Auto merge of #132629 - nnethercote:124141-preliminaries, r=petrochenkov

#124141 preliminaries

Preliminary changes required to start removing `Nonterminal` (https://github.com/rust-lang/rust/pull/124141).

r? `@petrochenkov`

compiler/rustc_ast/src/attr/mod.rs

@@ -457,7 +457,7 @@ impl MetaItemKind {
         tokens: &mut impl Iterator<Item = &'a TokenTree>,
     ) -> Option<MetaItemKind> {
         match tokens.next() {
-            Some(TokenTree::Delimited(.., Delimiter::Invisible, inner_tokens)) => {
+            Some(TokenTree::Delimited(.., Delimiter::Invisible(_), inner_tokens)) => {
                 MetaItemKind::name_value_from_tokens(&mut inner_tokens.trees())
             }
             Some(TokenTree::Token(token, _)) => {
@@ -605,7 +605,7 @@ impl MetaItemInner {
                 tokens.next();
                 return Some(MetaItemInner::Lit(lit));
             }
-            Some(TokenTree::Delimited(.., Delimiter::Invisible, inner_tokens)) => {
+            Some(TokenTree::Delimited(.., Delimiter::Invisible(_), inner_tokens)) => {
                 tokens.next();
                 return MetaItemInner::from_tokens(&mut inner_tokens.trees().peekable());
             }

compiler/rustc_ast/src/token.rs

@@ -42,11 +42,86 @@ pub enum BinOpToken {
     Shr,
 }
 
+// This type must not implement `Hash` due to the unusual `PartialEq` impl below.
+#[derive(Copy, Clone, Debug, Encodable, Decodable, HashStable_Generic)]
+pub enum InvisibleOrigin {
+    // From the expansion of a metavariable in a declarative macro.
+    MetaVar(MetaVarKind),
+
+    // Converted from `proc_macro::Delimiter` in
+    // `proc_macro::Delimiter::to_internal`, i.e. returned by a proc macro.
+    ProcMacro,
+
+    // Converted from `TokenKind::Interpolated` in
+    // `TokenStream::flatten_token`. Treated similarly to `ProcMacro`.
+    FlattenToken,
+}
+
+impl PartialEq for InvisibleOrigin {
+    #[inline]
+    fn eq(&self, _other: &InvisibleOrigin) -> bool {
+        // When we had AST-based nonterminals we couldn't compare them, and the
+        // old `Nonterminal` type had an `eq` that always returned false,
+        // resulting in this restriction:
+        // https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment
+        // This `eq` emulates that behaviour. We could consider lifting this
+        // restriction now but there are still cases involving invisible
+        // delimiters that make it harder than it first appears.
+        false
+    }
+}
+
+/// Annoyingly similar to `NonterminalKind`, but the slight differences are important.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
+pub enum MetaVarKind {
+    Item,
+    Block,
+    Stmt,
+    Pat(NtPatKind),
+    Expr {
+        kind: NtExprKind,
+        // This field is needed for `Token::can_begin_literal_maybe_minus`.
+        can_begin_literal_maybe_minus: bool,
+        // This field is needed for `Token::can_begin_string_literal`.
+        can_begin_string_literal: bool,
+    },
+    Ty,
+    Ident,
+    Lifetime,
+    Literal,
+    Meta,
+    Path,
+    Vis,
+    TT,
+}
+
+impl fmt::Display for MetaVarKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let sym = match self {
+            MetaVarKind::Item => sym::item,
+            MetaVarKind::Block => sym::block,
+            MetaVarKind::Stmt => sym::stmt,
+            MetaVarKind::Pat(PatParam { inferred: true } | PatWithOr) => sym::pat,
+            MetaVarKind::Pat(PatParam { inferred: false }) => sym::pat_param,
+            MetaVarKind::Expr { kind: Expr2021 { inferred: true } | Expr, .. } => sym::expr,
+            MetaVarKind::Expr { kind: Expr2021 { inferred: false }, .. } => sym::expr_2021,
+            MetaVarKind::Ty => sym::ty,
+            MetaVarKind::Ident => sym::ident,
+            MetaVarKind::Lifetime => sym::lifetime,
+            MetaVarKind::Literal => sym::literal,
+            MetaVarKind::Meta => sym::meta,
+            MetaVarKind::Path => sym::path,
+            MetaVarKind::Vis => sym::vis,
+            MetaVarKind::TT => sym::tt,
+        };
+        write!(f, "{sym}")
+    }
+}
+
 /// Describes how a sequence of token trees is delimited.
 /// Cannot use `proc_macro::Delimiter` directly because this
 /// structure should implement some additional traits.
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
-#[derive(Encodable, Decodable, Hash, HashStable_Generic)]
+#[derive(Copy, Clone, Debug, PartialEq, Encodable, Decodable, HashStable_Generic)]
 pub enum Delimiter {
     /// `( ... )`
     Parenthesis,
@@ -59,7 +134,34 @@ pub enum Delimiter {
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.
     /// Invisible delimiters might not survive roundtrip of a token stream through a string.
-    Invisible,
+    Invisible(InvisibleOrigin),
+}
+
+impl Delimiter {
+    // Should the parser skip these delimiters? Only happens for certain kinds
+    // of invisible delimiters. Ideally this function will eventually disappear
+    // and no invisible delimiters will be skipped.
+    #[inline]
+    pub fn skip(&self) -> bool {
+        match self {
+            Delimiter::Parenthesis | Delimiter::Bracket | Delimiter::Brace => false,
+            Delimiter::Invisible(InvisibleOrigin::MetaVar(_)) => false,
+            Delimiter::Invisible(InvisibleOrigin::FlattenToken | InvisibleOrigin::ProcMacro) => {
+                true
+            }
+        }
+    }
+
+    // This exists because `InvisibleOrigin`s should be compared. It is only used for assertions.
+    pub fn eq_ignoring_invisible_origin(&self, other: &Delimiter) -> bool {
+        match (self, other) {
+            (Delimiter::Parenthesis, Delimiter::Parenthesis) => true,
+            (Delimiter::Brace, Delimiter::Brace) => true,
+            (Delimiter::Bracket, Delimiter::Bracket) => true,
+            (Delimiter::Invisible(_), Delimiter::Invisible(_)) => true,
+            _ => false,
+        }
+    }
 }
 
