Add metavariables to `TokenDescription`.

Pasted metavariables are wrapped in invisible delimiters, which
pretty-print as empty strings, and changing that can break some proc
macros. But error messages saying "expected identifer, found ``" are
bad. So this commit adds support for metavariables in `TokenDescription`
so they print as "metavariable" in error messages, instead of "``".

It's not used meaningfully yet, but will be needed to get rid of
interpolated tokens.

1 files changed, 26 insertions, 15 deletions

diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 226f29f9900..0ed8d152d2d 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/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,
 };
@@ -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 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}`") }
+    let s = pprust::token_to_string(token).to_string();
+
+    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> {