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|
use clippy_utils::diagnostics::span_lint;
use clippy_utils::trait_ref_of_method;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::intravisit::nested_filter::{self as hir_nested_filter, NestedFilter};
use rustc_hir::intravisit::{
walk_fn_decl, walk_generic_param, walk_generics, walk_impl_item_ref, walk_item, walk_param_bound,
walk_poly_trait_ref, walk_trait_ref, walk_ty, Visitor,
};
use rustc_hir::FnRetTy::Return;
use rustc_hir::{
BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, Impl, ImplItem,
ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, ParamName, PolyTraitRef, PredicateOrigin,
TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WherePredicate,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::nested_filter as middle_nested_filter;
use rustc_middle::ty::TyCtxt;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::def_id::LocalDefId;
use rustc_span::source_map::Span;
use rustc_span::symbol::{kw, Ident, Symbol};
declare_clippy_lint! {
/// ### What it does
/// Checks for lifetime annotations which can be removed by
/// relying on lifetime elision.
///
/// ### Why is this bad?
/// The additional lifetimes make the code look more
/// complicated, while there is nothing out of the ordinary going on. Removing
/// them leads to more readable code.
///
/// ### Known problems
/// - We bail out if the function has a `where` clause where lifetimes
/// are mentioned due to potential false positives.
/// - Lifetime bounds such as `impl Foo + 'a` and `T: 'a` must be elided with the
/// placeholder notation `'_` because the fully elided notation leaves the type bound to `'static`.
///
/// ### Example
/// ```rust
/// // Unnecessary lifetime annotations
/// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
/// x
/// }
/// ```
///
/// Use instead:
/// ```rust
/// fn elided(x: &u8, y: u8) -> &u8 {
/// x
/// }
/// ```
#[clippy::version = "pre 1.29.0"]
pub NEEDLESS_LIFETIMES,
complexity,
"using explicit lifetimes for references in function arguments when elision rules \
would allow omitting them"
}
declare_clippy_lint! {
/// ### What it does
/// Checks for lifetimes in generics that are never used
/// anywhere else.
///
/// ### Why is this bad?
/// The additional lifetimes make the code look more
/// complicated, while there is nothing out of the ordinary going on. Removing
/// them leads to more readable code.
///
/// ### Example
/// ```rust
/// // unnecessary lifetimes
/// fn unused_lifetime<'a>(x: u8) {
/// // ..
/// }
/// ```
///
/// Use instead:
/// ```rust
/// fn no_lifetime(x: u8) {
/// // ...
/// }
/// ```
#[clippy::version = "pre 1.29.0"]
pub EXTRA_UNUSED_LIFETIMES,
complexity,
"unused lifetimes in function definitions"
}
declare_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
impl<'tcx> LateLintPass<'tcx> for Lifetimes {
fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
if let ItemKind::Fn(ref sig, generics, id) = item.kind {
check_fn_inner(cx, sig.decl, Some(id), None, generics, item.span, true);
} else if let ItemKind::Impl(impl_) = item.kind {
if !item.span.from_expansion() {
report_extra_impl_lifetimes(cx, impl_);
}
}
}
fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
if let ImplItemKind::Fn(ref sig, id) = item.kind {
let report_extra_lifetimes = trait_ref_of_method(cx, item.def_id).is_none();
check_fn_inner(
cx,
sig.decl,
Some(id),
None,
item.generics,
item.span,
report_extra_lifetimes,
);
}
}
fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
if let TraitItemKind::Fn(ref sig, ref body) = item.kind {
let (body, trait_sig) = match *body {
TraitFn::Required(sig) => (None, Some(sig)),
TraitFn::Provided(id) => (Some(id), None),
};
check_fn_inner(cx, sig.decl, body, trait_sig, item.generics, item.span, true);
}
}
}
/// The lifetime of a &-reference.
#[derive(PartialEq, Eq, Hash, Debug, Clone)]
enum RefLt {
Unnamed,
Static,
Named(LocalDefId),
}
fn check_fn_inner<'tcx>(
cx: &LateContext<'tcx>,
decl: &'tcx FnDecl<'_>,
body: Option<BodyId>,
trait_sig: Option<&[Ident]>,
generics: &'tcx Generics<'_>,
span: Span,
report_extra_lifetimes: bool,
) {
if span.from_expansion() || has_where_lifetimes(cx, generics) {
return;
}
let types = generics
.params
.iter()
.filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
for typ in types {
for pred in generics.bounds_for_param(cx.tcx.hir().local_def_id(typ.hir_id)) {
if pred.origin == PredicateOrigin::WhereClause {
// has_where_lifetimes checked that this predicate contains no lifetime.