 // Note that the suffix is *not* considered when deciding the `LitKind` in this
@@ -496,10 +598,11 @@ impl Token {
     /// **NB**: Take care when modifying this function, since it will change
     /// the stable set of tokens that are allowed to match an expr nonterminal.
     pub fn can_begin_expr(&self) -> bool {
+        use Delimiter::*;
         match self.uninterpolate().kind {
             Ident(name, is_raw)              =>
                 ident_can_begin_expr(name, self.span, is_raw), // value name or keyword
-            OpenDelim(..)                     | // tuple, array or block
+            OpenDelim(Parenthesis | Brace | Bracket) | // tuple, array or block
             Literal(..)                       | // literal
             Not                               | // operator not
             BinOp(Minus)                      | // unary minus
@@ -520,6 +623,12 @@ impl Token {
                     NtLiteral(..) |
                     NtPath(..)
                 ),
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
+                MetaVarKind::Block |
+                MetaVarKind::Expr { .. } |
+                MetaVarKind::Literal |
+                MetaVarKind::Path
+            ))) => true,
             _ => false,
         }
     }
@@ -553,6 +662,14 @@ impl Token {
                     | NtPath(..)
                     | NtTy(..)
                 ),
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
+                MetaVarKind::Expr { .. } |
+                MetaVarKind::Literal |
+                MetaVarKind::Meta |
+                MetaVarKind::Pat(_) |
+                MetaVarKind::Path |
+                MetaVarKind::Ty
+            ))) => true,
             _ => false,
         }
     }
@@ -573,6 +690,10 @@ impl Token {
             Lt | BinOp(Shl)             | // associated path
             PathSep                      => true, // global path
             Interpolated(ref nt) => matches!(&**nt, NtTy(..) | NtPath(..)),
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
+                MetaVarKind::Ty |
+                MetaVarKind::Path
+            ))) => true,
             // For anonymous structs or unions, which only appear in specific positions
             // (type of struct fields or union fields), we don't consider them as regular types
             _ => false,
@@ -585,6 +706,9 @@ impl Token {
             OpenDelim(Delimiter::Brace) | Literal(..) | BinOp(Minus) => true,
             Ident(name, IdentIsRaw::No) if name.is_bool_lit() => true,
             Interpolated(ref nt) => matches!(&**nt, NtExpr(..) | NtBlock(..) | NtLiteral(..)),
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(
+                MetaVarKind::Expr { .. } | MetaVarKind::Block | MetaVarKind::Literal,
+            ))) => true,
             _ => false,
         }
     }
@@ -641,6 +765,13 @@ impl Token {
                 },
                 _ => false,
             },
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(mv_kind))) => match mv_kind {
+                MetaVarKind::Literal => true,
+                MetaVarKind::Expr { can_begin_literal_maybe_minus, .. } => {
+                    can_begin_literal_maybe_minus
+                }
+                _ => false,
+            },
             _ => false,
         }
     }
@@ -656,6 +787,11 @@ impl Token {
                 },
                 _ => false,
             },
+            OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(mv_kind))) => match mv_kind {
+                MetaVarKind::Literal => true,
+                MetaVarKind::Expr { can_begin_string_literal, .. } => can_begin_string_literal,
+                _ => false,
+            },
             _ => false,
         }
     }
@@ -896,7 +1032,7 @@ impl PartialEq<TokenKind> for Token {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
 pub enum NtPatKind {
     // Matches or-patterns. Was written using `pat` in edition 2021 or later.
     PatWithOr,
@@ -906,7 +1042,7 @@ pub enum NtPatKind {
     PatParam { inferred: bool },
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
 pub enum NtExprKind {
     // Matches expressions using the post-edition 2024. Was written using
     // `expr` in edition 2024 or later.
@@ -933,7 +1069,7 @@ pub enum Nonterminal {
     NtVis(P<ast::Visibility>),
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, HashStable_Generic)]
 pub enum NonterminalKind {
     Item,
     Block,

