Add the iter_last_slice lint

CHANGELOG.md

@@ -6086,6 +6086,7 @@ Released 2018-09-13
 [`iter_filter_is_ok`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_filter_is_ok
 [`iter_filter_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_filter_is_some
 [`iter_kv_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_kv_map
+[`iter_last_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_last_slice
 [`iter_next_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_loop
 [`iter_next_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_slice
 [`iter_not_returning_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_not_returning_iterator

clippy_lints/src/declared_lints.rs

@@ -393,6 +393,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::methods::ITER_FILTER_IS_OK_INFO,
     crate::methods::ITER_FILTER_IS_SOME_INFO,
     crate::methods::ITER_KV_MAP_INFO,
+    crate::methods::ITER_LAST_SLICE_INFO,
     crate::methods::ITER_NEXT_SLICE_INFO,
     crate::methods::ITER_NTH_INFO,
     crate::methods::ITER_NTH_ZERO_INFO,

clippy_lints/src/methods/iter_last_slice.rs

@@ -0,0 +1,68 @@
+use super::ITER_LAST_SLICE;
+use super::utils::derefs_to_slice;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::higher;
+use clippy_utils::source::snippet_with_applicability;
+use rustc_ast::ast;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, caller_expr: &'tcx hir::Expr<'_>) {
+    if derefs_to_slice(cx, caller_expr, cx.typeck_results().expr_ty(caller_expr)).is_some() {
+        // caller is a Slice
+        if let hir::ExprKind::Index(caller_var, index_expr, _) = &caller_expr.kind
+            && let Some(higher::Range {
+                start: _,
+                end: Some(end_expr),
+                limits,
+            }) = higher::Range::hir(index_expr)
+            && let hir::ExprKind::Lit(end_lit) = &end_expr.kind
+            && let ast::LitKind::Int(end_idx, _) = end_lit.node
+        {
+            let mut applicability = Applicability::MachineApplicable;
+            let suggest = if end_idx == 0 {
+                // This corresponds to `$slice[..0].iter().last()`, which
+                // evaluates to `None`. Maybe this is worth its own lint?
+                format!(
+                    "{}.last()",
+                    snippet_with_applicability(cx, caller_var.span, "..", &mut applicability)
+                )
+            } else {
+                let idx = match limits {
+                    ast::RangeLimits::HalfOpen => end_idx.get() - 1,
+                    ast::RangeLimits::Closed => end_idx.get(),
+                };
+
+                format!(
+                    "{}.get({idx})",
+                    snippet_with_applicability(cx, caller_var.span, "..", &mut applicability)
+                )
+            };
+            span_lint_and_sugg(
+                cx,
+                ITER_LAST_SLICE,
+                expr.span,
+                "using `.iter().last()` on a slice without end index",
+                "try calling",
+                suggest,
+                applicability,
+            );
+        }
+    } else if super::iter_next_slice::is_vec_or_array(cx, caller_expr) {
+        // caller is a Vec or an Array
+        let mut applicability = Applicability::MachineApplicable;
+        span_lint_and_sugg(
+            cx,
+            ITER_LAST_SLICE,
+            expr.span,
+            "using `.iter().last()` on an array",
+            "try calling",
+            format!(
+                "{}.last()",
+                snippet_with_applicability(cx, caller_expr.span, "..", &mut applicability)
+            ),
+            applicability,
+        );
+    }
+}

clippy_lints/src/methods/iter_next_slice.rs

@@ -74,7 +74,7 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, cal
     }
 }
 
-fn is_vec_or_array<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) -> bool {
+pub(crate) fn is_vec_or_array<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) -> bool {
     is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(expr), sym::Vec)
         || matches!(&cx.typeck_results().expr_ty(expr).peel_refs().kind(), ty::Array(_, _))
 }

clippy_lints/src/methods/mod.rs

@@ -44,6 +44,7 @@ mod iter_cloned_collect;
 mod iter_count;
 mod iter_filter;
 mod iter_kv_map;
+mod iter_last_slice;
 mod iter_next_slice;
 mod iter_nth;
 mod iter_nth_zero;
@@ -1820,6 +1821,33 @@ declare_clippy_lint! {
     "using `.iter().next()` on a sliced array, which can be shortened to just `.get()`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `iter().last()` on a Slice or an Array.
+    ///
+    /// ### Why is this bad?
+    /// These can be shortened into `.last()`.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # let a = [1, 2, 3];
+    /// # let b = vec![1, 2, 3];
+    /// a[..2].iter().last();
+    /// b.iter().last();
+    /// ```
+    /// should be written as:
+    /// ```no_run
+    /// # let a = [1, 2, 3];
+    /// # let b = vec![1, 2, 3];
+    /// a.get(1);
+    /// b.get(0);
+    /// ```
+    #[clippy::version = "1.46.0"]
+    pub ITER_LAST_SLICE,
+    style,
+    "using `.iter().next()` on a slice, array or `Vec`, which can be shortened to just `.get()`"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Warns when using `push_str`/`insert_str` with a single-character string literal
@@ -4644,6 +4672,7 @@ impl_lint_pass!(Methods => [
     FLAT_MAP_IDENTITY,
     MAP_FLATTEN,
     ITERATOR_STEP_BY_ZERO,
+    ITER_LAST_SLICE,
     ITER_NEXT_SLICE,
     ITER_COUNT,
     ITER_NTH,
@@ -5228,17 +5257,27 @@ impl Methods {
                     }
                 },
                 (sym::last, []) => {
-                    if let Some((sym::cloned, recv2, [], _span2, _)) = method_call(recv) {
-                        iter_overeager_cloned::check(
-                            cx,
-                            expr,
-                            recv,
-                            recv2,
-                            iter_overeager_cloned::Op::LaterCloned,
-                            false,
-                        );
+                    let method_call = method_call(recv);
+
+                    match method_call {
+                        Some((sym::cloned, recv2, [], _, _)) => {
+                            iter_overeager_cloned::check(
+                                cx,
+                                expr,
+                                recv,
+                                recv2,
+                                iter_overeager_cloned::Op::LaterCloned,
+                                false,
+                            );
+                            double_ended_iterator_last::check(cx, expr, recv, call_span);
+                        },
+
+                        Some((sym::iter, recv2, [], _, _)) => iter_last_slice::check(cx, expr, recv2),
+
+                        _ => {
+                            double_ended_iterator_last::check(cx, expr, recv, call_span);
+                        },
                     }
-                    double_ended_iterator_last::check(cx, expr, recv, call_span);
                 },
                 (sym::len, []) => {
                     if let Some((prev_method @ (sym::as_bytes | sym::bytes), prev_recv, [], _, _)) = method_call(recv) {