continue;
}
for bound in pred.bounds {
let mut visitor = RefVisitor::new(cx);
walk_param_bound(&mut visitor, bound);
if visitor.lts.iter().any(|lt| matches!(lt, RefLt::Named(_))) {
return;
}
if let GenericBound::Trait(ref trait_ref, _) = *bound {
let params = &trait_ref
.trait_ref
.path
.segments
.last()
.expect("a path must have at least one segment")
.args;
if let Some(params) = *params {
let lifetimes = params.args.iter().filter_map(|arg| match arg {
GenericArg::Lifetime(lt) => Some(lt),
_ => None,
});
for bound in lifetimes {
if bound.name != LifetimeName::Static && !bound.is_elided() {
return;
}
}
}
}
}
}
}
if could_use_elision(cx, decl, body, trait_sig, generics.params) {
span_lint(
cx,
NEEDLESS_LIFETIMES,
span.with_hi(decl.output.span().hi()),
"explicit lifetimes given in parameter types where they could be elided \
(or replaced with `'_` if needed by type declaration)",
);
}
if report_extra_lifetimes {
self::report_extra_lifetimes(cx, decl, generics);
}
}
// elision doesn't work for explicit self types, see rust-lang/rust#69064
fn explicit_self_type<'tcx>(cx: &LateContext<'tcx>, func: &FnDecl<'tcx>, ident: Option<Ident>) -> bool {
if_chain! {
if let Some(ident) = ident;
if ident.name == kw::SelfLower;
if !func.implicit_self.has_implicit_self();
if let Some(self_ty) = func.inputs.first();
then {
let mut visitor = RefVisitor::new(cx);
visitor.visit_ty(self_ty);
!visitor.all_lts().is_empty()
} else {
false
}
}
}
fn could_use_elision<'tcx>(
cx: &LateContext<'tcx>,
func: &'tcx FnDecl<'_>,
body: Option<BodyId>,
trait_sig: Option<&[Ident]>,
named_generics: &'tcx [GenericParam<'_>],
) -> bool {
// There are two scenarios where elision works:
// * no output references, all input references have different LT
// * output references, exactly one input reference with same LT
// All lifetimes must be unnamed, 'static or defined without bounds on the
// level of the current item.
// check named LTs
let allowed_lts = allowed_lts_from(cx.tcx, named_generics);
// these will collect all the lifetimes for references in arg/return types
let mut input_visitor = RefVisitor::new(cx);
let mut output_visitor = RefVisitor::new(cx);
// extract lifetimes in input argument types
for arg in func.inputs {
input_visitor.visit_ty(arg);
}
// extract lifetimes in output type
if let Return(ty) = func.output {
output_visitor.visit_ty(ty);
}
for lt in named_generics {
input_visitor.visit_generic_param(lt);
}
if input_visitor.abort() || output_visitor.abort() {
return false;
}
let input_lts = input_visitor.lts;
let output_lts = output_visitor.lts;
if let Some(trait_sig) = trait_sig {
if explicit_self_type(cx, func, trait_sig.first().copied()) {
return false;
}
}
if let Some(body_id) = body {
let body = cx.tcx.hir().body(body_id);
let first_ident = body.params.first().and_then(|param| param.pat.simple_ident());
if explicit_self_type(cx, func, first_ident) {
return false;
}
let mut checker = BodyLifetimeChecker {
lifetimes_used_in_body: false,
};
checker.visit_expr(body.value);
if checker.lifetimes_used_in_body {
return false;
}
}
// check for lifetimes from higher scopes
for lt in input_lts.iter().chain(output_lts.iter()) {
if !allowed_lts.contains(lt) {
return false;
}
}
// check for higher-ranked trait bounds
if !input_visitor.nested_elision_site_lts.is_empty() || !output_visitor.nested_elision_site_lts.is_empty() {
let allowed_lts: FxHashSet<_> = allowed_lts
.iter()
.filter_map(|lt| match lt {
RefLt::Named(def_id) => Some(cx.tcx.item_name(def_id.to_def_id())),
_ => None,
})
.collect();
for lt in input_visitor.nested_elision_site_lts {
if let RefLt::Named(def_id) = lt {
if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
return false;
}
}
}
for lt in output_visitor.nested_elision_site_lts {
if let RefLt::Named(def_id) = lt {
if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
return false;
}
}
}
}
// no input lifetimes? easy case!
