108 lines
3.7 KiB
Rust
108 lines
3.7 KiB
Rust
//! lint on using `x.get(x.len() - 1)` instead of `x.last()`
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use clippy_utils::diagnostics::span_lint_and_sugg;
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use clippy_utils::source::snippet_with_applicability;
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use clippy_utils::ty::is_type_diagnostic_item;
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use clippy_utils::SpanlessEq;
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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};
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use rustc_span::source_map::Spanned;
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use rustc_span::sym;
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declare_clippy_lint! {
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/// **What it does:** Checks for using `x.get(x.len() - 1)` instead of
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/// `x.last()`.
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///
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/// **Why is this bad?** Using `x.last()` is easier to read and has the same
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/// result.
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///
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/// Note that using `x[x.len() - 1]` is semantically different from
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/// `x.last()`. Indexing into the array will panic on out-of-bounds
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/// accesses, while `x.get()` and `x.last()` will return `None`.
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///
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/// There is another lint (get_unwrap) that covers the case of using
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/// `x.get(index).unwrap()` instead of `x[index]`.
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///
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/// **Known problems:** None.
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///
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/// **Example:**
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///
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/// ```rust
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/// // Bad
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/// let x = vec![2, 3, 5];
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/// let last_element = x.get(x.len() - 1);
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///
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/// // Good
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/// let x = vec![2, 3, 5];
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/// let last_element = x.last();
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/// ```
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pub GET_LAST_WITH_LEN,
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complexity,
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"Using `x.get(x.len() - 1)` when `x.last()` is correct and simpler"
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}
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declare_lint_pass!(GetLastWithLen => [GET_LAST_WITH_LEN]);
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impl<'tcx> LateLintPass<'tcx> for GetLastWithLen {
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fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
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if_chain! {
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// Is a method call
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if let ExprKind::MethodCall(path, _, args, _) = expr.kind;
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// Method name is "get"
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if path.ident.name == sym!(get);
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// Argument 0 (the struct we're calling the method on) is a vector
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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
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if let Some(get_index_arg) = args.get(1);
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if let ExprKind::Binary(
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Spanned {
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node: BinOpKind::Sub,
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..
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},
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lhs,
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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;
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if arg_lhs_path.ident.name == sym!(len);
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if let Some(arg_lhs_struct) = lhs_args.get(0);
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// The two vectors referenced (x in x.get(...) and in x.len())
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if SpanlessEq::new(cx).eq_expr(struct_calling_on, arg_lhs_struct);
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// 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 {
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let mut applicability = Applicability::MachineApplicable;
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let vec_name = snippet_with_applicability(
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cx,
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struct_calling_on.span, "vec",
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&mut applicability,
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);
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span_lint_and_sugg(
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cx,
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GET_LAST_WITH_LEN,
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expr.span,
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&format!("accessing last element with `{0}.get({0}.len() - 1)`", vec_name),
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"try",
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format!("{}.last()", vec_name),
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applicability,
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);
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}
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}
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}
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}
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