Rollup merge of rust-lang#139102 - Zalathar:no-split, r=oli-obk

coverage: Avoid splitting spans during span extraction/refinement

This PR removes or simplifies some of the steps involved in extracting coverage-relevant spans from MIR, and preparing them for use in coverage instrumentation metadata.

A common theme is that we now try harder to avoid modifying or combining spans in non-trivial ways, because those modifications present the most risk for weird behaviour or ICEs.

The main changes are:
- When extracting spans from MIR call terminators, try to restrict them to just the function name.
- Instead of splitting spans around “holes”, just discard any span that overlaps with a hole.
- Instead of splitting macro-invocation spans into two parts, truncate them to just the macro name and subsequent `!`.

---

This results in a lot of tiny changes to the spans that end up in coverage metadata, and a few changes to coverage reports. Judging by test snapshots, these changes appear to be quite minor in practice.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mappings.rs

@@ -96,7 +96,7 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
         }
     } else {
         // Extract coverage spans from MIR statements/terminators as normal.
-        extract_refined_covspans(mir_body, hir_info, graph, &mut code_mappings);
+        extract_refined_covspans(tcx, mir_body, hir_info, graph, &mut code_mappings);
     }
 
     branch_pairs.extend(extract_branch_pairs(mir_body, hir_info, graph));

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -1,7 +1,9 @@
 use std::collections::VecDeque;
+use std::iter;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir;
+use rustc_middle::ty::TyCtxt;
 use rustc_span::{DesugaringKind, ExpnKind, MacroKind, Span};
 use tracing::{debug, debug_span, instrument};
 
@@ -11,8 +13,9 @@ use crate::coverage::{ExtractedHirInfo, mappings, unexpand};
 
 mod from_mir;
 
-pub(super) fn extract_refined_covspans(
-    mir_body: &mir::Body<'_>,
+pub(super) fn extract_refined_covspans<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mir_body: &mir::Body<'tcx>,
     hir_info: &ExtractedHirInfo,
     graph: &CoverageGraph,
     code_mappings: &mut impl Extend<mappings::CodeMapping>,
@@ -50,7 +53,7 @@ pub(super) fn extract_refined_covspans(
     // First, perform the passes that need macro information.
     covspans.sort_by(|a, b| graph.cmp_in_dominator_order(a.bcb, b.bcb));
     remove_unwanted_expansion_spans(&mut covspans);
-    split_visible_macro_spans(&mut covspans);
+    shrink_visible_macro_spans(tcx, &mut covspans);
 
     // We no longer need the extra information in `SpanFromMir`, so convert to `Covspan`.
     let mut covspans = covspans.into_iter().map(SpanFromMir::into_covspan).collect::<Vec<_>>();
@@ -83,9 +86,7 @@ pub(super) fn extract_refined_covspans(
     // Split the covspans into separate buckets that don't overlap any holes.
     let buckets = divide_spans_into_buckets(covspans, &holes);
 
-    for mut covspans in buckets {
-        // Make sure each individual bucket is internally sorted.
-        covspans.sort_by(compare_covspans);
+    for covspans in buckets {
         let _span = debug_span!("processing bucket", ?covspans).entered();
 
         let mut covspans = remove_unwanted_overlapping_spans(covspans);
@@ -129,82 +130,50 @@ fn remove_unwanted_expansion_spans(covspans: &mut Vec<SpanFromMir>) {
 }
 
 /// When a span corresponds to a macro invocation that is visible from the
-/// function body, split it into two parts. The first part covers just the
-/// macro name plus `!`, and the second part covers the rest of the macro
-/// invocation. This seems to give better results for code that uses macros.
-fn split_visible_macro_spans(covspans: &mut Vec<SpanFromMir>) {
-    let mut extra_spans = vec![];
-
-    covspans.retain(|covspan| {
-        let Some(ExpnKind::Macro(MacroKind::Bang, visible_macro)) = covspan.expn_kind else {
-            return true;
-        };
-
-        let split_len = visible_macro.as_str().len() as u32 + 1;
-        let (before, after) = covspan.span.split_at(split_len);
-        if !covspan.span.contains(before) || !covspan.span.contains(after) {
-            // Something is unexpectedly wrong with the split point.
-            // The debug assertion in `split_at` will have already caught this,
-            // but in release builds it's safer to do nothing and maybe get a
-            // bug report for unexpected coverage, rather than risk an ICE.
-            return true;
+/// function body, truncate it to just the macro name plus `!`.
+/// This seems to give better results for code that uses macros.
+fn shrink_visible_macro_spans(tcx: TyCtxt<'_>, covspans: &mut Vec<SpanFromMir>) {
+    let source_map = tcx.sess.source_map();
+
+    for covspan in covspans {
+        if matches!(covspan.expn_kind, Some(ExpnKind::Macro(MacroKind::Bang, _))) {
+            covspan.span = source_map.span_through_char(covspan.span, '!');
         }
-
-        extra_spans.push(SpanFromMir::new(before, covspan.expn_kind.clone(), covspan.bcb));
-        extra_spans.push(SpanFromMir::new(after, covspan.expn_kind.clone(), covspan.bcb));
-        false // Discard the original covspan that we just split.
-    });
-
-    // The newly-split spans are added at the end, so any previous sorting
-    // is not preserved.
-    covspans.extend(extra_spans);
+    }
 }
 
