Auto merge of #10096 - feniljain:fix-seek-rewind, r=xFrednet

fix: not suggest seek_to_start_instead_of_rewind when expr is used

changelog: [`seek_to_start_instead_of_rewind`]: No longer lints, if the return of `seek` is used.
[#10096](https://github.com/rust-lang/rust-clippy/pull/10096)
<!-- changelog_checked -->

Fixes #10065

clippy_lints/src/methods/seek_to_start_instead_of_rewind.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::ty::implements_trait;
-use clippy_utils::{get_trait_def_id, match_def_path, paths};
+use clippy_utils::{get_trait_def_id, is_expr_used_or_unified, match_def_path, paths};
 use rustc_ast::ast::{LitIntType, LitKind};
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind};
@@ -19,6 +19,10 @@ pub(super) fn check<'tcx>(
     // Get receiver type
     let ty = cx.typeck_results().expr_ty(recv).peel_refs();
 
+    if is_expr_used_or_unified(cx.tcx, expr) {
+        return;
+    }
+
     if let Some(seek_trait_id) = get_trait_def_id(cx, &paths::STD_IO_SEEK) &&
         implements_trait(cx, ty, seek_trait_id, &[]) &&
         let ExprKind::Call(func, args1) = arg.kind &&

tests/ui/seek_to_start_instead_of_rewind.fixed

@@ -70,6 +70,12 @@ fn seek_to_end<T: Seek>(t: &mut T) {
     t.seek(SeekFrom::End(0));
 }
 
+// This should NOT trigger clippy warning because
+// expr is used here
+fn seek_to_start_in_let<T: Seek>(t: &mut T) {
+    let a = t.seek(SeekFrom::Start(0)).unwrap();
+}
+
 fn main() {
     let mut f = OpenOptions::new()
         .write(true)

tests/ui/seek_to_start_instead_of_rewind.rs

@@ -70,6 +70,12 @@ fn seek_to_end<T: Seek>(t: &mut T) {
     t.seek(SeekFrom::End(0));
 }
 
+// This should NOT trigger clippy warning because
+// expr is used here
+fn seek_to_start_in_let<T: Seek>(t: &mut T) {
+    let a = t.seek(SeekFrom::Start(0)).unwrap();
+}
+
 fn main() {
     let mut f = OpenOptions::new()
         .write(true)

tests/ui/seek_to_start_instead_of_rewind.stderr

@@ -13,7 +13,7 @@ LL |     t.seek(SeekFrom::Start(0));
    |       ^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `rewind()`
 
 error: used `seek` to go to the start of the stream
-  --> $DIR/seek_to_start_instead_of_rewind.rs:128:7
+  --> $DIR/seek_to_start_instead_of_rewind.rs:134:7
    |
 LL |     f.seek(SeekFrom::Start(0));
    |       ^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `rewind()`