Implement get_last_with_len lint

This commit is contained in:
Harrison McCullough 2019-05-20 18:01:21 -06:00
parent 2cc23a5b18
commit f32c2fcb7e
7 changed files with 176 additions and 1 deletions

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@ -936,6 +936,7 @@ All notable changes to this project will be documented in this file.
[`for_loop_over_result`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loop_over_result
[`forget_copy`]: https://rust-lang.github.io/rust-clippy/master/index.html#forget_copy
[`forget_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#forget_ref
[`get_last_with_len`]: https://rust-lang.github.io/rust-clippy/master/index.html#get_last_with_len
[`get_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#get_unwrap
[`identity_conversion`]: https://rust-lang.github.io/rust-clippy/master/index.html#identity_conversion
[`identity_op`]: https://rust-lang.github.io/rust-clippy/master/index.html#identity_op

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@ -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 303 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
[There are 304 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:

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@ -0,0 +1,98 @@
//! lint on using `x.get(x.len() - 1)` instead of `x.last()`
use crate::utils::{match_type, paths, snippet_with_applicability, span_lint_and_sugg, SpanlessEq};
use if_chain::if_chain;
use rustc::hir::{BinOpKind, Expr, ExprKind};
use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use rustc::{declare_lint_pass, declare_tool_lint};
use rustc_errors::Applicability;
use syntax::ast::LitKind;
use syntax::source_map::Spanned;
use syntax::symbol::Symbol;
declare_clippy_lint! {
/// **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.
///
/// Note that using `x[x.len() - 1]` is semantically different from
/// `x.last()`. Indexing into the array will panic on out-of-bounds
/// accesses, while `x.get()` and `x.last()` will return `None`.
///
/// There is another lint (get_unwrap) that covers the case of using
/// `x.get(index).unwrap()` instead of `x[index]`.
///
/// **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();
/// ```
pub GET_LAST_WITH_LEN,
complexity,
"Using `x.get(x.len() - 1)` when `x.last()` is correct and simpler"
}
declare_lint_pass!(GetLastWithLen => [GET_LAST_WITH_LEN]);
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for GetLastWithLen {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
if_chain! {
// Is a method call
if let ExprKind::MethodCall(ref path, _, ref args) = expr.node;
// Method name is "get"
if path.ident.name == Symbol::intern("get");
// Argument 0 (the struct we're calling the method on) is a vector
if let Some(struct_calling_on) = args.get(0);
let struct_ty = cx.tables.expr_ty(struct_calling_on);
if match_type(cx, struct_ty, &paths::VEC);
// Argument to "get" is a subtraction
if let Some(get_index_arg) = args.get(1);
if let ExprKind::Binary(Spanned{node: BinOpKind::Sub, ..},
lhs, rhs) = &get_index_arg.node;
// LHS of subtraction is "x.len()"
if let ExprKind::MethodCall(arg_lhs_path, _, lhs_args) = &lhs.node;
if arg_lhs_path.ident.name == Symbol::intern("len");
if let Some(arg_lhs_struct) = lhs_args.get(0);
// The two vectors referenced (x in x.get(...) and in x.len())
if SpanlessEq::new(cx).eq_expr(struct_calling_on, arg_lhs_struct);
// RHS of subtraction is 1
if let ExprKind::Lit(rhs_lit) = &rhs.node;
if let LitKind::Int(rhs_value, ..) = rhs_lit.node;
if rhs_value == 1;
then {
let mut applicability = Applicability::MachineApplicable;
let vec_name = snippet_with_applicability(
cx,
struct_calling_on.span, "vec",
&mut applicability,
);
span_lint_and_sugg(
cx,
GET_LAST_WITH_LEN,
expr.span,
&format!("accessing last element with `{0}.get({0}.len() - 1)`", vec_name),
"try",
format!("{}.last()", vec_name),
applicability);
}
}
}
}

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@ -186,6 +186,7 @@ pub mod fallible_impl_from;
pub mod format;
pub mod formatting;
pub mod functions;
pub mod get_last_with_len;
pub mod identity_conversion;
pub mod identity_op;
pub mod if_not_else;
@ -492,6 +493,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
reg.register_late_lint_pass(box types::CharLitAsU8);
reg.register_late_lint_pass(box vec::UselessVec);
reg.register_late_lint_pass(box drop_bounds::DropBounds);
reg.register_late_lint_pass(box get_last_with_len::GetLastWithLen);
reg.register_late_lint_pass(box drop_forget_ref::DropForgetRef);
reg.register_late_lint_pass(box empty_enum::EmptyEnum);
reg.register_late_lint_pass(box types::AbsurdExtremeComparisons);
@ -710,6 +712,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
formatting::SUSPICIOUS_ELSE_FORMATTING,
functions::NOT_UNSAFE_PTR_ARG_DEREF,
functions::TOO_MANY_ARGUMENTS,
get_last_with_len::GET_LAST_WITH_LEN,
identity_conversion::IDENTITY_CONVERSION,
identity_op::IDENTITY_OP,
indexing_slicing::OUT_OF_BOUNDS_INDEXING,
@ -981,6 +984,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
explicit_write::EXPLICIT_WRITE,
format::USELESS_FORMAT,
functions::TOO_MANY_ARGUMENTS,
get_last_with_len::GET_LAST_WITH_LEN,
identity_conversion::IDENTITY_CONVERSION,
identity_op::IDENTITY_OP,
int_plus_one::INT_PLUS_ONE,

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@ -0,0 +1,31 @@
// run-rustfix
#![warn(clippy::get_last_with_len)]
fn dont_use_last() {
let x = vec![2, 3, 5];
let _ = x.last(); // ~ERROR Use x.last()
}
fn indexing_two_from_end() {
let x = vec![2, 3, 5];
let _ = x.get(x.len() - 2);
}
fn index_into_last() {
let x = vec![2, 3, 5];
let _ = x[x.len() - 1];
}
fn use_last_with_different_vec_length() {
let x = vec![2, 3, 5];
let y = vec!['a', 'b', 'c'];
let _ = x.get(y.len() - 1);
}
fn main() {
dont_use_last();
indexing_two_from_end();
index_into_last();
use_last_with_different_vec_length();
}

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@ -0,0 +1,31 @@
// run-rustfix
#![warn(clippy::get_last_with_len)]
fn dont_use_last() {
let x = vec![2, 3, 5];
let _ = x.get(x.len() - 1); // ~ERROR Use x.last()
}
fn indexing_two_from_end() {
let x = vec![2, 3, 5];
let _ = x.get(x.len() - 2);
}
fn index_into_last() {
let x = vec![2, 3, 5];
let _ = x[x.len() - 1];
}
fn use_last_with_different_vec_length() {
let x = vec![2, 3, 5];
let y = vec!['a', 'b', 'c'];
let _ = x.get(y.len() - 1);
}
fn main() {
dont_use_last();
indexing_two_from_end();
index_into_last();
use_last_with_different_vec_length();
}

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@ -0,0 +1,10 @@
error: accessing last element with `x.get(x.len() - 1)`
--> $DIR/get_last_with_len.rs:7:13
|
LL | let _ = x.get(x.len() - 1); // ~ERROR Use x.last()
| ^^^^^^^^^^^^^^^^^^ help: try: `x.last()`
|
= note: `-D clippy::get-last-with-len` implied by `-D warnings`
error: aborting due to previous error