609 lines
21 KiB
Rust
609 lines
21 KiB
Rust
use crate::utils::{
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fn_has_unsatisfiable_preds, has_drop, is_copy, match_def_path, match_type, paths, snippet_opt, span_lint_hir,
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span_lint_hir_and_then, walk_ptrs_ty_depth,
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};
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use if_chain::if_chain;
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use rustc_data_structures::{fx::FxHashMap, transitive_relation::TransitiveRelation};
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use rustc_errors::Applicability;
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use rustc_hir::intravisit::FnKind;
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use rustc_hir::{def_id, Body, FnDecl, HirId};
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use rustc_index::bit_set::{BitSet, HybridBitSet};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_middle::mir::{
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self, traversal,
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visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
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};
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use rustc_middle::ty::{self, fold::TypeVisitor, Ty};
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use rustc_mir::dataflow::BottomValue;
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use rustc_mir::dataflow::{Analysis, AnalysisDomain, GenKill, GenKillAnalysis, ResultsCursor};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::source_map::{BytePos, Span};
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use std::convert::TryFrom;
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macro_rules! unwrap_or_continue {
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($x:expr) => {
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match $x {
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Some(x) => x,
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None => continue,
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}
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};
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}
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declare_clippy_lint! {
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/// **What it does:** Checks for a redundant `clone()` (and its relatives) which clones an owned
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/// value that is going to be dropped without further use.
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///
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/// **Why is this bad?** It is not always possible for the compiler to eliminate useless
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/// allocations and deallocations generated by redundant `clone()`s.
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///
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/// **Known problems:**
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///
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/// False-negatives: analysis performed by this lint is conservative and limited.
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///
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/// **Example:**
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/// ```rust
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/// # use std::path::Path;
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/// # #[derive(Clone)]
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/// # struct Foo;
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/// # impl Foo {
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/// # fn new() -> Self { Foo {} }
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/// # }
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/// # fn call(x: Foo) {}
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/// {
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/// let x = Foo::new();
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/// call(x.clone());
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/// call(x.clone()); // this can just pass `x`
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/// }
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///
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/// ["lorem", "ipsum"].join(" ").to_string();
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///
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/// Path::new("/a/b").join("c").to_path_buf();
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/// ```
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pub REDUNDANT_CLONE,
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perf,
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"`clone()` of an owned value that is going to be dropped immediately"
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}
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declare_lint_pass!(RedundantClone => [REDUNDANT_CLONE]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for RedundantClone {
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#[allow(clippy::too_many_lines)]
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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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_: FnKind<'tcx>,
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_: &'tcx FnDecl<'_>,
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body: &'tcx Body<'_>,
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_: Span,
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_: HirId,
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) {
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let def_id = cx.tcx.hir().body_owner_def_id(body.id());
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// Building MIR for `fn`s with unsatisfiable preds results in ICE.
