Auto merge of #136000 - matthiaskrgr:rollup-j6ge32r, r=matthiaskrgr

Rollup of 6 pull requests

Successful merges:

 - #135873 (coverage: Prepare for upcoming changes to counter creation)
 - #135926 (Implement `needs-subprocess` directive, and cleanup a bunch of tests to use `needs-{subprocess,threads}`)
 - #135950 (Tidy Python improvements)
 - #135956 (Make `Vec::pop_if` a bit more presentable)
 - #135966 ([AIX] Allow different sized load and store in `tests/assembly/powerpc64-struct-abi.rs`)
 - #135983 (Doc difference between extend and extend_from_slice)

r? `@ghost`
`@rustbot` modify labels: rollup

6 files changed, 128 insertions, 164 deletions

diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs
index 8d397f63cc7..50ebde3292e 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters.rs
@@ -11,7 +11,9 @@ use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId,
 
 use crate::coverage::counters::balanced_flow::BalancedFlowGraph;
 use crate::coverage::counters::iter_nodes::IterNodes;
-use crate::coverage::counters::node_flow::{CounterTerm, MergedNodeFlowGraph, NodeCounters};
+use crate::coverage::counters::node_flow::{
+    CounterTerm, NodeCounters, make_node_counters, node_flow_data_for_balanced_graph,
+};
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};
 
 mod balanced_flow;
@@ -27,12 +29,12 @@ pub(super) fn make_bcb_counters(
 ) -> CoverageCounters {
     // Create the derived graphs that are necessary for subsequent steps.
     let balanced_graph = BalancedFlowGraph::for_graph(graph, |n| !graph[n].is_out_summable);
-    let merged_graph = MergedNodeFlowGraph::for_balanced_graph(&balanced_graph);
+    let node_flow_data = node_flow_data_for_balanced_graph(&balanced_graph);
 
     // Use those graphs to determine which nodes get physical counters, and how
     // to compute the execution counts of other nodes from those counters.
-    let nodes = make_node_counter_priority_list(graph, balanced_graph);
-    let node_counters = merged_graph.make_node_counters(&nodes);
+    let priority_list = make_node_flow_priority_list(graph, balanced_graph);
+    let node_counters = make_node_counters(&node_flow_data, &priority_list);
 
     // Convert the counters into a form suitable for embedding into MIR.
     transcribe_counters(&node_counters, bcb_needs_counter)
@@ -40,7 +42,7 @@ pub(super) fn make_bcb_counters(
 
 /// Arranges the nodes in `balanced_graph` into a list, such that earlier nodes
 /// take priority in being given a counter expression instead of a physical counter.
-fn make_node_counter_priority_list(
+fn make_node_flow_priority_list(
     graph: &CoverageGraph,
     balanced_graph: BalancedFlowGraph<&CoverageGraph>,
 ) -> Vec<BasicCoverageBlock> {
@@ -81,11 +83,11 @@ fn transcribe_counters(
     let mut new = CoverageCounters::with_num_bcbs(bcb_needs_counter.domain_size());
 
     for bcb in bcb_needs_counter.iter() {
-        // Our counter-creation algorithm doesn't guarantee that a counter
-        // expression starts or ends with a positive term, so partition the
+        // Our counter-creation algorithm doesn't guarantee that a node's list
+        // of terms starts or ends with a positive term, so partition the
         // counters into "positive" and "negative" lists for easier handling.
         let (mut pos, mut neg): (Vec<_>, Vec<_>) =
-            old.counter_expr(bcb).iter().partition_map(|&CounterTerm { node, op }| match op {
+            old.counter_terms[bcb].iter().partition_map(|&CounterTerm { node, op }| match op {
                 Op::Add => Either::Left(node),
                 Op::Subtract => Either::Right(node),
             });
diff --git a/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs b/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
index 610498c6c0e..3647c889937 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
@@ -8,18 +8,17 @@
 
 use rustc_data_structures::graph;
 use rustc_index::bit_set::DenseBitSet;
-use rustc_index::{Idx, IndexVec};
+use rustc_index::{Idx, IndexSlice, IndexVec};
 use rustc_middle::mir::coverage::Op;
-use smallvec::SmallVec;
 
 use crate::coverage::counters::iter_nodes::IterNodes;
-use crate::coverage::counters::union_find::{FrozenUnionFind, UnionFind};
+use crate::coverage::counters::union_find::UnionFind;
 
