token_str),
 143             String::new(),
 144             Applicability::MaybeIncorrect,
 145         );
 146         return Err(err);
 147     }
 148     Err(sess.span_diagnostic.struct_span_err(span, "expected identifier"))
 149 }
 150
 151 /// Tries to move the iterator forward returning `true` if there is a comma. If not, then the
 152 /// iterator is not modified and the result is `false`.
 153 fn try_eat_comma(iter: &mut Cursor) -> bool {
 154     if let Some(TokenTree::Token(token::Token { kind: token::Comma, .. })) = iter.look_ahead(0) {
 155         let _ = iter.next();
 156         return true;
 157     }
 158     false
 159 }