coverage: Add debugging flag -Zcoverage-options=no-mir-spans

When set, this flag skips the code that normally extracts coverage spans from
MIR statements and terminators. That sometimes makes it easier to debug branch
coverage and MC/DC coverage, because the coverage output is less noisy.

For internal debugging only. If other code changes would make it hard to keep
supporting this flag, remove it.

Author: Zalathar

Diff for: compiler/rustc_interface/src/tests.rs

@@ -761,7 +761,7 @@ fn test_unstable_options_tracking_hash() {
         })
     );
     tracked!(codegen_backend, Some("abc".to_string()));
-    tracked!(coverage_options, CoverageOptions { level: CoverageLevel::Mcdc });
+    tracked!(coverage_options, CoverageOptions { level: CoverageLevel::Mcdc, no_mir_spans: true });
     tracked!(crate_attr, vec!["abc".to_string()]);
     tracked!(cross_crate_inline_threshold, InliningThreshold::Always);
     tracked!(debug_info_for_profiling, true);

Diff for: compiler/rustc_mir_transform/src/coverage/mappings.rs

@@ -5,6 +5,7 @@ use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
 use rustc_middle::mir::coverage::{BlockMarkerId, BranchSpan, ConditionInfo, CoverageKind};
 use rustc_middle::mir::{self, BasicBlock, StatementKind};
+use rustc_middle::ty::TyCtxt;
 use rustc_span::Span;
 
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, START_BCB};
@@ -63,31 +64,35 @@ pub(super) struct ExtractedMappings {
 
 /// Extracts coverage-relevant spans from MIR, and associates them with
 /// their corresponding BCBs.
-pub(super) fn extract_all_mapping_info_from_mir(
-    mir_body: &mir::Body<'_>,
+pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    mir_body: &mir::Body<'tcx>,
     hir_info: &ExtractedHirInfo,
     basic_coverage_blocks: &CoverageGraph,
 ) -> ExtractedMappings {
-    if hir_info.is_async_fn {
+    let mut code_mappings = vec![];
+    let mut branch_pairs = vec![];
+    let mut mcdc_bitmap_bytes = 0;
+    let mut mcdc_branches = vec![];
+    let mut mcdc_decisions = vec![];
+
+    if hir_info.is_async_fn || tcx.sess.coverage_no_mir_spans() {
         // An async function desugars into a function that returns a future,
         // with the user code wrapped in a closure. Any spans in the desugared
         // outer function will be unhelpful, so just keep the signature span
         // and ignore all of the spans in the MIR body.
-        let mut mappings = ExtractedMappings::default();
+        //
+        // When debugging flag `-Zcoverage-options=no-mir-spans` is set, we need
+        // to give the same treatment to _all_ functions, because `llvm-cov`
+        // seems to ignore functions that don't have any ordinary code spans.
         if let Some(span) = hir_info.fn_sig_span_extended {
-            mappings.code_mappings.push(CodeMapping { span, bcb: START_BCB });
+            code_mappings.push(CodeMapping { span, bcb: START_BCB });
         }
-        return mappings;
+    } else {
+        // Extract coverage spans from MIR statements/terminators as normal.
+        extract_refined_covspans(mir_body, hir_info, basic_coverage_blocks, &mut code_mappings);
     }
 
-    let mut code_mappings = vec![];
-    let mut branch_pairs = vec![];
-    let mut mcdc_bitmap_bytes = 0;
-    let mut mcdc_branches = vec![];
-    let mut mcdc_decisions = vec![];
-
-    extract_refined_covspans(mir_body, hir_info, basic_coverage_blocks, &mut code_mappings);
-
     branch_pairs.extend(extract_branch_pairs(mir_body, hir_info, basic_coverage_blocks));
 
     extract_mcdc_mappings(

Diff for: compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -71,8 +71,12 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
 
     ////////////////////////////////////////////////////
     // Extract coverage spans and other mapping info from MIR.
-    let extracted_mappings =
-        mappings::extract_all_mapping_info_from_mir(mir_body, &hir_info, &basic_coverage_blocks);
+    let extracted_mappings = mappings::extract_all_mapping_info_from_mir(
+        tcx,
+        mir_body,
+        &hir_info,
+        &basic_coverage_blocks,
+    );
 
     ////////////////////////////////////////////////////
     // Create an optimized mix of `Counter`s and `Expression`s for the `CoverageGraph`. Ensure

Diff for: compiler/rustc_session/src/config.rs

@@ -149,7 +149,14 @@ pub enum InstrumentCoverage {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default)]
 pub struct CoverageOptions {
     pub level: CoverageLevel,
-    // Other boolean or enum-valued options might be added here.
+
+    /// `-Z coverage-options=no-mir-spans`: Don't extract block coverage spans
+    /// from MIR statements/terminators, making it easier to inspect/debug
+    /// branch and MC/DC coverage mappings.
+    ///
+    /// For internal debugging only. If other code changes would make it hard
+    /// to keep supporting this flag, remove it.
+    pub no_mir_spans: bool,
 }
 
 /// Controls whether branch coverage or MC/DC coverage is enabled.

