coverage: Replace the old span refiner with a single function

As more and more of the span refiner's functionality has been pulled out into
separate early passes, it has finally reached the point where we can remove the
rest of the old `SpansRefiner` code, and replace it with a single
modestly-sized function.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -1,6 +1,5 @@
-use rustc_middle::bug;
 use rustc_middle::mir;
-use rustc_span::{BytePos, Span};
+use rustc_span::Span;
 
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};
 use crate::coverage::mappings;
@@ -23,66 +22,14 @@ pub(super) fn extract_refined_covspans(
     let sorted_span_buckets =
         from_mir::mir_to_initial_sorted_coverage_spans(mir_body, hir_info, basic_coverage_blocks);
     for bucket in sorted_span_buckets {
-        let refined_spans = SpansRefiner::refine_sorted_spans(bucket);
+        let refined_spans = refine_sorted_spans(bucket);
         code_mappings.extend(refined_spans.into_iter().map(|RefinedCovspan { span, bcb }| {
             // Each span produced by the refiner represents an ordinary code region.
             mappings::CodeMapping { span, bcb }
         }));
     }
 }
 
-#[derive(Debug)]
-struct CurrCovspan {
-    span: Span,
-    bcb: BasicCoverageBlock,
-}
-
-impl CurrCovspan {
-    fn new(span: Span, bcb: BasicCoverageBlock) -> Self {
-        Self { span, bcb }
-    }
-
-    fn into_prev(self) -> PrevCovspan {
-        let Self { span, bcb } = self;
-        PrevCovspan { span, bcb, merged_spans: vec![span] }
-    }
-}
-
-#[derive(Debug)]
-struct PrevCovspan {
-    span: Span,
-    bcb: BasicCoverageBlock,
-    /// List of all the original spans from MIR that have been merged into this
-    /// span. Mainly used to precisely skip over gaps when truncating a span.
-    merged_spans: Vec<Span>,
-}
-
-impl PrevCovspan {
-    fn is_mergeable(&self, other: &CurrCovspan) -> bool {
-        self.bcb == other.bcb
-    }
-
-    fn merge_from(&mut self, other: &CurrCovspan) {
-        debug_assert!(self.is_mergeable(other));
-        self.span = self.span.to(other.span);
-        self.merged_spans.push(other.span);
-    }
-
-    fn cutoff_statements_at(mut self, cutoff_pos: BytePos) -> Option<RefinedCovspan> {
-        self.merged_spans.retain(|span| span.hi() <= cutoff_pos);
-        if let Some(max_hi) = self.merged_spans.iter().map(|span| span.hi()).max() {
-            self.span = self.span.with_hi(max_hi);
-        }
-
-        if self.merged_spans.is_empty() { None } else { Some(self.into_refined()) }
-    }
-
-    fn into_refined(self) -> RefinedCovspan {
-        let Self { span, bcb, merged_spans: _ } = self;
-        RefinedCovspan { span, bcb }
-    }
-}
-
 #[derive(Debug)]
 struct RefinedCovspan {
     span: Span,
@@ -100,164 +47,50 @@ impl RefinedCovspan {
     }
 }
 
