Rollup merge of rust-lang#70973 - ecstatic-morse:recursion-lint, r=jonas-schievink

Use forward traversal for unconditional recursion lint

While reviewing rust-lang#70822, I noted that rust-lang#54444 could be solved without requiring the predecessor graph and without allocating a `Vec<Span>` for every basic block. The unconditional recursion lint is not a performance bottleneck however, so I approved rust-lang#70822 as it was.

Nevertheless, I wanted to try implementing my idea using `TriColorDepthFirstSearch`, which is a DFS that can differentiate between [forward/tree edges and backward ones](https://en.wikipedia.org/wiki/Depth-first_search#Output_of_a_depth-first_search). I found this approach more straightforward than the existing one, so I'm opening this PR to see if it is desirable.

The pass is now just a DFS across the control-flow graph. We ignore false edges and false unwinds, as well as the successors of recursive calls, just like existing pass does. If we see a back-edge (loop) or a terminator that would cause us to yield control-flow back to the caller (`Return`, `Resume`, etc.), we know that the function does not unconditionally recurse.

r? @jonas-schievink

Author: Centril

src/librustc_data_structures/graph/iterate/mod.rs

@@ -209,7 +209,9 @@ where
                     // schedule its successors for examination.
                     self.stack.push(Event { node, becomes: Settled });
                     for succ in self.graph.successors(node) {
-                        self.stack.push(Event { node: succ, becomes: Visited });
+                        if !visitor.ignore_edge(node, succ) {
+                            self.stack.push(Event { node: succ, becomes: Visited });
+                        }
                     }
                 }
             }
@@ -255,16 +257,21 @@ where
     /// [CLR]: https://en.wikipedia.org/wiki/Introduction_to_Algorithms
     fn node_examined(
         &mut self,
-        _target: G::Node,
+        _node: G::Node,
         _prior_status: Option<NodeStatus>,
     ) -> ControlFlow<Self::BreakVal> {
         ControlFlow::Continue
     }
 
     /// Called after all nodes reachable from this one have been examined.
-    fn node_settled(&mut self, _target: G::Node) -> ControlFlow<Self::BreakVal> {
+    fn node_settled(&mut self, _node: G::Node) -> ControlFlow<Self::BreakVal> {
         ControlFlow::Continue
     }
+
+    /// Behave as if no edges exist from `source` to `target`.
+    fn ignore_edge(&mut self, _source: G::Node, _target: G::Node) -> bool {
+        false
+    }
 }
 
 /// This `TriColorVisitor` looks for back edges in a graph, which indicate that a cycle exists.

src/librustc_mir_build/build/mod.rs

@@ -178,11 +178,11 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyAndCache<'_> {
             build::construct_const(cx, body_id, return_ty, return_ty_span)
         };
 
+        lints::check(tcx, &body, def_id);
+
         let mut body = BodyAndCache::new(body);
         body.ensure_predecessors();
 
-        lints::check(tcx, &body.unwrap_read_only(), def_id);
-
         // The borrow checker will replace all the regions here with its own
         // inference variables. There's no point having non-erased regions here.
         // The exception is `body.user_type_annotations`, which is used unmodified

src/librustc_mir_build/lints.rs

@@ -1,15 +1,16 @@
+use rustc_data_structures::graph::iterate::{
+    ControlFlow, NodeStatus, TriColorDepthFirstSearch, TriColorVisitor,
+};
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::FnKind;
-use rustc_index::bit_set::BitSet;
-use rustc_index::vec::IndexVec;
 use rustc_middle::hir::map::blocks::FnLikeNode;
-use rustc_middle::mir::{BasicBlock, Body, ReadOnlyBodyAndCache, TerminatorKind, START_BLOCK};
-use rustc_middle::ty::subst::InternalSubsts;
+use rustc_middle::mir::{BasicBlock, Body, Operand, TerminatorKind};
+use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
 use rustc_middle::ty::{self, AssocItem, AssocItemContainer, Instance, TyCtxt};
 use rustc_session::lint::builtin::UNCONDITIONAL_RECURSION;
-use std::collections::VecDeque;
+use rustc_span::Span;
 