tests/ui/iter_last_slice.fixed

@@ -0,0 +1,34 @@
+#![warn(clippy::iter_last_slice)]
+#![allow(clippy::useless_vec)]
+
+fn main() {
+    // test code goes here
+    let s = [1, 2, 3];
+    let v = vec![1, 2, 3];
+
+    let _ = s.last();
+    //~^ iter_last_slice
+    // Should be replaced by s.last()
+
+    let _ = s.get(1);
+    //~^ iter_last_slice
+    // Should be replaced by s.get(1)
+    let _ = s.get(2);
+    //~^ iter_last_slice
+    // Should be replaced by s.get(2)
+
+    let _ = v.get(4);
+    //~^ iter_last_slice
+    // Should be replaced by v.get(4)
+    let _ = v.get(5);
+    //~^ iter_last_slice
+    // Should be replaced by v.get(5)
+
+    let _ = v.last();
+    //~^ iter_last_slice
+    // Should be replaced by v.last()
+
+    let o = Some(5);
+    o.iter().last();
+    // Shouldn't be linted since this is not a Slice or an Array
+}

tests/ui/iter_last_slice.rs

@@ -0,0 +1,34 @@
+#![warn(clippy::iter_last_slice)]
+#![allow(clippy::useless_vec)]
+
+fn main() {
+    // test code goes here
+    let s = [1, 2, 3];
+    let v = vec![1, 2, 3];
+
+    let _ = s.iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by s.last()
+
+    let _ = s[..2].iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by s.get(1)
+    let _ = s[..=2].iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by s.get(2)
+
+    let _ = v[..5].iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by v.get(4)
+    let _ = v[..=5].iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by v.get(5)
+
+    let _ = v.iter().last();
+    //~^ iter_last_slice
+    // Should be replaced by v.last()
+
+    let o = Some(5);
+    o.iter().last();
+    // Shouldn't be linted since this is not a Slice or an Array
+}

tests/ui/iter_last_slice.stderr

@@ -0,0 +1,41 @@
+error: using `.iter().last()` on an array
+  --> tests/ui/iter_last_slice.rs:9:13
+   |
+LL |     let _ = s.iter().last();
+   |             ^^^^^^^^^^^^^^^ help: try calling: `s.last()`
+   |
+   = note: `-D clippy::iter-last-slice` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::iter_last_slice)]`
+
+error: using `.iter().last()` on a slice without end index
+  --> tests/ui/iter_last_slice.rs:13:13
+   |
+LL |     let _ = s[..2].iter().last();
+   |             ^^^^^^^^^^^^^^^^^^^^ help: try calling: `s.get(1)`
+
+error: using `.iter().last()` on a slice without end index
+  --> tests/ui/iter_last_slice.rs:16:13
+   |
+LL |     let _ = s[..=2].iter().last();
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: try calling: `s.get(2)`
+
+error: using `.iter().last()` on a slice without end index
+  --> tests/ui/iter_last_slice.rs:20:13
+   |
+LL |     let _ = v[..5].iter().last();
+   |             ^^^^^^^^^^^^^^^^^^^^ help: try calling: `v.get(4)`
+
+error: using `.iter().last()` on a slice without end index
+  --> tests/ui/iter_last_slice.rs:23:13
+   |
+LL |     let _ = v[..=5].iter().last();
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: try calling: `v.get(5)`
+
+error: using `.iter().last()` on an array
+  --> tests/ui/iter_last_slice.rs:27:13
+   |
+LL |     let _ = v.iter().last();
+   |             ^^^^^^^^^^^^^^^ help: try calling: `v.last()`
+
+error: aborting due to 6 previous errors
+

tests/ui/iter_overeager_cloned.fixed

@@ -3,7 +3,8 @@
     dead_code,
     clippy::let_unit_value,
     clippy::useless_vec,
-    clippy::double_ended_iterator_last
+    clippy::double_ended_iterator_last,
+    clippy::iter_last_slice
 )]
 
 fn main() {

tests/ui/iter_overeager_cloned.rs

@@ -3,7 +3,8 @@
     dead_code,
     clippy::let_unit_value,
     clippy::useless_vec,
-    clippy::double_ended_iterator_last
+    clippy::double_ended_iterator_last,
+    clippy::iter_last_slice
 )]
 
 fn main() {