 #[cfg(test)]
 mod tests;
 
-/// View of some underlying graph, in which each node's successors have been
-/// merged into a single "supernode".
+/// Data representing a view of some underlying graph, in which each node's
+/// successors have been merged into a single "supernode".
 ///
 /// The resulting supernodes have no obvious meaning on their own.
 /// However, merging successor nodes means that a node's out-edges can all
@@ -30,10 +29,10 @@ mod tests;
 /// in the merged graph, it becomes possible to analyze the original node flows
 /// using techniques for analyzing edge flows.
 #[derive(Debug)]
-pub(crate) struct MergedNodeFlowGraph<Node: Idx> {
+pub(crate) struct NodeFlowData<Node: Idx> {
     /// Maps each node to the supernode that contains it, indicated by some
     /// arbitrary "root" node that is part of that supernode.
-    supernodes: FrozenUnionFind<Node>,
+    supernodes: IndexVec<Node, Node>,
     /// For each node, stores the single supernode that all of its successors
     /// have been merged into.
     ///
@@ -42,84 +41,71 @@ pub(crate) struct MergedNodeFlowGraph<Node: Idx> {
     succ_supernodes: IndexVec<Node, Node>,
 }
 
-impl<Node: Idx> MergedNodeFlowGraph<Node> {
-    /// Creates a "merged" view of an underlying graph.
-    ///
-    /// The given graph is assumed to have [“balanced flow”](balanced-flow),
-    /// though it does not necessarily have to be a `BalancedFlowGraph`.
-    ///
-    /// [balanced-flow]: `crate::coverage::counters::balanced_flow::BalancedFlowGraph`.
-    pub(crate) fn for_balanced_graph<G>(graph: G) -> Self
-    where
-        G: graph::DirectedGraph<Node = Node> + graph::Successors,
-    {
-        let mut supernodes = UnionFind::<G::Node>::new(graph.num_nodes());
-
-        // For each node, merge its successors into a single supernode, and
-        // arbitrarily choose one of those successors to represent all of them.
-        let successors = graph
-            .iter_nodes()
-            .map(|node| {
-                graph
-                    .successors(node)
-                    .reduce(|a, b| supernodes.unify(a, b))
-                    .expect("each node in a balanced graph must have at least one out-edge")
-            })
-            .collect::<IndexVec<G::Node, G::Node>>();
-
-        // Now that unification is complete, freeze the supernode forest,
-        // and resolve each arbitrarily-chosen successor to its canonical root.
-        // (This avoids having to explicitly resolve them later.)
-        let supernodes = supernodes.freeze();
-        let succ_supernodes = successors.into_iter().map(|succ| supernodes.find(succ)).collect();
-
-        Self { supernodes, succ_supernodes }
-    }
-
-    fn num_nodes(&self) -> usize {
-        self.succ_supernodes.len()
-    }
+/// Creates a "merged" view of an underlying graph.
+///
+/// The given graph is assumed to have [“balanced flow”](balanced-flow),
+/// though it does not necessarily have to be a `BalancedFlowGraph`.
+///
+/// [balanced-flow]: `crate::coverage::counters::balanced_flow::BalancedFlowGraph`.
+pub(crate) fn node_flow_data_for_balanced_graph<G>(graph: G) -> NodeFlowData<G::Node>
+where
+    G: graph::Successors,
+{
+    let mut supernodes = UnionFind::<G::Node>::new(graph.num_nodes());
+
+    // For each node, merge its successors into a single supernode, and
+    // arbitrarily choose one of those successors to represent all of them.
+    let successors = graph
+        .iter_nodes()
+        .map(|node| {
+            graph
+                .successors(node)
+                .reduce(|a, b| supernodes.unify(a, b))
+                .expect("each node in a balanced graph must have at least one out-edge")
+        })
+        .collect::<IndexVec<G::Node, G::Node>>();
+
+    // Now that unification is complete, take a snapshot of the supernode forest,
+    // and resolve each arbitrarily-chosen successor to its canonical root.
+    // (This avoids having to explicitly resolve them later.)
+    let supernodes = supernodes.snapshot();
+    let succ_supernodes = successors.into_iter().map(|succ| supernodes[succ]).collect();
+
+    NodeFlowData { supernodes, succ_supernodes }
+}
 