compiler/rustc_ast/src/tokenstream.rs

@@ -24,7 +24,7 @@ use rustc_span::{DUMMY_SP, Span, SpanDecoder, SpanEncoder, Symbol, sym};
 
 use crate::ast::{AttrStyle, StmtKind};
 use crate::ast_traits::{HasAttrs, HasTokens};
-use crate::token::{self, Delimiter, Nonterminal, Token, TokenKind};
+use crate::token::{self, Delimiter, InvisibleOrigin, Nonterminal, Token, TokenKind};
 use crate::{AttrVec, Attribute};
 
 /// Part of a `TokenStream`.
@@ -484,13 +484,13 @@ impl TokenStream {
             token::NtLifetime(ident, is_raw) => TokenTree::Delimited(
                 DelimSpan::from_single(token.span),
                 DelimSpacing::new(Spacing::JointHidden, spacing),
-                Delimiter::Invisible,
+                Delimiter::Invisible(InvisibleOrigin::FlattenToken),
                 TokenStream::token_alone(token::Lifetime(ident.name, is_raw), ident.span),
             ),
             token::Interpolated(ref nt) => TokenTree::Delimited(
                 DelimSpan::from_single(token.span),
                 DelimSpacing::new(Spacing::JointHidden, spacing),
-                Delimiter::Invisible,
+                Delimiter::Invisible(InvisibleOrigin::FlattenToken),
                 TokenStream::from_nonterminal_ast(&nt).flattened(),
             ),
             _ => TokenTree::Token(token.clone(), spacing),

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -942,9 +942,8 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             token::CloseDelim(Delimiter::Bracket) => "]".into(),
             token::OpenDelim(Delimiter::Brace) => "{".into(),
             token::CloseDelim(Delimiter::Brace) => "}".into(),
-            token::OpenDelim(Delimiter::Invisible) | token::CloseDelim(Delimiter::Invisible) => {
-                "".into()
-            }
+            token::OpenDelim(Delimiter::Invisible(_))
+            | token::CloseDelim(Delimiter::Invisible(_)) => "".into(),
             token::Pound => "#".into(),
             token::Dollar => "$".into(),
             token::Question => "?".into(),

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 
-use rustc_ast::token::{self, Token, TokenKind};
+use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::{Applicability, Diag, DiagCtxtHandle, DiagMessage};
 use rustc_macros::Subdiagnostic;
@@ -68,7 +68,9 @@ pub(super) fn failed_to_match_macro(
 
     if let MatcherLoc::Token { token: expected_token } = &remaining_matcher
         && (matches!(expected_token.kind, TokenKind::Interpolated(_))
-            || matches!(token.kind, TokenKind::Interpolated(_)))
+            || matches!(token.kind, TokenKind::Interpolated(_))
+            || matches!(expected_token.kind, TokenKind::OpenDelim(Delimiter::Invisible(_)))
+            || matches!(token.kind, TokenKind::OpenDelim(Delimiter::Invisible(_))))
     {
         err.note("captured metavariables except for `:tt`, `:ident` and `:lifetime` cannot be compared to other tokens");
         err.note("see <https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment> for more information");