 /// Uses the holes to divide the given covspans into buckets, such that:
-/// - No span in any hole overlaps a bucket (truncating the spans if necessary).
+/// - No span in any hole overlaps a bucket (discarding spans if necessary).
 /// - The spans in each bucket are strictly after all spans in previous buckets,
 ///   and strictly before all spans in subsequent buckets.
 ///
-/// The resulting buckets are sorted relative to each other, but might not be
-/// internally sorted.
+/// The lists of covspans and holes must be sorted.
+/// The resulting buckets are sorted relative to each other, and each bucket's
+/// contents are sorted.
 #[instrument(level = "debug")]
 fn divide_spans_into_buckets(input_covspans: Vec<Covspan>, holes: &[Hole]) -> Vec<Vec<Covspan>> {
     debug_assert!(input_covspans.is_sorted_by(|a, b| compare_spans(a.span, b.span).is_le()));
     debug_assert!(holes.is_sorted_by(|a, b| compare_spans(a.span, b.span).is_le()));
 
-    // Now we're ready to start carving holes out of the initial coverage spans,
-    // and grouping them in buckets separated by the holes.
+    // Now we're ready to start grouping spans into buckets separated by holes.
 
     let mut input_covspans = VecDeque::from(input_covspans);
-    let mut fragments = vec![];
 
     // For each hole:
     // - Identify the spans that are entirely or partly before the hole.
-    // - Put those spans in a corresponding bucket, truncated to the start of the hole.
-    // - If one of those spans also extends after the hole, put the rest of it
-    //   in a "fragments" vector that is processed by the next hole.
+    // - Discard any that overlap with the hole.
+    // - Add the remaining identified spans to the corresponding bucket.
     let mut buckets = (0..holes.len()).map(|_| vec![]).collect::<Vec<_>>();
     for (hole, bucket) in holes.iter().zip(&mut buckets) {
-        let fragments_from_prev = std::mem::take(&mut fragments);
-
-        // Only inspect spans that precede or overlap this hole,
-        // leaving the rest to be inspected by later holes.
-        // (This relies on the spans and holes both being sorted.)
-        let relevant_input_covspans =
-            drain_front_while(&mut input_covspans, |c| c.span.lo() < hole.span.hi());
-
-        for covspan in fragments_from_prev.into_iter().chain(relevant_input_covspans) {
-            let (before, after) = covspan.split_around_hole_span(hole.span);
-            bucket.extend(before);
-            fragments.extend(after);
-        }
+        bucket.extend(
+            drain_front_while(&mut input_covspans, |c| c.span.lo() < hole.span.hi())
+                .filter(|c| !c.span.overlaps(hole.span)),
+        );
     }
 
-    // After finding the spans before each hole, any remaining fragments/spans
-    // form their own final bucket, after the final hole.
+    // Any remaining spans form their own final bucket, after the final hole.
     // (If there were no holes, this will just be all of the initial spans.)
-    fragments.extend(input_covspans);
-    buckets.push(fragments);
+    buckets.push(Vec::from(input_covspans));
 
     buckets
 }
@@ -215,7 +184,7 @@ fn drain_front_while<'a, T>(
     queue: &'a mut VecDeque<T>,
     mut pred_fn: impl FnMut(&T) -> bool,
 ) -> impl Iterator<Item = T> {
-    std::iter::from_fn(move || if pred_fn(queue.front()?) { queue.pop_front() } else { None })
+    iter::from_fn(move || queue.pop_front_if(|x| pred_fn(x)))
 }
 
