rust/clippy_lints/src/implied_bounds_in_impls.rs
2024-05-30 22:52:33 +02:00

354 lines
15 KiB
Rust

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::source::snippet;
use rustc_errors::{Applicability, SuggestionStyle};
use rustc_hir::def_id::DefId;
use rustc_hir::{
GenericArg, GenericBound, GenericBounds, ItemKind, PredicateOrigin, TraitBoundModifier, TyKind, AssocItemConstraint,
WherePredicate,
};
use rustc_hir_analysis::lower_ty;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, ClauseKind, Generics, Ty, TyCtxt};
use rustc_session::declare_lint_pass;
use rustc_span::Span;
declare_clippy_lint! {
/// ### What it does
/// Looks for bounds in `impl Trait` in return position that are implied by other bounds.
/// This can happen when a trait is specified that another trait already has as a supertrait
/// (e.g. `fn() -> impl Deref + DerefMut<Target = i32>` has an unnecessary `Deref` bound,
/// because `Deref` is a supertrait of `DerefMut`)
///
/// ### Why is this bad?
/// Specifying more bounds than necessary adds needless complexity for the reader.
///
/// ### Limitations
/// This lint does not check for implied bounds transitively. Meaning that
/// it doesn't check for implied bounds from supertraits of supertraits
/// (e.g. `trait A {} trait B: A {} trait C: B {}`, then having an `fn() -> impl A + C`)
///
/// ### Example
/// ```no_run
/// # use std::ops::{Deref,DerefMut};
/// fn f() -> impl Deref<Target = i32> + DerefMut<Target = i32> {
/// // ^^^^^^^^^^^^^^^^^^^ unnecessary bound, already implied by the `DerefMut` trait bound
/// Box::new(123)
/// }
/// ```
/// Use instead:
/// ```no_run
/// # use std::ops::{Deref,DerefMut};
/// fn f() -> impl DerefMut<Target = i32> {
/// Box::new(123)
/// }
/// ```
#[clippy::version = "1.74.0"]
pub IMPLIED_BOUNDS_IN_IMPLS,
complexity,
"specifying bounds that are implied by other bounds in `impl Trait` type"
}
declare_lint_pass!(ImpliedBoundsInImpls => [IMPLIED_BOUNDS_IN_IMPLS]);
fn emit_lint(
cx: &LateContext<'_>,
poly_trait: &rustc_hir::PolyTraitRef<'_>,
bounds: GenericBounds<'_>,
index: usize,
// The constraints that were implied, used for suggestion purposes since removing a bound with
// associated types means we might need to then move it to a different bound.
implied_constraints: &[AssocItemConstraint<'_>],
bound: &ImplTraitBound<'_>,
) {
let implied_by = snippet(cx, bound.span, "..");
span_lint_and_then(
cx,
IMPLIED_BOUNDS_IN_IMPLS,
poly_trait.span,
format!("this bound is already specified as the supertrait of `{implied_by}`"),
|diag| {
// If we suggest removing a bound, we may also need to extend the span
// to include the `+` token that is ahead or behind,
// so we don't end up with something like `impl + B` or `impl A + `
let implied_span_extended = if let Some(next_bound) = bounds.get(index + 1) {
poly_trait.span.to(next_bound.span().shrink_to_lo())
} else if index > 0
&& let Some(prev_bound) = bounds.get(index - 1)
{
prev_bound.span().shrink_to_hi().to(poly_trait.span.shrink_to_hi())
} else {
poly_trait.span
};
let mut sugg = vec![(implied_span_extended, String::new())];
// We also might need to include associated item constraints that were specified in the implied bound,
// but omitted in the implied-by bound:
// `fn f() -> impl Deref<Target = u8> + DerefMut`
// If we're going to suggest removing `Deref<..>`, we'll need to put `<Target = u8>` on `DerefMut`
let omitted_constraints: Vec<_> = implied_constraints
.iter()
.filter(|constraint| !bound.constraints.iter().any(|c| c.ident == constraint.ident))
.collect();
if !omitted_constraints.is_empty() {
// `<>` needs to be added if there aren't yet any generic arguments or constraints
let needs_angle_brackets = bound.args.is_empty() && bound.constraints.is_empty();
let insert_span = match (bound.args, bound.constraints) {
([.., arg], [.., constraint]) => arg.span().max(constraint.span).shrink_to_hi(),
([.., arg], []) => arg.span().shrink_to_hi(),
([], [.., constraint]) => constraint.span.shrink_to_hi(),
([], []) => bound.span.shrink_to_hi(),
};
let mut constraints_sugg = if needs_angle_brackets {
"<".to_owned()
} else {
// If angle brackets aren't needed (i.e., there are already generic arguments or constraints),
// we need to add a comma:
// `impl A<B, C >`
// ^ if we insert `Assoc=i32` without a comma here, that'd be invalid syntax:
// `impl A<B, C Assoc=i32>`
", ".to_owned()
};
for (index, constraint) in omitted_constraints.into_iter().enumerate() {
if index > 0 {
constraints_sugg += ", ";
}
constraints_sugg += &snippet(cx, constraint.span, "..");
}
if needs_angle_brackets {
constraints_sugg += ">";
}
sugg.push((insert_span, constraints_sugg));
}
diag.multipart_suggestion_with_style(
"try removing this bound",
sugg,
Applicability::MachineApplicable,
SuggestionStyle::ShowAlways,
);
},
);
}
/// Tries to "resolve" a type.