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -693,7 +693,7 @@ fn has_compile_error_macro(rhs: &mbe::TokenTree) -> bool {
                     && let mbe::TokenTree::Token(bang) = bang
                     && let TokenKind::Not = bang.kind
                     && let mbe::TokenTree::Delimited(.., del) = args
-                    && del.delim != Delimiter::Invisible
+                    && !del.delim.skip()
                 {
                     true
                 } else {

compiler/rustc_expand/src/mbe/quoted.rs

@@ -165,11 +165,12 @@ fn parse_tree<'a>(
             // during parsing.
             let mut next = outer_trees.next();
             let mut trees: Box<dyn Iterator<Item = &tokenstream::TokenTree>>;
-            if let Some(tokenstream::TokenTree::Delimited(.., Delimiter::Invisible, tts)) = next {
+            match next {
+                Some(tokenstream::TokenTree::Delimited(.., delim, tts)) if delim.skip() => {
                     trees = Box::new(tts.trees());
                     next = trees.next();
-            } else {
-                trees = Box::new(outer_trees);
+                }
+                _ => trees = Box::new(outer_trees),
             }
 
             match next {

compiler/rustc_expand/src/proc_macro_server.rs

@@ -38,7 +38,7 @@ impl FromInternal<token::Delimiter> for Delimiter {
             token::Delimiter::Parenthesis => Delimiter::Parenthesis,
             token::Delimiter::Brace => Delimiter::Brace,
             token::Delimiter::Bracket => Delimiter::Bracket,
-            token::Delimiter::Invisible => Delimiter::None,
+            token::Delimiter::Invisible(_) => Delimiter::None,
         }
     }
 }
@@ -49,7 +49,7 @@ impl ToInternal<token::Delimiter> for Delimiter {
             Delimiter::Parenthesis => token::Delimiter::Parenthesis,
             Delimiter::Brace => token::Delimiter::Brace,
             Delimiter::Bracket => token::Delimiter::Bracket,
-            Delimiter::None => token::Delimiter::Invisible,
+            Delimiter::None => token::Delimiter::Invisible(token::InvisibleOrigin::ProcMacro),
         }
     }
 }

compiler/rustc_parse/messages.ftl

@@ -216,6 +216,9 @@ parse_expected_identifier_found_doc_comment = expected identifier, found doc com
 parse_expected_identifier_found_doc_comment_str = expected identifier, found doc comment `{$token}`
 parse_expected_identifier_found_keyword = expected identifier, found keyword
 parse_expected_identifier_found_keyword_str = expected identifier, found keyword `{$token}`
+parse_expected_identifier_found_metavar = expected identifier, found metavariable
+# This one deliberately doesn't print a token.
+parse_expected_identifier_found_metavar_str = expected identifier, found metavariable
 parse_expected_identifier_found_reserved_identifier = expected identifier, found reserved identifier
 parse_expected_identifier_found_reserved_identifier_str = expected identifier, found reserved identifier `{$token}`
 parse_expected_identifier_found_reserved_keyword = expected identifier, found reserved keyword
@@ -227,6 +230,8 @@ parse_expected_mut_or_const_in_raw_pointer_type = expected `mut` or `const` keyw
 
 parse_expected_semi_found_doc_comment_str = expected `;`, found doc comment `{$token}`
 parse_expected_semi_found_keyword_str = expected `;`, found keyword `{$token}`
+# This one deliberately doesn't print a token.
+parse_expected_semi_found_metavar_str = expected `;`, found metavariable
 parse_expected_semi_found_reserved_identifier_str = expected `;`, found reserved identifier `{$token}`
 parse_expected_semi_found_reserved_keyword_str = expected `;`, found reserved keyword `{$token}`
 parse_expected_semi_found_str = expected `;`, found `{$token}`
@@ -864,6 +869,8 @@ parse_unexpected_token_after_not_logical = use `!` to perform logical negation
 parse_unexpected_token_after_struct_name = expected `where`, `{"{"}`, `(`, or `;` after struct name
 parse_unexpected_token_after_struct_name_found_doc_comment = expected `where`, `{"{"}`, `(`, or `;` after struct name, found doc comment `{$token}`
 parse_unexpected_token_after_struct_name_found_keyword = expected `where`, `{"{"}`, `(`, or `;` after struct name, found keyword `{$token}`
+# This one deliberately doesn't print a token.
+parse_unexpected_token_after_struct_name_found_metavar = expected `where`, `{"{"}`, `(`, or `;` after struct name, found metavar
 parse_unexpected_token_after_struct_name_found_other = expected `where`, `{"{"}`, `(`, or `;` after struct name, found `{$token}`
 
 parse_unexpected_token_after_struct_name_found_reserved_identifier = expected `where`, `{"{"}`, `(`, or `;` after struct name, found reserved identifier `{$token}`

compiler/rustc_parse/src/errors.rs

@@ -1086,6 +1086,8 @@ pub(crate) enum ExpectedIdentifierFound {
     ReservedKeyword(#[primary_span] Span),
     #[label(parse_expected_identifier_found_doc_comment)]
     DocComment(#[primary_span] Span),
+    #[label(parse_expected_identifier_found_metavar)]
+    MetaVar(#[primary_span] Span),
     #[label(parse_expected_identifier)]
     Other(#[primary_span] Span),
 }
@@ -1099,6 +1101,7 @@ impl ExpectedIdentifierFound {
             Some(TokenDescription::Keyword) => ExpectedIdentifierFound::Keyword,
             Some(TokenDescription::ReservedKeyword) => ExpectedIdentifierFound::ReservedKeyword,
             Some(TokenDescription::DocComment) => ExpectedIdentifierFound::DocComment,
+            Some(TokenDescription::MetaVar(_)) => ExpectedIdentifierFound::MetaVar,
             None => ExpectedIdentifierFound::Other,
         })(span)
     }
@@ -1117,6 +1120,7 @@ impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for ExpectedIdentifier {
     fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, G> {
         let token_descr = TokenDescription::from_token(&self.token);
 