Diff for: compiler/rustc_session/src/options.rs

@@ -395,7 +395,8 @@ mod desc {
     pub const parse_optimization_fuel: &str = "crate=integer";
     pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str = parse_bool;
-    pub const parse_coverage_options: &str = "`block` | `branch` | `condition` | `mcdc`";
+    pub const parse_coverage_options: &str =
+        "`block` | `branch` | `condition` | `mcdc` | `no-mir-spans`";
     pub const parse_instrument_xray: &str = "either a boolean (`yes`, `no`, `on`, `off`, etc), or a comma separated list of settings: `always` or `never` (mutually exclusive), `ignore-loops`, `instruction-threshold=N`, `skip-entry`, `skip-exit`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
     pub const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
@@ -963,6 +964,7 @@ mod parse {
                 "branch" => slot.level = CoverageLevel::Branch,
                 "condition" => slot.level = CoverageLevel::Condition,
                 "mcdc" => slot.level = CoverageLevel::Mcdc,
+                "no-mir-spans" => slot.no_mir_spans = true,
                 _ => return false,
             }
         }

Diff for: compiler/rustc_session/src/session.rs

@@ -363,6 +363,11 @@ impl Session {
             && self.opts.unstable_opts.coverage_options.level >= CoverageLevel::Mcdc
     }
 
+    /// True if `-Zcoverage-options=no-mir-spans` was passed.
+    pub fn coverage_no_mir_spans(&self) -> bool {
+        self.opts.unstable_opts.coverage_options.no_mir_spans
+    }
+
     pub fn is_sanitizer_cfi_enabled(&self) -> bool {
         self.opts.unstable_opts.sanitizer.contains(SanitizerSet::CFI)
     }

Diff for: tests/coverage/branch/no-mir-spans.cov-map

@@ -0,0 +1,52 @@
+Function name: no_mir_spans::while_cond
+Raw bytes (16): 0x[01, 01, 00, 02, 01, 10, 01, 00, 11, 20, 05, 09, 04, 0b, 00, 10]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 16, 1) to (start + 0, 17)
+- Branch { true: Counter(1), false: Counter(2) } at (prev + 4, 11) to (start + 0, 16)
+    true  = c1
+    false = c2
+
+Function name: no_mir_spans::while_cond_not
+Raw bytes (16): 0x[01, 01, 00, 02, 01, 19, 01, 00, 15, 20, 09, 05, 04, 0b, 00, 14]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 0
+Number of file 0 mappings: 2
+- Code(Counter(0)) at (prev + 25, 1) to (start + 0, 21)
+- Branch { true: Counter(2), false: Counter(1) } at (prev + 4, 11) to (start + 0, 20)
+    true  = c2
+    false = c1
+
+Function name: no_mir_spans::while_op_and
+Raw bytes (25): 0x[01, 01, 01, 09, 0d, 03, 01, 22, 01, 00, 13, 20, 09, 05, 05, 0b, 00, 10, 20, 02, 0d, 00, 14, 00, 19]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 1
+- expression 0 operands: lhs = Counter(2), rhs = Counter(3)
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 34, 1) to (start + 0, 19)
+- Branch { true: Counter(2), false: Counter(1) } at (prev + 5, 11) to (start + 0, 16)
+    true  = c2
+    false = c1
+- Branch { true: Expression(0, Sub), false: Counter(3) } at (prev + 0, 20) to (start + 0, 25)
+    true  = (c2 - c3)
+    false = c3
+
+Function name: no_mir_spans::while_op_or
+Raw bytes (25): 0x[01, 01, 01, 09, 0d, 03, 01, 2d, 01, 00, 12, 20, 05, 09, 05, 0b, 00, 10, 20, 0d, 02, 00, 14, 00, 19]
+Number of files: 1
+- file 0 => global file 1
+Number of expressions: 1
+- expression 0 operands: lhs = Counter(2), rhs = Counter(3)
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 45, 1) to (start + 0, 18)
+- Branch { true: Counter(1), false: Counter(2) } at (prev + 5, 11) to (start + 0, 16)
+    true  = c1
+    false = c2
+- Branch { true: Counter(3), false: Expression(0, Sub) } at (prev + 0, 20) to (start + 0, 25)
+    true  = c3
+    false = (c2 - c3)
+

