SERVER-55204 Error on fallthrough unless the attribute is used

33 files changed, 89 insertions, 26 deletions

diff --git a/SConstruct b/SConstruct
index 408693bf142..6aa3b3aded5 100644
--- a/SConstruct
+++ b/SConstruct
@@ -2899,18 +2899,23 @@ def doConfigure(myenv):
 
     def AddToCFLAGSIfSupported(env, flag):
         return AddFlagIfSupported(env, 'C', '.c', flag, False, CFLAGS=[flag])
+    env.AddMethod(AddToCFLAGSIfSupported)
 
     def AddToCCFLAGSIfSupported(env, flag):
         return AddFlagIfSupported(env, 'C', '.c', flag, False, CCFLAGS=[flag])
+    env.AddMethod(AddToCCFLAGSIfSupported)
 
     def AddToCXXFLAGSIfSupported(env, flag):
         return AddFlagIfSupported(env, 'C++', '.cpp', flag, False, CXXFLAGS=[flag])
+    env.AddMethod(AddToCXXFLAGSIfSupported)
 
     def AddToLINKFLAGSIfSupported(env, flag):
         return AddFlagIfSupported(env, 'C', '.c', flag, True, LINKFLAGS=[flag])
+    env.AddMethod(AddToLINKFLAGSIfSupported)
 
     def AddToSHLINKFLAGSIfSupported(env, flag):
         return AddFlagIfSupported(env, 'C', '.c', flag, True, SHLINKFLAGS=[flag])
+    env.AddMethod(AddToSHLINKFLAGSIfSupported)
 
     if myenv.ToolchainIs('gcc', 'clang'):
         # This tells clang/gcc to use the gold linker if it is available - we prefer the gold linker
diff --git a/src/SConscript b/src/SConscript
index b046e91173c..25f9eb93146 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -8,6 +8,8 @@ import SCons
 Import('env')
 Import('module_sconscripts')
 
+env = env.Clone()
+
 # Add any "global" dependencies here. This is where we make every build node
 # depend on a list of other build nodes, such as an allocator or libunwind
 # or libstdx or similar.
@@ -21,6 +23,12 @@ env.AppendUnique(
 # that we need in the mongo sconscript
 env.SConscript('third_party/SConscript', exports=['env'])
 
+# Ensure our subsequent modifications are not shared with the
+# third_party env. Normally that SConscript would have done a clone,
+# so handled that isolation itself, but it doesn't since it needs to
+# alter the env in ways that we need to use up here.
+env = env.Clone()
+
 # Inject common dependencies from third_party globally for all core mongo code
 # and modules. Ideally, pcre wouldn't be here, but enough things require it
 # now that it seems hopeless to remove it.
@@ -33,6 +41,21 @@ env.InjectThirdParty(libraries=[
     'variant',
 ])
 
+
+# It would be somewhat better if this could be applied down in
+# `src/mongo/SConscript`, since the goal of doing it here rather than
+# up in SConstruct is to only enforce this rule for code that we wrote
+# and exclude third party sources. However, doing it in
+# `src/mongo/SConscript`` would also exclude applying it for modules,
+# and we do want this enabled for enterprise.
+
+if env.ToolchainIs('gcc'):
+    # With GCC, use the implicit fallthrough flag variant that doesn't
+    # care about your feeble attempts to use comments to explain yourself.
+    env.AddToCCFLAGSIfSupported('-Wimplicit-fallthrough=5')
+elif env.ToolchainIs('clang'):
+    env.AddToCCFLAGSIfSupported('-Wimplicit-fallthrough')
+
 # Run the core mongodb SConscript.
 env.SConscript('mongo/SConscript', exports=['env'])
 