-crate fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &ReadOnlyBodyAndCache<'_, 'tcx>, def_id: DefId) {
+crate fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, def_id: DefId) {
     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 
     if let Some(fn_like_node) = FnLikeNode::from_node(tcx.hir().get(hir_id)) {
@@ -18,158 +19,133 @@ crate fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &ReadOnlyBodyAndCache<'_, 'tcx>, d
             return;
         }
 
-        check_fn_for_unconditional_recursion(tcx, body, def_id);
-    }
-}
-
-fn check_fn_for_unconditional_recursion<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    body: &ReadOnlyBodyAndCache<'_, 'tcx>,
-    def_id: DefId,
-) {
-    let self_calls = find_blocks_calling_self(tcx, &body, def_id);
-
-    // Stores a list of `Span`s for every basic block. Those are the spans of self-calls where we
-    // know that one of them will definitely be reached. If the list is empty, the block either
-    // wasn't processed yet or will not always go to a self-call.
-    let mut results = IndexVec::from_elem_n(vec![], body.basic_blocks().len());
-
-    // We start the analysis at the self calls and work backwards.
-    let mut queue: VecDeque<_> = self_calls.iter().collect();
-
-    while let Some(bb) = queue.pop_front() {
-        if !results[bb].is_empty() {
-            // Already propagated.
-            continue;
-        }
-
-        let locations = if self_calls.contains(bb) {
-            // `bb` *is* a self-call.
-            // We don't look at successors here because they are irrelevant here and we don't want
-            // to lint them (eg. `f(); f()` should only lint the first call).
-            vec![bb]
-        } else {
-            // If *all* successors of `bb` lead to a self-call, emit notes at all of their
-            // locations.
-
-            // Determine all "relevant" successors. We ignore successors only reached via unwinding.
-            let terminator = body[bb].terminator();
-            let relevant_successors = match &terminator.kind {
-                TerminatorKind::Call { destination: None, .. }
-                | TerminatorKind::Yield { .. }
-                | TerminatorKind::GeneratorDrop => None.into_iter().chain(&[]),
-                TerminatorKind::SwitchInt { targets, .. } => None.into_iter().chain(targets),
-                TerminatorKind::Goto { target }
-                | TerminatorKind::Drop { target, .. }
-                | TerminatorKind::DropAndReplace { target, .. }
-                | TerminatorKind::Assert { target, .. }
-                | TerminatorKind::FalseEdges { real_target: target, .. }
-                | TerminatorKind::FalseUnwind { real_target: target, .. }
-                | TerminatorKind::Call { destination: Some((_, target)), .. } => {
-                    Some(target).into_iter().chain(&[])
-                }
-                TerminatorKind::Resume
-                | TerminatorKind::Abort
-                | TerminatorKind::Return
-                | TerminatorKind::Unreachable => {
-                    // We propagate backwards, so these should never be encountered here.
-                    unreachable!("unexpected terminator {:?}", terminator.kind)
-                }
-            };
-
-            // If all our successors are known to lead to self-calls, then we do too.
-            let all_are_self_calls =
-                relevant_successors.clone().all(|&succ| !results[succ].is_empty());
-
-            if all_are_self_calls {
-                // We'll definitely lead to a self-call. Merge all call locations of the successors
-                // for linting them later.
-                relevant_successors.flat_map(|&succ| results[succ].iter().copied()).collect()
-            } else {
-                // At least 1 successor does not always lead to a self-call, so we also don't.
-                vec![]
+        // If this is trait/impl method, extract the trait's substs.
+        let trait_substs = match tcx.opt_associated_item(def_id) {
+            Some(AssocItem {
+                container: AssocItemContainer::TraitContainer(trait_def_id), ..
+            }) => {
+                let trait_substs_count = tcx.generics_of(trait_def_id).count();
+                &InternalSubsts::identity_for_item(tcx, def_id)[..trait_substs_count]
             }
+            _ => &[],
         };
 