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if fn_has_unsatisfiable_preds(cx, def_id) {
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return;
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}
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let mir = cx.tcx.optimized_mir(def_id);
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let mir_read_only = mir.unwrap_read_only();
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let maybe_storage_live_result = MaybeStorageLive
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.into_engine(cx.tcx, mir, def_id)
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.iterate_to_fixpoint()
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.into_results_cursor(mir);
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let mut possible_borrower = {
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let mut vis = PossibleBorrowerVisitor::new(cx, mir);
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vis.visit_body(&mir_read_only);
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vis.into_map(cx, maybe_storage_live_result)
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};
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for (bb, bbdata) in mir.basic_blocks().iter_enumerated() {
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let terminator = bbdata.terminator();
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if terminator.source_info.span.from_expansion() {
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continue;
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}
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// Give up on loops
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if terminator.successors().any(|s| *s == bb) {
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continue;
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}
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let (fn_def_id, arg, arg_ty, clone_ret) =
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unwrap_or_continue!(is_call_with_ref_arg(cx, mir, &terminator.kind));
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let from_borrow = match_def_path(cx, fn_def_id, &paths::CLONE_TRAIT_METHOD)
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|| match_def_path(cx, fn_def_id, &paths::TO_OWNED_METHOD)
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|| (match_def_path(cx, fn_def_id, &paths::TO_STRING_METHOD) && match_type(cx, arg_ty, &paths::STRING));
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let from_deref = !from_borrow
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&& (match_def_path(cx, fn_def_id, &paths::PATH_TO_PATH_BUF)
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|| match_def_path(cx, fn_def_id, &paths::OS_STR_TO_OS_STRING));
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if !from_borrow && !from_deref {
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continue;
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}
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// `{ cloned = &arg; clone(move cloned); }` or `{ cloned = &arg; to_path_buf(cloned); }`
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let (cloned, cannot_move_out) = unwrap_or_continue!(find_stmt_assigns_to(cx, mir, arg, from_borrow, bb));
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let loc = mir::Location {
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block: bb,
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statement_index: bbdata.statements.len(),
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};
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// `Local` to be cloned, and a local of `clone` call's destination
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let (local, ret_local) = if from_borrow {
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// `res = clone(arg)` can be turned into `res = move arg;`
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// if `arg` is the only borrow of `cloned` at this point.
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if cannot_move_out || !possible_borrower.only_borrowers(&[arg], cloned, loc) {
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continue;
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}
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(cloned, clone_ret)
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} else {
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// `arg` is a reference as it is `.deref()`ed in the previous block.
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// Look into the predecessor block and find out the source of deref.
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let ps = mir_read_only.predecessors_for(bb);
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if ps.len() != 1 {
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continue;
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}
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let pred_terminator = mir[ps[0]].terminator();
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// receiver of the `deref()` call
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let (pred_arg, deref_clone_ret) = if_chain! {
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if let Some((pred_fn_def_id, pred_arg, pred_arg_ty, res)) =
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is_call_with_ref_arg(cx, mir, &pred_terminator.kind);
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if res == cloned;
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if match_def_path(cx, pred_fn_def_id, &paths::DEREF_TRAIT_METHOD);
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if match_type(cx, pred_arg_ty, &paths::PATH_BUF)
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|| match_type(cx, pred_arg_ty, &paths::OS_STRING);
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then {
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(pred_arg, res)
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} else {
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continue;
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}
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};
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let (local, cannot_move_out) =
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unwrap_or_continue!(find_stmt_assigns_to(cx, mir, pred_arg, true, ps[0]));
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let loc = mir::Location {
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block: bb,
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statement_index: mir.basic_blocks()[bb].statements.len(),
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};
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// This can be turned into `res = move local` if `arg` and `cloned` are not borrowed
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// at the last statement:
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//
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// ```
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// pred_arg = &local;
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// cloned = deref(pred_arg);
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// arg = &cloned;
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// StorageDead(pred_arg);
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// res = to_path_buf(cloned);
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// ```
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if cannot_move_out || !possible_borrower.only_borrowers(&[arg, cloned], local, loc) {
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continue;
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}
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(local, deref_clone_ret)
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};
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let is_temp = mir_read_only.local_kind(ret_local) == mir::LocalKind::Temp;
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// 1. `local` can be moved out if it is not used later.
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// 2. If `ret_local` is a temporary and is neither consumed nor mutated, we can remove this `clone`
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// call anyway.