diff --git a/src/mongo/bson/bson_comparator_interface_base.cpp b/src/mongo/bson/bson_comparator_interface_base.cpp
index 96aaf1575a7..e0d85f3e926 100644
--- a/src/mongo/bson/bson_comparator_interface_base.cpp
+++ b/src/mongo/bson/bson_comparator_interface_base.cpp
@@ -109,6 +109,7 @@ void BSONComparatorInterfaceBase<T>::hashCombineBSONElement(
             // Else, fall through and convert the decimal to a double and hash.
             // At this point the decimal fits into the range of doubles, is infinity, or is NaN,
             // which doubles have a cheaper representation for.
+            [[fallthrough]];
         }
         case mongo::NumberDouble:
         case mongo::NumberLong:
diff --git a/src/mongo/bson/bsonelement.cpp b/src/mongo/bson/bsonelement.cpp
index 430b259699b..dd331399950 100644
--- a/src/mongo/bson/bsonelement.cpp
+++ b/src/mongo/bson/bsonelement.cpp
@@ -209,6 +209,7 @@ BSONObj BSONElement::_jsonStringGenerator(const Generator& g,
                 break;
             }
             // fall through if scope is empty
+            [[fallthrough]];
         }
         case Code:
             g.writeCode(buffer, _asCode());
diff --git a/src/mongo/client/cyrus_sasl_client_session.cpp b/src/mongo/client/cyrus_sasl_client_session.cpp
index 84ae1ab0b5a..c92c4927d26 100644
--- a/src/mongo/client/cyrus_sasl_client_session.cpp
+++ b/src/mongo/client/cyrus_sasl_client_session.cpp
@@ -296,7 +296,7 @@ Status CyrusSaslClientSession::step(StringData inputData, std::string* outputDat
     switch (result) {
         case SASL_OK:
             _success = true;
-        // Fall through
+            [[fallthrough]];
         case SASL_CONTINUE:
             *outputData = std::string(output, outputSize);
             return Status::OK();
diff --git a/src/mongo/db/catalog/collection_catalog.cpp b/src/mongo/db/catalog/collection_catalog.cpp
index 1c938446046..30a14810133 100644
--- a/src/mongo/db/catalog/collection_catalog.cpp
+++ b/src/mongo/db/catalog/collection_catalog.cpp
@@ -157,6 +157,7 @@ public:
                         catalog.registerCollection(opCtx, uuid, std::move(collection));
                     });
                     // Fallthrough to the createCollection case to finish committing the collection.
+                    [[fallthrough]];
                 }
                 case UncommittedCatalogUpdates::Entry::Action::kCreatedCollection: {
                     auto collPtr = entry.collection.get();
diff --git a/src/mongo/db/exec/document_value/value.cpp b/src/mongo/db/exec/document_value/value.cpp
index 248514180f0..97573281dbb 100644
--- a/src/mongo/db/exec/document_value/value.cpp
+++ b/src/mongo/db/exec/document_value/value.cpp
@@ -908,6 +908,7 @@ void Value::hash_combine(size_t& seed,
             // Else, fall through and convert the decimal to a double and hash.
             // At this point the decimal fits into the range of doubles, is infinity, or is NaN,
             // which doubles have a cheaper representation for.
+            [[fallthrough]];
         }
         // This converts all numbers to doubles, which ignores the low-order bits of
         // NumberLongs > 2**53 and precise decimal numbers without double representations,
diff --git a/src/mongo/db/exec/plan_cache_util.cpp b/src/mongo/db/exec/plan_cache_util.cpp
index 166e01ba6d6..14745c4d128 100644
--- a/src/mongo/db/exec/plan_cache_util.cpp
+++ b/src/mongo/db/exec/plan_cache_util.cpp
@@ -182,6 +182,7 @@ plan_cache_debug_info::DebugInfoSBE buildDebugInfo(const QuerySolution* solution
