add test case for not whole length, move sugg into variable

Author: y21

clippy_lints/src/methods/drain_collect.rs

@@ -66,17 +66,19 @@ pub(super) fn check(cx: &LateContext<'_>, args: &[Expr<'_>], expr: &Expr<'_>, re
             .or_else(|| check_collections(cx, expr_ty, recv_ty_no_refs))
     {
         let recv = snippet(cx, recv.span, "<expr>");
+        let sugg = if let ty::Ref(..) = recv_ty.kind() {
+            format!("std::mem::take({recv})")
+        } else {
+            format!("std::mem::take(&mut {recv})")
+        };
+
         span_lint_and_sugg(
             cx,
             DRAIN_COLLECT,
             expr.span,
             &format!("you seem to be trying to move all elements into a new `{typename}`"),
             "consider using `mem::take`",
-            if let ty::Ref(..) = recv_ty.kind() {
-                format!("std::mem::take({recv})")
-            } else {
-                format!("std::mem::take(&mut {recv})")
-            },
+            sugg,
             Applicability::MachineApplicable,
         );
     }

clippy_lints/src/methods/mod.rs

@@ -3233,7 +3233,7 @@ declare_clippy_lint! {
     /// When using `mem::take`, the old collection is replaced with an empty one and ownership of
     /// the old collection is returned.
     ///
-    /// ### Drawback
+    /// ### Known issues
     /// `mem::take(&mut vec)` is almost equivalent to `vec.drain(..).collect()`, except that
     /// it also moves the **capacity**. The user might have explicitly written it this way
     /// to keep the capacity on the original `Vec`.

tests/ui/drain_collect.fixed

@@ -70,4 +70,8 @@ fn string_dont_lint(b: &mut String) -> HashSet<char> {
     b.drain(..).collect()
 }
 
+fn not_whole_length(v: &mut Vec<i32>) -> Vec<i32> {
+    v.drain(1..).collect()
+}
+
 fn main() {}

tests/ui/drain_collect.rs

@@ -70,4 +70,8 @@ fn string_dont_lint(b: &mut String) -> HashSet<char> {
     b.drain(..).collect()
 }
 
+fn not_whole_length(v: &mut Vec<i32>) -> Vec<i32> {
+    v.drain(1..).collect()
+}
+
 fn main() {}