fix #104088, Slightly improve error message for invalid identifier

compiler/rustc_parse/src/errors.rs

@@ -1205,6 +1205,14 @@ pub(crate) struct SelfParamNotFirst {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parser_invalid_identifier_with_leading_number)]
+pub(crate) struct InvalidIdentiferStartsWithNumber {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parser_const_generic_without_braces)]
 pub(crate) struct ConstGenericWithoutBraces {

compiler/rustc_parse/src/parser/stmt.rs

@@ -10,8 +10,8 @@ use super::{
 use crate::errors::{
     AssignmentElseNotAllowed, CompoundAssignmentExpressionInLet, ConstLetMutuallyExclusive,
     DocCommentDoesNotDocumentAnything, ExpectedStatementAfterOuterAttr, InvalidCurlyInLetElse,
-    InvalidExpressionInLetElse, InvalidVariableDeclaration, InvalidVariableDeclarationSub,
-    WrapExpressionInParentheses,
+    InvalidExpressionInLetElse, InvalidIdentiferStartsWithNumber, InvalidVariableDeclaration,
+    InvalidVariableDeclarationSub, WrapExpressionInParentheses,
 };
 use crate::maybe_whole;
 
@@ -264,6 +264,7 @@ impl<'a> Parser<'a> {
             self.bump();
         }
 
+        self.report_invalid_identifier_error()?;
         let (pat, colon) = self.parse_pat_before_ty(None, RecoverComma::Yes, "`let` bindings")?;
 
         let (err, ty) = if colon {
@@ -355,6 +356,18 @@ impl<'a> Parser<'a> {
         Ok(P(ast::Local { ty, pat, kind, id: DUMMY_NODE_ID, span: lo.to(hi), attrs, tokens: None }))
     }
 
+    /// report error for `let 1x = 123`
+    pub fn report_invalid_identifier_error(&mut self) -> PResult<'a, ()> {
+        if let token::Literal(lit) = self.token.uninterpolate().kind &&
+            let Err(_) = rustc_ast::Lit::from_token(&self.token) &&
+            (lit.kind == token::LitKind::Integer || lit.kind == token::LitKind::Float) &&
+            self.look_ahead(1, |t| matches!(t.kind, token::Eq) || matches!(t.kind, token::Colon ) ) {
+                let err = self.sess.create_err(InvalidIdentiferStartsWithNumber { span: self.token.span });
+                return Err(err);
+        }
+        Ok(())
+    }
+
     fn check_let_else_init_bool_expr(&self, init: &ast::Expr) {
         if let ast::ExprKind::Binary(op, ..) = init.kind {
             if op.node.lazy() {