-/// Converts the initial set of coverage spans (one per MIR `Statement` or `Terminator`) into a
-/// minimal set of coverage spans, using the BCB CFG to determine where it is safe and useful to:
-///
-///  * Remove duplicate source code coverage regions
-///  * Merge spans that represent continuous (both in source code and control flow), non-branching
-///    execution
-struct SpansRefiner {
-    /// The initial set of coverage spans, sorted by `Span` (`lo` and `hi`) and by relative
-    /// dominance between the `BasicCoverageBlock`s of equal `Span`s.
-    sorted_spans_iter: std::vec::IntoIter<SpanFromMir>,
-
-    /// The current coverage span to compare to its `prev`, to possibly merge, discard,
-    /// or cause `prev` to be modified or discarded.
-    /// If `curr` is not discarded or merged, it becomes `prev` for the next iteration.
-    some_curr: Option<CurrCovspan>,
-
-    /// The coverage span from a prior iteration; typically assigned from that iteration's `curr`.
-    /// If that `curr` was discarded, `prev` retains its value from the previous iteration.
-    some_prev: Option<PrevCovspan>,
-
-    /// The final coverage spans to add to the coverage map. A `Counter` or `Expression`
-    /// will also be injected into the MIR for each BCB that has associated spans.
-    refined_spans: Vec<RefinedCovspan>,
-}
-
-impl SpansRefiner {
-    /// Takes the initial list of (sorted) spans extracted from MIR, and "refines"
-    /// them by merging compatible adjacent spans, removing redundant spans,
-    /// and carving holes in spans when they overlap in unwanted ways.
-    fn refine_sorted_spans(sorted_spans: Vec<SpanFromMir>) -> Vec<RefinedCovspan> {
-        let sorted_spans_len = sorted_spans.len();
-        let this = Self {
-            sorted_spans_iter: sorted_spans.into_iter(),
-            some_curr: None,
-            some_prev: None,
-            refined_spans: Vec::with_capacity(sorted_spans_len),
-        };
-
-        this.to_refined_spans()
-    }
-
-    /// Iterate through the sorted coverage spans, and return the refined list of merged and
-    /// de-duplicated spans.
-    fn to_refined_spans(mut self) -> Vec<RefinedCovspan> {
-        while self.next_coverage_span() {
-            // For the first span we don't have `prev` set, so most of the
-            // span-processing steps don't make sense yet.
-            if self.some_prev.is_none() {
-                debug!("  initial span");
-                continue;
-            }
-
-            // The remaining cases assume that `prev` and `curr` are set.
-            let prev = self.prev();
-            let curr = self.curr();
-
-            if prev.is_mergeable(curr) {
-                debug!(?prev, "curr will be merged into prev");
-                let curr = self.take_curr();
-                self.prev_mut().merge_from(&curr);
-            } else if prev.span.hi() <= curr.span.lo() {
-                debug!(
-                    "  different bcbs and disjoint spans, so keep curr for next iter, and add prev={prev:?}",
-                );
-                let prev = self.take_prev().into_refined();
-                self.refined_spans.push(prev);
-            } else {
-                self.cutoff_prev_at_overlapping_curr();
-            }
-        }
-
-        // There is usually a final span remaining in `prev` after the loop ends,
-        // so add it to the output as well.
-        if let Some(prev) = self.some_prev.take() {
-            debug!("    AT END, adding last prev={prev:?}");
-            self.refined_spans.push(prev.into_refined());
-        }
-
-        // Do one last merge pass, to simplify the output.
-        self.refined_spans.dedup_by(|b, a| {
-            if a.is_mergeable(b) {
-                debug!(?a, ?b, "merging list-adjacent refined spans");
-                a.merge_from(b);
-                true
-            } else {
+/// Takes one of the buckets of (sorted) spans extracted from MIR, and "refines"
+/// those spans by removing spans that overlap in unwanted ways, and by merging
+/// compatible adjacent spans.
+#[instrument(level = "debug")]
+fn refine_sorted_spans(sorted_spans: Vec<SpanFromMir>) -> Vec<RefinedCovspan> {
+    // Holds spans that have been read from the input vector, but haven't yet
+    // been committed to the output vector.
+    let mut pending = vec![];
+    let mut refined = vec![];
+
+    for covspan in sorted_spans {
+        pending.retain(|prev: &SpanFromMir| {
+            if prev.span.hi() <= covspan.span.lo() {
+                // There's no overlap between the previous/current covspans,
+                // so move the previous one into the refined list.
+                refined.push(RefinedCovspan { span: prev.span, bcb: prev.bcb });
                 false
+            } else {
+                // Otherwise, retain the previous covspan only if it has the
+                // same BCB. This tends to discard long outer spans that enclose
+                // smaller inner spans with different control flow.
+                prev.bcb == covspan.bcb
             }
         });
-
-        self.refined_spans
-    }
-
-    #[track_caller]
-    fn curr(&self) -> &CurrCovspan {
-        self.some_curr.as_ref().unwrap_or_else(|| bug!("some_curr is None (curr)"))
-    }
-
-    /// If called, then the next call to `next_coverage_span()` will *not* update `prev` with the
-    /// `curr` coverage span.
-    #[track_caller]
-    fn take_curr(&mut self) -> CurrCovspan {
-        self.some_curr.take().unwrap_or_else(|| bug!("some_curr is None (take_curr)"))
-    }
-
-    #[track_caller]
-    fn prev(&self) -> &PrevCovspan {
-        self.some_prev.as_ref().unwrap_or_else(|| bug!("some_prev is None (prev)"))
-    }
-
-    #[track_caller]
-    fn prev_mut(&mut self) -> &mut PrevCovspan {
-        self.some_prev.as_mut().unwrap_or_else(|| bug!("some_prev is None (prev_mut)"))
+        pending.push(covspan);
     }
 
-    #[track_caller]
-    fn take_prev(&mut self) -> PrevCovspan {
-        self.some_prev.take().unwrap_or_else(|| bug!("some_prev is None (take_prev)"))
+    // Drain the rest of the pending list into the refined list.
+    for prev in pending {
+        refined.push(RefinedCovspan { span: prev.span, bcb: prev.bcb });
     }
 
-    /// Advance `prev` to `curr` (if any), and `curr` to the next coverage span in sorted order.
-    fn next_coverage_span(&mut self) -> bool {
-        if let Some(curr) = self.some_curr.take() {
-            self.some_prev = Some(curr.into_prev());
-        }
-        if let Some(SpanFromMir { span, bcb, .. }) = self.sorted_spans_iter.next() {
-            // This code only sees sorted spans after hole-carving, so there should
-            // be no way for `curr` to start before `prev`.
-            if let Some(prev) = &self.some_prev {
-                debug_assert!(prev.span.lo() <= span.lo());
-            }
-            self.some_curr = Some(CurrCovspan::new(span, bcb));
-            debug!(?self.some_prev, ?self.some_curr, "next_coverage_span");
+    // Do one last merge pass, to simplify the output.
+    debug!(?refined, "before merge");
+    refined.dedup_by(|b, a| {
+        if a.is_mergeable(b) {
+            debug!(?a, ?b, "merging list-adjacent refined spans");
+            a.merge_from(b);
             true
         } else {
             false
         }
-    }
+    });
+    debug!(?refined, "after merge");
 
-    /// `curr` overlaps `prev`. If `prev`s span extends left of `curr`s span, keep _only_
-    /// statements that end before `curr.lo()` (if any), and add the portion of the
-    /// combined span for those statements. Any other statements have overlapping spans
-    /// that can be ignored because `curr` and/or other upcoming statements/spans inside
-    /// the overlap area will produce their own counters. This disambiguation process
-    /// avoids injecting multiple counters for overlapping spans, and the potential for
-    /// double-counting.
-    fn cutoff_prev_at_overlapping_curr(&mut self) {
-        debug!(
-            "  different bcbs, overlapping spans, so ignore/drop pending and only add prev \
-            if it has statements that end before curr; prev={:?}",
-            self.prev()
-        );
-
-        let curr_span = self.curr().span;
-        if let Some(prev) = self.take_prev().cutoff_statements_at(curr_span.lo()) {
-            debug!("after cutoff, adding {prev:?}");
-            self.refined_spans.push(prev);
-        } else {
-            debug!("prev was eliminated by cutoff");
-        }
-    }
+    refined
 }