Fix yet another FP in `USELESS_LET_IF_SEQ`

The block expression before the assignment must be `None`.

clippy_lints/src/let_if_seq.rs

@@ -150,6 +150,7 @@ impl<'a, 'tcx, 'v> hir::intravisit::Visitor<'v> for UsedVisitor<'a, 'tcx> {
 
 fn check_assign<'e>(cx: &LateContext, decl: hir::def_id::DefId, block: &'e hir::Block) -> Option<&'e hir::Expr> {
     if_let_chain! {[
+        block.expr.is_none(),
         let Some(expr) = block.stmts.iter().last(),
         let hir::StmtSemi(ref expr, _) = expr.node,
         let hir::ExprAssign(ref var, ref value) = expr.node,

tests/compile-fail/let_if_seq.rs

@@ -98,6 +98,15 @@ fn main() {
         toto = 2;
     }
 
+    // found in libcore, the inner if is not a statement but the block's expr
+    let mut ch = b'x';
+    if f() {
+        ch = b'*';
+        if f() {
+            ch = b'?';
+        }
+    }
+
     // baz needs to be mut
     let mut baz = 0;
     //~^ ERROR `if _ { .. } else { .. }` is an expression