Rollup merge of #136053 - Zalathar:defer-counters, r=saethlin

coverage: Defer part of counter-creation until codegen

Follow-up to #135481 and #135873.

One of the pleasant properties of the new counter-assignment algorithm is that we can stop partway through the process, store the intermediate state in MIR, and then resume the rest of the algorithm during codegen. This lets it take into account which parts of the control-flow graph were eliminated by MIR opts, resulting in fewer physical counters and simpler counter expressions.

Those improvements end up completely obsoleting much larger chunks of code that were previously responsible for cleaning up the coverage metadata after MIR opts, while also doing a more thorough cleanup job.

(That change also unlocks some further simplifications that I've kept out of this PR to limit its scope.)

1 files changed, 1 insertions, 24 deletions

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 9d80b3af42d..91ed54b8b59 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters/node_flow.rs
@@ -9,6 +9,7 @@
 use rustc_data_structures::graph;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{Idx, IndexSlice, IndexVec};
+pub(crate) use rustc_middle::mir::coverage::NodeFlowData;
 use rustc_middle::mir::coverage::Op;
 
 use crate::coverage::counters::union_find::UnionFind;
@@ -16,30 +17,6 @@ use crate::coverage::counters::union_find::UnionFind;
 #[cfg(test)]
 mod tests;
 
-/// 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
-/// be combined into a single out-edge, whose flow is the same as the flow
-/// (execution count) of its corresponding node in the original graph.
-///
-/// With all node flows now in the original graph now represented as edge flows
-/// in the merged graph, it becomes possible to analyze the original node flows
-/// using techniques for analyzing edge flows.
-#[derive(Debug)]
-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: IndexVec<Node, Node>,
-    /// For each node, stores the single supernode that all of its successors
-    /// have been merged into.
-    ///
-    /// (Note that each node in a supernode can potentially have a _different_
-    /// successor supernode from its peers.)
-    succ_supernodes: IndexVec<Node, Node>,
-}
-
 /// Creates a "merged" view of an underlying graph.
 ///
 /// The given graph is assumed to have [“balanced flow”](balanced-flow),