rust-clippy/clippy_lints/src/needless_pass_by_value.rs

339 lines
13 KiB
Rust

use clippy_utils::diagnostics::{multispan_sugg, span_lint_and_then};
use clippy_utils::ptr::get_spans;
use clippy_utils::source::{snippet, snippet_opt};
use clippy_utils::ty::{implements_trait, is_copy, is_type_diagnostic_item};
use clippy_utils::{get_trait_def_id, is_self, paths};
use if_chain::if_chain;
use rustc_ast::ast::Attribute;
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::{Applicability, DiagnosticBuilder};
use rustc_hir::intravisit::FnKind;
use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
use rustc_hir::{HirIdMap, HirIdSet};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::mir::FakeReadCause;
use rustc_middle::ty::{self, TypeFoldable};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::kw;
use rustc_span::{sym, Span};
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::traits;
use rustc_trait_selection::traits::misc::can_type_implement_copy;
use rustc_typeck::expr_use_visitor as euv;
use std::borrow::Cow;
declare_clippy_lint! {
/// ### What it does
/// Checks for functions taking arguments by value, but not
/// consuming them in its
/// body.
///
/// ### Why is this bad?
/// Taking arguments by reference is more flexible and can
/// sometimes avoid
/// unnecessary allocations.
///
/// ### Known problems
/// * This lint suggests taking an argument by reference,
/// however sometimes it is better to let users decide the argument type
/// (by using `Borrow` trait, for example), depending on how the function is used.
///
/// ### Example
/// ```rust
/// fn foo(v: Vec<i32>) {
/// assert_eq!(v.len(), 42);
/// }
/// ```
/// should be
/// ```rust
/// fn foo(v: &[i32]) {
/// assert_eq!(v.len(), 42);
/// }
/// ```
pub NEEDLESS_PASS_BY_VALUE,
pedantic,
"functions taking arguments by value, but not consuming them in its body"
}
declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
macro_rules! need {
($e: expr) => {
if let Some(x) = $e {
x
} else {
return;
}
};
}
impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
#[allow(clippy::too_many_lines)]
fn check_fn(
&mut self,
cx: &LateContext<'tcx>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl<'_>,
body: &'tcx Body<'_>,
span: Span,
hir_id: HirId,
) {
if span.from_expansion() {
return;
}
match kind {
FnKind::ItemFn(.., header, _) => {
let attrs = cx.tcx.hir().attrs(hir_id);
if header.abi != Abi::Rust || requires_exact_signature(attrs) {
return;
}
},
FnKind::Method(..) => (),
FnKind::Closure => return,
}
// Exclude non-inherent impls
if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
if matches!(
item.kind,
ItemKind::Impl(Impl { of_trait: Some(_), .. }) | ItemKind::Trait(..)
) {
return;
}
}
// Allow `Borrow` or functions to be taken by value
let allowed_traits = [
need!(cx.tcx.lang_items().fn_trait()),
need!(cx.tcx.lang_items().fn_once_trait()),
need!(cx.tcx.lang_items().fn_mut_trait()),
need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
];
let sized_trait = need!(cx.tcx.lang_items().sized_trait());
let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
.filter(|p| !p.is_global())
.filter_map(|obligation| {
// Note that we do not want to deal with qualified predicates here.
match obligation.predicate.kind().no_bound_vars() {
Some(ty::PredicateKind::Trait(pred, _)) if pred.def_id() != sized_trait => Some(pred),
_ => None,
}
})
.collect::<Vec<_>>();
// Collect moved variables and spans which will need dereferencings from the
// function body.
let MovedVariablesCtxt {
moved_vars,
spans_need_deref,
..
} = {
let mut ctx = MovedVariablesCtxt::default();
cx.tcx.infer_ctxt().enter(|infcx| {
euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.typeck_results())
.consume_body(body);
});
ctx
};
let fn_sig = cx.tcx.fn_sig(fn_def_id);
let fn_sig = cx.tcx.erase_late_bound_regions(fn_sig);
for (idx, ((input, &ty), arg)) in decl.inputs.iter().zip(fn_sig.inputs()).zip(body.params).enumerate() {
// All spans generated from a proc-macro invocation are the same...
if span == input.span {
return;
}
// Ignore `self`s.
if idx == 0 {
if let PatKind::Binding(.., ident, _) = arg.pat.kind {
if ident.name == kw::SelfLower {
continue;
}
}
}
//
// * Exclude a type that is specifically bounded by `Borrow`.
// * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
// `serde::Serialize`)
let (implements_borrow_trait, all_borrowable_trait) = {
let preds = preds.iter().filter(|t| t.self_ty() == ty).collect::<Vec<_>>();
(
preds.iter().any(|t| cx.tcx.is_diagnostic_item(sym::Borrow, t.def_id())),
!preds.is_empty() && {
let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
preds.iter().all(|t| {
let ty_params = t.trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
implements_trait(cx, ty_empty_region, t.def_id(), &ty_params)
})
},
)
};
if_chain! {
if !is_self(arg);
if !ty.is_mutable_ptr();
if !is_copy(cx, ty);
if !allowed_traits.iter().any(|&t| implements_trait(cx, ty, t, &[]));
if !implements_borrow_trait;
if !all_borrowable_trait;
if let PatKind::Binding(mode, canonical_id, ..) = arg.pat.kind;
if !moved_vars.contains(&canonical_id);
then {
if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
continue;
}
// Dereference suggestion
let sugg = |diag: &mut DiagnosticBuilder<'_>| {
if let ty::Adt(def, ..) = ty.kind() {
if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
if can_type_implement_copy(cx.tcx, cx.param_env, ty).is_ok() {
diag.span_help(span, "consider marking this type as `Copy`");
}
}
}
let deref_span = spans_need_deref.get(&canonical_id);
if_chain! {
if is_type_diagnostic_item(cx, ty, sym::vec_type);
if let Some(clone_spans) =
get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
if let TyKind::Path(QPath::Resolved(_, path)) = input.kind;
if let Some(elem_ty) = path.segments.iter()
.find(|seg| seg.ident.name == sym::Vec)
.and_then(|ps| ps.args.as_ref())
.map(|params| params.args.iter().find_map(|arg| match arg {
GenericArg::Type(ty) => Some(ty),
_ => None,
}).unwrap());
then {
let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
diag.span_suggestion(
input.span,
"consider changing the type to",
slice_ty,
Applicability::Unspecified,
);
for (span, suggestion) in clone_spans {
diag.span_suggestion(
span,
&snippet_opt(cx, span)
.map_or(
"change the call to".into(),
|x| Cow::from(format!("change `{}` to", x)),
),
suggestion.into(),
Applicability::Unspecified,
);
}
// cannot be destructured, no need for `*` suggestion
assert!(deref_span.is_none());
return;
}
}
if is_type_diagnostic_item(cx, ty, sym::string_type) {
if let Some(clone_spans) =
get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
diag.span_suggestion(
input.span,
"consider changing the type to",
"&str".to_string(),
Applicability::Unspecified,
);
for (span, suggestion) in clone_spans {
diag.span_suggestion(
span,
&snippet_opt(cx, span)
.map_or(
"change the call to".into(),
|x| Cow::from(format!("change `{}` to", x))
),
suggestion.into(),
Applicability::Unspecified,
);
}
assert!(deref_span.is_none());
return;
}
}
let mut spans = vec![(input.span, format!("&{}", snippet(cx, input.span, "_")))];
// Suggests adding `*` to dereference the added reference.
if let Some(deref_span) = deref_span {
spans.extend(
deref_span
.iter()
.copied()
.map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
);
spans.sort_by_key(|&(span, _)| span);
}
multispan_sugg(diag, "consider taking a reference instead", spans);
};
span_lint_and_then(
cx,
NEEDLESS_PASS_BY_VALUE,
input.span,
"this argument is passed by value, but not consumed in the function body",
sugg,
);
}
}
}
}
}
/// Functions marked with these attributes must have the exact signature.
fn requires_exact_signature(attrs: &[Attribute]) -> bool {
attrs.iter().any(|attr| {
[sym::proc_macro, sym::proc_macro_attribute, sym::proc_macro_derive]
.iter()
.any(|&allow| attr.has_name(allow))
})
}
#[derive(Default)]
struct MovedVariablesCtxt {
moved_vars: HirIdSet,
/// Spans which need to be prefixed with `*` for dereferencing the
/// suggested additional reference.
spans_need_deref: HirIdMap<FxHashSet<Span>>,
}
impl MovedVariablesCtxt {
fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
if let euv::PlaceBase::Local(vid) = cmt.place.base {
self.moved_vars.insert(vid);
}
}
}
impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, _: HirId) {
self.move_common(cmt);
}
fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId, _: ty::BorrowKind) {}
fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: HirId) {}
fn fake_read(&mut self, _: rustc_typeck::expr_use_visitor::Place<'tcx>, _: FakeReadCause, _: HirId) {}
}