652 lines
22 KiB
Rust
652 lines
22 KiB
Rust
use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
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use clippy_utils::trait_ref_of_method;
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use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use rustc_hir::intravisit::nested_filter::{self as hir_nested_filter, NestedFilter};
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use rustc_hir::intravisit::{
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walk_fn_decl, walk_generic_arg, walk_generic_param, walk_generics, walk_impl_item_ref, walk_item, walk_param_bound,
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walk_poly_trait_ref, walk_trait_ref, walk_ty, Visitor,
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};
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use rustc_hir::lang_items;
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use rustc_hir::FnRetTy::Return;
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use rustc_hir::{
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BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, Impl, ImplItem,
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ImplItemKind, Item, ItemKind, Lifetime, LifetimeName, LifetimeParamKind, PolyTraitRef, PredicateOrigin, TraitFn,
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TraitItem, TraitItemKind, Ty, TyKind, WherePredicate,
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};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::hir::nested_filter as middle_nested_filter;
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use rustc_middle::ty::TyCtxt;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::def_id::LocalDefId;
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use rustc_span::source_map::Span;
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use rustc_span::symbol::{kw, Ident, Symbol};
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for lifetime annotations which can be removed by
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/// relying on lifetime elision.
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///
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/// ### Why is this bad?
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/// The additional lifetimes make the code look more
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/// complicated, while there is nothing out of the ordinary going on. Removing
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/// them leads to more readable code.
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///
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/// ### Known problems
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/// - We bail out if the function has a `where` clause where lifetimes
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/// are mentioned due to potential false positives.
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/// - Lifetime bounds such as `impl Foo + 'a` and `T: 'a` must be elided with the
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/// placeholder notation `'_` because the fully elided notation leaves the type bound to `'static`.
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///
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/// ### Example
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/// ```rust
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/// // Unnecessary lifetime annotations
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/// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
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/// x
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/// }
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/// ```
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///
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/// Use instead:
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/// ```rust
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/// fn elided(x: &u8, y: u8) -> &u8 {
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/// x
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/// }
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/// ```
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#[clippy::version = "pre 1.29.0"]
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pub NEEDLESS_LIFETIMES,
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complexity,
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"using explicit lifetimes for references in function arguments when elision rules \
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would allow omitting them"
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}
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declare_clippy_lint! {
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/// ### What it does
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/// Checks for lifetimes in generics that are never used
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/// anywhere else.
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///
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/// ### Why is this bad?
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/// The additional lifetimes make the code look more
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/// complicated, while there is nothing out of the ordinary going on. Removing
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/// them leads to more readable code.
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///
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/// ### Example
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/// ```rust
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/// // unnecessary lifetimes
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/// fn unused_lifetime<'a>(x: u8) {
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/// // ..
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/// }
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/// ```
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///
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/// Use instead:
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/// ```rust
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/// fn no_lifetime(x: u8) {
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/// // ...
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/// }
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/// ```
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#[clippy::version = "pre 1.29.0"]
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pub EXTRA_UNUSED_LIFETIMES,
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complexity,
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"unused lifetimes in function definitions"
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}
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declare_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
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impl<'tcx> LateLintPass<'tcx> for Lifetimes {
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fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
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if let ItemKind::Fn(ref sig, generics, id) = item.kind {
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check_fn_inner(cx, sig.decl, Some(id), None, generics, item.span, true);
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} else if let ItemKind::Impl(impl_) = item.kind {
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if !item.span.from_expansion() {
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report_extra_impl_lifetimes(cx, impl_);
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}
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}
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}
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fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
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if let ImplItemKind::Fn(ref sig, id) = item.kind {
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let report_extra_lifetimes = trait_ref_of_method(cx, item.owner_id.def_id).is_none();
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check_fn_inner(
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cx,
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sig.decl,
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Some(id),
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None,
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item.generics,
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item.span,
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report_extra_lifetimes,
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);
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}
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}
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fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
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if let TraitItemKind::Fn(ref sig, ref body) = item.kind {
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let (body, trait_sig) = match *body {
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TraitFn::Required(sig) => (None, Some(sig)),
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TraitFn::Provided(id) => (Some(id), None),
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};
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check_fn_inner(cx, sig.decl, body, trait_sig, item.generics, item.span, true);
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}
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}
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}
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/// The lifetime of a &-reference.