+        let mut add_token = true;
         let mut diag = Diag::new(dcx, level, match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
                 fluent::parse_expected_identifier_found_reserved_identifier_str
@@ -1128,10 +1132,16 @@ impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for ExpectedIdentifier {
             Some(TokenDescription::DocComment) => {
                 fluent::parse_expected_identifier_found_doc_comment_str
             }
+            Some(TokenDescription::MetaVar(_)) => {
+                add_token = false;
+                fluent::parse_expected_identifier_found_metavar_str
+            }
             None => fluent::parse_expected_identifier_found_str,
         });
         diag.span(self.span);
+        if add_token {
             diag.arg("token", self.token);
+        }
 
         if let Some(sugg) = self.suggest_raw {
             sugg.add_to_diag(&mut diag);
@@ -1171,6 +1181,7 @@ impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for ExpectedSemi {
     fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, G> {
         let token_descr = TokenDescription::from_token(&self.token);
 
+        let mut add_token = true;
         let mut diag = Diag::new(dcx, level, match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
                 fluent::parse_expected_semi_found_reserved_identifier_str
@@ -1180,10 +1191,16 @@ impl<'a, G: EmissionGuarantee> Diagnostic<'a, G> for ExpectedSemi {
                 fluent::parse_expected_semi_found_reserved_keyword_str
             }
             Some(TokenDescription::DocComment) => fluent::parse_expected_semi_found_doc_comment_str,
+            Some(TokenDescription::MetaVar(_)) => {
+                add_token = false;
+                fluent::parse_expected_semi_found_metavar_str
+            }
             None => fluent::parse_expected_semi_found_str,
         });
         diag.span(self.span);
+        if add_token {
             diag.arg("token", self.token);
+        }
 
         if let Some(unexpected_token_label) = self.unexpected_token_label {
             diag.span_label(unexpected_token_label, fluent::parse_label_unexpected_token);
@@ -1925,6 +1942,12 @@ pub(crate) enum UnexpectedTokenAfterStructName {
         span: Span,
         token: Token,
     },
+    #[diag(parse_unexpected_token_after_struct_name_found_metavar)]
+    MetaVar {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+    },
     #[diag(parse_unexpected_token_after_struct_name_found_other)]
     Other {
         #[primary_span]
@@ -1941,6 +1964,7 @@ impl UnexpectedTokenAfterStructName {
             Some(TokenDescription::Keyword) => Self::Keyword { span, token },
             Some(TokenDescription::ReservedKeyword) => Self::ReservedKeyword { span, token },
             Some(TokenDescription::DocComment) => Self::DocComment { span, token },
+            Some(TokenDescription::MetaVar(_)) => Self::MetaVar { span },
             None => Self::Other { span, token },
         }
     }

compiler/rustc_parse/src/lexer/tokentrees.rs

@@ -43,11 +43,19 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
         let mut buf = Vec::new();
         loop {
             match self.token.kind {
-                token::OpenDelim(delim) => buf.push(match self.lex_token_tree_open_delim(delim) {
+                token::OpenDelim(delim) => {
+                    // Invisible delimiters cannot occur here because `TokenTreesReader` parses
+                    // code directly from strings, with no macro expansion involved.
+                    debug_assert!(!matches!(delim, Delimiter::Invisible(_)));
+                    buf.push(match self.lex_token_tree_open_delim(delim) {
                         Ok(val) => val,
                         Err(errs) => return (open_spacing, TokenStream::new(buf), Err(errs)),
-                }),
+                    })
+                }
                 token::CloseDelim(delim) => {
+                    // Invisible delimiters cannot occur here because `TokenTreesReader` parses
+                    // code directly from strings, with no macro expansion involved.
+                    debug_assert!(!matches!(delim, Delimiter::Invisible(_)));
                     return (
                         open_spacing,
                         TokenStream::new(buf),