/// The index passed to this function must start with `Self=0`, i.e. it must be a valid
/// type parameter index.
/// If the index is out of bounds, it means that the generic parameter has a default type.
fn try_resolve_type<'tcx>(
tcx: TyCtxt<'tcx>,
args: &'tcx [GenericArg<'tcx>],
generics: &'tcx Generics,
index: usize,
) -> Option<Ty<'tcx>> {
match args.get(index - 1) {
Some(GenericArg::Type(ty)) => Some(lower_ty(tcx, ty)),
Some(_) => None,
None => Some(tcx.type_of(generics.own_params[index].def_id).skip_binder()),
}
}
/// This function tries to, for all generic type parameters in a supertrait predicate `trait ...<U>:
/// GenericTrait<U>`, check if the substituted type in the implied-by bound matches with what's
/// substituted in the implied bound.
///
/// Consider this example.
/// ```rust,ignore
/// trait GenericTrait<T> {}
/// trait GenericSubTrait<T, U, V>: GenericTrait<U> {}
/// ^^^^^^^^^^^^^^^ trait_predicate_args: [Self#0, U#2]
/// (the Self#0 is implicit: `<Self as GenericTrait<U>>`)
/// impl GenericTrait<i32> for () {}
/// impl GenericSubTrait<(), i32, ()> for () {}
/// impl GenericSubTrait<(), i64, ()> for () {}
///
/// fn f() -> impl GenericTrait<i32> + GenericSubTrait<(), i64, ()> {
/// ^^^ implied_args ^^^^^^^^^^^ implied_by_args
/// (we are interested in `i64` specifically, as that
/// is what `U` in `GenericTrait<U>` is substituted with)
/// }
/// ```
/// Here i32 != i64, so this will return false.
fn is_same_generics<'tcx>(
tcx: TyCtxt<'tcx>,
trait_predicate_args: &'tcx [ty::GenericArg<'tcx>],
implied_by_args: &'tcx [GenericArg<'tcx>],
implied_args: &'tcx [GenericArg<'tcx>],
implied_by_def_id: DefId,
implied_def_id: DefId,
) -> bool {
// Get the generics of the two traits to be able to get default generic parameter.
let implied_by_generics = tcx.generics_of(implied_by_def_id);
let implied_generics = tcx.generics_of(implied_def_id);
trait_predicate_args
.iter()
.enumerate()
.skip(1) // skip `Self` implicit arg
.all(|(arg_index, arg)| {
if [
implied_by_generics.host_effect_index,
implied_generics.host_effect_index,
]
.contains(&Some(arg_index))
{
// skip host effect params in determining whether generics are same
return true;
}
if let Some(ty) = arg.as_type() {
if let &ty::Param(ty::ParamTy { index, .. }) = ty.kind()
// `index == 0` means that it's referring to `Self`,
// in which case we don't try to substitute it
&& index != 0
&& let Some(ty_a) = try_resolve_type(tcx, implied_by_args, implied_by_generics, index as usize)
&& let Some(ty_b) = try_resolve_type(tcx, implied_args, implied_generics, arg_index)
{
ty_a == ty_b
} else if let Some(ty_b) = try_resolve_type(tcx, implied_args, implied_generics, arg_index) {
ty == ty_b
} else {
false
}
} else {
false
}
})
}
struct ImplTraitBound<'tcx> {
/// The span of the bound in the `impl Trait` type
span: Span,
/// The predicates defined in the trait referenced by this bound. This also contains the actual
/// supertrait bounds
predicates: &'tcx [(ty::Clause<'tcx>, Span)],
/// The `DefId` of the trait being referenced by this bound
trait_def_id: DefId,
/// The generic arguments on the `impl Trait` bound
args: &'tcx [GenericArg<'tcx>],
/// The associated item constraints of this bound
constraints: &'tcx [AssocItemConstraint<'tcx>],
}
/// Given an `impl Trait` type, gets all the supertraits from each bound ("implied bounds").
///
/// For `impl Deref + DerefMut + Eq` this returns `[Deref, PartialEq]`.