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#[derive(PartialEq, Eq, Hash, Debug, Clone)]
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enum RefLt {
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Unnamed,
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Static,
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Named(LocalDefId),
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}
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fn check_fn_inner<'tcx>(
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cx: &LateContext<'tcx>,
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decl: &'tcx FnDecl<'_>,
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body: Option<BodyId>,
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trait_sig: Option<&[Ident]>,
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generics: &'tcx Generics<'_>,
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span: Span,
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report_extra_lifetimes: bool,
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) {
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if span.from_expansion() || has_where_lifetimes(cx, generics) {
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return;
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}
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let types = generics
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.params
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.iter()
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.filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
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for typ in types {
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for pred in generics.bounds_for_param(cx.tcx.hir().local_def_id(typ.hir_id)) {
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if pred.origin == PredicateOrigin::WhereClause {
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// has_where_lifetimes checked that this predicate contains no lifetime.
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continue;
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}
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for bound in pred.bounds {
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let mut visitor = RefVisitor::new(cx);
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walk_param_bound(&mut visitor, bound);
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if visitor.lts.iter().any(|lt| matches!(lt, RefLt::Named(_))) {
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return;
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}
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if let GenericBound::Trait(ref trait_ref, _) = *bound {
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let params = &trait_ref
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.trait_ref
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.path
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.segments
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.last()
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.expect("a path must have at least one segment")
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.args;
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if let Some(params) = *params {
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let lifetimes = params.args.iter().filter_map(|arg| match arg {
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GenericArg::Lifetime(lt) => Some(lt),
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_ => None,
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});
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for bound in lifetimes {
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if !bound.is_static() && !bound.is_elided() {
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return;
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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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}
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if let Some(elidable_lts) = could_use_elision(cx, decl, body, trait_sig, generics.params) {
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let lts = elidable_lts
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.iter()
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// In principle, the result of the call to `Node::ident` could be `unwrap`ped, as `DefId` should refer to a
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// `Node::GenericParam`.
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.filter_map(|&(def_id, _)| cx.tcx.hir().get_by_def_id(def_id).ident())
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.map(|ident| ident.to_string())
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.collect::<Vec<_>>()
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.join(", ");
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span_lint_and_then(
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cx,
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NEEDLESS_LIFETIMES,
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span.with_hi(decl.output.span().hi()),
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&format!("the following explicit lifetimes could be elided: {lts}"),
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|diag| {
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if let Some(span) = elidable_lts.iter().find_map(|&(_, span)| span) {
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diag.span_help(span, "replace with `'_` in generic arguments such as here");
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}
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},
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);
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}
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if report_extra_lifetimes {
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self::report_extra_lifetimes(cx, decl, generics);
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}
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}
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// elision doesn't work for explicit self types, see rust-lang/rust#69064
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fn explicit_self_type<'tcx>(cx: &LateContext<'tcx>, func: &FnDecl<'tcx>, ident: Option<Ident>) -> bool {
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if_chain! {
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if let Some(ident) = ident;
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if ident.name == kw::SelfLower;
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if !func.implicit_self.has_implicit_self();
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if let Some(self_ty) = func.inputs.first();
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then {
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let mut visitor = RefVisitor::new(cx);
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visitor.visit_ty(self_ty);
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!visitor.all_lts().is_empty()
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} else {
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false
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}
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}
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}
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fn could_use_elision<'tcx>(
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cx: &LateContext<'tcx>,
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func: &'tcx FnDecl<'_>,
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body: Option<BodyId>,
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trait_sig: Option<&[Ident]>,
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named_generics: &'tcx [GenericParam<'_>],
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) -> Option<Vec<(LocalDefId, Option<Span>)>> {
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// There are two scenarios where elision works:
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// * no output references, all input references have different LT
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// * output references, exactly one input reference with same LT
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// All lifetimes must be unnamed, 'static or defined without bounds on the
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// level of the current item.