 /// Takes one of the buckets of (sorted) spans extracted from MIR, and "refines"
@@ -258,22 +227,6 @@ struct Covspan {
 }
 
 impl Covspan {
-    /// Splits this covspan into 0-2 parts:
-    /// - The part that is strictly before the hole span, if any.
-    /// - The part that is strictly after the hole span, if any.
-    fn split_around_hole_span(&self, hole_span: Span) -> (Option<Self>, Option<Self>) {
-        let before = try {
-            let span = self.span.trim_end(hole_span)?;
-            Self { span, ..*self }
-        };
-        let after = try {
-            let span = self.span.trim_start(hole_span)?;
-            Self { span, ..*self }
-        };
-
-        (before, after)
-    }
-
     /// If `self` and `other` can be merged (i.e. they have the same BCB),
     /// mutates `self.span` to also include `other.span` and returns true.
     ///

compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs

@@ -120,22 +120,20 @@ fn filtered_terminator_span(terminator: &Terminator<'_>) -> Option<Span> {
         // an `if condition { block }` has a span that includes the executed block, if true,
         // but for coverage, the code region executed, up to *and* through the SwitchInt,
         // actually stops before the if's block.)
-        TerminatorKind::Unreachable // Unreachable blocks are not connected to the MIR CFG
+        TerminatorKind::Unreachable
         | TerminatorKind::Assert { .. }
         | TerminatorKind::Drop { .. }
         | TerminatorKind::SwitchInt { .. }
-        // For `FalseEdge`, only the `real` branch is taken, so it is similar to a `Goto`.
         | TerminatorKind::FalseEdge { .. }
         | TerminatorKind::Goto { .. } => None,
 
         // Call `func` operand can have a more specific span when part of a chain of calls
-        TerminatorKind::Call { ref func, .. }
-        | TerminatorKind::TailCall { ref func, .. } => {
+        TerminatorKind::Call { ref func, .. } | TerminatorKind::TailCall { ref func, .. } => {
             let mut span = terminator.source_info.span;
-            if let mir::Operand::Constant(box constant) = func {
-                if constant.span.lo() > span.lo() {
-                    span = span.with_lo(constant.span.lo());
-                }
+            if let mir::Operand::Constant(constant) = func
+                && span.contains(constant.span)
+            {
+                span = constant.span;
             }
             Some(span)
         }
@@ -147,9 +145,7 @@ fn filtered_terminator_span(terminator: &Terminator<'_>) -> Option<Span> {
         | TerminatorKind::Yield { .. }
         | TerminatorKind::CoroutineDrop
         | TerminatorKind::FalseUnwind { .. }
-        | TerminatorKind::InlineAsm { .. } => {
-            Some(terminator.source_info.span)
-        }
+        | TerminatorKind::InlineAsm { .. } => Some(terminator.source_info.span),
     }
 }
 

compiler/rustc_mir_transform/src/lib.rs

@@ -12,6 +12,7 @@
 #![feature(map_try_insert)]
 #![feature(never_type)]
 #![feature(try_blocks)]
+#![feature(vec_deque_pop_if)]
 #![feature(yeet_expr)]
 // tidy-alphabetical-end
 

tests/coverage/abort.cov-map

@@ -34,14 +34,14 @@ Number of file 0 mappings: 13
 Highest counter ID seen: c4
 
 Function name: abort::might_abort
-Raw bytes (21): 0x[01, 01, 01, 01, 05, 03, 01, 03, 01, 01, 14, 05, 02, 09, 01, 24, 02, 02, 0c, 03, 02]
+Raw bytes (21): 0x[01, 01, 01, 01, 05, 03, 01, 03, 01, 01, 14, 05, 02, 09, 01, 0f, 02, 02, 0c, 03, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 1
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 Number of file 0 mappings: 3
 - Code(Counter(0)) at (prev + 3, 1) to (start + 1, 20)
-- Code(Counter(1)) at (prev + 2, 9) to (start + 1, 36)
+- Code(Counter(1)) at (prev + 2, 9) to (start + 1, 15)
 - Code(Expression(0, Sub)) at (prev + 2, 12) to (start + 3, 2)
     = (c0 - c1)
 Highest counter ID seen: c1

