coverage: Skip spans that can't be un-expanded back to the function body

When we extract coverage spans from MIR, we try to "un-expand" them back to spans that are inside the function's body span. In cases where that doesn't succeed, the current code just swaps in the entire body span instead. But that tends to result in coverage spans that are completely unrelated to the control flow of the affected code, so it's better to just discard those spans.
rust-lang · Dec 2, 2023 · d800cd1 · d800cd1
1 parent 8c2b577
commit d800cd1
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Showing 7 changed files with 28 additions and 28 deletions.
diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -63,14 +63,14 @@ fn bcb_to_initial_coverage_spans<'a, 'tcx>(
 
         let statement_spans = data.statements.iter().filter_map(move |statement| {
             let expn_span = filtered_statement_span(statement)?;
-            let span = function_source_span(expn_span, body_span);
+            let span = unexpand_into_body_span(expn_span, body_span)?;
 
             Some(CoverageSpan::new(span, expn_span, bcb, is_closure(statement)))
         });
 
         let terminator_span = Some(data.terminator()).into_iter().filter_map(move |terminator| {
             let expn_span = filtered_terminator_span(terminator)?;
-            let span = function_source_span(expn_span, body_span);
+            let span = unexpand_into_body_span(expn_span, body_span)?;
 
             Some(CoverageSpan::new(span, expn_span, bcb, false))
         });
@@ -180,14 +180,14 @@ fn filtered_terminator_span(terminator: &Terminator<'_>) -> Option<Span> {
 /// Returns an extrapolated span (pre-expansion[^1]) corresponding to a range
 /// within the function's body source. This span is guaranteed to be contained
 /// within, or equal to, the `body_span`. If the extrapolated span is not
-/// contained within the `body_span`, the `body_span` is returned.
+/// contained within the `body_span`, `None` is returned.
 ///
 /// [^1]Expansions result from Rust syntax including macros, syntactic sugar,
 /// etc.).
 #[inline]
-fn function_source_span(span: Span, body_span: Span) -> Span {
+fn unexpand_into_body_span(span: Span, body_span: Span) -> Option<Span> {
     use rustc_span::source_map::original_sp;
 
     let original_span = original_sp(span, body_span).with_ctxt(body_span.ctxt());
-    if body_span.contains(original_span) { original_span } else { body_span }
+    body_span.contains(original_span).then_some(original_span)
 }
diff --git a/tests/coverage/async.cov-map b/tests/coverage/async.cov-map
@@ -74,28 +74,28 @@ Number of file 0 mappings: 6
     = ((c0 + c1) - c1)
 
 Function name: async::executor::block_on::VTABLE::{closure#0}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 72, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 72, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 114, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 114, 17) to (start + 0, 49)
 
 Function name: async::executor::block_on::VTABLE::{closure#1}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 73, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 73, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 115, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 115, 17) to (start + 0, 49)
 
 Function name: async::executor::block_on::VTABLE::{closure#2}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 74, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 74, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 116, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 116, 17) to (start + 0, 49)
 
 Function name: async::executor::block_on::VTABLE::{closure#3}
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 75, 11, 00, 13]

diff --git a/tests/coverage/async2.cov-map b/tests/coverage/async2.cov-map
@@ -78,28 +78,28 @@ Number of file 0 mappings: 6
     = ((c0 + c1) - c1)
 
 Function name: async2::executor::block_on::VTABLE::{closure#0}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 2b, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 2b, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 43, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 43, 17) to (start + 0, 49)
 
 Function name: async2::executor::block_on::VTABLE::{closure#1}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 2c, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 2c, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 44, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 44, 17) to (start + 0, 49)
 
 Function name: async2::executor::block_on::VTABLE::{closure#2}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 2d, 11, 00, 33]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 2d, 11, 00, 31]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 45, 17) to (start + 0, 51)
+- Code(Counter(0)) at (prev + 45, 17) to (start + 0, 49)
 
 Function name: async2::executor::block_on::VTABLE::{closure#3}
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 2e, 11, 00, 13]

diff --git a/tests/coverage/inline.cov-map b/tests/coverage/inline.cov-map
@@ -15,12 +15,12 @@ Number of file 0 mappings: 5
     = ((c0 + c1) - c1)
 
 Function name: inline::error
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 31, 01, 02, 02]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 31, 01, 01, 14]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 49, 1) to (start + 2, 2)
+- Code(Counter(0)) at (prev + 49, 1) to (start + 1, 20)
 
 Function name: inline::length::<char>
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 1e, 01, 02, 02]

diff --git a/tests/coverage/inline.coverage b/tests/coverage/inline.coverage
@@ -50,5 +50,5 @@
    LL|       |#[inline(always)]
    LL|      0|fn error() {
    LL|      0|    panic!("error");
-   LL|      0|}
+   LL|       |}
 
diff --git a/tests/coverage/unreachable.cov-map b/tests/coverage/unreachable.cov-map
@@ -1,24 +1,24 @@
 Function name: unreachable::UNREACHABLE_CLOSURE::{closure#0}
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 0f, 27, 00, 49]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0f, 27, 00, 47]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 15, 39) to (start + 0, 73)
+- Code(Counter(0)) at (prev + 15, 39) to (start + 0, 71)
 
 Function name: unreachable::unreachable_function
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 11, 01, 02, 02]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 11, 01, 01, 25]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 17, 1) to (start + 2, 2)
+- Code(Counter(0)) at (prev + 17, 1) to (start + 1, 37)
 
 Function name: unreachable::unreachable_intrinsic
-Raw bytes (9): 0x[01, 01, 00, 01, 01, 16, 01, 02, 02]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 16, 01, 01, 2c]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 0
 Number of file 0 mappings: 1
-- Code(Counter(0)) at (prev + 22, 1) to (start + 2, 2)
+- Code(Counter(0)) at (prev + 22, 1) to (start + 1, 44)
 
diff --git a/tests/coverage/unreachable.coverage b/tests/coverage/unreachable.coverage
@@ -16,12 +16,12 @@
    LL|       |
    LL|      0|fn unreachable_function() {
    LL|      0|    unsafe { unreachable_unchecked() }
-   LL|      0|}
+   LL|       |}
    LL|       |
    LL|       |// Use an intrinsic to more reliably trigger unreachable-propagation.
    LL|      0|fn unreachable_intrinsic() {
    LL|      0|    unsafe { std::intrinsics::unreachable() }
-   LL|      0|}
+   LL|       |}
    LL|       |
    LL|       |#[coverage(off)]
    LL|       |fn main() {