Suggest quoting unquoted idents in attrs

compiler/rustc_parse/messages.ftl

@@ -407,6 +407,7 @@ parse_invalid_logical_operator = `{$incorrect}` is not a logical operator
     .use_pipe_pipe_for_disjunction = use `||` to perform logical disjunction
 
 parse_invalid_meta_item = expected unsuffixed literal or identifier, found `{$token}`
+    .suggestion = surround the identifier with quotation marks to parse it as a string
 
 parse_invalid_unicode_escape = invalid unicode character escape
     .label = invalid escape

compiler/rustc_parse/src/errors.rs

@@ -971,6 +971,17 @@ pub(crate) struct InvalidMetaItem {
     #[primary_span]
     pub span: Span,
     pub token: Token,
+    #[subdiagnostic]
+    pub sugg: Option<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)]

compiler/rustc_parse/src/parser/attr.rs

@@ -1,4 +1,4 @@
-use crate::errors::{InvalidMetaItem, SuffixedLiteralInAttribute};
+use crate::errors::{InvalidMetaItem, InvalidMetaItemSuggQuoteIdent, SuffixedLiteralInAttribute};
 use crate::fluent_generated as fluent;
 
 use super::{AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle};
@@ -416,9 +416,19 @@ 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();
+        let sugg = 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();
+            Some(InvalidMetaItemSuggQuoteIdent { before, after })
+        } else {
+            None
+        };
+
+        Err(self.dcx().create_err(InvalidMetaItem { span: token.span, token, sugg }))
     }
 }
 

tests/ui/parser/attribute/attr-unquoted-ident.fixed

@@ -0,0 +1,15 @@
+// compile-flags: -Zdeduplicate-diagnostics=yes
+// run-rustfix
+
+fn main() {
+    #[cfg(key="foo")]
+    //~^ ERROR expected unsuffixed literal or identifier, found `foo`
+    //~| HELP surround the identifier with quotation marks to parse it as a string
+    println!();
+    #[cfg(key="bar")]
+    println!();
+    #[cfg(key="foo bar baz")]
+    //~^ ERROR expected unsuffixed literal or identifier, found `foo`
+    //~| HELP surround the identifier with quotation marks to parse it as a string
+    println!();
+}

tests/ui/parser/attribute/attr-unquoted-ident.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Zdeduplicate-diagnostics=yes
+// run-rustfix
+
+fn main() {
+    #[cfg(key=foo)]
+    //~^ ERROR expected unsuffixed literal or identifier, found `foo`
+    //~| HELP surround the identifier with quotation marks to parse it as a string
+    println!();
+    #[cfg(key="bar")]
+    println!();
+    #[cfg(key=foo bar baz)]
+    //~^ ERROR expected unsuffixed literal or identifier, found `foo`
+    //~| HELP surround the identifier with quotation marks to parse it as a string
+    println!();
+}

tests/ui/parser/attribute/attr-unquoted-ident.stderr

@@ -0,0 +1,24 @@
+error: expected unsuffixed literal or identifier, found `foo`
+  --> $DIR/attr-unquoted-ident.rs:5:15
+   |
+LL |     #[cfg(key=foo)]
+   |               ^^^
+   |
+help: surround the identifier with quotation marks to parse it as a string
+   |
+LL |     #[cfg(key="foo")]
+   |               +   +
+
+error: expected unsuffixed literal or identifier, found `foo`
+  --> $DIR/attr-unquoted-ident.rs:11:15
+   |
+LL |     #[cfg(key=foo bar baz)]
+   |               ^^^
+   |
+help: surround the identifier with quotation marks to parse it as a string
+   |
+LL |     #[cfg(key="foo bar baz")]
+   |               +           +
+
+error: aborting due to 2 previous errors
+