if input_lts.is_empty() {
false
} else if output_lts.is_empty() {
// no output lifetimes, check distinctness of input lifetimes
// only unnamed and static, ok
let unnamed_and_static = input_lts.iter().all(|lt| *lt == RefLt::Unnamed || *lt == RefLt::Static);
if unnamed_and_static {
return false;
}
// we have no output reference, so we only need all distinct lifetimes
input_lts.len() == unique_lifetimes(&input_lts)
} else {
// we have output references, so we need one input reference,
// and all output lifetimes must be the same
if unique_lifetimes(&output_lts) > 1 {
return false;
}
if input_lts.len() == 1 {
match (&input_lts[0], &output_lts[0]) {
(&RefLt::Named(n1), &RefLt::Named(n2)) if n1 == n2 => true,
(&RefLt::Named(_), &RefLt::Unnamed) => true,
_ => false, /* already elided, different named lifetimes
* or something static going on */
}
} else {
false
}
}
}
fn allowed_lts_from(tcx: TyCtxt<'_>, named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
let mut allowed_lts = FxHashSet::default();
for par in named_generics.iter() {
if let GenericParamKind::Lifetime { .. } = par.kind {
allowed_lts.insert(RefLt::Named(tcx.hir().local_def_id(par.hir_id)));
}
}
allowed_lts.insert(RefLt::Unnamed);
allowed_lts.insert(RefLt::Static);
allowed_lts
}
/// Number of unique lifetimes in the given vector.
#[must_use]
fn unique_lifetimes(lts: &[RefLt]) -> usize {
lts.iter().collect::<FxHashSet<_>>().len()
}
const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
/// A visitor usable for `rustc_front::visit::walk_ty()`.
struct RefVisitor<'a, 'tcx> {
cx: &'a LateContext<'tcx>,
lts: Vec<RefLt>,
nested_elision_site_lts: Vec<RefLt>,
unelided_trait_object_lifetime: bool,
}
impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
fn new(cx: &'a LateContext<'tcx>) -> Self {
Self {
cx,
lts: Vec::new(),
nested_elision_site_lts: Vec::new(),
unelided_trait_object_lifetime: false,
}
}
fn record(&mut self, lifetime: &Option<Lifetime>) {
if let Some(ref lt) = *lifetime {
if lt.name == LifetimeName::Static {
self.lts.push(RefLt::Static);
} else if let LifetimeName::Param(_, ParamName::Fresh) = lt.name {
// Fresh lifetimes generated should be ignored.
self.lts.push(RefLt::Unnamed);
} else if lt.is_elided() {
self.lts.push(RefLt::Unnamed);
} else if let LifetimeName::Param(def_id, _) = lt.name {
self.lts.push(RefLt::Named(def_id));
} else {
self.lts.push(RefLt::Unnamed);
}
} else {
self.lts.push(RefLt::Unnamed);
}
}
fn all_lts(&self) -> Vec<RefLt> {
self.lts
.iter()
.chain(self.nested_elision_site_lts.iter())
.cloned()
.collect::<Vec<_>>()
}
fn abort(&self) -> bool {
self.unelided_trait_object_lifetime
}
}
impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
self.record(&Some(*lifetime));
}
fn visit_poly_trait_ref(&mut self, poly_tref: &'tcx PolyTraitRef<'tcx>) {
let trait_ref = &poly_tref.trait_ref;
if CLOSURE_TRAIT_BOUNDS.iter().any(|&item| {
self.cx
.tcx
.lang_items()
.require(item)
.map_or(false, |id| Some(id) == trait_ref.trait_def_id())
}) {
let mut sub_visitor = RefVisitor::new(self.cx);
sub_visitor.visit_trait_ref(trait_ref);
self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
} else {
walk_poly_trait_ref(self, poly_tref);
}
}
fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
match ty.kind {
TyKind::OpaqueDef(item, bounds, _) => {
let map = self.cx.tcx.hir();
let item = map.item(item);
let len = self.lts.len();
walk_item(self, item);
self.lts.truncate(len);
self.lts.extend(bounds.iter().filter_map(|bound| match bound {
GenericArg::Lifetime(l) => Some(if let LifetimeName::Param(def_id, _) = l.name {
RefLt::Named(def_id)
} else {
RefLt::Unnamed
}),
_ => None,
}));
},
TyKind::BareFn(&BareFnTy { decl, .. }) => {
let mut sub_visitor = RefVisitor::new(self.cx);
sub_visitor.visit_fn_decl(decl);
self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
},
TyKind::TraitObject(bounds, ref 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),
}
}
}
/// 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.name.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 { .. } => 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 { .. } => 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.name.ident().name != kw::UnderscoreLifetime && lifetime.name.ident().name != kw::StaticLifetime {
self.lifetimes_used_in_body = true;
}
}
}
|