                 } else {
                     secondaryStats.collectionScans++;
                 }
+                [[fallthrough]];
             }
             default:
                 break;
diff --git a/src/mongo/db/exec/sbe/vm/arith.cpp b/src/mongo/db/exec/sbe/vm/arith.cpp
index e4c67775ad8..89dc552f7f1 100644
--- a/src/mongo/db/exec/sbe/vm/arith.cpp
+++ b/src/mongo/db/exec/sbe/vm/arith.cpp
@@ -144,6 +144,7 @@ std::tuple<bool, value::TypeTags, value::Value> genericArithmeticOp(value::TypeT
                         false, value::TypeTags::NumberInt32, value::bitcastFrom<int32_t>(result)};
                 }
                 // The result does not fit into int32_t so fallthru to the wider type.
+                [[fallthrough]];
             }
             case value::TypeTags::NumberInt64: {
                 int64_t result;
@@ -154,6 +155,7 @@ std::tuple<bool, value::TypeTags, value::Value> genericArithmeticOp(value::TypeT
                         false, value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(result)};
                 }
                 // The result does not fit into int64_t so fallthru to the wider type.
+                [[fallthrough]];
             }
             case value::TypeTags::NumberDecimal: {
                 Decimal128 result;
diff --git a/src/mongo/db/exec/sbe/vm/vm.cpp b/src/mongo/db/exec/sbe/vm/vm.cpp
index a7e69fb7dee..e3e8adbffcb 100644
--- a/src/mongo/db/exec/sbe/vm/vm.cpp
+++ b/src/mongo/db/exec/sbe/vm/vm.cpp
@@ -2365,7 +2365,7 @@ std::tuple<bool, value::TypeTags, value::Value> ByteCode::builtinDoubleDoubleSum
                                 value::TypeTags::NumberInt32,
                                 value::bitcastFrom<int32_t>(result)};
                     }
-                    // Fall through to the larger type.
+                    [[fallthrough]];  // To the larger type
                 }
                 case value::TypeTags::NumberInt64: {
                     if (sum.fitsLong()) {
@@ -2373,7 +2373,7 @@ std::tuple<bool, value::TypeTags, value::Value> ByteCode::builtinDoubleDoubleSum
                                 value::TypeTags::NumberInt64,
                                 value::bitcastFrom<int64_t>(sum.getLong())};
                     }
-                    // Fall through to the larger type.
+                    [[fallthrough]];  // To the larger type.
                 }
                 case value::TypeTags::NumberDouble: {
                     return {false,
diff --git a/src/mongo/db/exec/trial_stage.cpp b/src/mongo/db/exec/trial_stage.cpp
index 32d303c8f7d..66442c21eda 100644
--- a/src/mongo/db/exec/trial_stage.cpp
+++ b/src/mongo/db/exec/trial_stage.cpp
@@ -119,6 +119,7 @@ PlanStage::StageState TrialStage::_workTrialPlan(WorkingSetID* out) {
             // Increment the 'advanced' count and fall through into NEED_TIME so that we check for
             // the end of the trial period and assess the results for both NEED_TIME and ADVANCED.
             ++_specificStats.trialAdvanced;
+            [[fallthrough]];
         }
         case PlanStage::NEED_TIME:
             // Check whether we have completed the evaluation phase.
diff --git a/src/mongo/db/index/wildcard_key_generator.cpp b/src/mongo/db/index/wildcard_key_generator.cpp
index e43b2a21eb0..6aa2aa64d7b 100644
--- a/src/mongo/db/index/wildcard_key_generator.cpp
+++ b/src/mongo/db/index/wildcard_key_generator.cpp
@@ -156,6 +156,7 @@ void WildcardKeyGenerator::_traverseWildcard(SharedBufferFragmentBuilder& pooled
 
