Rollup merge of #144663 - Zalathar:empty-span, r=petrochenkov

coverage: Re-land "Enlarge empty spans during MIR instrumentation"

This allows us to assume that coverage spans will only be discarded during codegen in very unusual situations.

---

This seemingly-simple change has a rather messy history:
- rust-lang/rust#140847
- rust-lang/rust#141650
- rust-lang/rust#144298
- rust-lang/rust#144480

Since then, a number of related changes have landed that should make it reasonable to try again:
- rust-lang/rust#144530
- rust-lang/rust#144560
- rust-lang/rust#144616

In particular, we have multiple fixes/mitigations, and a confirmed regression test for the original bug that is not triggered by re-landing the changes in this PR.

9 files changed, 17 insertions, 47 deletions

diff --git a/tests/coverage/async_closure.cov-map b/tests/coverage/async_closure.cov-map
index 9f8dc8d6cbb..53128dd7a48 100644
--- a/tests/coverage/async_closure.cov-map
+++ b/tests/coverage/async_closure.cov-map
@@ -37,32 +37,29 @@ Number of file 0 mappings: 8
 Highest counter ID seen: c0
 
 Function name: async_closure::main::{closure#0}
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 0b, 22, 00, 23, 01, 00, 23, 00, 24]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0b, 22, 00, 24]
 Number of files: 1
 - file 0 => $DIR/async_closure.rs
 Number of expressions: 0
-Number of file 0 mappings: 2
-- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 35)
-- Code(Counter(0)) at (prev + 0, 35) to (start + 0, 36)
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 36)
 Highest counter ID seen: c0
 
 Function name: async_closure::main::{closure#0}
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 0b, 22, 00, 23, 01, 00, 23, 00, 24]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0b, 22, 00, 24]
 Number of files: 1
 - file 0 => $DIR/async_closure.rs
 Number of expressions: 0
-Number of file 0 mappings: 2
-- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 35)
-- Code(Counter(0)) at (prev + 0, 35) to (start + 0, 36)
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 36)
 Highest counter ID seen: c0
 
 Function name: async_closure::main::{closure#0}::{closure#0}::<i16>
-Raw bytes (14): 0x[01, 01, 00, 02, 01, 0b, 22, 00, 23, 01, 00, 23, 00, 24]
+Raw bytes (9): 0x[01, 01, 00, 01, 01, 0b, 22, 00, 24]
 Number of files: 1
 - file 0 => $DIR/async_closure.rs
 Number of expressions: 0
-Number of file 0 mappings: 2
-- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 35)
-- Code(Counter(0)) at (prev + 0, 35) to (start + 0, 36)
+Number of file 0 mappings: 1
+- Code(Counter(0)) at (prev + 11, 34) to (start + 0, 36)
 Highest counter ID seen: c0
 
