diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
index 53ae3448150b4..cc39a2319311d 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
@@ -102,8 +102,6 @@ pub enum RegionKind {
 
     /// A DecisionRegion represents a top-level boolean expression and is
     /// associated with a variable length bitmap index and condition number.
-    /// FIXME(dprn): Remove unused variables.
-    #[allow(dead_code)]
     MCDCDecisionRegion = 5,
 
     /// A Branch Region can be extended to include IDs to facilitate MC/DC.
@@ -208,6 +206,15 @@ impl CounterMappingRegion {
                 end_col,
                 None,
             ),
+            MappingKind::MCDCDecision { bitmap_idx, num_conditions } => Self::decision_region(
+                bitmap_idx,
+                num_conditions,
+                local_file_id,
+                start_line,
+                start_col,
+                end_line,
+                end_col,
+            ),
         }
     }
 
@@ -263,7 +270,6 @@ impl CounterMappingRegion {
         }
     }
 
-    #[allow(dead_code)]
     pub(crate) fn decision_region(
         bitmap_idx: u32,
         num_conditions: u32,
@@ -283,11 +289,7 @@ impl CounterMappingRegion {
             end_line,
             end_col,
             kind: RegionKind::MCDCDecisionRegion,
-            mcdc_params: MCDCParameters {
-                bitmap_idx,
-                num_conditions,
-                .. Default::default()
-            },
+            mcdc_params: MCDCParameters { bitmap_idx, num_conditions, ..Default::default() },
         }
     }
 
diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
index ca9d892de0aac..8b56f48bae271 100644
--- a/compiler/rustc_middle/src/mir/coverage.rs
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -287,16 +287,21 @@ pub enum MappingKind {
     Code(CovTerm),
     /// Associates a branch region with separate counters for true and false.
     Branch { true_term: CovTerm, false_term: CovTerm },
+    /// FIXME(dprn): doc
+    /// Associates an MCDC Decision with
+    MCDCDecision { bitmap_idx: u32, num_conditions: u32 },
 }
 
 impl MappingKind {
     /// Iterator over all coverage terms in this mapping kind.
     pub fn terms(&self) -> impl Iterator<Item = CovTerm> {
-        let one = |a| std::iter::once(a).chain(None);
-        let two = |a, b| std::iter::once(a).chain(Some(b));
+        let zero = || std::iter::empty().chain(None).chain(None);
+        let one = |a| std::iter::empty().chain(Some(a)).chain(None);
+        let two = |a, b| std::iter::empty().chain(Some(a)).chain(Some(b));
         match *self {
             Self::Code(term) => one(term),
             Self::Branch { true_term, false_term } => two(true_term, false_term),
+            Self::MCDCDecision { .. } => zero(),
         }
     }
 
@@ -308,6 +313,9 @@ impl MappingKind {
             Self::Branch { true_term, false_term } => {
                 Self::Branch { true_term: map_fn(true_term), false_term: map_fn(false_term) }
             }
+            Self::MCDCDecision { bitmap_idx, num_conditions } => {
+                Self::MCDCDecision { bitmap_idx, num_conditions }
+            }
         }
     }
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 46d55be07a20a..739feeec71ed2 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -150,6 +150,9 @@ fn create_mappings<'tcx>(
                     true_term: term_for_bcb(true_bcb),
                     false_term: term_for_bcb(false_bcb),
                 },
+                BcbMappingKind::MCDCDecision { bitmap_idx, num_conditions } => {
+                    MappingKind::MCDCDecision { bitmap_idx, num_conditions }
+                }
             };
             let code_region = make_code_region(source_map, file_name, span, body_span)?;
             Some(Mapping { kind, code_region })
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index 672de1fbe60fd..24f3f0efd2f0b 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans.rs
@@ -9,12 +9,21 @@ use crate::coverage::ExtractedHirInfo;
 
 mod from_mir;
 
+// FIXME(dprn): Remove allow dead code
+#[allow(dead_code)]
 #[derive(Clone, Copy, Debug)]
 pub(super) enum BcbMappingKind {
     /// Associates an ordinary executable code span with its corresponding BCB.
     Code(BasicCoverageBlock),
     /// Associates a branch span with BCBs for its true and false arms.
-    Branch { true_bcb: BasicCoverageBlock, false_bcb: BasicCoverageBlock },
+    Branch {
+        true_bcb: BasicCoverageBlock,
+        false_bcb: BasicCoverageBlock,
+    },
+    MCDCDecision {
+        bitmap_idx: u32,
+        num_conditions: u32,
+    },
 }
 
 #[derive(Debug)]
@@ -92,6 +101,7 @@ pub(super) fn generate_coverage_spans(
                 insert(true_bcb);
                 insert(false_bcb);
             }
+            BcbMappingKind::MCDCDecision { .. } => (),
         }
     }