-        if !locations.is_empty() {
-            // This is a newly confirmed-to-always-reach-self-call block.
-            results[bb] = locations;
-
-            // Propagate backwards through the CFG.
-            debug!("propagate loc={:?} in {:?} -> {:?}", results[bb], bb, body.predecessors()[bb]);
-            queue.extend(body.predecessors()[bb].iter().copied());
+        let mut vis = Search { tcx, body, def_id, reachable_recursive_calls: vec![], trait_substs };
+        if let Some(NonRecursive) = TriColorDepthFirstSearch::new(&body).run_from_start(&mut vis) {
+            return;
         }
-    }
-
-    debug!("unconditional recursion results: {:?}", results);
 
-    let self_call_locations = &mut results[START_BLOCK];
-    self_call_locations.sort();
-    self_call_locations.dedup();
+        vis.reachable_recursive_calls.sort();
 
-    if !self_call_locations.is_empty() {
         let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
         let sp = tcx.sess.source_map().guess_head_span(tcx.hir().span(hir_id));
         tcx.struct_span_lint_hir(UNCONDITIONAL_RECURSION, hir_id, sp, |lint| {
             let mut db = lint.build("function cannot return without recursing");
             db.span_label(sp, "cannot return without recursing");
             // offer some help to the programmer.
-            for bb in self_call_locations {
-                let span = body.basic_blocks()[*bb].terminator().source_info.span;
-                db.span_label(span, "recursive call site");
+            for call_span in vis.reachable_recursive_calls {
+                db.span_label(call_span, "recursive call site");
             }
             db.help("a `loop` may express intention better if this is on purpose");
             db.emit();
         });
     }
 }
 
-/// Finds blocks with `Call` terminators that would end up calling back into the same method.
-fn find_blocks_calling_self<'tcx>(
+struct NonRecursive;
+
+struct Search<'mir, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    body: &Body<'tcx>,
+    body: &'mir Body<'tcx>,
     def_id: DefId,
-) -> BitSet<BasicBlock> {
-    let param_env = tcx.param_env(def_id);
+    trait_substs: &'tcx [GenericArg<'tcx>],
 