compiler/rustc_parse/src/parser/attr_wrapper.rs

@@ -510,8 +510,8 @@ fn make_attr_token_stream(
             FlatToken::Token((Token { kind: TokenKind::CloseDelim(delim), span }, spacing)) => {
                 let frame_data = mem::replace(&mut stack_top, stack_rest.pop().unwrap());
                 let (open_delim, open_sp, open_spacing) = frame_data.open_delim_sp.unwrap();
-                assert_eq!(
-                    open_delim, delim,
+                assert!(
+                    open_delim.eq_ignoring_invisible_origin(&delim),
                     "Mismatched open/close delims: open={open_delim:?} close={span:?}"
                 );
                 let dspan = DelimSpan::from_pair(open_sp, span);

compiler/rustc_parse/src/parser/expr.rs

@@ -49,7 +49,7 @@ pub(super) enum DestructuredFloat {
     /// 1.2 | 1.2e3
     MiddleDot(Symbol, Span, Span, Symbol, Span),
     /// Invalid
-    Error(ErrorGuaranteed),
+    Error,
 }
 
 impl<'a> Parser<'a> {
@@ -1005,7 +1005,7 @@ impl<'a> Parser<'a> {
                             self.mk_expr_tuple_field_access(lo, ident1_span, base, sym1, None);
                         self.mk_expr_tuple_field_access(lo, ident2_span, base1, sym2, suffix)
                     }
-                    DestructuredFloat::Error(_) => base,
+                    DestructuredFloat::Error => base,
                 })
             }
             _ => {
@@ -1015,7 +1015,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn error_unexpected_after_dot(&self) -> ErrorGuaranteed {
+    fn error_unexpected_after_dot(&self) {
         let actual = pprust::token_to_string(&self.token);
         let span = self.token.span;
         let sm = self.psess.source_map();
@@ -1025,7 +1025,7 @@ impl<'a> Parser<'a> {
             }
             _ => (span, actual),
         };
-        self.dcx().emit_err(errors::UnexpectedTokenAfterDot { span, actual })
+        self.dcx().emit_err(errors::UnexpectedTokenAfterDot { span, actual });
     }
 
     /// We need an identifier or integer, but the next token is a float.
@@ -1111,8 +1111,8 @@ impl<'a> Parser<'a> {
             // 1.2e+3 | 1.2e-3
             [IdentLike(_), Punct('.'), IdentLike(_), Punct('+' | '-'), IdentLike(_)] => {
                 // See the FIXME about `TokenCursor` above.
-                let guar = self.error_unexpected_after_dot();
-                DestructuredFloat::Error(guar)
+                self.error_unexpected_after_dot();
+                DestructuredFloat::Error
             }
             _ => panic!("unexpected components in a float token: {components:?}"),
         }
@@ -1178,7 +1178,7 @@ impl<'a> Parser<'a> {
                                 fields.insert(start_idx, Ident::new(symbol2, span2));
                                 fields.insert(start_idx, Ident::new(symbol1, span1));
                             }
-                            DestructuredFloat::Error(_) => {
+                            DestructuredFloat::Error => {
                                 trailing_dot = None;
                                 fields.insert(start_idx, Ident::new(symbol, self.prev_token.span));
                             }
@@ -3591,11 +3591,19 @@ impl<'a> Parser<'a> {
                         && !self.token.is_reserved_ident()
                         && self.look_ahead(1, |t| {
                             AssocOp::from_token(t).is_some()
-                                || matches!(t.kind, token::OpenDelim(_))
+                                || matches!(
+                                    t.kind,
+                                    token::OpenDelim(
+                                        Delimiter::Parenthesis
+                                            | Delimiter::Bracket
+                                            | Delimiter::Brace
+                                    )
+                                )
                                 || *t == token::Dot
                         })
                     {
-                        // Looks like they tried to write a shorthand, complex expression.
+                        // Looks like they tried to write a shorthand, complex expression,
+                        // E.g.: `n + m`, `f(a)`, `a[i]`, `S { x: 3 }`, or `x.y`.
                         e.span_suggestion_verbose(
                             self.token.span.shrink_to_lo(),
                             "try naming a field",

