rust-clippy/clippy_lints/src/lifetimes.rs
2017-08-25 19:38:28 +10:00

444 lines
14 KiB
Rust

use reexport::*;
use rustc::lint::*;
use rustc::hir::def::Def;
use rustc::hir::*;
use rustc::hir::intravisit::{Visitor, walk_ty, walk_ty_param_bound, walk_fn_decl, walk_generics, NestedVisitorMap};
use std::collections::{HashSet, HashMap};
use syntax::codemap::Span;
use utils::{in_external_macro, span_lint, last_path_segment};
use syntax::symbol::keywords;
/// **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
/// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 { x }
/// ```
declare_lint! {
pub NEEDLESS_LIFETIMES,
Warn,
"using explicit lifetimes for references in function arguments when elision rules \
would allow omitting them"
}
/// **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
/// fn unused_lifetime<'a>(x: u8) { .. }
/// ```
declare_lint! {
pub UNUSED_LIFETIMES,
Warn,
"unused lifetimes in function definitions"
}
#[derive(Copy, Clone)]
pub struct LifetimePass;
impl LintPass for LifetimePass {
fn get_lints(&self) -> LintArray {
lint_array!(NEEDLESS_LIFETIMES, UNUSED_LIFETIMES)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for LifetimePass {
fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx Item) {
if let ItemFn(ref decl, _, _, _, ref generics, id) = item.node {
check_fn_inner(cx, decl, Some(id), generics, item.span);
}
}
fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx ImplItem) {
if let ImplItemKind::Method(ref sig, id) = item.node {
check_fn_inner(cx, &sig.decl, Some(id), &sig.generics, item.span);
}
}
fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx TraitItem) {
if let TraitItemKind::Method(ref sig, ref body) = item.node {
let body = match *body {
TraitMethod::Required(_) => None,
TraitMethod::Provided(id) => Some(id),
};
check_fn_inner(cx, &sig.decl, body, &sig.generics, item.span);
}
}
}
/// The lifetime of a &-reference.
#[derive(PartialEq, Eq, Hash, Debug)]
enum RefLt {
Unnamed,
Static,
Named(Name),
}
fn check_fn_inner<'a, 'tcx>(
cx: &LateContext<'a, 'tcx>,
decl: &'tcx FnDecl,
body: Option<BodyId>,
generics: &'tcx Generics,
span: Span,
) {
if in_external_macro(cx, span) || has_where_lifetimes(cx, &generics.where_clause) {
return;
}
let mut bounds_lts = Vec::new();
for typ in &generics.ty_params {
for bound in &typ.bounds {
if let TraitTyParamBound(ref trait_ref, _) = *bound {
let bounds = &trait_ref
.trait_ref
.path
.segments
.last()
.expect("a path must have at least one segment")
.parameters
.lifetimes;
for bound in bounds {
if bound.name != "'static" && !bound.is_elided() {
return;
}
bounds_lts.push(bound);
}
}
}
}
if could_use_elision(cx, decl, body, &generics.lifetimes, bounds_lts) {
span_lint(
cx,
NEEDLESS_LIFETIMES,
span,
"explicit lifetimes given in parameter types where they could be elided",
);
}
report_extra_lifetimes(cx, decl, generics);
}
fn could_use_elision<'a, 'tcx: 'a>(
cx: &LateContext<'a, 'tcx>,
func: &'tcx FnDecl,
body: Option<BodyId>,
named_lts: &'tcx [LifetimeDef],
bounds_lts: Vec<&'tcx Lifetime>,
) -> 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_lts);
// 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);
}
let input_lts = match input_visitor.into_vec() {
Some(lts) => lts_from_bounds(lts, bounds_lts.into_iter()),
None => return false,
};
let output_lts = match output_visitor.into_vec() {
Some(val) => val,
None => return false,
};
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_lts: &[LifetimeDef]) -> HashSet<RefLt> {
let mut allowed_lts = HashSet::new();
for lt in named_lts {
if lt.bounds.is_empty() {
allowed_lts.insert(RefLt::Named(lt.lifetime.name));
}
}
allowed_lts.insert(RefLt::Unnamed);
allowed_lts.insert(RefLt::Static);
allowed_lts
}
fn lts_from_bounds<'a, T: Iterator<Item = &'a Lifetime>>(mut vec: Vec<RefLt>, bounds_lts: T) -> Vec<RefLt> {
for lt in bounds_lts {
if lt.name != "'static" {
vec.push(RefLt::Named(lt.name));
}
}
vec
}
/// Number of unique lifetimes in the given vector.
