rust-clippy/clippy_lints/src/needless_pass_by_value.rs

448 lines
17 KiB
Rust

// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use crate::rustc::hir::intravisit::FnKind;
use crate::rustc::hir::*;
use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
use crate::rustc::middle::expr_use_visitor as euv;
use crate::rustc::middle::mem_categorization as mc;
use crate::rustc::traits;
use crate::rustc::ty::{self, RegionKind, TypeFoldable};
use crate::rustc::{declare_tool_lint, lint_array};
use crate::rustc_data_structures::fx::{FxHashMap, FxHashSet};
use crate::rustc_errors::Applicability;
use crate::rustc_target::spec::abi::Abi;
use crate::syntax::ast::NodeId;
use crate::syntax::errors::DiagnosticBuilder;
use crate::syntax_pos::Span;
use crate::utils::ptr::get_spans;
use crate::utils::{
get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths, snippet,
snippet_opt, span_lint_and_then,
};
use if_chain::if_chain;
use matches::matches;
use std::borrow::Cow;
/// **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
/// fn foo(v: &[i32]) {
/// assert_eq!(v.len(), 42);
/// }
/// ```
declare_clippy_lint! {
pub NEEDLESS_PASS_BY_VALUE,
pedantic,
"functions taking arguments by value, but not consuming them in its body"
}
pub struct NeedlessPassByValue;
impl LintPass for NeedlessPassByValue {
fn get_lints(&self) -> LintArray {
lint_array![NEEDLESS_PASS_BY_VALUE]
}
}
macro_rules! need {
($e: expr) => {
if let Some(x) = $e {
x
} else {
return;
}
};
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl,
body: &'tcx Body,
span: Span,
node_id: NodeId,
) {
if in_macro(span) {
return;
}
match kind {
FnKind::ItemFn(.., header, _, attrs) => {
if header.abi != Abi::Rust {
return;
}
for a in attrs {
if a.meta_item_list().is_some() && a.name() == "proc_macro_derive" {
return;
}
}
},
FnKind::Method(..) => (),
_ => return,
}
// Exclude non-inherent impls
if let Some(Node::Item(item)) = cx.tcx.hir().find(cx.tcx.hir().get_parent_node(node_id)) {
if matches!(item.node, ItemKind::Impl(_, _, _, _, Some(_), _, _) |
ItemKind::Trait(..))
{
return;
}
}
// Allow `Borrow` or functions to be taken by value
let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
let whitelisted_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(node_id);
let preds = traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
.filter(|p| !p.is_global())
.filter_map(|pred| {
if let ty::Predicate::Trait(poly_trait_ref) = pred {
if poly_trait_ref.def_id() == sized_trait || poly_trait_ref.skip_binder().has_escaping_bound_vars()
{
return None;
}
Some(poly_trait_ref)
} else {
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::new(cx);
let region_scope_tree = &cx.tcx.region_scope_tree(fn_def_id);
euv::ExprUseVisitor::new(&mut ctx, cx.tcx, cx.param_env, region_scope_tree, cx.tables, None)
.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.arguments).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.node {
if ident.as_str() == "self" {
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.skip_binder().self_ty() == ty)
.collect::<Vec<_>>();
(
preds.iter().any(|t| t.def_id() == borrow_trait),
!preds.is_empty()
&& preds.iter().all(|t| {
let ty_params = &t
.skip_binder()
.trait_ref
.substs
.iter()
.skip(1)
.cloned()
.collect::<Vec<_>>();
implements_trait(cx, cx.tcx.mk_imm_ref(&RegionKind::ReErased, ty), t.def_id(), ty_params)
}),
)
};
if_chain! {
if !is_self(arg);
if !ty.is_mutable_pointer();
if !is_copy(cx, ty);
if !whitelisted_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.node;
if !moved_vars.contains(&canonical_id);
then {
if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
continue;
}
// Dereference suggestion
let sugg = |db: &mut DiagnosticBuilder<'_>| {
if let ty::Adt(def, ..) = ty.sty {
if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
if cx.param_env.can_type_implement_copy(cx.tcx, ty).is_ok() {
db.span_help(span, "consider marking this type as Copy");
}
}
}
let deref_span = spans_need_deref.get(&canonical_id);
if_chain! {
if match_type(cx, ty, &paths::VEC);
if let Some(clone_spans) =
get_spans(cx, Some(body.id()), idx, &[("clone", ".to_owned()")]);
if let TyKind::Path(QPath::Resolved(_, ref path)) = input.node;
if let Some(elem_ty) = path.segments.iter()
.find(|seg| seg.ident.name == "Vec")
.and_then(|ps| ps.args.as_ref())
.map(|params| params.args.iter().find_map(|arg| match arg {
GenericArg::Type(ty) => Some(ty),
GenericArg::Lifetime(_) => None,
}).unwrap());
then {
let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
db.span_suggestion_with_applicability(
input.span,
"consider changing the type to",
slice_ty,
Applicability::Unspecified,
);
for (span, suggestion) in clone_spans {
db.span_suggestion_with_applicability(
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 match_type(cx, ty, &paths::STRING) {
if let Some(clone_spans) =
get_spans(cx, Some(body.id()), idx, &[("clone", ".to_string()"), ("as_str", "")]) {
db.span_suggestion_with_applicability(
input.span,
"consider changing the type to",
"&str".to_string(),
Applicability::Unspecified,
);
for (span, suggestion) in clone_spans {
db.span_suggestion_with_applicability(
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()
.cloned()
.map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))),
);
spans.sort_by_key(|&(span, _)| span);
}
multispan_sugg(db, "consider taking a reference instead".to_string(), 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,
);
}
}
}
}
}
struct MovedVariablesCtxt<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
moved_vars: FxHashSet<NodeId>,
/// Spans which need to be prefixed with `*` for dereferencing the
/// suggested additional reference.
