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lifetimes.rs
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lifetimes.rs
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use crate::utils::paths;
use crate::utils::{get_trait_def_id, in_macro, span_lint, trait_ref_of_method};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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,
};
use rustc_hir::FnRetTy::Return;
use rustc_hir::{
BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, ImplItem,
ImplItemKind, Item, ItemKind, Lifetime, LifetimeName, ParamName, PolyTraitRef, TraitBoundModifier, TraitFn,
TraitItem, TraitItemKind, Ty, TyKind, WhereClause, WherePredicate,
};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::hir::map::Map;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
use rustc_span::symbol::{kw, Symbol};
use std::iter::FromIterator;
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:** Potential false negatives: we bail out if the function
/// has a `where` clause where lifetimes are mentioned.
///
/// **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
/// }
/// ```
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.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// // Bad: unnecessary lifetimes
/// fn unused_lifetime<'a>(x: u8) {
/// // ..
/// }
///
/// // Good
/// fn no_lifetime(x: u8) {
/// // ...
/// }
/// ```
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, 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<'_>) {
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>,
decl: &'tcx FnDecl<'_>,
body: Option<BodyId>,
generics: &'tcx Generics<'_>,
span: Span,
report_extra_lifetimes: bool,
) {
if in_macro(span) || has_where_lifetimes(cx, &generics.where_clause) {
return;
}
let types = generics
.params
.iter()
.filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
for typ in types {
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(ref 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) {
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);
}
}
fn could_use_elision<'tcx>(
cx: &LateContext<'tcx>,
func: &'tcx FnDecl<'_>,
body: Option<BodyId>,
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
for arg in func.inputs {
input_visitor.visit_ty(arg);
}
// extract lifetimes in output type
if let Return(ref 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(&FxHashSet::from_iter(
input_visitor
.nested_elision_site_lts
.iter()
.chain(output_visitor.nested_elision_site_lts.iter())
.cloned()
.filter(|v| matches!(v, RefLt::Named(_))),
))
.next()
.is_some()
{
return false;
}
let input_lts = input_visitor.lts;
let output_lts = output_visitor.lts;
if let Some(body_id) = body {
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
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(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() {
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]
fn unique_lifetimes(lts: &[RefLt]) -> usize {
lts.iter().collect::<FxHashSet<_>>().len()
}
const CLOSURE_TRAIT_BOUNDS: [&[&str]; 3] = [&paths::FN, &paths::FN_MUT, &paths::FN_ONCE];
/// 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.
} else if lt.is_elided() {
self.lts.push(RefLt::Unnamed);
} else {
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> {
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(|trait_path| trait_ref.trait_def_id() == get_trait_def_id(self.cx, trait_path))
{
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);
}
}
fn visit_ty(&mut self, ty: &'tcx Ty<'_>) {
match ty.kind {
TyKind::OpaqueDef(item, _) => {
let map = self.cx.tcx.hir();
let item = map.expect_item(item.id);
walk_item(self, item);
walk_ty(self, ty);
},
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, TraitBoundModifier::None);
}
return;
},
_ => (),
}
walk_ty(self, ty);
}
fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}
/// 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 {
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.lts.is_empty() {
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
return visitor.all_lts().iter().any(|it| !allowed_lts.contains(it));
},
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 {
map: FxHashMap<Symbol, Span>,
}
impl<'tcx> Visitor<'tcx> for LifetimeChecker {
type Map = Map<'tcx>;
// 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) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}
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 { map: 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",
);
}
}
struct BodyLifetimeChecker {
lifetimes_used_in_body: bool,
}
impl<'tcx> Visitor<'tcx> for BodyLifetimeChecker {
type Map = Map<'tcx>;
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
if lifetime.name.ident().name != kw::Invalid && lifetime.name.ident().name != kw::StaticLifetime {
self.lifetimes_used_in_body = true;
}
}
fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
NestedVisitorMap::None
}
}