.last() to .next_back() requires a mutable receiver

In the case where `iter` is a `DoubleEndedIterator`, replacing a call to
`iter.last()` (which consumes `iter`) by `iter.next_back()` (which
requires a mutable reference to `iter`) cannot be done when `iter` is a
non-mutable binding which is not a mutable reference.

clippy_lints/src/methods/double_ended_iterator_last.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::is_trait_method;
 use clippy_utils::ty::implements_trait;
+use clippy_utils::{is_mutable, is_trait_method};
 use rustc_errors::Applicability;
 use rustc_hir::Expr;
 use rustc_lint::LateContext;
@@ -27,6 +27,8 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &'_ Expr<'_>, self_expr: &'_ Exp
         && let Some(last_def) = cx.tcx.provided_trait_methods(item).find(|m| m.name.as_str() == "last")
         // if the resolved method is the same as the provided definition
         && fn_def.def_id() == last_def.def_id
+        // if `selx_expr` is a reference, it is mutable because it is used for `.last()`
+        && (is_mutable(cx, self_expr) || cx.typeck_results().expr_ty(self_expr).is_ref())
     {
         span_lint_and_sugg(
             cx,

tests/ui/double_ended_iterator_last.fixed

@@ -51,3 +51,23 @@ fn main() {
     }
     let _ = CustomLast.last();
 }
+
+fn issue_14139() {
+    let mut index = [true, true, false, false, false, true].iter();
+    let subindex = index.by_ref().take(3);
+    let _ = subindex.last(); // No lint, `subindex` is not mutable
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let _ = subindex.next_back(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let subindex = &mut subindex;
+    let _ = subindex.next_back(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let subindex = &mut subindex;
+    let _ = subindex.next_back(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+}

tests/ui/double_ended_iterator_last.rs

@@ -51,3 +51,23 @@ fn main() {
     }
     let _ = CustomLast.last();
 }
+
+fn issue_14139() {
+    let mut index = [true, true, false, false, false, true].iter();
+    let subindex = index.by_ref().take(3);
+    let _ = subindex.last(); // No lint, `subindex` is not mutable
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let _ = subindex.last(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let subindex = &mut subindex;
+    let _ = subindex.last(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+
+    let mut index = [true, true, false, false, false, true].iter();
+    let mut subindex = index.by_ref().take(3);
+    let subindex = &mut subindex;
+    let _ = subindex.last(); //~ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+}

tests/ui/double_ended_iterator_last.stderr

@@ -13,5 +13,23 @@ error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly
 LL |     let _ = DeIterator.last();
    |                        ^^^^^^ help: try: `next_back()`
 
-error: aborting due to 2 previous errors
+error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
+  --> tests/ui/double_ended_iterator_last.rs:62:22
+   |
+LL |     let _ = subindex.last();
+   |                      ^^^^^^ help: try: `next_back()`
+
+error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
+  --> tests/ui/double_ended_iterator_last.rs:67:22
+   |
+LL |     let _ = subindex.last();
+   |                      ^^^^^^ help: try: `next_back()`
+
+error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
+  --> tests/ui/double_ended_iterator_last.rs:72:22
+   |
+LL |     let _ = subindex.last();
+   |                      ^^^^^^ help: try: `next_back()`
+
+error: aborting due to 5 previous errors