Fixes #8128

changelog: Fix error suggestion of skip(..).next() for immutable variable.

clippy_lints/src/methods/iter_skip_next.rs

@@ -1,8 +1,10 @@
-use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::is_trait_method;
+use clippy_utils::path_to_local;
 use clippy_utils::source::snippet;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
+use rustc_hir::{BindingAnnotation, Node, PatKind};
 use rustc_lint::LateContext;
 use rustc_span::sym;
 
@@ -11,14 +13,34 @@ use super::ITER_SKIP_NEXT;
 pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>, arg: &hir::Expr<'_>) {
     // lint if caller of skip is an Iterator
     if is_trait_method(cx, expr, sym::Iterator) {
-        span_lint_and_sugg(
+        let mut application = Applicability::MachineApplicable;
+        span_lint_and_then(
             cx,
             ITER_SKIP_NEXT,
             expr.span.trim_start(recv.span).unwrap(),
             "called `skip(..).next()` on an iterator",
-            "use `nth` instead",
-            format!(".nth({})", snippet(cx, arg.span, "..")),
-            Applicability::MachineApplicable,
+            |diag| {
+                if_chain! {
+                    if let Some(id) = path_to_local(recv);
+                    if let Node::Binding(pat) = cx.tcx.hir().get(id);
+                    if let PatKind::Binding(ann, _, _, _)  = pat.kind;
+                    if ann != BindingAnnotation::Mutable;
+                    then {
+                        application = Applicability::Unspecified;
+                        diag.span_help(
+                            pat.span,
+                            &format!("for this change `{}` has to be mutable", snippet(cx, pat.span, "..")),
+                        );
+                    }
+                }
+
+                diag.span_suggestion(
+                    expr.span.trim_start(recv.span).unwrap(),
+                    "use `nth` instead",
+                    format!(".nth({})", snippet(cx, arg.span, "..")),
+                    application,
+                );
+            },
         );
     }
 }