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// check named LTs
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let allowed_lts = allowed_lts_from(cx.tcx, named_generics);
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// these will collect all the lifetimes for references in arg/return types
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let mut input_visitor = RefVisitor::new(cx);
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let mut output_visitor = RefVisitor::new(cx);
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// extract lifetimes in input argument types
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for arg in func.inputs {
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input_visitor.visit_ty(arg);
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}
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// extract lifetimes in output type
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if let Return(ty) = func.output {
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output_visitor.visit_ty(ty);
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}
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for lt in named_generics {
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input_visitor.visit_generic_param(lt);
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}
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if input_visitor.abort() || output_visitor.abort() {
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return None;
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}
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let input_lts = input_visitor.lts;
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let output_lts = output_visitor.lts;
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if let Some(trait_sig) = trait_sig {
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if explicit_self_type(cx, func, trait_sig.first().copied()) {
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return None;
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}
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}
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if let Some(body_id) = body {
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let body = cx.tcx.hir().body(body_id);
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let first_ident = body.params.first().and_then(|param| param.pat.simple_ident());
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if explicit_self_type(cx, func, first_ident) {
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return None;
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}
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let mut checker = BodyLifetimeChecker {
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lifetimes_used_in_body: false,
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};
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checker.visit_expr(body.value);
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if checker.lifetimes_used_in_body {
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return None;
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}
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}
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// check for lifetimes from higher scopes
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for lt in input_lts.iter().chain(output_lts.iter()) {
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if !allowed_lts.contains(lt) {
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return None;
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}
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}
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// check for higher-ranked trait bounds
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if !input_visitor.nested_elision_site_lts.is_empty() || !output_visitor.nested_elision_site_lts.is_empty() {
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let allowed_lts: FxHashSet<_> = allowed_lts
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.iter()
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.filter_map(|lt| match lt {
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RefLt::Named(def_id) => Some(cx.tcx.item_name(def_id.to_def_id())),
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_ => None,
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})
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.collect();
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for lt in input_visitor.nested_elision_site_lts {
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if let RefLt::Named(def_id) = lt {
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if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
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return None;
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}
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}
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}
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for lt in output_visitor.nested_elision_site_lts {
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if let RefLt::Named(def_id) = lt {
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if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
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return None;
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}
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}
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}
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}
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// A lifetime can be newly elided if:
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// - It occurs only once among the inputs.
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// - If there are multiple input lifetimes, then the newly elided lifetime does not occur among the
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// outputs (because eliding such an lifetime would create an ambiguity).
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let elidable_lts = named_lifetime_occurrences(&input_lts)
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.into_iter()
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.filter_map(|(def_id, occurrences)| {
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if occurrences == 1 && (input_lts.len() == 1 || !output_lts.contains(&RefLt::Named(def_id))) {
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Some((
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def_id,
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input_visitor
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.lifetime_generic_arg_spans
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.get(&def_id)
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.or_else(|| output_visitor.lifetime_generic_arg_spans.get(&def_id))
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.copied(),
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))
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} else {
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None
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}
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})
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.collect::<Vec<_>>();
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if elidable_lts.is_empty() {
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None
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} else {
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Some(elidable_lts)
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}
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}
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fn allowed_lts_from(tcx: TyCtxt<'_>, named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
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let mut allowed_lts = FxHashSet::default();
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for par in named_generics.iter() {
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if let GenericParamKind::Lifetime { .. } = par.kind {
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allowed_lts.insert(RefLt::Named(tcx.hir().local_def_id(par.hir_id)));
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}
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}
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allowed_lts.insert(RefLt::Unnamed);
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allowed_lts.insert(RefLt::Static);
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allowed_lts
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}
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/// Number of times each named lifetime occurs in the given slice. Returns a vector to preserve
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/// relative order.
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#[must_use]
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fn named_lifetime_occurrences(lts: &[RefLt]) -> Vec<(LocalDefId, usize)> {
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let mut occurrences = Vec::new();
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for lt in lts {
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if let &RefLt::Named(curr_def_id) = lt {
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if let Some(pair) = occurrences
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.iter_mut()
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.find(|(prev_def_id, _)| *prev_def_id == curr_def_id)
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{
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pair.1 += 1;
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} else {
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occurrences.push((curr_def_id, 1));
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}
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}
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}
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occurrences
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}
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/// A visitor usable for `rustc_front::visit::walk_ty()`.
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struct RefVisitor<'a, 'tcx> {
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cx: &'a LateContext<'tcx>,
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lts: Vec<RefLt>,
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lifetime_generic_arg_spans: FxHashMap<LocalDefId, Span>,
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nested_elision_site_lts: Vec<RefLt>,
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unelided_trait_object_lifetime: bool,
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}
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impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
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fn new(cx: &'a LateContext<'tcx>) -> Self {
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Self {
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cx,
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lts: Vec::new(),
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lifetime_generic_arg_spans: FxHashMap::default(),
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nested_elision_site_lts: Vec::new(),
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unelided_trait_object_lifetime: false,
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}
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}
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fn record(&mut self, lifetime: &Option<Lifetime>) {
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if let Some(ref lt) = *lifetime {
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if lt.is_static() {
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self.lts.push(RefLt::Static);
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} else if lt.is_anonymous() {
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// Fresh lifetimes generated should be ignored.