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let (used, consumed_or_mutated) = traversal::ReversePostorder::new(&mir, bb).skip(1).fold(
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(false, !is_temp),
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|(used, consumed), (tbb, tdata)| {
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// Short-circuit
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if (used && consumed) ||
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// Give up on loops
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tdata.terminator().successors().any(|s| *s == bb)
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{
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return (true, true);
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}
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let mut vis = LocalUseVisitor {
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used: (local, false),
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consumed_or_mutated: (ret_local, false),
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};
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vis.visit_basic_block_data(tbb, tdata);
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(used || vis.used.1, consumed || vis.consumed_or_mutated.1)
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},
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);
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if !used || !consumed_or_mutated {
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let span = terminator.source_info.span;
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let scope = terminator.source_info.scope;
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let node = mir.source_scopes[scope]
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.local_data
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.as_ref()
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.assert_crate_local()
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.lint_root;
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if_chain! {
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if let Some(snip) = snippet_opt(cx, span);
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if let Some(dot) = snip.rfind('.');
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then {
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let sugg_span = span.with_lo(
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span.lo() + BytePos(u32::try_from(dot).unwrap())
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);
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let mut app = Applicability::MaybeIncorrect;
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let mut call_snip = &snip[dot + 1..];
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// Machine applicable when `call_snip` looks like `foobar()`
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if call_snip.ends_with("()") {
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call_snip = call_snip[..call_snip.len()-2].trim();
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if call_snip.as_bytes().iter().all(|b| b.is_ascii_alphabetic() || *b == b'_') {
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app = Applicability::MachineApplicable;
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}
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}
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span_lint_hir_and_then(cx, REDUNDANT_CLONE, node, sugg_span, "redundant clone", |db| {
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db.span_suggestion(
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sugg_span,
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"remove this",
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String::new(),
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app,
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);
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if used {
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db.span_note(
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span,
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"cloned value is neither consumed nor mutated",
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);
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} else {
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db.span_note(
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span.with_hi(span.lo() + BytePos(u32::try_from(dot).unwrap())),
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"this value is dropped without further use",
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);
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}
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});
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} else {
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span_lint_hir(cx, REDUNDANT_CLONE, node, span, "redundant clone");
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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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/// If `kind` is `y = func(x: &T)` where `T: !Copy`, returns `(DefId of func, x, T, y)`.
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fn is_call_with_ref_arg<'tcx>(
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cx: &LateContext<'_, 'tcx>,
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mir: &'tcx mir::Body<'tcx>,
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kind: &'tcx mir::TerminatorKind<'tcx>,
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) -> Option<(def_id::DefId, mir::Local, Ty<'tcx>, mir::Local)> {
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if_chain! {
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if let mir::TerminatorKind::Call { func, args, destination, .. } = kind;
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if args.len() == 1;
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if let mir::Operand::Move(mir::Place { local, .. }) = &args[0];
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if let ty::FnDef(def_id, _) = func.ty(&*mir, cx.tcx).kind;
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if let (inner_ty, 1) = walk_ptrs_ty_depth(args[0].ty(&*mir, cx.tcx));
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if !is_copy(cx, inner_ty);
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then {
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Some((def_id, *local, inner_ty, destination.as_ref().map(|(dest, _)| dest)?.as_local()?))
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} else {
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None
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}
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}
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}
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type CannotMoveOut = bool;
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/// Finds the first `to = (&)from`, and returns
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/// ``Some((from, whether `from` cannot be moved out))``.
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fn find_stmt_assigns_to<'tcx>(
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cx: &LateContext<'_, 'tcx>,
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mir: &mir::Body<'tcx>,
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to_local: mir::Local,
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by_ref: bool,
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bb: mir::BasicBlock,
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) -> Option<(mir::Local, CannotMoveOut)> {
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let rvalue = mir.basic_blocks()[bb].statements.iter().rev().find_map(|stmt| {
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if let mir::StatementKind::Assign(box (mir::Place { local, .. }, v)) = &stmt.kind {
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return if *local == to_local { Some(v) } else { None };
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}
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None
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})?;
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match (by_ref, &*rvalue) {
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(true, mir::Rvalue::Ref(_, _, place)) | (false, mir::Rvalue::Use(mir::Operand::Copy(place))) => {
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base_local_and_movability(cx, mir, *place)
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},
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(false, mir::Rvalue::Ref(_, _, place)) => {
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if let [mir::ProjectionElem::Deref] = place.as_ref().projection {
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base_local_and_movability(cx, mir, *place)
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} else {
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None
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}
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},
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_ => None,
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}
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}
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/// Extracts and returns the undermost base `Local` of given `place`. Returns `place` itself
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/// if it is already a `Local`.