                 // Add an entry for the multi-key path, and then fall through to BSONType::Object.
                 _addMultiKey(pooledBufferBuilder, *path, multikeyPaths);
+                [[fallthrough]];
 
             case BSONType::Object:
                 if (_addKeyForEmptyLeaf(pooledBufferBuilder, elem, *path, keys, id))
diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 4b79d54f7c2..80a2dc60488 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -211,6 +211,7 @@ DocumentSource::GetNextResult DocumentSourceGroup::getNextSpilled() {
         switch (numAccumulators) {  // mirrors switch in spill()
             case 1:                 // Single accumulators serialize as a single Value.
                 _currentAccumulators[0]->process(_firstPartOfNextGroup.second, true);
+                [[fallthrough]];
             case 0:  // No accumulators so no Values.
                 break;
             default: {  // Multiple accumulators serialize as an array of Values.
diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index 76ec9a5debb..f6487b8e108 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -398,11 +398,11 @@ Value ExpressionAdd::evaluate(const Document& root, Variables* variables) const
             dassert(nonDecimalTotal.isInteger());
             if (nonDecimalTotal.fitsLong())
                 return Value(nonDecimalTotal.getLong());
-        // Fallthrough.
+            [[fallthrough]];
         case NumberInt:
             if (nonDecimalTotal.fitsLong())
                 return Value::createIntOrLong(nonDecimalTotal.getLong());
-        // Fallthrough.
+            [[fallthrough]];
         case NumberDouble:
             return Value(nonDecimalTotal.getDouble());
         default:
diff --git a/src/mongo/db/query/canonical_query_encoder.cpp b/src/mongo/db/query/canonical_query_encoder.cpp
index f41f6827694..ee6efa914b1 100644
--- a/src/mongo/db/query/canonical_query_encoder.cpp
+++ b/src/mongo/db/query/canonical_query_encoder.cpp
@@ -133,7 +133,7 @@ void encodeUserString(StringData s, BuilderType* builder) {
                 } else {
                     *builder << '\\';
                 }
-            // Fall through to default case.
+                [[fallthrough]];
             default:
                 if constexpr (hasAppendChar<BuilderType>) {
                     builder->appendChar(c);
diff --git a/src/mongo/db/query/fle/server_rewrite.cpp b/src/mongo/db/query/fle/server_rewrite.cpp
index a69b06bc397..38e757b9123 100644
--- a/src/mongo/db/query/fle/server_rewrite.cpp
+++ b/src/mongo/db/query/fle/server_rewrite.cpp
@@ -463,6 +463,7 @@ std::unique_ptr<MatchExpression> FLEQueryRewriter::_rewrite(MatchExpression* exp
             if (rewritten) {
                 return std::make_unique<ExprMatchExpression>(rewritten.release(), expCtx());
             }
+            [[fallthrough]];
         }
         default:
             return nullptr;
diff --git a/src/mongo/db/query/optimizer/cascades/logical_props_derivation.cpp b/src/mongo/db/query/optimizer/cascades/logical_props_derivation.cpp
index 510bad791e3..617d5a2074c 100644
--- a/src/mongo/db/query/optimizer/cascades/logical_props_derivation.cpp
+++ b/src/mongo/db/query/optimizer/cascades/logical_props_derivation.cpp
@@ -85,6 +85,7 @@ static void populateDistributionPaths(const PartialSchemaRequirements& req,
                 distributions.emplace(distributionAndPaths._type,
                                       std::move(distributionProjections));
             }
+            [[fallthrough]];
         }
 
         default:
diff --git a/src/mongo/db/query/optimizer/rewrites/const_eval.cpp b/src/mongo/db/query/optimizer/rewrites/const_eval.cpp
index 723bd29400d..e680406c006 100644
--- a/src/mongo/db/query/optimizer/rewrites/const_eval.cpp
+++ b/src/mongo/db/query/optimizer/rewrites/const_eval.cpp
@@ -415,6 +415,7 @@ void ConstEval::transport(ABT& n, const BinaryOp& op, ABT& lhs, ABT& rhs) {
                     }
                 }
             }
+            [[fallthrough]];
         }
 
