rust-clippy/clippy_lints/src/needless_pass_by_value.rs

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use crate::utils::ptr::get_spans;
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use crate::utils::{
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get_trait_def_id, implements_trait, is_copy, is_self, is_type_diagnostic_item, multispan_sugg, paths, snippet,
snippet_opt, span_lint_and_then,
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};
use if_chain::if_chain;
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use rustc_ast::ast::Attribute;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use rustc_errors::{Applicability, DiagnosticBuilder};
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use rustc_hir::intravisit::FnKind;
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use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, ItemKind, Node, PatKind, QPath, TyKind};
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use rustc_infer::infer::TyCtxtInferExt;
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use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::{self, TypeFoldable};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::Span;
use rustc_target::spec::abi::Abi;
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use rustc_trait_selection::traits;
use rustc_trait_selection::traits::misc::can_type_implement_copy;
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use rustc_typeck::expr_use_visitor as euv;
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use std::borrow::Cow;
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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);
/// }
/// ```
///
/// ```rust
/// // should be
/// 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"
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}
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declare_lint_pass!(NeedlessPassByValue => [NEEDLESS_PASS_BY_VALUE]);
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macro_rules! need {
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($e: expr) => {
if let Some(x) = $e {
x
} else {
return;
}
};
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}
impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
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#[allow(clippy::too_many_lines)]
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fn check_fn(
&mut self,
cx: &LateContext<'tcx>,
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kind: FnKind<'tcx>,
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decl: &'tcx FnDecl<'_>,
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body: &'tcx Body<'_>,
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span: Span,
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hir_id: HirId,
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) {
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if span.from_expansion() {
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return;
}
match kind {
FnKind::ItemFn(.., header, _, attrs) => {
if header.abi != Abi::Rust || requires_exact_signature(attrs) {
return;
}
},
FnKind::Method(..) => (),
FnKind::Closure(..) => return,
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}
// Exclude non-inherent impls
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if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(hir_id)) {
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if matches!(item.kind, ItemKind::Impl{ of_trait: Some(_), .. } |
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ItemKind::Trait(..))
{
return;
}
}
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// Allow `Borrow` or functions to be taken by value
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let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
let allowed_traits = [
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need!(cx.tcx.lang_items().fn_trait()),
need!(cx.tcx.lang_items().fn_once_trait()),
need!(cx.tcx.lang_items().fn_mut_trait()),
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need!(get_trait_def_id(cx, &paths::RANGE_ARGUMENT_TRAIT)),
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];
let sized_trait = need!(cx.tcx.lang_items().sized_trait());
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let fn_def_id = cx.tcx.hir().local_def_id(hir_id);
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let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds().iter())
.filter(|p| !p.is_global())
.filter_map(|obligation| {
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if let ty::PredicateKind::Trait(poly_trait_ref, _) = obligation.predicate.kind() {
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if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
{
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return None;
}
Some(poly_trait_ref)
} else {
None
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}
})
.collect::<Vec<_>>();
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// 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();
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cx.tcx.infer_ctxt().enter(|infcx| {
euv::ExprUseVisitor::new(&mut ctx, &infcx, fn_def_id, cx.param_env, cx.tables()).consume_body(body);
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});
ctx
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};
let fn_sig = cx.tcx.fn_sig(fn_def_id);
let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
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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 {
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if let PatKind::Binding(.., ident, _) = arg.pat.kind {
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if ident.as_str() == "self" {
continue;
}
}
}
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//
// * Exclude a type that is specifically bounded by `Borrow`.
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// * Exclude a type whose reference also fulfills its bound. (e.g., `std::convert::AsRef`,
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// `serde::Serialize`)
let (implements_borrow_trait, all_borrowable_trait) = {
let preds = preds
.iter()
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.filter(|t| t.skip_binder().self_ty() == ty)
.collect::<Vec<_>>();
(
preds.iter().any(|t| t.def_id() == borrow_trait),
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!preds.is_empty() && {
let ty_empty_region = cx.tcx.mk_imm_ref(cx.tcx.lifetimes.re_root_empty, ty);
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preds.iter().all(|t| {
let ty_params = &t.skip_binder().trait_ref.substs.iter().skip(1).collect::<Vec<_>>();
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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;
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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) {
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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) =
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get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
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if let TyKind::Path(QPath::Resolved(_, ref path)) = input.kind;
if let Some(elem_ty) = path.segments.iter()
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.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(
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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;
}
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}
if is_type_diagnostic_item(cx, ty, sym!(string_type)) {
if let Some(clone_spans) =
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get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
diag.span_suggestion(
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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;
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}
}
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()
.cloned()
.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,
);
}
}
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}
}
}
/// Functions marked with these attributes must have the exact signature.
fn requires_exact_signature(attrs: &[Attribute]) -> bool {
attrs.iter().any(|attr| {
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[sym!(proc_macro), sym!(proc_macro_attribute), sym!(proc_macro_derive)]
.iter()
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.any(|&allow| attr.check_name(allow))
})
}
#[derive(Default)]
struct MovedVariablesCtxt {
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moved_vars: FxHashSet<HirId>,
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/// Spans which need to be prefixed with `*` for dereferencing the
/// suggested additional reference.
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spans_need_deref: FxHashMap<HirId, FxHashSet<Span>>,
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}
impl MovedVariablesCtxt {
fn move_common(&mut self, cmt: &euv::PlaceWithHirId<'_>) {
if let euv::PlaceBase::Local(vid) = cmt.place.base {
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self.moved_vars.insert(vid);
}
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}
}
impl<'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt {
fn consume(&mut self, cmt: &euv::PlaceWithHirId<'tcx>, mode: euv::ConsumeMode) {
if let euv::ConsumeMode::Move = mode {
self.move_common(cmt);
}
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}
fn borrow(&mut self, _: &euv::PlaceWithHirId<'tcx>, _: ty::BorrowKind) {}
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fn mutate(&mut self, _: &euv::PlaceWithHirId<'tcx>) {}
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}