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///
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/// Also reports whether given `place` cannot be moved out.
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fn base_local_and_movability<'tcx>(
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cx: &LateContext<'_, 'tcx>,
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mir: &mir::Body<'tcx>,
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place: mir::Place<'tcx>,
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) -> Option<(mir::Local, CannotMoveOut)> {
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use rustc_middle::mir::PlaceRef;
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// Dereference. You cannot move things out from a borrowed value.
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let mut deref = false;
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// Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
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let mut field = false;
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let PlaceRef { local, mut projection } = place.as_ref();
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while let [base @ .., elem] = projection {
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projection = base;
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deref |= matches!(elem, mir::ProjectionElem::Deref);
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field |= matches!(elem, mir::ProjectionElem::Field(..))
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&& has_drop(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
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}
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Some((local, deref || field))
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}
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struct LocalUseVisitor {
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used: (mir::Local, bool),
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consumed_or_mutated: (mir::Local, bool),
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}
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impl<'tcx> mir::visit::Visitor<'tcx> for LocalUseVisitor {
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fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
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let statements = &data.statements;
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for (statement_index, statement) in statements.iter().enumerate() {
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self.visit_statement(statement, mir::Location { block, statement_index });
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}
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self.visit_terminator(
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data.terminator(),
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mir::Location {
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block,
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statement_index: statements.len(),
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},
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);
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}
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fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, _: mir::Location) {
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let local = place.local;
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if local == self.used.0
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&& !matches!(ctx, PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_))
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{
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self.used.1 = true;
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}
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if local == self.consumed_or_mutated.0 {
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match ctx {
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PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
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| PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
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self.consumed_or_mutated.1 = true;
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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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/// Determines liveness of each local purely based on `StorageLive`/`Dead`.
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#[derive(Copy, Clone)]
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struct MaybeStorageLive;
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impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
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type Idx = mir::Local;
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const NAME: &'static str = "maybe_storage_live";
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fn bits_per_block(&self, body: &mir::Body<'tcx>) -> usize {
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body.local_decls.len()
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}
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fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut BitSet<Self::Idx>) {
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for arg in body.args_iter() {
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state.insert(arg);
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}
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}
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}
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impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
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fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
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match stmt.kind {
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mir::StatementKind::StorageLive(l) => trans.gen(l),
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mir::StatementKind::StorageDead(l) => trans.kill(l),
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_ => (),
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}
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}
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fn terminator_effect(
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&self,
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_trans: &mut impl GenKill<Self::Idx>,
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_terminator: &mir::Terminator<'tcx>,
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_loc: mir::Location,
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) {
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}
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fn call_return_effect(
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&self,
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_in_out: &mut impl GenKill<Self::Idx>,
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_block: mir::BasicBlock,
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_func: &mir::Operand<'tcx>,
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_args: &[mir::Operand<'tcx>],
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_return_place: mir::Place<'tcx>,
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) {
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// Nothing to do when a call returns successfully
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}
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}
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impl BottomValue for MaybeStorageLive {
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/// bottom = dead
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const BOTTOM_VALUE: bool = false;
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}
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/// Collects the possible borrowers of each local.
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/// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
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/// possible borrowers of `a`.