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self.lts.push(RefLt::Unnamed);
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} else if let LifetimeName::Param(def_id) = lt.res {
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self.lts.push(RefLt::Named(def_id));
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}
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} else {
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self.lts.push(RefLt::Unnamed);
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}
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}
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fn all_lts(&self) -> Vec<RefLt> {
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self.lts
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.iter()
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.chain(self.nested_elision_site_lts.iter())
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.cloned()
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.collect::<Vec<_>>()
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}
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fn abort(&self) -> bool {
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self.unelided_trait_object_lifetime
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}
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}
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impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
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// for lifetimes as parameters of generics
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fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
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self.record(&Some(*lifetime));
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}
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fn visit_poly_trait_ref(&mut self, poly_tref: &'tcx PolyTraitRef<'tcx>) {
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let trait_ref = &poly_tref.trait_ref;
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if let Some(id) = trait_ref.trait_def_id() && lang_items::FN_TRAITS.iter().any(|&item| {
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self.cx.tcx.lang_items().get(item) == Some(id)
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}) {
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let mut sub_visitor = RefVisitor::new(self.cx);
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sub_visitor.visit_trait_ref(trait_ref);
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self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
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} else {
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walk_poly_trait_ref(self, poly_tref);
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}
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}
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fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
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match ty.kind {
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TyKind::OpaqueDef(item, bounds, _) => {
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let map = self.cx.tcx.hir();
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let item = map.item(item);
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let len = self.lts.len();
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walk_item(self, item);
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self.lts.truncate(len);
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self.lts.extend(bounds.iter().filter_map(|bound| match bound {
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GenericArg::Lifetime(l) => Some(if let LifetimeName::Param(def_id) = l.res {
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RefLt::Named(def_id)
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} else {
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RefLt::Unnamed
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}),
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_ => None,
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}));
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},
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TyKind::BareFn(&BareFnTy { decl, .. }) => {
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let mut sub_visitor = RefVisitor::new(self.cx);
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sub_visitor.visit_fn_decl(decl);
|
|
self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
|
|
},
|
|
TyKind::TraitObject(bounds, lt, _) => {
|
|
if !lt.is_elided() {
|
|
self.unelided_trait_object_lifetime = true;
|
|
}
|
|
for bound in bounds {
|
|
self.visit_poly_trait_ref(bound);
|
|
}
|
|
},
|
|
_ => walk_ty(self, ty),
|
|
}
