Introduce rustc_lexer::is_ident and use it in couple of places

Cargo.lock

@@ -3257,6 +3257,7 @@ dependencies = [
  "rustc_data_structures",
  "rustc_errors",
  "rustc_feature",
+ "rustc_lexer",
  "rustc_macros",
  "rustc_serialize",
  "rustc_session",

src/librustc_attr/Cargo.toml

@@ -16,6 +16,7 @@ rustc_errors = { path = "../librustc_errors" }
 rustc_span = { path = "../librustc_span" }
 rustc_data_structures = { path = "../librustc_data_structures" }
 rustc_feature = { path = "../librustc_feature" }
+rustc_lexer = { path = "../librustc_lexer" }
 rustc_macros = { path = "../librustc_macros" }
 rustc_session = { path = "../librustc_session" }
 rustc_ast = { path = "../librustc_ast" }

src/librustc_attr/builtin.rs

@@ -20,6 +20,7 @@ enum AttrError {
     MultipleItem(String),
     UnknownMetaItem(String, &'static [&'static str]),
     MissingSince,
+    NonIdentFeature,
     MissingFeature,
     MultipleStabilityLevels,
     UnsupportedLiteral(&'static str, /* is_bytestr */ bool),
@@ -40,6 +41,9 @@ fn handle_errors(sess: &ParseSess, span: Span, error: AttrError) {
         AttrError::MissingSince => {
             struct_span_err!(diag, span, E0542, "missing 'since'").emit();
         }
+        AttrError::NonIdentFeature => {
+            struct_span_err!(diag, span, E0546, "'feature' is not an identifier").emit();
+        }
         AttrError::MissingFeature => {
             struct_span_err!(diag, span, E0546, "missing 'feature'").emit();
         }
@@ -344,6 +348,14 @@ where
 
                     match (feature, reason, issue) {
                         (Some(feature), reason, Some(_)) => {
+                            if !rustc_lexer::is_ident(&feature.as_str()) {
+                                handle_errors(
+                                    &sess.parse_sess,
+                                    attr.span,
+                                    AttrError::NonIdentFeature,
+                                );
+                                continue;
+                            }
                             let level = Unstable { reason, issue: issue_num, is_soft };
                             if sym::unstable == meta_name {
                                 stab = Some(Stability { level, feature });

src/librustc_expand/proc_macro_server.rs

@@ -319,18 +319,10 @@ pub struct Ident {
 }
 
 impl Ident {
-    fn is_valid(string: &str) -> bool {
-        let mut chars = string.chars();
-        if let Some(start) = chars.next() {
-            rustc_lexer::is_id_start(start) && chars.all(rustc_lexer::is_id_continue)
-        } else {
-            false
-        }
-    }
     fn new(sess: &ParseSess, sym: Symbol, is_raw: bool, span: Span) -> Ident {
         let sym = nfc_normalize(&sym.as_str());
         let string = sym.as_str();
-        if !Self::is_valid(&string) {
+        if !rustc_lexer::is_ident(&string) {
             panic!("`{:?}` is not a valid identifier", string)
         }
         if is_raw && !sym.can_be_raw() {

src/librustc_lexer/src/lib.rs

@@ -274,6 +274,16 @@ pub fn is_id_continue(c: char) -> bool {
         || (c > '\x7f' && unicode_xid::UnicodeXID::is_xid_continue(c))
 }
 
+/// The passed string is lexically an identifier.
+pub fn is_ident(string: &str) -> bool {
+    let mut chars = string.chars();
+    if let Some(start) = chars.next() {
+        is_id_start(start) && chars.all(is_id_continue)
+    } else {
+        false
+    }
+}
+
 impl Cursor<'_> {
     /// Parses a token from the input string.
     fn advance_token(&mut self) -> Token {