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