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struct PossibleBorrowerVisitor<'a, 'tcx> {
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possible_borrower: TransitiveRelation<mir::Local>,
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body: &'a mir::Body<'tcx>,
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cx: &'a LateContext<'a, 'tcx>,
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}
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impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
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fn new(cx: &'a LateContext<'a, 'tcx>, body: &'a mir::Body<'tcx>) -> Self {
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Self {
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possible_borrower: TransitiveRelation::default(),
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cx,
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body,
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}
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}
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fn into_map(
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self,
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cx: &LateContext<'a, 'tcx>,
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maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
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) -> PossibleBorrowerMap<'a, 'tcx> {
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let mut map = FxHashMap::default();
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for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
|
|
if is_copy(cx, self.body.local_decls[row].ty) {
|
|
continue;
|
|
}
|
|
|
|
let borrowers = self.possible_borrower.reachable_from(&row);
|
|
if !borrowers.is_empty() {
|
|
let mut bs = HybridBitSet::new_empty(self.body.local_decls.len());
|
|
for &c in borrowers {
|
|
if c != mir::Local::from_usize(0) {
|
|
bs.insert(c);
|
|
}
|
|
}
|
|
|
|
if !bs.is_empty() {
|
|
map.insert(row, bs);
|
|
}
|
|
}
|
|
}
|
|
|
|
let bs = BitSet::new_empty(self.body.local_decls.len());
|
|
PossibleBorrowerMap {
|
|
map,
|
|
maybe_live,
|
|
bitset: (bs.clone(), bs),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
|
|
fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
|
|
let lhs = place.local;
|
|
match rvalue {
|
|
mir::Rvalue::Ref(_, _, borrowed) => {
|
|
self.possible_borrower.add(borrowed.local, lhs);
|
|
},
|
|
other => {
|
|
if !ContainsRegion.visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty) {
|
|
return;
|
|
}
|
|
rvalue_locals(other, |rhs| {
|
|
if lhs != rhs {
|
|
self.possible_borrower.add(rhs, lhs);
|
|
}
|
|
});
|
|
},
|
|
}
|
|
}
|
|
|
|
fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
|
|
if let mir::TerminatorKind::Call {
|
|
args,
|
|
destination: Some((mir::Place { local: dest, .. }, _)),
|
|
..
|
|
} = &terminator.kind
|
|
{
|
|
// If the call returns something with lifetimes,
|
|
// let's conservatively assume the returned value contains lifetime of all the arguments.
|
|
// For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
|
|
if !ContainsRegion.visit_ty(&self.body.local_decls[*dest].ty) {
|
|
return;
|
|
}
|
|
|
|
for op in args {
|
|
match op {
|
|
mir::Operand::Copy(p) | mir::Operand::Move(p) => {
|
|
self.possible_borrower.add(p.local, *dest);
|
|
},
|
|
_ => (),
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
struct ContainsRegion;
|
|
|
|
impl TypeVisitor<'_> for ContainsRegion {
|
|
fn visit_region(&mut self, _: ty::Region<'_>) -> bool {
|
|
true
|
|
}
|
|
}
|
|
|
|
fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
|
|
use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
|
|
|
|
let mut visit_op = |op: &mir::Operand<'_>| match op {
|
|
mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
|
|
_ => (),
|
|
};
|
|
|
|
match rvalue {
|
|
Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
|
|
Aggregate(_, ops) => ops.iter().for_each(visit_op),
|
|
BinaryOp(_, lhs, rhs) | CheckedBinaryOp(_, lhs, rhs) => {
|
|
visit_op(lhs);
|
|
visit_op(rhs);
|
|
},
|
|
_ => (),
|
|
}
|
|
}
|
|
|
|
/// Result of `PossibleBorrowerVisitor`.
|
|
struct PossibleBorrowerMap<'a, 'tcx> {
|
|
/// Mapping `Local -> its possible borrowers`
|
|
map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
|
|
maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
|
|
// Caches to avoid allocation of `BitSet` on every query
|
|
bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
|
|
}
|
|
|
|
impl PossibleBorrowerMap<'_, '_> {
|
|
/// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
|
|
fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
|
|
self.maybe_live.seek_after(at);
|
|
|
|
self.bitset.0.clear();
|
|
let maybe_live = &mut self.maybe_live;
|
|
if let Some(bitset) = self.map.get(&borrowed) {
|
|
for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
|
|
self.bitset.0.insert(b);
|
|
}
|
|
} else {
|
|
return false;
|
|
}
|
|
|
|
self.bitset.1.clear();
|
|
for b in borrowers {
|
|
self.bitset.1.insert(*b);
|
|
}
|
|
|
|
self.bitset.0 == self.bitset.1
|
|
}
|
|
}
|