-    fn is_supernode(&self, node: Node) -> bool {
-        self.supernodes.find(node) == node
+/// Uses the graph information in `node_flow_data`, together with a given
+/// permutation of all nodes in the graph, to create physical counters and
+/// counter expressions for each node in the underlying graph.
+///
+/// The given list must contain exactly one copy of each node in the
+/// underlying balanced-flow graph. The order of nodes is used as a hint to
+/// influence counter allocation:
+/// - Earlier nodes are more likely to receive counter expressions.
+/// - Later nodes are more likely to receive physical counters.
+pub(crate) fn make_node_counters<Node: Idx>(
+    node_flow_data: &NodeFlowData<Node>,
+    priority_list: &[Node],
+) -> NodeCounters<Node> {
+    let mut builder = SpantreeBuilder::new(node_flow_data);
+
+    for &node in priority_list {
+        builder.visit_node(node);
     }
 
-    /// Using the information in this merged graph, together with a given
-    /// permutation of all nodes in the graph, to create physical counters and
-    /// counter expressions for each node in the underlying graph.
-    ///
-    /// The given list must contain exactly one copy of each node in the
-    /// underlying balanced-flow graph. The order of nodes is used as a hint to
-    /// influence counter allocation:
-    /// - Earlier nodes are more likely to receive counter expressions.
-    /// - Later nodes are more likely to receive physical counters.
-    pub(crate) fn make_node_counters(&self, all_nodes_permutation: &[Node]) -> NodeCounters<Node> {
-        let mut builder = SpantreeBuilder::new(self);
-
-        for &node in all_nodes_permutation {
-            builder.visit_node(node);
-        }
-
-        NodeCounters { counter_exprs: builder.finish() }
-    }
+    NodeCounters { counter_terms: builder.finish() }
 }
 
 /// End result of allocating physical counters and counter expressions for the
 /// nodes of a graph.
 #[derive(Debug)]
 pub(crate) struct NodeCounters<Node: Idx> {
-    counter_exprs: IndexVec<Node, CounterExprVec<Node>>,
-}
-
-impl<Node: Idx> NodeCounters<Node> {
     /// For the given node, returns the finished list of terms that represent
     /// its physical counter or counter expression. Always non-empty.
     ///
-    /// If a node was given a physical counter, its "expression" will contain
+    /// If a node was given a physical counter, the term list will contain
     /// that counter as its sole element.
-    pub(crate) fn counter_expr(&self, this: Node) -> &[CounterTerm<Node>] {
-        self.counter_exprs[this].as_slice()
-    }
+    pub(crate) counter_terms: IndexVec<Node, Vec<CounterTerm<Node>>>,
 }
 
 #[derive(Debug)]
@@ -146,12 +132,11 @@ pub(crate) struct CounterTerm<Node> {
     pub(crate) node: Node,
 }
 
-/// Stores the list of counter terms that make up a node's counter expression.
-type CounterExprVec<Node> = SmallVec<[CounterTerm<Node>; 2]>;
-
 #[derive(Debug)]
 struct SpantreeBuilder<'a, Node: Idx> {
-    graph: &'a MergedNodeFlowGraph<Node>,
+    supernodes: &'a IndexSlice<Node, Node>,
+    succ_supernodes: &'a IndexSlice<Node, Node>,
+
     is_unvisited: DenseBitSet<Node>,
     /// Links supernodes to each other, gradually forming a spanning tree of
     /// the merged-flow graph.
@@ -163,26 +148,32 @@ struct SpantreeBuilder<'a, Node: Idx> {
     yank_buffer: Vec<Node>,
     /// An in-progress counter expression for each node. Each expression is
     /// initially empty, and will be filled in as relevant nodes are visited.
-    counter_exprs: IndexVec<Node, CounterExprVec<Node>>,
+    counter_terms: IndexVec<Node, Vec<CounterTerm<Node>>>,
 }
 
 impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
-    fn new(graph: &'a MergedNodeFlowGraph<Node>) -> Self {
-        let num_nodes = graph.num_nodes();
+    fn new(node_flow_data: &'a NodeFlowData<Node>) -> Self {
+        let NodeFlowData { supernodes, succ_supernodes } = node_flow_data;
+        let num_nodes = supernodes.len();
         Self {
-            graph,
+            supernodes,
+            succ_supernodes,
             is_unvisited: DenseBitSet::new_filled(num_nodes),
             span_edges: IndexVec::from_fn_n(|_| None, num_nodes),
             yank_buffer: vec![],
-            counter_exprs: IndexVec::from_fn_n(|_| SmallVec::new(), num_nodes),
+            counter_terms: IndexVec::from_fn_n(|_| vec![], num_nodes),
         }
     }
 
+    fn is_supernode(&self, node: Node) -> bool {
+        self.supernodes[node] == node
+    }
+
     /// Given a supernode, finds the supernode that is the "root" of its
     /// spantree component. Two nodes that have the same spantree root are
     /// connected in the spantree.
     fn spantree_root(&self, this: Node) -> Node {
-        debug_assert!(self.graph.is_supernode(this));
+        debug_assert!(self.is_supernode(this));
 
         match self.span_edges[this] {
             None => this,
@@ -193,7 +184,7 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
     /// Rotates edges in the spantree so that `this` is the root of its
     /// spantree component.
     fn yank_to_spantree_root(&mut self, this: Node) {
-        debug_assert!(self.graph.is_supernode(this));
+        debug_assert!(self.is_supernode(this));
 
         // The rotation is done iteratively, by first traversing from `this` to
         // its root and storing the path in a buffer, and then traversing the
@@ -235,12 +226,12 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
 
         // Get the supernode containing `this`, and make it the root of its
         // component of the spantree.
-        let this_supernode = self.graph.supernodes.find(this);
+        let this_supernode = self.supernodes[this];
         self.yank_to_spantree_root(this_supernode);
 
         // Get the supernode containing all of this's successors.
-        let succ_supernode = self.graph.succ_supernodes[this];
-        debug_assert!(self.graph.is_supernode(succ_supernode));
+        let succ_supernode = self.succ_supernodes[this];
+        debug_assert!(self.is_supernode(succ_supernode));
 
         // If two supernodes are already connected in the spantree, they will
         // have the same spantree root. (Each supernode is connected to itself.)
@@ -268,8 +259,8 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
             // `this_supernode`.
 
             // Instead of setting `this.measure = true` as in the original paper,
-            // we just add the node's ID to its own "expression".
-            self.counter_exprs[this].push(CounterTerm { node: this, op: Op::Add });
+            // we just add the node's ID to its own list of terms.
+            self.counter_terms[this].push(CounterTerm { node: this, op: Op::Add });
 
             // Walk the spantree from `this.successor` back to `this`. For each
             // spantree edge along the way, add this node's physical counter to
@@ -279,7 +270,7 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
                 let &SpantreeEdge { is_reversed, claiming_node, span_parent } =
                     self.span_edges[curr].as_ref().unwrap();
                 let op = if is_reversed { Op::Subtract } else { Op::Add };
-                self.counter_exprs[claiming_node].push(CounterTerm { node: this, op });
+                self.counter_terms[claiming_node].push(CounterTerm { node: this, op });
 
                 curr = span_parent;
             }
@@ -288,19 +279,20 @@ impl<'a, Node: Idx> SpantreeBuilder<'a, Node> {
 