spans_need_deref: FxHashMap<NodeId, FxHashSet<Span>>,
}
impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
Self {
cx,
moved_vars: FxHashSet::default(),
spans_need_deref: FxHashMap::default(),
}
}
fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: &mc::cmt_<'tcx>) {
let cmt = unwrap_downcast_or_interior(cmt);
if let mc::Categorization::Local(vid) = cmt.cat {
self.moved_vars.insert(vid);
}
}
fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>) {
let cmt = unwrap_downcast_or_interior(cmt);
if let mc::Categorization::Local(vid) = cmt.cat {
let mut id = matched_pat.id;
loop {
let parent = self.cx.tcx.hir().get_parent_node(id);
if id == parent {
// no parent
return;
}
id = parent;
if let Some(node) = self.cx.tcx.hir().find(id) {
match node {
Node::Expr(e) => {
// `match` and `if let`
if let ExprKind::Match(ref c, ..) = e.node {
self.spans_need_deref
.entry(vid)
.or_insert_with(FxHashSet::default)
.insert(c.span);
}
},
Node::Stmt(s) => {
// `let <pat> = x;`
if_chain! {
if let StmtKind::Decl(ref decl, _) = s.node;
if let DeclKind::Local(ref local) = decl.node;
then {
self.spans_need_deref
.entry(vid)
.or_insert_with(FxHashSet::default)
.insert(local.init
.as_ref()
.map(|e| e.span)
.expect("`let` stmt without init aren't caught by match_pat"));
}
}
},
_ => {},
}
}
}
}
}
}
impl<'a, 'tcx> euv::Delegate<'tcx> for MovedVariablesCtxt<'a, 'tcx> {
fn consume(&mut self, consume_id: NodeId, consume_span: Span, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
if let euv::ConsumeMode::Move(_) = mode {
self.move_common(consume_id, consume_span, cmt);
}
}
fn matched_pat(&mut self, matched_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::MatchMode) {
if let euv::MatchMode::MovingMatch = mode {
self.move_common(matched_pat.id, matched_pat.span, cmt);
} else {
self.non_moving_pat(matched_pat, cmt);
}
}
fn consume_pat(&mut self, consume_pat: &Pat, cmt: &mc::cmt_<'tcx>, mode: euv::ConsumeMode) {
if let euv::ConsumeMode::Move(_) = mode {
self.move_common(consume_pat.id, consume_pat.span, cmt);
}
}
fn borrow(
&mut self,
_: NodeId,
_: Span,
_: &mc::cmt_<'tcx>,
_: ty::Region<'_>,
_: ty::BorrowKind,
_: euv::LoanCause,
) {
}
fn mutate(&mut self, _: NodeId, _: Span, _: &mc::cmt_<'tcx>, _: euv::MutateMode) {}
fn decl_without_init(&mut self, _: NodeId, _: Span) {}
}
fn unwrap_downcast_or_interior<'a, 'tcx>(mut cmt: &'a mc::cmt_<'tcx>) -> mc::cmt_<'tcx> {
loop {
match cmt.cat {
mc::Categorization::Downcast(ref c, _) | mc::Categorization::Interior(ref c, _) => {
cmt = c;
},
_ => return (*cmt).clone(),
}
}
}