rust-clippy/clippy_lints/src/lifetimes.rs

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use clippy_utils::diagnostics::span_lint;
use clippy_utils::trait_ref_of_method;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use rustc_hir::intravisit::{
walk_fn_decl, walk_generic_param, walk_generics, walk_item, walk_param_bound, walk_poly_trait_ref, walk_ty,
NestedVisitorMap, Visitor,
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};
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use rustc_hir::FnRetTy::Return;
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use rustc_hir::{
BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem,
ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, ParamName, PolyTraitRef, TraitBoundModifier,
TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WhereClause, WherePredicate,
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};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::map::Map;
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
use rustc_span::symbol::{kw, Symbol};
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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
/// // Bad: unnecessary lifetime annotations
/// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
/// x
/// }
///
/// // Good
/// 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"
}
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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
/// // Bad: unnecessary lifetimes
/// fn unused_lifetime<'a>(x: u8) {
/// // ..
/// }
///
/// // Good
/// fn no_lifetime(x: u8) {
/// // ...
/// }
/// ```
#[clippy::version = "pre 1.29.0"]
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pub EXTRA_UNUSED_LIFETIMES,
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complexity,
"unused lifetimes in function definitions"
}
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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<'_>) {
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if let ItemKind::Fn(ref sig, ref generics, id) = item.kind {
check_fn_inner(cx, sig.decl, Some(id), generics, item.span, true);
}
}
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.hir_id()).is_none();
check_fn_inner(
cx,
sig.decl,
Some(id),
&item.generics,
item.span,
report_extra_lifetimes,
);
}
}
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 {
let body = match *body {
TraitFn::Required(_) => None,
TraitFn::Provided(id) => Some(id),
};
check_fn_inner(cx, sig.decl, body, &item.generics, item.span, true);
}
}
}
/// The lifetime of a &-reference.
#[derive(PartialEq, Eq, Hash, Debug, Clone)]
enum RefLt {
Unnamed,
Static,
Named(Symbol),
}
fn check_fn_inner<'tcx>(
cx: &LateContext<'tcx>,
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decl: &'tcx FnDecl<'_>,
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body: Option<BodyId>,
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generics: &'tcx Generics<'_>,
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span: Span,
report_extra_lifetimes: bool,
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) {
if span.from_expansion() || has_where_lifetimes(cx, &generics.where_clause) {
return;
}
let types = generics
.params
.iter()
.filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
for typ in types {
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for bound in typ.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, generics.params) {
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span_lint(
cx,
NEEDLESS_LIFETIMES,
span.with_hi(decl.output.span().hi()),
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"explicit lifetimes given in parameter types where they could be elided \
(or replaced with `'_` if needed by type declaration)",
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);
}
if report_extra_lifetimes {
self::report_extra_lifetimes(cx, decl, generics);
}
}
fn could_use_elision<'tcx>(
cx: &LateContext<'tcx>,
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func: &'tcx FnDecl<'_>,
body: Option<BodyId>,
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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(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
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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;
}
if allowed_lts
.intersection(
&input_visitor
.nested_elision_site_lts
.iter()
.chain(output_visitor.nested_elision_site_lts.iter())
.cloned()
.filter(|v| matches!(v, RefLt::Named(_)))
.collect(),
)
.next()
.is_some()
{
return false;
}
let input_lts = input_visitor.lts;
let output_lts = output_visitor.lts;
if let Some(body_id) = body {
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let mut checker = BodyLifetimeChecker {
lifetimes_used_in_body: false,
};
checker.visit_expr(&cx.tcx.hir().body(body_id).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;
}
}
// 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
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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,
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_ => false, /* already elided, different named lifetimes
* or something static going on */
}
} else {
false
}
}
}
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fn allowed_lts_from(named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
let mut allowed_lts = FxHashSet::default();
for par in named_generics.iter() {
if let GenericParamKind::Lifetime { .. } = par.kind {
if par.bounds.is_empty() {
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allowed_lts.insert(RefLt::Named(par.name.ident().name));
}
}
}
allowed_lts.insert(RefLt::Unnamed);
allowed_lts.insert(RefLt::Static);
allowed_lts
}
/// Number of unique lifetimes in the given vector.
#[must_use]
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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>,
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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 {
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Self {
cx,
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lts: Vec::new(),
nested_elision_site_lts: Vec::new(),
unelided_trait_object_lifetime: false,
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}
}
fn record(&mut self, lifetime: &Option<Lifetime>) {
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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.
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} else if lt.is_elided() {
self.lts.push(RefLt::Unnamed);
} else {
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self.lts.push(RefLt::Named(lt.name.ident().name));
}
} 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> {
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type Map = Map<'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>, tbm: TraitBoundModifier) {
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, tbm);
}
}
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fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
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match ty.kind {
TyKind::OpaqueDef(item, bounds) => {
let map = self.cx.tcx.hir();
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let item = map.item(item);
walk_item(self, item);
walk_ty(self, ty);
self.lts.extend(bounds.iter().filter_map(|bound| match bound {
GenericArg::Lifetime(l) => Some(RefLt::Named(l.name.ident().name)),
_ => None,
}));
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},
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());
return;
},
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, TraitBoundModifier::None);
}
return;
},
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_ => (),
}
walk_ty(self, ty);
}
fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}
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/// 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>, where_clause: &'tcx WhereClause<'_>) -> bool {
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for predicate in where_clause.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() {
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return true;
}
// if the bounds define new lifetimes, they are fine to occur
let allowed_lts = allowed_lts_from(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;
}
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},
WherePredicate::EqPredicate(ref pred) => {
let mut visitor = RefVisitor::new(cx);
walk_ty(&mut visitor, pred.lhs_ty);
walk_ty(&mut visitor, pred.rhs_ty);
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if !visitor.lts.is_empty() {
return true;
}
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},
}
}
false
}
struct LifetimeChecker {
map: FxHashMap<Symbol, Span>,
}
impl<'tcx> Visitor<'tcx> for LifetimeChecker {
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type Map = Map<'tcx>;
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
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self.map.remove(&lifetime.name.ident().name);
}
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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) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}
fn report_extra_lifetimes<'tcx>(cx: &LateContext<'tcx>, func: &'tcx FnDecl<'_>, generics: &'tcx Generics<'_>) {
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let hs = generics
.params
.iter()
.filter_map(|par| match par.kind {
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GenericParamKind::Lifetime { .. } => Some((par.name.ident().name, par.span)),
_ => None,
})
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.collect();
let mut checker = LifetimeChecker { map: hs };
walk_generics(&mut checker, generics);
walk_fn_decl(&mut checker, func);
for &v in checker.map.values() {
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span_lint(
cx,
EXTRA_UNUSED_LIFETIMES,
v,
"this lifetime isn't used in the function definition",
);
}
}
struct BodyLifetimeChecker {
lifetimes_used_in_body: bool,
}
impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
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type Map = Map<'tcx>;
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
if lifetime.name.ident().name != kw::Empty && lifetime.name.ident().name != kw::StaticLifetime {
self.lifetimes_used_in_body = true;
}
}
fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
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}