tests/coverage/assert-ne.cov-map

@@ -1,12 +1,12 @@
 Function name: assert_ne::main
-Raw bytes (28): 0x[01, 01, 02, 01, 05, 01, 09, 04, 01, 08, 01, 03, 1c, 05, 04, 0d, 00, 13, 02, 02, 0d, 00, 13, 06, 03, 05, 01, 02]
+Raw bytes (28): 0x[01, 01, 02, 01, 05, 01, 09, 04, 01, 08, 01, 03, 15, 05, 04, 0d, 00, 13, 02, 02, 0d, 00, 13, 06, 03, 05, 01, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 2
 - expression 0 operands: lhs = Counter(0), rhs = Counter(1)
 - expression 1 operands: lhs = Counter(0), rhs = Counter(2)
 Number of file 0 mappings: 4
-- Code(Counter(0)) at (prev + 8, 1) to (start + 3, 28)
+- Code(Counter(0)) at (prev + 8, 1) to (start + 3, 21)
 - Code(Counter(1)) at (prev + 4, 13) to (start + 0, 19)
 - Code(Expression(0, Sub)) at (prev + 2, 13) to (start + 0, 19)
     = (c0 - c1)

tests/coverage/assert-ne.coverage

@@ -7,7 +7,7 @@
    LL|       |
    LL|      1|fn main() {
    LL|      1|    assert_ne!(
-   LL|      1|        Foo(5), // Make sure this expression's span isn't lost.
+   LL|      1|        black_box(Foo(5)), // Make sure this expression's span isn't lost.
    LL|      1|        if black_box(false) {
    LL|      0|            Foo(0) //
    LL|       |        } else {

tests/coverage/assert-ne.rs

@@ -7,7 +7,7 @@ struct Foo(u32);
 
 fn main() {
     assert_ne!(
-        Foo(5), // Make sure this expression's span isn't lost.
+        black_box(Foo(5)), // Make sure this expression's span isn't lost.
         if black_box(false) {
             Foo(0) //
         } else {

tests/coverage/assert_not.cov-map

@@ -1,13 +1,13 @@
 Function name: assert_not::main
-Raw bytes (29): 0x[01, 01, 00, 05, 01, 06, 01, 01, 12, 01, 02, 05, 00, 14, 01, 01, 05, 00, 14, 01, 01, 05, 00, 16, 01, 01, 01, 00, 02]
+Raw bytes (29): 0x[01, 01, 00, 05, 01, 06, 01, 01, 11, 01, 02, 05, 00, 13, 01, 01, 05, 00, 13, 01, 01, 05, 00, 15, 01, 01, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 5
-- Code(Counter(0)) at (prev + 6, 1) to (start + 1, 18)
-- Code(Counter(0)) at (prev + 2, 5) to (start + 0, 20)
-- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 20)
-- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 22)
+- Code(Counter(0)) at (prev + 6, 1) to (start + 1, 17)
+- Code(Counter(0)) at (prev + 2, 5) to (start + 0, 19)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 19)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 21)
 - Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
 Highest counter ID seen: c0
 

tests/coverage/async_block.cov-map

@@ -1,5 +1,5 @@
 Function name: async_block::main
-Raw bytes (36): 0x[01, 01, 01, 05, 01, 06, 01, 07, 01, 00, 0b, 02, 01, 09, 00, 0a, 05, 00, 0e, 00, 13, 02, 00, 14, 01, 16, 02, 07, 0a, 02, 06, 01, 03, 01, 00, 02]
+Raw bytes (36): 0x[01, 01, 01, 05, 01, 06, 01, 07, 01, 00, 0b, 02, 01, 09, 00, 0a, 05, 00, 0e, 00, 13, 02, 01, 0d, 00, 13, 02, 07, 09, 00, 22, 01, 02, 01, 00, 02]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 1
@@ -9,11 +9,11 @@ Number of file 0 mappings: 6
 - Code(Expression(0, Sub)) at (prev + 1, 9) to (start + 0, 10)
     = (c1 - c0)
 - Code(Counter(1)) at (prev + 0, 14) to (start + 0, 19)
-- Code(Expression(0, Sub)) at (prev + 0, 20) to (start + 1, 22)
+- Code(Expression(0, Sub)) at (prev + 1, 13) to (start + 0, 19)
     = (c1 - c0)
-- Code(Expression(0, Sub)) at (prev + 7, 10) to (start + 2, 6)
+- Code(Expression(0, Sub)) at (prev + 7, 9) to (start + 0, 34)
     = (c1 - c0)
-- Code(Counter(0)) at (prev + 3, 1) to (start + 0, 2)
+- Code(Counter(0)) at (prev + 2, 1) to (start + 0, 2)
 Highest counter ID seen: c1
 
 Function name: async_block::main::{closure#0}