         default:
diff --git a/src/mongo/db/query/sbe_stage_builder.cpp b/src/mongo/db/query/sbe_stage_builder.cpp
index db4e9a50e23..573fa246ea3 100644
--- a/src/mongo/db/query/sbe_stage_builder.cpp
+++ b/src/mongo/db/query/sbe_stage_builder.cpp
@@ -3088,6 +3088,7 @@ std::pair<std::unique_ptr<sbe::PlanStage>, PlanStageSlots> SlotBasedStageBuilder
                 childReqs.setIsBuildingUnionForTailableCollScan(true);
                 return makeUnionForTailableCollScan(root, childReqs);
             }
+            [[fallthrough]];
         default:
             break;
     }
diff --git a/src/mongo/db/repl/oplog.cpp b/src/mongo/db/repl/oplog.cpp
index b4a1d1e88d4..6a45d15edce 100644
--- a/src/mongo/db/repl/oplog.cpp
+++ b/src/mongo/db/repl/oplog.cpp
@@ -2025,8 +2025,8 @@ Status applyCommand_inlock(OperationContext* opCtx,
                                 "error"_attr = status,
                                 "oplogEntry"_attr = redact(entry.toBSONForLogging()));
                 }
+                [[fallthrough]];
             }
-            // fallthrough
             case ErrorCodes::OK:
                 done = true;
                 break;
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index 01818cca93b..9ae47741942 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -3604,7 +3604,8 @@ Status ReplicationCoordinatorImpl::_doReplSetReconfig(OperationContext* opCtx,
                           "Node not yet initialized; use the replSetInitiate command");
         case kConfigReplicationDisabled:
             invariant(
-                false);  // should be unreachable due to !_settings.usingReplSets() check above
+                false);       // should be unreachable due to !_settings.usingReplSets() check above
+            [[fallthrough]];  // Placate clang.
         case kConfigInitiating:
         case kConfigReconfiguring:
         case kConfigHBReconfiguring:
@@ -4575,7 +4576,7 @@ void ReplicationCoordinatorImpl::_performPostMemberStateUpdateAction(
             break;
         case kActionRollbackOrRemoved:
             _externalState->closeConnections();
-        /* FALLTHROUGH */
+            [[fallthrough]];
         case kActionSteppedDown:
             _externalState->onStepDownHook();
             ReplicaSetAwareServiceRegistry::get(_service).onStepDown();
diff --git a/src/mongo/db/repl/tenant_migration_donor_access_blocker.cpp b/src/mongo/db/repl/tenant_migration_donor_access_blocker.cpp
index d74890d7e65..141b2c1e6a5 100644
--- a/src/mongo/db/repl/tenant_migration_donor_access_blocker.cpp
+++ b/src/mongo/db/repl/tenant_migration_donor_access_blocker.cpp
@@ -94,6 +94,7 @@ Status TenantMigrationDonorAccessBlocker::checkIfCanWrite(Timestamp writeTs) {
             // As a sanity check, we track the highest allowed write timestamp to ensure no
             // writes are allowed with a timestamp higher than the block timestamp.
             _highestAllowedWriteTimestamp = std::max(writeTs, _highestAllowedWriteTimestamp);
+            [[fallthrough]];
         case BlockerState::State::kAborted:
             return Status::OK();
         case BlockerState::State::kBlockWrites:
diff --git a/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp b/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
index ccda6697c83..1b9f825d3d9 100644
--- a/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
+++ b/src/mongo/db/s/migration_chunk_cloner_source_legacy.cpp
@@ -428,8 +428,8 @@ void MigrationChunkClonerSourceLegacy::cancelClone(OperationContext* opCtx) noex
                       "Failed to cancel migration",
                       "error"_attr = redact(status));
             }
+            [[fallthrough]];
         }
-        // Intentional fall through
         case kNew:
             _cleanup();
             break;
diff --git a/src/mongo/db/service_entry_point_common.cpp b/src/mongo/db/service_entry_point_common.cpp
index be2b96c0657..43894e827c5 100644
--- a/src/mongo/db/service_entry_point_common.cpp
+++ b/src/mongo/db/service_entry_point_common.cpp
@@ -2103,9 +2103,10 @@ std::unique_ptr<HandleRequest::OpRunner> HandleRequest::makeOpRunner() {
         case dbQuery:
             if (!executionContext->nsString().isCommand())
                 return std::make_unique<QueryOpRunner>(this);
-            // FALLTHROUGH: it's a query containing a command. Ideally, we'd like to let through
-            // only hello|isMaster commands but at this point the command hasn't been parsed yet, so
-            // we don't know what it is.
+            // Fallthrough because it's a query containing a command. Ideally, we'd like to let
+            // through only hello|isMaster commands but at this point the command hasn't been parsed
+            // yet, so we don't know what it is.
+            [[fallthrough]];
         case dbMsg:
             return std::make_unique<CommandOpRunner>(this);
         case dbGetMore:
diff --git a/src/mongo/db/storage/key_string.cpp b/src/mongo/db/storage/key_string.cpp
index 908bd51908a..d4a9e8efde3 100644
--- a/src/mongo/db/storage/key_string.cpp
+++ b/src/mongo/db/storage/key_string.cpp
@@ -1658,7 +1658,7 @@ void toBsonValue(uint8_t ctype,
         case CType::kNumericNegativeLargeMagnitude:
             inverted = !inverted;
             isNegative = true;
-        // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
         case CType::kNumericPositiveLargeMagnitude: {
             const uint8_t originalType = typeBits->readNumeric();
             keyStringAssert(31231,
@@ -1725,7 +1725,7 @@ void toBsonValue(uint8_t ctype,
         case CType::kNumericNegativeSmallMagnitude:
             inverted = !inverted;
             isNegative = true;
-            // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
 