diff --git a/tests/coverage/try-in-macro.attr.cov-map b/tests/coverage/try-in-macro.attr.cov-map
index 7111e89637c..1b6c97cf145 100644
--- a/tests/coverage/try-in-macro.attr.cov-map
+++ b/tests/coverage/try-in-macro.attr.cov-map
@@ -1,12 +1,3 @@
-Function name: <try_in_macro::MyEnum as try_in_macro::Arbitrary>::try_size_hint
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 1e, 2a, 00, 2b]
-Number of files: 1
-- file 0 => $DIR/try-in-macro.rs
-Number of expressions: 0
-Number of file 0 mappings: 1
-- Code(Zero) at (prev + 30, 42) to (start + 0, 43)
-Highest counter ID seen: (none)
-
 Function name: try_in_macro::main
 Raw bytes (19): 0x[01, 01, 00, 03, 01, 29, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
 Number of files: 1
diff --git a/tests/coverage/try-in-macro.bang.cov-map b/tests/coverage/try-in-macro.bang.cov-map
index 80bd91a993c..1b6c97cf145 100644
--- a/tests/coverage/try-in-macro.bang.cov-map
+++ b/tests/coverage/try-in-macro.bang.cov-map
@@ -1,12 +1,3 @@
-Function name: <try_in_macro::MyEnum as try_in_macro::Arbitrary>::try_size_hint
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 23, 1c, 00, 1d]
-Number of files: 1
-- file 0 => $DIR/try-in-macro.rs
-Number of expressions: 0
-Number of file 0 mappings: 1
-- Code(Zero) at (prev + 35, 28) to (start + 0, 29)
-Highest counter ID seen: (none)
-
 Function name: try_in_macro::main
 Raw bytes (19): 0x[01, 01, 00, 03, 01, 29, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
 Number of files: 1
diff --git a/tests/coverage/try-in-macro.derive.cov-map b/tests/coverage/try-in-macro.derive.cov-map
index 6646b6693ba..1b6c97cf145 100644
--- a/tests/coverage/try-in-macro.derive.cov-map
+++ b/tests/coverage/try-in-macro.derive.cov-map
@@ -1,12 +1,3 @@
-Function name: <try_in_macro::MyEnum as try_in_macro::Arbitrary>::try_size_hint
-Raw bytes (9): 0x[01, 01, 00, 01, 00, 26, 38, 00, 39]
-Number of files: 1
-- file 0 => $DIR/try-in-macro.rs
-Number of expressions: 0
-Number of file 0 mappings: 1
-- Code(Zero) at (prev + 38, 56) to (start + 0, 57)
-Highest counter ID seen: (none)
-
 Function name: try_in_macro::main
 Raw bytes (19): 0x[01, 01, 00, 03, 01, 29, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
 Number of files: 1
diff --git a/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff b/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
index d465b8bded2..fa88211383a 100644
--- a/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
+++ b/tests/mir-opt/coverage/branch_match_arms.main.InstrumentCoverage.diff
@@ -40,7 +40,7 @@
 +     coverage Code { bcb: bcb5 } => $DIR/branch_match_arms.rs:19:17: 19:18 (#0);
 +     coverage Code { bcb: bcb5 } => $DIR/branch_match_arms.rs:19:23: 19:30 (#0);
 +     coverage Code { bcb: bcb5 } => $DIR/branch_match_arms.rs:19:31: 19:32 (#0);
-+     coverage Code { bcb: bcb2 } => $DIR/branch_match_arms.rs:21:2: 21:2 (#0);
++     coverage Code { bcb: bcb2 } => $DIR/branch_match_arms.rs:21:1: 21:2 (#0);
 + 
       bb0: {
 +         Coverage::VirtualCounter(bcb0);
diff --git a/tests/mir-opt/coverage/instrument_coverage.bar.InstrumentCoverage.diff b/tests/mir-opt/coverage/instrument_coverage.bar.InstrumentCoverage.diff
index cf6d85abd80..9b6d2b22087 100644
--- a/tests/mir-opt/coverage/instrument_coverage.bar.InstrumentCoverage.diff
+++ b/tests/mir-opt/coverage/instrument_coverage.bar.InstrumentCoverage.diff
@@ -6,7 +6,7 @@
   
