rust-clippy/clippy_lints/src/needless_pass_by_value.rs

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use rustc::hir::*;
use rustc::hir::intravisit::FnKind;
use rustc::lint::*;
use rustc::ty::{self, TypeFoldable};
use rustc::traits;
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use rustc::middle::expr_use_visitor as euv;
use rustc::middle::mem_categorization as mc;
use syntax::ast::NodeId;
use syntax_pos::Span;
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use syntax::errors::DiagnosticBuilder;
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use utils::{get_trait_def_id, implements_trait, in_macro, is_copy, is_self, match_type, multispan_sugg, paths,
snippet, span_lint_and_then};
use std::collections::{HashMap, HashSet};
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/// **What it does:** Checks for functions taking arguments by value, but not
/// consuming them in its
/// body.
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///
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/// **Why is this bad?** Taking arguments by reference is more flexible and can
/// sometimes avoid
/// unnecessary allocations.
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///
/// **Known problems:** Hopefully none.
///
/// **Example:**
/// ```rust
/// fn foo(v: Vec<i32>) {
/// assert_eq!(v.len(), 42);
/// }
/// ```
declare_lint! {
pub NEEDLESS_PASS_BY_VALUE,
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Warn,
"functions taking arguments by value, but not consuming them in its body"
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}
pub struct NeedlessPassByValue;
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impl LintPass for NeedlessPassByValue {
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fn get_lints(&self) -> LintArray {
lint_array![NEEDLESS_PASS_BY_VALUE]
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}
}
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macro_rules! need {
($e: expr) => { if let Some(x) = $e { x } else { return; } };
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
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fn check_fn(
&mut self,
cx: &LateContext<'a, 'tcx>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl,
body: &'tcx Body,
span: Span,
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node_id: NodeId,
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) {
if in_macro(span) {
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return;
}
match kind {
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FnKind::ItemFn(.., attrs) => for a in attrs {
if_let_chain!{[
a.meta_item_list().is_some(),
let Some(name) = a.name(),
name == "proc_macro_derive",
], {
return;
}}
},
_ => return,
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}
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// Allows these to be passed by value.
let fn_trait = need!(cx.tcx.lang_items().fn_trait());
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let asref_trait = need!(get_trait_def_id(cx, &paths::ASREF_TRAIT));
let borrow_trait = need!(get_trait_def_id(cx, &paths::BORROW_TRAIT));
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let fn_def_id = cx.tcx.hir.local_def_id(node_id);
let preds: Vec<ty::Predicate> = {
traits::elaborate_predicates(cx.tcx, cx.param_env.caller_bounds.to_vec())
.filter(|p| !p.is_global())
.collect()
};
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// Collect moved variables and spans which will need dereferencings from the
// function body.
let MovedVariablesCtxt {
moved_vars,
spans_need_deref,
..
} = {
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let mut ctx = MovedVariablesCtxt::new(cx);
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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).consume_body(body);
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 ((input, &ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
// Determines whether `ty` implements `Borrow<U>` (U != ty) specifically.
// This is needed due to the `Borrow<T> for T` blanket impl.
