302 lines
11 KiB
Rust
302 lines
11 KiB
Rust
use clippy_utils::diagnostics::span_lint_and_sugg;
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use clippy_utils::source::snippet_with_context;
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use clippy_utils::ty::peel_mid_ty_refs;
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use clippy_utils::{get_parent_node, in_macro, is_allowed};
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use rustc_ast::util::parser::PREC_PREFIX;
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use rustc_errors::Applicability;
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use rustc_hir::{BorrowKind, Expr, ExprKind, HirId, MatchSource, Mutability, Node, UnOp};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::ty::{self, Ty, TyCtxt, TyS, TypeckResults};
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use rustc_session::{declare_tool_lint, impl_lint_pass};
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use rustc_span::{symbol::sym, Span};
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declare_clippy_lint! {
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/// **What it does:** Checks for explicit `deref()` or `deref_mut()` method calls.
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///
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/// **Why is this bad?** Dereferencing by `&*x` or `&mut *x` is clearer and more concise,
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/// when not part of a method chain.
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///
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/// **Example:**
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/// ```rust
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/// use std::ops::Deref;
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/// let a: &mut String = &mut String::from("foo");
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/// let b: &str = a.deref();
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/// ```
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/// Could be written as:
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/// ```rust
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/// let a: &mut String = &mut String::from("foo");
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/// let b = &*a;
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/// ```
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///
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/// This lint excludes
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/// ```rust,ignore
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/// let _ = d.unwrap().deref();
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/// ```
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pub EXPLICIT_DEREF_METHODS,
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pedantic,
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"Explicit use of deref or deref_mut method while not in a method chain."
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}
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impl_lint_pass!(Dereferencing => [
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EXPLICIT_DEREF_METHODS,
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]);
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#[derive(Default)]
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pub struct Dereferencing {
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state: Option<(State, StateData)>,
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// While parsing a `deref` method call in ufcs form, the path to the function is itself an
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// expression. This is to store the id of that expression so it can be skipped when
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// `check_expr` is called for it.
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skip_expr: Option<HirId>,
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}
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struct StateData {
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/// Span of the top level expression
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span: Span,
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/// The required mutability
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target_mut: Mutability,
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}
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enum State {
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// Any number of deref method calls.
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DerefMethod {
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// The number of calls in a sequence which changed the referenced type
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ty_changed_count: usize,
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is_final_ufcs: bool,
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},
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}
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// A reference operation considered by this lint pass
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enum RefOp {
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Method(Mutability),
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Deref,
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AddrOf,
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}
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impl<'tcx> LateLintPass<'tcx> for Dereferencing {
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fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
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// Skip path expressions from deref calls. e.g. `Deref::deref(e)`
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if Some(expr.hir_id) == self.skip_expr.take() {
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return;
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}
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// Stop processing sub expressions when a macro call is seen
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if in_macro(expr.span) {
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if let Some((state, data)) = self.state.take() {
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report(cx, expr, state, data);
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}
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return;
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}
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let typeck = cx.typeck_results();
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let (kind, sub_expr) = if let Some(x) = try_parse_ref_op(cx.tcx, typeck, expr) {
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x
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} else {
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// The whole chain of reference operations has been seen
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if let Some((state, data)) = self.state.take() {
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report(cx, expr, state, data);
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}
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return;
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};
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match (self.state.take(), kind) {
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(None, kind) => {
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let parent = get_parent_node(cx.tcx, expr.hir_id);
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let expr_ty = typeck.expr_ty(expr);
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match kind {
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RefOp::Method(target_mut)
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if !is_allowed(cx, EXPLICIT_DEREF_METHODS, expr.hir_id)
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&& is_linted_explicit_deref_position(parent, expr.hir_id, expr.span) =>
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{
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self.state = Some((
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State::DerefMethod {
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ty_changed_count: if deref_method_same_type(expr_ty, typeck.expr_ty(sub_expr)) {
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0
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} else {
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1
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},
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is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
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},
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StateData {
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span: expr.span,
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target_mut,
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},
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));
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}
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_ => (),
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}
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},
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(Some((State::DerefMethod { ty_changed_count, .. }, data)), RefOp::Method(_)) => {
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self.state = Some((
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State::DerefMethod {
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ty_changed_count: if deref_method_same_type(typeck.expr_ty(expr), typeck.expr_ty(sub_expr)) {
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ty_changed_count
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} else {
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ty_changed_count + 1
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},
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is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
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},
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data,
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));
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},
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(Some((state, data)), _) => report(cx, expr, state, data),
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}
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}
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}
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fn try_parse_ref_op(
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tcx: TyCtxt<'tcx>,
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typeck: &'tcx TypeckResults<'_>,
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expr: &'tcx Expr<'_>,
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) -> Option<(RefOp, &'tcx Expr<'tcx>)> {
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let (def_id, arg) = match expr.kind {
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ExprKind::MethodCall(_, _, [arg], _) => (typeck.type_dependent_def_id(expr.hir_id)?, arg),
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ExprKind::Call(
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Expr {
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kind: ExprKind::Path(path),
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hir_id,
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..
