Correctly check for variable use in useless_let_if_seq

clippy_lints/src/let_if_seq.rs

@@ -69,6 +69,11 @@ impl LateLintPass for LetIfSeq {
                 let Some(def) = cx.tcx.def_map.borrow().get(&decl.pat.id),
                 let hir::StmtExpr(ref if_, _) = expr.node,
                 let hir::ExprIf(ref cond, ref then, ref else_) = if_.node,
+                {
+                    let mut v = UsedVisitor { cx: cx, id: def.def_id(), used: false };
+                    hir::intravisit::walk_expr(&mut v, cond);
+                    !v.used
+                },
                 let Some(value) = check_assign(cx, def.def_id(), then),
             ], {
                 let span = codemap::mk_sp(stmt.span.lo, if_.span.hi);
@@ -163,13 +168,13 @@ fn check_assign<'e>(cx: &LateContext, decl: hir::def_id::DefId, block: &'e hir::
 
         for s in block.stmts.iter().take(block.stmts.len()-1) {
             hir::intravisit::walk_stmt(&mut v, s);
+
+            if v.used {
+                return None;
+            }
         }
 
-        return if v.used {
-            None
-        } else {
-            Some(value)
-        };
+        return Some(value);
     }}
 
     None

tests/compile-fail/let_if_seq.rs

@@ -6,6 +6,14 @@
 
 fn f() -> bool { true }
 
+fn issue975() -> String {
+    let mut udn = "dummy".to_string();
+    if udn.starts_with("uuid:") {
+        udn = String::from(&udn[5..]);
+    }
+    udn
+}
+
 fn early_return() -> u8 {
     // FIXME: we could extend the lint to include such cases:
     let foo;
@@ -21,6 +29,7 @@ fn early_return() -> u8 {
 
 fn main() {
     early_return();
+    issue975();
 
     let mut foo = 0;
     //~^ ERROR `if _ { .. } else { .. }` is an expression