Start writing use_last test

Getting unresolved import error for if_chain.

Author: HarrisonMc555

CHANGELOG.md

@@ -1016,6 +1016,7 @@ All notable changes to this project will be documented in this file.
 [`unused_label`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_label
 [`unused_unit`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
 [`use_debug`]: https://rust-lang.github.io/rust-clippy/master/index.html#use_debug
+[`use_last`]: https://rust-lang.github.io/rust-clippy/master/index.html#use_last
 [`use_self`]: https://rust-lang.github.io/rust-clippy/master/index.html#use_self
 [`used_underscore_binding`]: https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_binding
 [`useless_asref`]: https://rust-lang.github.io/rust-clippy/master/index.html#useless_asref

README.md

@@ -7,7 +7,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 296 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 297 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -198,6 +198,7 @@ pub mod unsafe_removed_from_name;
 pub mod unused_io_amount;
 pub mod unused_label;
 pub mod unwrap;
+pub mod use_last;
 pub mod use_self;
 pub mod vec;
 pub mod wildcard_dependencies;
@@ -785,6 +786,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
         unused_io_amount::UNUSED_IO_AMOUNT,
         unused_label::UNUSED_LABEL,
+        use_last::USE_LAST,
         vec::USELESS_VEC,
         write::PRINTLN_EMPTY_STRING,
         write::PRINT_LITERAL,
@@ -948,6 +950,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         types::UNNECESSARY_CAST,
         types::VEC_BOX,
         unused_label::UNUSED_LABEL,
+        use_last::USE_LAST,
         zero_div_zero::ZERO_DIVIDED_BY_ZERO,
     ]);
 

clippy_lints/src/use_last.rs

@@ -0,0 +1,68 @@
+//! lint on using `x.get(x.len() - 1)` instead of `x.last()`
+
+use rustc::lint::{in_external_macro, LateContext, LateLintPass, LintArray, LintContext, LintPass};
+use rustc::{declare_tool_lint, lint_array};
+use if_chain::if_chain;
+// use syntax::ast::*;
+use rustc::hir::*;
+use crate::utils::{last_path_segment, match_def_path, paths, snippet, span_lint, span_lint_and_then};
+use crate::utils::{opt_def_id, sugg};
+use if_chain::if_chain;
+use rustc::ty::{self, Ty};
+use rustc_errors::Applicability;
+use std::borrow::Cow;
+use syntax::ast;
+
+/// **What it does:** Checks for using `x.get(x.len() - 1)` instead of `x.last()`.
+///
+/// **Why is this bad?** Using `x.last()` is easier to read and has the same result.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+///
+/// ```rust
+/// // Bad
+/// let x = vec![2, 3, 5];
+/// let last_element = x.get(x.len() - 1);
+///
+/// // Good
+/// let x = vec![2, 3, 5];
+/// let last_element = x.last();
+/// ```
+
+declare_clippy_lint! {
+    pub USE_LAST,
+    complexity,
+    "using `x.get(x.len() - 1)` instead of `x.last()`"
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct UseLast;
+
+impl LintPass for UseLast {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(USE_LAST)
+    }
+
+    fn name(&self) -> &'static str {
+        "UseLast"
+    }
+}
+
+impl LateLintPass for UseLast {
+    fn check_expr(&mut self, cx: &LateContext<'_>, mut item: &Expr) {
+        if_chain! {
+            if let StmtKind::Local(ref local) = stmt.node;
+            if let Some(ref init) = local.init
+                if let ExprKind::MethodCall(ref method_name, ref generics, ref args) = init.node;
+            // unimplemented: `ExprKind::MethodCall` is not further destructured at the moment
+            if let PatKind::Binding(BindingAnnotation::Unannotated, _, name, None) = local.pat.node;
+            if name.node.as_str() == "last_element";
+            then {
+                // report your lint here
+            }
+        }
+        
+    }
+}

tests/ui/use_last.rs

@@ -0,0 +1,22 @@
+#![warn(clippy::all)]
+#![warn(clippy::use_last)]
+
+fn dont_use_last() -> Option<i32> {
+    let x = vec![2, 3, 5];
+    #[clippy::author]
+    let last_element = x.get(x.len() - 1); // ~ERROR Use _.last()
+    last_element.map(|val| val + 1) // To avoid warnings
+}
+
+fn index_into_last() -> i32 {
+    let x = vec![2, 3, 5];
+    let last_element = x[x.len() - 1];
+    last_element + 1 // To avoid warnings
+}
+
+fn main() {
+    // let expected_value: &i32 = &5;
+    let expected_value: i32 = 5;
+    assert_eq!(dont_use_last(), Some(expected_value));
+    assert_eq!(index_into_last(), 5);
+}