|
|
}
|
|
|
|
fn visit_generic_arg(&mut self, generic_arg: &'tcx GenericArg<'tcx>) {
|
|
if let GenericArg::Lifetime(l) = generic_arg && let LifetimeName::Param(def_id) = l.res {
|
|
self.lifetime_generic_arg_spans.entry(def_id).or_insert(l.ident.span);
|
|
}
|
|
walk_generic_arg(self, generic_arg);
|
|
}
|
|
}
|
|
|
|
/// Are any lifetimes mentioned in the `where` clause? If so, we don't try to
|
|
/// reason about elision.
|
|
fn has_where_lifetimes<'tcx>(cx: &LateContext<'tcx>, generics: &'tcx Generics<'_>) -> bool {
|
|
for predicate in generics.predicates {
|
|
match *predicate {
|
|
WherePredicate::RegionPredicate(..) => return true,
|
|
WherePredicate::BoundPredicate(ref pred) => {
|
|
// a predicate like F: Trait or F: for<'a> Trait<'a>
|
|
let mut visitor = RefVisitor::new(cx);
|
|
// walk the type F, it may not contain LT refs
|
|
walk_ty(&mut visitor, pred.bounded_ty);
|
|
if !visitor.all_lts().is_empty() {
|
|
return true;
|
|
}
|
|
// if the bounds define new lifetimes, they are fine to occur
|
|
let allowed_lts = allowed_lts_from(cx.tcx, pred.bound_generic_params);
|
|
// now walk the bounds
|
|
for bound in pred.bounds.iter() {
|
|
walk_param_bound(&mut visitor, bound);
|
|
}
|
|
// and check that all lifetimes are allowed
|
|
if visitor.all_lts().iter().any(|it| !allowed_lts.contains(it)) {
|
|
return true;
|
|
}
|
|
},
|
|
WherePredicate::EqPredicate(ref pred) => {
|
|
let mut visitor = RefVisitor::new(cx);
|
|
walk_ty(&mut visitor, pred.lhs_ty);
|
|
walk_ty(&mut visitor, pred.rhs_ty);
|
|
if !visitor.lts.is_empty() {
|
|
return true;
|
|
}
|
|
},
|
|
}
|
|
}
|
|
false
|
|
}
|
|
|
|
struct LifetimeChecker<'cx, 'tcx, F> {
|
|
cx: &'cx LateContext<'tcx>,
|
|
map: FxHashMap<Symbol, Span>,
|
|
phantom: std::marker::PhantomData<F>,
|
|
}
|
|
|
|
impl<'cx, 'tcx, F> LifetimeChecker<'cx, 'tcx, F> {
|
|
fn new(cx: &'cx LateContext<'tcx>, map: FxHashMap<Symbol, Span>) -> LifetimeChecker<'cx, 'tcx, F> {
|
|
Self {
|
|
cx,
|
|
map,
|
|
phantom: std::marker::PhantomData,
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<'cx, 'tcx, F> Visitor<'tcx> for LifetimeChecker<'cx, 'tcx, F>
|
|
where
|
|
F: NestedFilter<'tcx>,
|
|
{
|
|
type Map = rustc_middle::hir::map::Map<'tcx>;
|
|
type NestedFilter = F;
|
|
|
|
// for lifetimes as parameters of generics
|
|
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
|
|
self.map.remove(&lifetime.ident.name);
|
|
}
|
|
|
|
fn visit_generic_param(&mut self, param: &'tcx GenericParam<'_>) {
|
|
// don't actually visit `<'a>` or `<'a: 'b>`
|
|
// we've already visited the `'a` declarations and
|
|
// don't want to spuriously remove them
|
|
// `'b` in `'a: 'b` is useless unless used elsewhere in
|
|
// a non-lifetime bound
|
|
if let GenericParamKind::Type { .. } = param.kind {
|
|
walk_generic_param(self, param);
|
|
}
|
|
}
|
|
|
|
fn nested_visit_map(&mut self) -> Self::Map {
|
|
self.cx.tcx.hir()
|
|
}
|
|
}
|
|
|
|
fn report_extra_lifetimes<'tcx>(cx: &LateContext<'tcx>, func: &'tcx FnDecl<'_>, generics: &'tcx Generics<'_>) {
|
|
let hs = generics
|
|
.params
|
|
.iter()
|
|
.filter_map(|par| match par.kind {
|
|
GenericParamKind::Lifetime {
|
|
kind: LifetimeParamKind::Explicit,
|
|
} => Some((par.name.ident().name, par.span)),
|
|
_ => None,
|
|
})
|
|
.collect();
|
|
let mut checker = LifetimeChecker::<hir_nested_filter::None>::new(cx, hs);
|
|
|
|
walk_generics(&mut checker, generics);
|
|
walk_fn_decl(&mut checker, func);
|
|
|
|
for &v in checker.map.values() {
|
|
span_lint(
|
|
cx,
|
|
EXTRA_UNUSED_LIFETIMES,
|
|
v,
|
|
"this lifetime isn't used in the function definition",
|
|
);
|
|
}
|
|
}
|
|
|
|
fn report_extra_impl_lifetimes<'tcx>(cx: &LateContext<'tcx>, impl_: &'tcx Impl<'_>) {
|
|
let hs = impl_
|
|
.generics
|
|
.params
|
|
.iter()
|
|
.filter_map(|par| match par.kind {
|
|
GenericParamKind::Lifetime {
|
|
kind: LifetimeParamKind::Explicit,
|
|
} => Some((par.name.ident().name, par.span)),
|
|
_ => None,
|
|
})
|
|
.collect();
|
|
let mut checker = LifetimeChecker::<middle_nested_filter::All>::new(cx, hs);
|
|
|
|
walk_generics(&mut checker, impl_.generics);
|
|
if let Some(ref trait_ref) = impl_.of_trait {
|
|
walk_trait_ref(&mut checker, trait_ref);
|
|
}
|
|
walk_ty(&mut checker, impl_.self_ty);
|
|
for item in impl_.items {
|
|
walk_impl_item_ref(&mut checker, item);
|
|
}
|
|
|
|
for &v in checker.map.values() {
|
|
span_lint(cx, EXTRA_UNUSED_LIFETIMES, v, "this lifetime isn't used in the impl");
|
|
}
|
|
}
|
|
|
|
struct BodyLifetimeChecker {
|
|
lifetimes_used_in_body: bool,
|
|
}
|
|
|
|
impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
|
|
// for lifetimes as parameters of generics
|
|
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
|
|
if !lifetime.is_anonymous() && lifetime.ident.name != kw::StaticLifetime {
|
|
self.lifetimes_used_in_body = true;
|
|
}
|
|
}
|
|
}
|