Diff for: tests/coverage/branch/no-mir-spans.coverage

@@ -0,0 +1,77 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2021
+   LL|       |//@ compile-flags: -Zcoverage-options=branch,no-mir-spans
+   LL|       |//@ llvm-cov-flags: --show-branches=count
+   LL|       |
+   LL|       |// Tests the behaviour of the `-Zcoverage-options=no-mir-spans` debugging flag.
+   LL|       |// The actual code below is just some non-trivial code copied from another test
+   LL|       |// (`while.rs`), and has no particular significance.
+   LL|       |
+   LL|       |macro_rules! no_merge {
+   LL|       |    () => {
+   LL|       |        for _ in 0..1 {}
+   LL|       |    };
+   LL|       |}
+   LL|       |
+   LL|      1|fn while_cond() {
+   LL|       |    no_merge!();
+   LL|       |
+   LL|       |    let mut a = 8;
+   LL|       |    while a > 0 {
+  ------------------
+  |  Branch (LL:11): [True: 8, False: 1]
+  ------------------
+   LL|       |        a -= 1;
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|      1|fn while_cond_not() {
+   LL|       |    no_merge!();
+   LL|       |
+   LL|       |    let mut a = 8;
+   LL|       |    while !(a == 0) {
+  ------------------
+  |  Branch (LL:11): [True: 8, False: 1]
+  ------------------
+   LL|       |        a -= 1;
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|      1|fn while_op_and() {
+   LL|       |    no_merge!();
+   LL|       |
+   LL|       |    let mut a = 8;
+   LL|       |    let mut b = 4;
+   LL|       |    while a > 0 && b > 0 {
+  ------------------
+  |  Branch (LL:11): [True: 5, False: 0]
+  |  Branch (LL:20): [True: 4, False: 1]
+  ------------------
+   LL|       |        a -= 1;
+   LL|       |        b -= 1;
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|      1|fn while_op_or() {
+   LL|       |    no_merge!();
+   LL|       |
+   LL|       |    let mut a = 4;
+   LL|       |    let mut b = 8;
+   LL|       |    while a > 0 || b > 0 {
+  ------------------
+  |  Branch (LL:11): [True: 4, False: 5]
+  |  Branch (LL:20): [True: 4, False: 1]
+  ------------------
+   LL|       |        a -= 1;
+   LL|       |        b -= 1;
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |fn main() {
+   LL|       |    while_cond();
+   LL|       |    while_cond_not();
+   LL|       |    while_op_and();
+   LL|       |    while_op_or();
+   LL|       |}
+

Diff for: tests/coverage/branch/no-mir-spans.rs

@@ -0,0 +1,62 @@
+#![feature(coverage_attribute)]
+//@ edition: 2021
+//@ compile-flags: -Zcoverage-options=branch,no-mir-spans
+//@ llvm-cov-flags: --show-branches=count
+
+// Tests the behaviour of the `-Zcoverage-options=no-mir-spans` debugging flag.
+// The actual code below is just some non-trivial code copied from another test
+// (`while.rs`), and has no particular significance.
+
+macro_rules! no_merge {
+    () => {
+        for _ in 0..1 {}
+    };
+}
+
+fn while_cond() {
+    no_merge!();
+
+    let mut a = 8;
+    while a > 0 {
+        a -= 1;
+    }
+}
+
+fn while_cond_not() {
+    no_merge!();
+
+    let mut a = 8;
+    while !(a == 0) {
+        a -= 1;
+    }
+}
+
+fn while_op_and() {
+    no_merge!();
+
+    let mut a = 8;
+    let mut b = 4;
+    while a > 0 && b > 0 {
+        a -= 1;
+        b -= 1;
+    }
+}
+
+fn while_op_or() {
+    no_merge!();
+
+    let mut a = 4;
+    let mut b = 8;
+    while a > 0 || b > 0 {
+        a -= 1;
+        b -= 1;
+    }
+}
+
+#[coverage(off)]
+fn main() {
+    while_cond();
+    while_cond_not();
+    while_op_and();
+    while_op_or();
+}

Diff for: tests/ui/instrument-coverage/coverage-options.bad.stderr

@@ -1,2 +1,2 @@
-error: incorrect value `bad` for unstable option `coverage-options` - `block` | `branch` | `condition` | `mcdc` was expected
+error: incorrect value `bad` for unstable option `coverage-options` - `block` | `branch` | `condition` | `mcdc` | `no-mir-spans` was expected