compiler/rustc_parse/src/parser/mod.rs

@@ -21,7 +21,9 @@ pub(crate) use item::FnParseMode;
 pub use pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use path::PathStyle;
 use rustc_ast::ptr::P;
-use rustc_ast::token::{self, Delimiter, IdentIsRaw, Nonterminal, Token, TokenKind};
+use rustc_ast::token::{
+    self, Delimiter, IdentIsRaw, InvisibleOrigin, MetaVarKind, Nonterminal, Token, TokenKind,
+};
 use rustc_ast::tokenstream::{
     AttrsTarget, DelimSpacing, DelimSpan, Spacing, TokenStream, TokenTree, TokenTreeCursor,
 };
@@ -317,7 +319,7 @@ impl TokenCursor {
                             spacing,
                             delim,
                         ));
-                        if delim != Delimiter::Invisible {
+                        if !delim.skip() {
                             return (Token::new(token::OpenDelim(delim), sp.open), spacing.open);
                         }
                         // No open delimiter to return; continue on to the next iteration.
@@ -326,7 +328,7 @@ impl TokenCursor {
             } else if let Some((tree_cursor, span, spacing, delim)) = self.stack.pop() {
                 // We have exhausted this token stream. Move back to its parent token stream.
                 self.tree_cursor = tree_cursor;
-                if delim != Delimiter::Invisible {
+                if !delim.skip() {
                     return (Token::new(token::CloseDelim(delim), span.close), spacing.close);
                 }
                 // No close delimiter to return; continue on to the next iteration.
@@ -410,6 +412,12 @@ pub(super) enum TokenDescription {
     Keyword,
     ReservedKeyword,
     DocComment,
+
+    // Expanded metavariables are wrapped in invisible delimiters which aren't
+    // pretty-printed. In error messages we must handle these specially
+    // otherwise we get confusing things in messages like "expected `(`, found
+    // ``". It's better to say e.g. "expected `(`, found type metavariable".
+    MetaVar(MetaVarKind),
 }
 
 impl TokenDescription {
@@ -419,26 +427,29 @@ impl TokenDescription {
             _ if token.is_used_keyword() => Some(TokenDescription::Keyword),
             _ if token.is_unused_keyword() => Some(TokenDescription::ReservedKeyword),
             token::DocComment(..) => Some(TokenDescription::DocComment),
+            token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(kind))) => {
+                Some(TokenDescription::MetaVar(kind))
+            }
             _ => None,
         }
     }
 }
 
 pub fn token_descr(token: &Token) -> String {
-    let name = pprust::token_to_string(token).to_string();
+    let s = pprust::token_to_string(token).to_string();
 
-    let kind = match (TokenDescription::from_token(token), &token.kind) {
-        (Some(TokenDescription::ReservedIdentifier), _) => Some("reserved identifier"),
-        (Some(TokenDescription::Keyword), _) => Some("keyword"),
-        (Some(TokenDescription::ReservedKeyword), _) => Some("reserved keyword"),
-        (Some(TokenDescription::DocComment), _) => Some("doc comment"),
-        (None, TokenKind::NtIdent(..)) => Some("identifier"),
-        (None, TokenKind::NtLifetime(..)) => Some("lifetime"),
-        (None, TokenKind::Interpolated(node)) => Some(node.descr()),
-        (None, _) => None,
-    };
-
-    if let Some(kind) = kind { format!("{kind} `{name}`") } else { format!("`{name}`") }
+    match (TokenDescription::from_token(token), &token.kind) {
+        (Some(TokenDescription::ReservedIdentifier), _) => format!("reserved identifier `{s}`"),
+        (Some(TokenDescription::Keyword), _) => format!("keyword `{s}`"),
+        (Some(TokenDescription::ReservedKeyword), _) => format!("reserved keyword `{s}`"),
+        (Some(TokenDescription::DocComment), _) => format!("doc comment `{s}`"),
+        // Deliberately doesn't print `s`, which is empty.
+        (Some(TokenDescription::MetaVar(kind)), _) => format!("`{kind}` metavariable"),
+        (None, TokenKind::NtIdent(..)) => format!("identifier `{s}`"),
+        (None, TokenKind::NtLifetime(..)) => format!("lifetime `{s}`"),
+        (None, TokenKind::Interpolated(node)) => format!("{} `{s}`", node.descr()),
+        (None, _) => format!("`{s}`"),
+    }
 }
 
 impl<'a> Parser<'a> {
@@ -1163,7 +1174,7 @@ impl<'a> Parser<'a> {
         }
         debug_assert!(!matches!(
             next.0.kind,
-            token::OpenDelim(Delimiter::Invisible) | token::CloseDelim(Delimiter::Invisible)
+            token::OpenDelim(delim) | token::CloseDelim(delim) if delim.skip()
         ));
         self.inlined_bump_with(next)
     }
@@ -1187,7 +1198,7 @@ impl<'a> Parser<'a> {
                     match tree {
                         TokenTree::Token(token, _) => return looker(token),
                         &TokenTree::Delimited(dspan, _, delim, _) => {
-                            if delim != Delimiter::Invisible {
+                            if !delim.skip() {
                                 return looker(&Token::new(token::OpenDelim(delim), dspan.open));
                             }
                         }
@@ -1197,7 +1208,7 @@ impl<'a> Parser<'a> {
                     // The tree cursor lookahead went (one) past the end of the
                     // current token tree. Try to return a close delimiter.
                     if let Some(&(_, span, _, delim)) = self.token_cursor.stack.last()
-                        && delim != Delimiter::Invisible
+                        && !delim.skip()
                     {
                         // We are not in the outermost token stream, so we have
                         // delimiters. Also, those delimiters are not skipped.
@@ -1216,7 +1227,7 @@ impl<'a> Parser<'a> {
             token = cursor.next().0;
             if matches!(
                 token.kind,
-                token::OpenDelim(Delimiter::Invisible) | token::CloseDelim(Delimiter::Invisible)
+                token::OpenDelim(delim) | token::CloseDelim(delim) if delim.skip()
             ) {
                 continue;
             }