 +     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage.rs:27:1: 27:17 (#0);
 +     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage.rs:28:5: 28:9 (#0);
-+     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage.rs:29:2: 29:2 (#0);
++     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage.rs:29:1: 29:2 (#0);
 + 
       bb0: {
 +         Coverage::VirtualCounter(bcb0);
diff --git a/tests/mir-opt/coverage/instrument_coverage.main.InstrumentCoverage.diff b/tests/mir-opt/coverage/instrument_coverage.main.InstrumentCoverage.diff
index 980c5e202ff..b2bb2375aee 100644
--- a/tests/mir-opt/coverage/instrument_coverage.main.InstrumentCoverage.diff
+++ b/tests/mir-opt/coverage/instrument_coverage.main.InstrumentCoverage.diff
@@ -10,8 +10,8 @@
 +     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage.rs:13:1: 13:10 (#0);
 +     coverage Code { bcb: bcb1 } => $DIR/instrument_coverage.rs:15:12: 15:15 (#0);
 +     coverage Code { bcb: bcb2 } => $DIR/instrument_coverage.rs:16:13: 16:18 (#0);
-+     coverage Code { bcb: bcb3 } => $DIR/instrument_coverage.rs:17:10: 17:10 (#0);
-+     coverage Code { bcb: bcb2 } => $DIR/instrument_coverage.rs:19:2: 19:2 (#0);
++     coverage Code { bcb: bcb3 } => $DIR/instrument_coverage.rs:17:9: 17:10 (#0);
++     coverage Code { bcb: bcb2 } => $DIR/instrument_coverage.rs:19:1: 19:2 (#0);
 + 
       bb0: {
 +         Coverage::VirtualCounter(bcb0);
diff --git a/tests/mir-opt/coverage/instrument_coverage_cleanup.main.CleanupPostBorrowck.diff b/tests/mir-opt/coverage/instrument_coverage_cleanup.main.CleanupPostBorrowck.diff
index b707cd41788..2eb78c08ee8 100644
--- a/tests/mir-opt/coverage/instrument_coverage_cleanup.main.CleanupPostBorrowck.diff
+++ b/tests/mir-opt/coverage/instrument_coverage_cleanup.main.CleanupPostBorrowck.diff
@@ -10,8 +10,8 @@
       coverage Code { bcb: bcb0 } => $DIR/instrument_coverage_cleanup.rs:13:1: 13:10 (#0);
       coverage Code { bcb: bcb0 } => $DIR/instrument_coverage_cleanup.rs:14:8: 14:36 (#0);
       coverage Code { bcb: bcb3 } => $DIR/instrument_coverage_cleanup.rs:14:37: 14:39 (#0);
-      coverage Code { bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:39: 14:39 (#0);
-      coverage Code { bcb: bcb2 } => $DIR/instrument_coverage_cleanup.rs:15:2: 15:2 (#0);
+      coverage Code { bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:38: 14:39 (#0);
+      coverage Code { bcb: bcb2 } => $DIR/instrument_coverage_cleanup.rs:15:1: 15:2 (#0);
       coverage Branch { true_bcb: bcb3, false_bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:8: 14:36 (#0);
   
       bb0: {
diff --git a/tests/mir-opt/coverage/instrument_coverage_cleanup.main.InstrumentCoverage.diff b/tests/mir-opt/coverage/instrument_coverage_cleanup.main.InstrumentCoverage.diff
index 239b845c231..0c1bc24b6dc 100644
--- a/tests/mir-opt/coverage/instrument_coverage_cleanup.main.InstrumentCoverage.diff
+++ b/tests/mir-opt/coverage/instrument_coverage_cleanup.main.InstrumentCoverage.diff
@@ -10,8 +10,8 @@
 +     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage_cleanup.rs:13:1: 13:10 (#0);
 +     coverage Code { bcb: bcb0 } => $DIR/instrument_coverage_cleanup.rs:14:8: 14:36 (#0);
 +     coverage Code { bcb: bcb3 } => $DIR/instrument_coverage_cleanup.rs:14:37: 14:39 (#0);
-+     coverage Code { bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:39: 14:39 (#0);
-+     coverage Code { bcb: bcb2 } => $DIR/instrument_coverage_cleanup.rs:15:2: 15:2 (#0);
++     coverage Code { bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:38: 14:39 (#0);
++     coverage Code { bcb: bcb2 } => $DIR/instrument_coverage_cleanup.rs:15:1: 15:2 (#0);
 +     coverage Branch { true_bcb: bcb3, false_bcb: bcb1 } => $DIR/instrument_coverage_cleanup.rs:14:8: 14:36 (#0);
 + 
       bb0: {