rust-clippy/clippy_lints/src/get_last_with_len.rs

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//! lint on using `x.get(x.len() - 1)` instead of `x.last()`
use crate::utils::{span_lint_and_sugg, SpanlessEq};
use clippy_utils::source::snippet_with_applicability;
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use clippy_utils::ty::is_type_diagnostic_item;
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use if_chain::if_chain;
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use rustc_ast::ast::LitKind;
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use rustc_errors::Applicability;
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use rustc_hir::{BinOpKind, Expr, ExprKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Spanned;
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use rustc_span::sym;
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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<'tcx> LateLintPass<'tcx> for GetLastWithLen {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
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if_chain! {
// Is a method call
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if let ExprKind::MethodCall(ref path, _, ref args, _) = expr.kind;
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// Method name is "get"
if path.ident.name == sym!(get);
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// Argument 0 (the struct we're calling the method on) is a vector
if let Some(struct_calling_on) = args.get(0);
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let struct_ty = cx.typeck_results().expr_ty(struct_calling_on);
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if is_type_diagnostic_item(cx, struct_ty, sym::vec_type);
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// Argument to "get" is a subtraction
if let Some(get_index_arg) = args.get(1);
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if let ExprKind::Binary(
Spanned {
node: BinOpKind::Sub,
..
},
lhs,
rhs,
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) = &get_index_arg.kind;
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// LHS of subtraction is "x.len()"
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if let ExprKind::MethodCall(arg_lhs_path, _, lhs_args, _) = &lhs.kind;
if arg_lhs_path.ident.name == sym!(len);
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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
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if let ExprKind::Lit(rhs_lit) = &rhs.kind;
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if let LitKind::Int(1, ..) = rhs_lit.node;
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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),
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applicability,
);
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}
}
}
}