compiler/rustc_mir_build/src/lints.rs

   1 use rustc_data_structures::graph::iterate::{
   2     NodeStatus, TriColorDepthFirstSearch, TriColorVisitor,
   3 };
   4 use rustc_hir::intravisit::FnKind;
   5 use rustc_middle::hir::map::blocks::FnLikeNode;
   6 use rustc_middle::mir::{BasicBlock, Body, Operand, TerminatorKind};
   7 use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
   8 use rustc_middle::ty::{self, AssocItem, AssocItemContainer, Instance, TyCtxt};
   9 use rustc_session::lint::builtin::UNCONDITIONAL_RECURSION;
  10 use rustc_span::Span;
  11 use std::ops::ControlFlow;
  12
  13 crate fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
  14     let def_id = body.source.def_id().expect_local();
  15     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
  16
  17     if let Some(fn_like_node) = FnLikeNode::from_node(tcx.hir().get(hir_id)) {
  18         if let FnKind::Closure(_) = fn_like_node.kind() {
  19             // closures can't recur, so they don't matter.
  20             return;
  21         }
  22
  23         // If this is trait/impl method, extract the trait's substs.
  24         let trait_substs = match tcx.opt_associated_item(def_id.to_def_id()) {
  25             Some(AssocItem {
  26                 container: AssocItemContainer::TraitContainer(trait_def_id), ..
  27             }) => {
  28                 let trait_substs_count = tcx.generics_of(*trait_def_id).count();
  29                 &InternalSubsts::identity_for_item(tcx, def_id.to_def_id())[..trait_substs_count]
  30             }
  31             _ => &[],
  32         };
  33
  34         let mut vis = Search { tcx, body, reachable_recursive_calls: vec![], trait_substs };
  35         if let Some(NonRecursive) = TriColorDepthFirstSearch::new(&body).run_from_start(&mut vis) {
  36             return;
  37         }
  38
  39         vis.reachable_recursive_calls.sort();
  40
  41         let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
  42         let sp = tcx.sess.source_map().guess_head_span(tcx.hir().span_with_body(hir_id));
  43         tcx.struct_span_lint_hir(UNCONDITIONAL_RECURSION, hir_id, sp, |lint| {
  44             let mut db = lint.build("function cannot return without recursing");
  45             db.span_label(sp, "cannot return without recursing");
  46             // offer some help to the programmer.
  47             for call_span in vis.reachable_recursive_calls {
  48                 db.span_label(call_span, "recursive call site");
  49             }
  50             db.help("a `loop` may express intention better if this is on purpose");
  51             db.emit();
  52         });
  53     }
  54 }
  55
  56 struct NonRecursive;
  57
  58 struct Search<'mir, 'tcx> {
  59     tcx: TyCtxt<'tcx>,
  60     body: &'mir Body<'tcx>,
  61     trait_substs: &'tcx [GenericArg<'tcx>],
  62
  63     reachable_recursive_calls: Vec<Span>,
  64 }
  65
  66 impl<'mir, 'tcx> Search<'mir, 'tcx> {
  67     /// Returns `true` if `func` refers to the function we are searching in.
  68     fn is_recursive_call(&self, func: &Operand<'tcx>) -> bool {
  69         let Search { tcx, body, trait_substs, .. } = *self;
  70         let caller = body.source.def_id();
  71         let param_env = tcx.param_env(caller);
  72
  73         let func_ty = func.ty(body, tcx);
  74         if let ty::FnDef(callee, substs) = *func_ty.kind() {
  75             let normalized_substs = tcx.normalize_erasing_regions(param_env, substs);
  76             let (callee, call_substs) = if let Ok(Some(instance)) =
  77                 Instance::resolve(tcx, param_env, callee, normalized_substs)
  78             {
  79                 (instance.def_id(), instance.substs)
  80             } else {
  81                 (callee, normalized_substs)
  82             };
  83
  84             // FIXME(#57965): Make this work across function boundaries
  85
  86             // If this is a trait fn, the substs on the trait have to match, or we might be
  87             // calling into an entirely different method (for example, a call from the default
  88             // method in the trait to `<A as Trait<B>>::method`, where `A` and/or `B` are
  89             // specific types).
  90             return callee == caller && &call_substs[..trait_substs.len()] == trait_substs;
  91         }
  92
  93         false
  94     }
  95 }
  96
  97 impl<'mir, 'tcx> TriColorVisitor<&'mir Body<'tcx>> for Search<'mir, 'tcx> {
  98     type BreakVal = NonRecursive;
  99
 100     fn node_examined(
 101         &mut self,
 102         bb: BasicBlock,
 103         prior_status: Option<NodeStatus>,
 104     ) -> ControlFlow<Self::BreakVal> {
 105         // Back-edge in the CFG (loop).
 106         if let Some(NodeStatus::Visited) = prior_status {
 107             return ControlFlow::Break(NonRecursive);
 108         }
 109
 110         match self.body[bb].terminator().kind {
 111             // These terminators return control flow to the caller.
 112             TerminatorKind::Abort
 113             | TerminatorKind::GeneratorDrop
 114             | TerminatorKind::Resume
 115             | TerminatorKind::Return
 116             | TerminatorKind::Unreachable
 117             | TerminatorKind::Yield { .. } => ControlFlow::Break(NonRecursive),
 118
 119             // A diverging InlineAsm is treated as non-recursing
 120             TerminatorKind::InlineAsm { destination, .. } => {
 121                 if destination.is_some() {
 122                     ControlFlow::CONTINUE
 123                 } else {
 124                     ControlFlow::Break(NonRecursive)
 125                 }
 126             }
 127
 128             // These do not.
 129             TerminatorKind::Assert { .. }
 130             | TerminatorKind::Call { .. }
 131             | TerminatorKind::Drop { .. }
 132             | TerminatorKind::DropAndReplace { .. }
 133             | TerminatorKind::FalseEdge { .. }
 134             | TerminatorKind::FalseUnwind { .. }
 135             | TerminatorKind::Goto { .. }
 136             | TerminatorKind::SwitchInt { .. } => ControlFlow::CONTINUE,
 137         }
 138     }
 139
 140     fn node_settled(&mut self, bb: BasicBlock) -> ControlFlow<Self::BreakVal> {
 141         // When we examine a node for the last time, remember it if it is a recursive call.
 142         let terminator = self.body[bb].terminator();
 143         if let TerminatorKind::Call { func, .. } = &terminator.kind {
 144             if self.is_recursive_call(func) {
 145                 self.reachable_recursive_calls.push(terminator.source_info.span);
 146             }
 147         }
 148
 149         ControlFlow::CONTINUE
 150     }
 151
 152     fn ignore_edge(&mut self, bb: BasicBlock, target: BasicBlock) -> bool {
 153         // Don't traverse successors of recursive calls or false CFG edges.
 154         match self.body[bb].terminator().kind {
 155             TerminatorKind::Call { ref func, .. } => self.is_recursive_call(func),
 156
 157             TerminatorKind::FalseUnwind { unwind: Some(imaginary_target), .. }
 158             | TerminatorKind::FalseEdge { imaginary_target, .. } => imaginary_target == target,
 159
 160             _ => false,
 161         }
 162     }
 163 }