-    // If this is trait/impl method, extract the trait's substs.
-    let trait_substs_count = match tcx.opt_associated_item(def_id) {
-        Some(AssocItem { container: AssocItemContainer::TraitContainer(trait_def_id), .. }) => {
-            tcx.generics_of(trait_def_id).count()
+    reachable_recursive_calls: Vec<Span>,
+}
+
+impl<'mir, 'tcx> Search<'mir, 'tcx> {
+    /// Returns `true` if `func` refers to the function we are searching in.
+    fn is_recursive_call(&self, func: &Operand<'tcx>) -> bool {
+        let Search { tcx, body, def_id, trait_substs, .. } = *self;
+        let param_env = tcx.param_env(def_id);
+
+        let func_ty = func.ty(body, tcx);
+        if let ty::FnDef(fn_def_id, substs) = func_ty.kind {
+            let (call_fn_id, call_substs) =
+                if let Some(instance) = Instance::resolve(tcx, param_env, fn_def_id, substs) {
+                    (instance.def_id(), instance.substs)
+                } else {
+                    (fn_def_id, substs)
+                };
+
+            // FIXME(#57965): Make this work across function boundaries
+
+            // If this is a trait fn, the substs on the trait have to match, or we might be
+            // calling into an entirely different method (for example, a call from the default
+            // method in the trait to `<A as Trait<B>>::method`, where `A` and/or `B` are
+            // specific types).
+            return call_fn_id == def_id && &call_substs[..trait_substs.len()] == trait_substs;
+        }
+
+        false
+    }
+}
+
+impl<'mir, 'tcx> TriColorVisitor<&'mir Body<'tcx>> for Search<'mir, 'tcx> {
+    type BreakVal = NonRecursive;
+
+    fn node_examined(
+        &mut self,
+        bb: BasicBlock,
+        prior_status: Option<NodeStatus>,
+    ) -> ControlFlow<Self::BreakVal> {
+        // Back-edge in the CFG (loop).
+        if let Some(NodeStatus::Visited) = prior_status {
+            return ControlFlow::Break(NonRecursive);
+        }
+
+        match self.body[bb].terminator().kind {
+            // These terminators return control flow to the caller.
+            TerminatorKind::Abort
+            | TerminatorKind::GeneratorDrop
+            | TerminatorKind::Resume
+            | TerminatorKind::Return
+            | TerminatorKind::Unreachable
+            | TerminatorKind::Yield { .. } => ControlFlow::Break(NonRecursive),
+
+            // These do not.
+            TerminatorKind::Assert { .. }
+            | TerminatorKind::Call { .. }
+            | TerminatorKind::Drop { .. }
+            | TerminatorKind::DropAndReplace { .. }
+            | TerminatorKind::FalseEdges { .. }
+            | TerminatorKind::FalseUnwind { .. }
+            | TerminatorKind::Goto { .. }
+            | TerminatorKind::SwitchInt { .. } => ControlFlow::Continue,
         }
-        _ => 0,
-    };
-    let trait_substs = &InternalSubsts::identity_for_item(tcx, def_id)[..trait_substs_count];
-
-    let mut self_calls = BitSet::new_empty(body.basic_blocks().len());
-
-    for (bb, data) in body.basic_blocks().iter_enumerated() {
-        if let TerminatorKind::Call { func, .. } = &data.terminator().kind {
-            let func_ty = func.ty(body, tcx);
-
-            if let ty::FnDef(fn_def_id, substs) = func_ty.kind {
-                let (call_fn_id, call_substs) =
-                    if let Some(instance) = Instance::resolve(tcx, param_env, fn_def_id, substs) {
-                        (instance.def_id(), instance.substs)
-                    } else {
-                        (fn_def_id, substs)
-                    };
-
-                // FIXME(#57965): Make this work across function boundaries
-
-                // If this is a trait fn, the substs on the trait have to match, or we might be
-                // calling into an entirely different method (for example, a call from the default
-                // method in the trait to `<A as Trait<B>>::method`, where `A` and/or `B` are
-                // specific types).
-                let is_self_call =
-                    call_fn_id == def_id && &call_substs[..trait_substs.len()] == trait_substs;
-
-                if is_self_call {
-                    self_calls.insert(bb);
-                }
+    }
+
+    fn node_settled(&mut self, bb: BasicBlock) -> ControlFlow<Self::BreakVal> {
+        // When we examine a node for the last time, remember it if it is a recursive call.
+        let terminator = self.body[bb].terminator();
+        if let TerminatorKind::Call { func, .. } = &terminator.kind {
+            if self.is_recursive_call(func) {
+                self.reachable_recursive_calls.push(terminator.source_info.span);
             }
         }
+
+        ControlFlow::Continue
     }
 
-    self_calls
+    fn ignore_edge(&mut self, bb: BasicBlock, target: BasicBlock) -> bool {
+        // Don't traverse successors of recursive calls or false CFG edges.
+        match self.body[bb].terminator().kind {
+            TerminatorKind::Call { ref func, .. } => self.is_recursive_call(func),
+
+            TerminatorKind::FalseUnwind { unwind: Some(imaginary_target), .. }
+            | TerminatorKind::FalseEdges { imaginary_target, .. } => imaginary_target == target,
+
+            _ => false,
+        }
+    }
 }