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -3,7 +3,9 @@ use rustc_ast::ptr::P;
 use rustc_ast::token::Nonterminal::*;
 use rustc_ast::token::NtExprKind::*;
 use rustc_ast::token::NtPatKind::*;
-use rustc_ast::token::{self, Delimiter, NonterminalKind, Token};
+use rustc_ast::token::{
+    self, Delimiter, InvisibleOrigin, MetaVarKind, Nonterminal, NonterminalKind, Token,
+};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::PResult;
@@ -22,7 +24,28 @@ impl<'a> Parser<'a> {
     #[inline]
     pub fn nonterminal_may_begin_with(kind: NonterminalKind, token: &Token) -> bool {
         /// Checks whether the non-terminal may contain a single (non-keyword) identifier.
-        fn may_be_ident(nt: &token::Nonterminal) -> bool {
+        fn may_be_ident(kind: MetaVarKind) -> bool {
+            match kind {
+                MetaVarKind::Stmt
+                | MetaVarKind::Pat(_)
+                | MetaVarKind::Expr { .. }
+                | MetaVarKind::Ty
+                | MetaVarKind::Literal // `true`, `false`
+                | MetaVarKind::Meta
+                | MetaVarKind::Path => true,
+
+                MetaVarKind::Item
+                | MetaVarKind::Block
+                | MetaVarKind::Vis => false,
+
+                MetaVarKind::Ident
+                | MetaVarKind::Lifetime
+                | MetaVarKind::TT => unreachable!(),
+            }
+        }
+
+        /// Old variant of `may_be_ident`. Being phased out.
+        fn nt_may_be_ident(nt: &Nonterminal) -> bool {
             match nt {
                 NtStmt(_)
                 | NtPat(_)
@@ -69,7 +92,8 @@ impl<'a> Parser<'a> {
                 | token::Ident(..)
                 | token::NtIdent(..)
                 | token::NtLifetime(..)
-                | token::Interpolated(_) => true,
+                | token::Interpolated(_)
+                | token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(_))) => true,
                 _ => token.can_begin_type(),
             },
             NonterminalKind::Block => match &token.kind {
@@ -79,11 +103,29 @@ impl<'a> Parser<'a> {
                     NtBlock(_) | NtStmt(_) | NtExpr(_) | NtLiteral(_) => true,
                     NtItem(_) | NtPat(_) | NtTy(_) | NtMeta(_) | NtPath(_) | NtVis(_) => false,
                 },
+                token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(k))) => match k {
+                    MetaVarKind::Block
+                    | MetaVarKind::Stmt
+                    | MetaVarKind::Expr { .. }
+                    | MetaVarKind::Literal => true,
+                    MetaVarKind::Item
+                    | MetaVarKind::Pat(_)
+                    | MetaVarKind::Ty
+                    | MetaVarKind::Meta
+                    | MetaVarKind::Path
+                    | MetaVarKind::Vis => false,
+                    MetaVarKind::Lifetime | MetaVarKind::Ident | MetaVarKind::TT => {
+                        unreachable!()
+                    }
+                },
                 _ => false,
             },
             NonterminalKind::Path | NonterminalKind::Meta => match &token.kind {
                 token::PathSep | token::Ident(..) | token::NtIdent(..) => true,
-                token::Interpolated(nt) => may_be_ident(nt),
+                token::Interpolated(nt) => nt_may_be_ident(nt),
+                token::OpenDelim(Delimiter::Invisible(InvisibleOrigin::MetaVar(kind))) => {
+                    may_be_ident(*kind)
+                }
                 _ => false,
             },
             NonterminalKind::Pat(pat_kind) => token.can_begin_pattern(pat_kind),

src/tools/rustfmt/src/macros.rs

@@ -620,7 +620,7 @@ fn delim_token_to_str(
                 ("{ ", " }")
             }
         }
-        Delimiter::Invisible => unreachable!(),
+        Delimiter::Invisible(_) => unreachable!(),
     };
     if use_multiple_lines {
         let indent_str = shape.indent.to_string_with_newline(context.config);