     /// Asserts that all nodes have been visited, and returns the computed
     /// counter expressions (made up of physical counters) for each node.
-    fn finish(self) -> IndexVec<Node, CounterExprVec<Node>> {
-        let Self { graph, is_unvisited, span_edges, yank_buffer: _, counter_exprs } = self;
+    fn finish(self) -> IndexVec<Node, Vec<CounterTerm<Node>>> {
+        let Self { ref span_edges, ref is_unvisited, ref counter_terms, .. } = self;
         assert!(is_unvisited.is_empty(), "some nodes were never visited: {is_unvisited:?}");
         debug_assert!(
             span_edges
                 .iter_enumerated()
-                .all(|(node, span_edge)| { span_edge.is_some() <= graph.is_supernode(node) }),
+                .all(|(node, span_edge)| { span_edge.is_some() <= self.is_supernode(node) }),
             "only supernodes can have a span edge",
         );
         debug_assert!(
-            counter_exprs.iter().all(|expr| !expr.is_empty()),
-            "after visiting all nodes, every node should have a non-empty expression",
+            counter_terms.iter().all(|terms| !terms.is_empty()),
+            "after visiting all nodes, every node should have at least one term",
         );
-        counter_exprs
+
+        self.counter_terms
     }
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/counters/node_flow/tests.rs b/compiler/rustc_mir_transform/src/coverage/counters/node_flow/tests.rs
index 9e7f754523d..b509a14514b 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters/node_flow/tests.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters/node_flow/tests.rs
@@ -4,10 +4,12 @@ use rustc_data_structures::graph::vec_graph::VecGraph;
 use rustc_index::Idx;
 use rustc_middle::mir::coverage::Op;
 
-use super::{CounterTerm, MergedNodeFlowGraph, NodeCounters};
+use crate::coverage::counters::node_flow::{
+    CounterTerm, NodeCounters, NodeFlowData, make_node_counters, node_flow_data_for_balanced_graph,
+};
 
-fn merged_node_flow_graph<G: graph::Successors>(graph: G) -> MergedNodeFlowGraph<G::Node> {
-    MergedNodeFlowGraph::for_balanced_graph(graph)
+fn node_flow_data<G: graph::Successors>(graph: G) -> NodeFlowData<G::Node> {
+    node_flow_data_for_balanced_graph(graph)
 }
 
 fn make_graph<Node: Idx + Ord>(num_nodes: usize, edge_pairs: Vec<(Node, Node)>) -> VecGraph<Node> {
@@ -30,8 +32,8 @@ fn example_driver() {
         (4, 0),
     ]);
 
-    let merged = merged_node_flow_graph(&graph);
-    let counters = merged.make_node_counters(&[3, 1, 2, 0, 4]);
+    let node_flow_data = node_flow_data(&graph);
+    let counters = make_node_counters(&node_flow_data, &[3, 1, 2, 0, 4]);
 
     assert_eq!(format_counter_expressions(&counters), &[
         // (comment to force vertical formatting for clarity)
@@ -53,12 +55,12 @@ fn format_counter_expressions<Node: Idx>(counters: &NodeCounters<Node>) -> Vec<S
     };
 
     counters
-        .counter_exprs
+        .counter_terms
         .indices()
         .map(|node| {
-            let mut expr = counters.counter_expr(node).iter().collect::<Vec<_>>();
-            expr.sort_by_key(|item| item.node.index());
-            format!("[{node:?}]: {}", expr.into_iter().map(format_item).join(" "))
+            let mut terms = counters.counter_terms[node].iter().collect::<Vec<_>>();
+            terms.sort_by_key(|item| item.node.index());
+            format!("[{node:?}]: {}", terms.into_iter().map(format_item).join(" "))
         })
         .collect()
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/counters/union_find.rs b/compiler/rustc_mir_transform/src/coverage/counters/union_find.rs
index 2da4f5f5fce..a826a953fa6 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters/union_find.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters/union_find.rs
@@ -88,29 +88,9 @@ impl<Key: Idx> UnionFind<Key> {
         a
     }
 
