Suggest quoting unquoted idents in attrs

compiler/rustc_parse/src/errors.rs

@@ -973,6 +973,25 @@ pub(crate) struct InvalidMetaItem {
     pub token: Token,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_invalid_meta_item_unquoted_ident)]
+pub(crate) struct InvalidMetaItemUnquotedIdent {
+    #[primary_span]
+    pub span: Span,
+    pub token: Token,
+    #[subdiagnostic]
+    pub sugg: InvalidMetaItemSuggQuoteIdent,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
+pub(crate) struct InvalidMetaItemSuggQuoteIdent {
+    #[suggestion_part(code = "\"")]
+    pub before: Span,
+    #[suggestion_part(code = "\"")]
+    pub after: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[suggestion(
     parse_sugg_escape_identifier,

compiler/rustc_parse/src/parser/attr.rs

@@ -1,4 +1,7 @@
-use crate::errors::{InvalidMetaItem, SuffixedLiteralInAttribute};
+use crate::errors::{
+    InvalidMetaItem, InvalidMetaItemSuggQuoteIdent, InvalidMetaItemUnquotedIdent,
+    SuffixedLiteralInAttribute,
+};
 use crate::fluent_generated as fluent;
 
 use super::{AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle};
@@ -417,9 +420,26 @@ impl<'a> Parser<'a> {
             Err(err) => err.cancel(),
         }
 
-        Err(self
-            .dcx()
-            .create_err(InvalidMetaItem { span: self.token.span, token: self.token.clone() }))
+        let token = self.token.clone();
+
+        // Check for unquoted idents in meta items, e.g.: #[cfg(key = foo)]
+        // `from_expansion()` ensures we don't suggest for cases such as
+        // `#[cfg(feature = $expr)]` in macros
+        if self.prev_token == token::Eq && !self.token.span.from_expansion() {
+            let before = self.token.span.shrink_to_lo();
+            while matches!(self.token.kind, token::Ident(..)) {
+                self.bump();
+            }
+            let after = self.prev_token.span.shrink_to_hi();
+            let sugg = InvalidMetaItemSuggQuoteIdent { before, after };
+            return Err(self.dcx().create_err(InvalidMetaItemUnquotedIdent {
+                span: token.span,
+                token,
+                sugg,
+            }));
+        }
+
+        Err(self.dcx().create_err(InvalidMetaItem { span: token.span, token }))
     }
 }