/// The `Deref` comes from `DerefMut` because `trait DerefMut: Deref {}`, and `PartialEq` comes from
/// `Eq`.
fn collect_supertrait_bounds<'tcx>(cx: &LateContext<'tcx>, bounds: GenericBounds<'tcx>) -> Vec<ImplTraitBound<'tcx>> {
bounds
.iter()
.filter_map(|bound| {
if let GenericBound::Trait(poly_trait, TraitBoundModifier::None) = bound
&& let [.., path] = poly_trait.trait_ref.path.segments
&& poly_trait.bound_generic_params.is_empty()
&& let Some(trait_def_id) = path.res.opt_def_id()
&& let predicates = cx.tcx.super_predicates_of(trait_def_id).predicates
// If the trait has no supertrait, there is no need to collect anything from that bound
&& !predicates.is_empty()
{
Some(ImplTraitBound {
predicates,
args: path.args.map_or([].as_slice(), |p| p.args),
constraints: path.args.map_or([].as_slice(), |p| p.constraints),
trait_def_id,
span: bound.span(),
})
} else {
None
}
})
.collect()
}
/// Given a bound in an `impl Trait` type, looks for a trait in the set of supertraits (previously
/// collected in [`collect_supertrait_bounds`]) that matches (same trait and generic arguments).
fn find_bound_in_supertraits<'a, 'tcx>(
cx: &LateContext<'tcx>,
trait_def_id: DefId,
args: &'tcx [GenericArg<'tcx>],
bounds: &'a [ImplTraitBound<'tcx>],
) -> Option<&'a ImplTraitBound<'tcx>> {
bounds.iter().find(|bound| {
bound.predicates.iter().any(|(clause, _)| {
if let ClauseKind::Trait(tr) = clause.kind().skip_binder()
&& tr.def_id() == trait_def_id
{
is_same_generics(
cx.tcx,
tr.trait_ref.args,
bound.args,
args,
bound.trait_def_id,
trait_def_id,
)
} else {
false
}
})
})
}
fn check<'tcx>(cx: &LateContext<'tcx>, bounds: GenericBounds<'tcx>) {
if bounds.len() == 1 {
// Very often there is only a single bound, e.g. `impl Deref<..>`, in which case
// we can avoid doing a bunch of stuff unnecessarily; there will trivially be
// no duplicate bounds
return;
}
let supertraits = collect_supertrait_bounds(cx, bounds);
// Lint all bounds in the `impl Trait` type that we've previously also seen in the set of
// supertraits of each of the bounds.
// This involves some extra logic when generic arguments are present, since
// simply comparing trait `DefId`s won't be enough. We also need to compare the generics.
for (index, bound) in bounds.iter().enumerate() {
if let GenericBound::Trait(poly_trait, TraitBoundModifier::None) = bound
&& let [.., path] = poly_trait.trait_ref.path.segments
&& let implied_args = path.args.map_or([].as_slice(), |a| a.args)
&& let implied_constraints = path.args.map_or([].as_slice(), |a| a.constraints)
&& let Some(def_id) = poly_trait.trait_ref.path.res.opt_def_id()
&& let Some(bound) = find_bound_in_supertraits(cx, def_id, implied_args, &supertraits)
// If the implied bound has a type binding that also exists in the implied-by trait,
// then we shouldn't lint. See #11880 for an example.
&& let assocs = cx.tcx.associated_items(bound.trait_def_id)
&& !implied_constraints.iter().any(|constraint| {
assocs
.filter_by_name_unhygienic(constraint.ident.name)
.next()
.is_some_and(|assoc| assoc.kind == ty::AssocKind::Type)
})
{
emit_lint(cx, poly_trait, bounds, index, implied_constraints, bound);
}
}
}
impl<'tcx> LateLintPass<'tcx> for ImpliedBoundsInImpls {
fn check_generics(&mut self, cx: &LateContext<'tcx>, generics: &rustc_hir::Generics<'tcx>) {
for predicate in generics.predicates {
if let WherePredicate::BoundPredicate(predicate) = predicate
// In theory, the origin doesn't really matter,
// we *could* also lint on explicit where clauses written out by the user,
// not just impl trait desugared ones, but that contradicts with the lint name...
&& let PredicateOrigin::ImplTrait = predicate.origin
{
check(cx, predicate.bounds);
}
}
}
fn check_ty(&mut self, cx: &LateContext<'_>, ty: &rustc_hir::Ty<'_>) {
if let TyKind::OpaqueDef(item_id, ..) = ty.kind
&& let item = cx.tcx.hir().item(item_id)
&& let ItemKind::OpaqueTy(opaque_ty) = item.kind
{
check(cx, opaque_ty.bounds);
}
}
}