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},
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[arg],
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) => (typeck.qpath_res(path, *hir_id).opt_def_id()?, arg),
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ExprKind::Unary(UnOp::Deref, sub_expr) if !typeck.expr_ty(sub_expr).is_unsafe_ptr() => {
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return Some((RefOp::Deref, sub_expr));
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},
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ExprKind::AddrOf(BorrowKind::Ref, _, sub_expr) => return Some((RefOp::AddrOf, sub_expr)),
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_ => return None,
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};
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if tcx.is_diagnostic_item(sym::deref_method, def_id) {
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Some((RefOp::Method(Mutability::Not), arg))
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} else if tcx.trait_of_item(def_id)? == tcx.lang_items().deref_mut_trait()? {
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Some((RefOp::Method(Mutability::Mut), arg))
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} else {
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None
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}
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}
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// Checks whether the type for a deref call actually changed the type, not just the mutability of
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// the reference.
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fn deref_method_same_type(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
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match (result_ty.kind(), arg_ty.kind()) {
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(ty::Ref(_, result_ty, _), ty::Ref(_, arg_ty, _)) => TyS::same_type(result_ty, arg_ty),
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// The result type for a deref method is always a reference
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// Not matching the previous pattern means the argument type is not a reference
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// This means that the type did change
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_ => false,
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}
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}
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// Checks whether the parent node is a suitable context for switching from a deref method to the
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// deref operator.
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fn is_linted_explicit_deref_position(parent: Option<Node<'_>>, child_id: HirId, child_span: Span) -> bool {
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let parent = match parent {
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Some(Node::Expr(e)) if e.span.ctxt() == child_span.ctxt() => e,
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_ => return true,
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};
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match parent.kind {
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// Leave deref calls in the middle of a method chain.
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// e.g. x.deref().foo()
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ExprKind::MethodCall(_, _, [self_arg, ..], _) if self_arg.hir_id == child_id => false,
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// Leave deref calls resulting in a called function
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// e.g. (x.deref())()
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ExprKind::Call(func_expr, _) if func_expr.hir_id == child_id => false,
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// Makes an ugly suggestion
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// e.g. *x.deref() => *&*x
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ExprKind::Unary(UnOp::Deref, _)
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// Postfix expressions would require parens
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| ExprKind::Match(_, _, MatchSource::TryDesugar | MatchSource::AwaitDesugar)
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| ExprKind::Field(..)
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| ExprKind::Index(..)
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| ExprKind::Err => false,
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ExprKind::Box(..)
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| ExprKind::ConstBlock(..)
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| ExprKind::Array(_)
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| ExprKind::Call(..)
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| ExprKind::MethodCall(..)
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| ExprKind::Tup(..)
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| ExprKind::Binary(..)
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| ExprKind::Unary(..)
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| ExprKind::Lit(..)
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| ExprKind::Cast(..)
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| ExprKind::Type(..)
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| ExprKind::DropTemps(..)
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| ExprKind::If(..)
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| ExprKind::Loop(..)
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| ExprKind::Match(..)
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| ExprKind::Closure(..)
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| ExprKind::Block(..)
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| ExprKind::Assign(..)
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| ExprKind::AssignOp(..)
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| ExprKind::Path(..)
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| ExprKind::AddrOf(..)
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| ExprKind::Break(..)
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| ExprKind::Continue(..)
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| ExprKind::Ret(..)
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| ExprKind::InlineAsm(..)
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| ExprKind::LlvmInlineAsm(..)
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| ExprKind::Struct(..)
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| ExprKind::Repeat(..)
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| ExprKind::Yield(..) => true,
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}
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}
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#[allow(clippy::needless_pass_by_value)]
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fn report(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data: StateData) {
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match state {
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State::DerefMethod {
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ty_changed_count,
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is_final_ufcs,
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} => {
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let mut app = Applicability::MachineApplicable;
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let (expr_str, expr_is_macro_call) = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app);
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let ty = cx.typeck_results().expr_ty(expr);
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let (_, ref_count) = peel_mid_ty_refs(ty);
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let deref_str = if ty_changed_count >= ref_count && ref_count != 0 {
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// a deref call changing &T -> &U requires two deref operators the first time
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// this occurs. One to remove the reference, a second to call the deref impl.
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"*".repeat(ty_changed_count + 1)
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} else {
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"*".repeat(ty_changed_count)
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};
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let addr_of_str = if ty_changed_count < ref_count {
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// Check if a reborrow from &mut T -> &T is required.
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if data.target_mut == Mutability::Not && matches!(ty.kind(), ty::Ref(_, _, Mutability::Mut)) {
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"&*"
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} else {
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""
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}
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} else if data.target_mut == Mutability::Mut {
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"&mut "
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} else {
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"&"
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};
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let expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence().order() < PREC_PREFIX {
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format!("({})", expr_str)
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} else {
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expr_str.into_owned()
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};
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span_lint_and_sugg(
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cx,
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EXPLICIT_DEREF_METHODS,
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data.span,
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match data.target_mut {
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Mutability::Not => "explicit `deref` method call",
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Mutability::Mut => "explicit `deref_mut` method call",
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},
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"try this",
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format!("{}{}{}", addr_of_str, deref_str, expr_str),
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app,
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);
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},
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}
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}
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