-    /// Creates a snapshot of this disjoint-set forest that can no longer be
-    /// mutated, but can be queried without mutation.
-    pub(crate) fn freeze(&mut self) -> FrozenUnionFind<Key> {
-        // Just resolve each key to its actual root.
-        let roots = self.table.indices().map(|key| self.find(key)).collect();
-        FrozenUnionFind { roots }
-    }
-}
-
-/// Snapshot of a disjoint-set forest that can no longer be mutated, but can be
-/// queried in O(1) time without mutation.
-///
-/// This is really just a wrapper around a direct mapping from keys to roots,
-/// but with a [`Self::find`] method that resembles [`UnionFind::find`].
-#[derive(Debug)]
-pub(crate) struct FrozenUnionFind<Key: Idx> {
-    roots: IndexVec<Key, Key>,
-}
-
-impl<Key: Idx> FrozenUnionFind<Key> {
-    /// Returns the "root" key of the disjoint-set containing the given key.
-    /// If two keys have the same root, they belong to the same set.
-    pub(crate) fn find(&self, key: Key) -> Key {
-        self.roots[key]
+    /// Takes a "snapshot" of the current state of this disjoint-set forest, in
+    /// the form of a vector that directly maps each key to its current root.
+    pub(crate) fn snapshot(&mut self) -> IndexVec<Key, Key> {
+        self.table.indices().map(|key| self.find(key)).collect()
     }
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 19568735df7..b8aa76a7dbe 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -180,7 +180,12 @@ fn create_mappings(
     ));
 
     for (decision, branches) in mcdc_mappings {
-        let num_conditions = branches.len() as u16;
+        // FIXME(#134497): Previously it was possible for some of these branch
+        // conversions to fail, in which case the remaining branches in the
+        // decision would be degraded to plain `MappingKind::Branch`.
+        // The changes in #134497 made that failure impossible, because the
+        // fallible step was deferred to codegen. But the corresponding code
+        // in codegen wasn't updated to detect the need for a degrade step.
         let conditions = branches
             .into_iter()
             .map(
@@ -206,24 +211,13 @@ fn create_mappings(
             )
             .collect::<Vec<_>>();
 
-        if conditions.len() == num_conditions as usize {
-            // LLVM requires end index for counter mapping regions.
-            let kind = MappingKind::MCDCDecision(DecisionInfo {
-                bitmap_idx: (decision.bitmap_idx + decision.num_test_vectors) as u32,
-                num_conditions,
-            });
-            let span = decision.span;
-            mappings.extend(std::iter::once(Mapping { kind, span }).chain(conditions.into_iter()));
-        } else {
-            mappings.extend(conditions.into_iter().map(|mapping| {
-                let MappingKind::MCDCBranch { true_term, false_term, mcdc_params: _ } =
-                    mapping.kind
-                else {
-                    unreachable!("all mappings here are MCDCBranch as shown above");
-                };
-                Mapping { kind: MappingKind::Branch { true_term, false_term }, span: mapping.span }
-            }))
-        }
+        // LLVM requires end index for counter mapping regions.
+        let kind = MappingKind::MCDCDecision(DecisionInfo {
+            bitmap_idx: (decision.bitmap_idx + decision.num_test_vectors) as u32,
+            num_conditions: u16::try_from(conditions.len()).unwrap(),
+        });
+        let span = decision.span;
+        mappings.extend(std::iter::once(Mapping { kind, span }).chain(conditions.into_iter()));
     }
 
     mappings
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index 3e7cf8541c2..5e7b46182dc 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -87,15 +87,9 @@ fn coverage_attr_on(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
 fn coverage_ids_info<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance_def: ty::InstanceKind<'tcx>,
-) -> CoverageIdsInfo {
+) -> Option<CoverageIdsInfo> {
     let mir_body = tcx.instance_mir(instance_def);
-
-    let Some(fn_cov_info) = mir_body.function_coverage_info.as_deref() else {
-        return CoverageIdsInfo {
-            counters_seen: DenseBitSet::new_empty(0),
-            zero_expressions: DenseBitSet::new_empty(0),
-        };
-    };
+    let fn_cov_info = mir_body.function_coverage_info.as_deref()?;
 
     let mut counters_seen = DenseBitSet::new_empty(fn_cov_info.num_counters);
     let mut expressions_seen = DenseBitSet::new_filled(fn_cov_info.expressions.len());
@@ -129,7 +123,7 @@ fn coverage_ids_info<'tcx>(
     let zero_expressions =
         identify_zero_expressions(fn_cov_info, &counters_seen, &expressions_seen);
 
-    CoverageIdsInfo { counters_seen, zero_expressions }
+    Some(CoverageIdsInfo { counters_seen, zero_expressions })
 }
 
 fn all_coverage_in_mir_body<'a, 'tcx>(