fn unique_lifetimes(lts: &[RefLt]) -> usize {
lts.iter().collect::<HashSet<_>>().len()
}
/// A visitor usable for `rustc_front::visit::walk_ty()`.
struct RefVisitor<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
lts: Vec<RefLt>,
abort: bool,
}
impl<'v, 't> RefVisitor<'v, 't> {
fn new(cx: &'v LateContext<'v, 't>) -> Self {
Self {
cx: cx,
lts: Vec::new(),
abort: false,
}
}
fn record(&mut self, lifetime: &Option<Lifetime>) {
if let Some(ref lt) = *lifetime {
if lt.name == "'static" {
self.lts.push(RefLt::Static);
} else if lt.is_elided() {
self.lts.push(RefLt::Unnamed);
} else {
self.lts.push(RefLt::Named(lt.name));
}
} else {
self.lts.push(RefLt::Unnamed);
}
}
fn into_vec(self) -> Option<Vec<RefLt>> {
if self.abort { None } else { Some(self.lts) }
}
fn collect_anonymous_lifetimes(&mut self, qpath: &QPath, ty: &Ty) {
let last_path_segment = &last_path_segment(qpath).parameters;
if !last_path_segment.parenthesized && last_path_segment.lifetimes.is_empty() {
let hir_id = self.cx.tcx.hir.node_to_hir_id(ty.id);
match self.cx.tables.qpath_def(qpath, hir_id) {
Def::TyAlias(def_id) |
Def::Struct(def_id) => {
let generics = self.cx.tcx.generics_of(def_id);
for _ in generics.regions.as_slice() {
self.record(&None);
}
},
Def::Trait(def_id) => {
let trait_def = self.cx.tcx.trait_def(def_id);
for _ in &self.cx.tcx.generics_of(trait_def.def_id).regions {
self.record(&None);
}
},
_ => (),
}
}
}
}
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_ty(&mut self, ty: &'tcx Ty) {
match ty.node {
TyRptr(ref lt, _) if lt.is_elided() => {
self.record(&None);
},
TyPath(ref path) => {
self.collect_anonymous_lifetimes(path, ty);
},
TyImplTrait(ref param_bounds) => {
for bound in param_bounds {
if let RegionTyParamBound(_) = *bound {
self.record(&None);
}
}
},
TyTraitObject(ref bounds, ref lt) => {
if !lt.is_elided() {
self.abort = true;
}
for bound in bounds {
self.visit_poly_trait_ref(bound, TraitBoundModifier::None);
}
return;
},
_ => (),
}
walk_ty(self, ty);
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}
/// Are any lifetimes mentioned in the `where` clause? If yes, we don't try to
/// reason about elision.
fn has_where_lifetimes<'a, 'tcx: 'a>(cx: &LateContext<'a, '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_lifetimes);
// now walk the bounds
for bound in pred.bounds.iter() {
walk_ty_param_bound(&mut visitor, bound);
}
// and check that all lifetimes are allowed
match visitor.into_vec() {
None => return false,
Some(lts) => {
for lt in lts {
if !allowed_lts.contains(&lt) {
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 {
map: HashMap<Name, Span>,
}
impl<'tcx> Visitor<'tcx> for LifetimeChecker {
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
self.map.remove(&lifetime.name);
}
fn visit_lifetime_def(&mut self, _: &'tcx LifetimeDef) {
// 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
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}
fn report_extra_lifetimes<'a, 'tcx: 'a>(cx: &LateContext<'a, 'tcx>, func: &'tcx FnDecl, generics: &'tcx Generics) {
let hs = generics
.lifetimes
.iter()
.map(|lt| (lt.lifetime.name, lt.lifetime.span))
.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, 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 {
// for lifetimes as parameters of generics
fn visit_lifetime(&mut self, lifetime: &'tcx Lifetime) {
if lifetime.name != keywords::Invalid.name() && lifetime.name != "'static" {
self.lifetimes_used_in_body = true;
}
}
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
}