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let implements_borrow_trait = preds
.iter()
.filter_map(|pred| if let ty::Predicate::Trait(ref poly_trait_ref) = *pred {
Some(poly_trait_ref.skip_binder())
} else {
None
})
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.filter(|tpred| tpred.def_id() == borrow_trait && tpred.self_ty() == ty)
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.any(|tpred| {
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tpred
.input_types()
.nth(1)
.expect("Borrow trait must have an parameter") != ty
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});
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if_let_chain! {[
!is_self(arg),
!ty.is_mutable_pointer(),
!is_copy(cx, ty),
!implements_trait(cx, ty, fn_trait, &[]),
!implements_trait(cx, ty, asref_trait, &[]),
!implements_borrow_trait,
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let PatKind::Binding(mode, canonical_id, ..) = arg.pat.node,
!moved_vars.contains(&canonical_id),
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], {
// Note: `toplevel_ref_arg` warns if `BindByRef`
if mode == BindingAnnotation::Mutable || mode == BindingAnnotation::RefMut {
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continue;
}
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// Suggestion logic
let sugg = |db: &mut DiagnosticBuilder| {
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let deref_span = spans_need_deref.get(&canonical_id);
if_let_chain! {[
match_type(cx, ty, &paths::VEC),
let TyPath(QPath::Resolved(_, ref path)) = input.node,
let Some(elem_ty) = path.segments.iter()
.find(|seg| seg.name == "Vec")
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.map(|ps| &ps.parameters.types[0]),
], {
let slice_ty = format!("&[{}]", snippet(cx, elem_ty.span, "_"));
db.span_suggestion(input.span,
"consider changing the type to",
slice_ty);
assert!(deref_span.is_none());
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return; // `Vec` and `String` cannot be destructured - no need for `*` suggestion
}}
if match_type(cx, ty, &paths::STRING) {
db.span_suggestion(input.span,
"consider changing the type to",
"&str".to_string());
assert!(deref_span.is_none());
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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 {
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spans.extend(deref_span.iter().cloned()
.map(|span| (span, format!("*{}", snippet(cx, span, "<expr>")))));
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spans.sort_by_key(|&(span, _)| span);
}
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multispan_sugg(db, "consider taking a reference instead".to_string(), spans);
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};
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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}}
}
}
}
struct MovedVariablesCtxt<'a, 'tcx: 'a> {
cx: &'a LateContext<'a, 'tcx>,
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moved_vars: HashSet<NodeId>,
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/// Spans which need to be prefixed with `*` for dereferencing the
/// suggested additional
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/// reference.
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spans_need_deref: HashMap<NodeId, HashSet<Span>>,
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}
impl<'a, 'tcx> MovedVariablesCtxt<'a, 'tcx> {
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fn new(cx: &'a LateContext<'a, 'tcx>) -> Self {
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Self {
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cx: cx,
moved_vars: HashSet::new(),
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spans_need_deref: HashMap::new(),
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}
}
fn move_common(&mut self, _consume_id: NodeId, _span: Span, cmt: mc::cmt<'tcx>) {
let cmt = unwrap_downcast_or_interior(cmt);
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if let mc::Categorization::Local(vid) = cmt.cat {
self.moved_vars.insert(vid);
}
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}
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fn non_moving_pat(&mut self, matched_pat: &Pat, cmt: mc::cmt<'tcx>) {
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let cmt = unwrap_downcast_or_interior(cmt);
if_let_chain! {[
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 {
map::Node::NodeExpr(e) => {
// `match` and `if let`
if let ExprMatch(ref c, ..) = e.node {
self.spans_need_deref
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.entry(vid)
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.or_insert_with(HashSet::new)
.insert(c.span);
}
}
map::Node::NodeStmt(s) => {
// `let <pat> = x;`
if_let_chain! {[
let StmtDecl(ref decl, _) = s.node,
let DeclLocal(ref local) = decl.node,
], {
self.spans_need_deref
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.entry(vid)
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.or_insert_with(HashSet::new)
.insert(local.init
.as_ref()
.map(|e| e.span)
.expect("`let` stmt without init aren't caught by match_pat"));
}}
}
_ => {}
}
}
}
}}
}
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}
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);
}
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}
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 {
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self.non_moving_pat(matched_pat, cmt);
}
}
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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);
}
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}
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fn borrow(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: ty::Region, _: ty::BorrowKind, _: euv::LoanCause) {}
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fn mutate(&mut self, _: NodeId, _: Span, _: mc::cmt<'tcx>, _: euv::MutateMode) {}
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fn decl_without_init(&mut self, _: NodeId, _: Span) {}
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}
fn unwrap_downcast_or_interior(mut cmt: mc::cmt) -> mc::cmt {
loop {
match cmt.cat.clone() {
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mc::Categorization::Downcast(c, _) | mc::Categorization::Interior(c, _) => {
cmt = c;
},
_ => return cmt,
}
}
}