         case CType::kNumericPositiveSmallMagnitude: {
             const uint8_t originalType = typeBits->readNumeric();
@@ -1856,7 +1856,7 @@ void toBsonValue(uint8_t ctype,
         case CType::kNumericNegative1ByteInt:
             inverted = !inverted;
             isNegative = true;
-            // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
 
         case CType::kNumericPositive1ByteInt:
         case CType::kNumericPositive2ByteInt:
@@ -2105,7 +2105,7 @@ void filterKeyFromKeyString(uint8_t ctype, BufReader* reader, bool inverted, Ver
         case CType::kNumericNegativeLargeMagnitude:
             inverted = !inverted;
             isNegative = true;
-        // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
         case CType::kNumericPositiveLargeMagnitude: {
             uint64_t encoded = readType<uint64_t>(reader, inverted);
             encoded = endian::bigToNative(encoded);
@@ -2131,7 +2131,7 @@ void filterKeyFromKeyString(uint8_t ctype, BufReader* reader, bool inverted, Ver
         case CType::kNumericNegativeSmallMagnitude:
             inverted = !inverted;
             isNegative = true;
-            // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
 
         case CType::kNumericPositiveSmallMagnitude: {
             uint64_t encoded = readType<uint64_t>(reader, inverted);
@@ -2193,7 +2193,7 @@ void filterKeyFromKeyString(uint8_t ctype, BufReader* reader, bool inverted, Ver
         case CType::kNumericNegative1ByteInt:
             inverted = !inverted;
             isNegative = true;
-            // fallthrough (format is the same as positive, but inverted)
+            [[fallthrough]];  // format is the same as positive, but inverted
 
         case CType::kNumericPositive1ByteInt:
         case CType::kNumericPositive2ByteInt:
@@ -2440,7 +2440,7 @@ void TypeBits::appendZero(uint8_t zeroType) {
                 break;
             }
             zeroType = kV1NegativeDoubleZero;
-        // fallthrough for 5-bit encodings
+            [[fallthrough]];  // fallthrough for 5-bit encodings
         case kDecimalZero0xxx:
         case kDecimalZero1xxx:
         case kDecimalZero2xxx:
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
index ca2e5d8c1b4..969db98506e 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.cpp
@@ -634,7 +634,7 @@ void WiredTigerRecoveryUnit::_txnOpen() {
                 _readAtTimestamp = _beginTransactionAtAllDurableTimestamp(session);
                 break;
             }
-            // Intentionally continue to the next case to read at the _readAtTimestamp.
+            [[fallthrough]];  // Continue to the next case to read at the _readAtTimestamp.
         }
         case ReadSource::kProvided: {
             WiredTigerBeginTxnBlock txnOpen(
diff --git a/src/mongo/embedded/periodic_runner_embedded.cpp b/src/mongo/embedded/periodic_runner_embedded.cpp
index 68c94fa541d..87adf0842ce 100644
--- a/src/mongo/embedded/periodic_runner_embedded.cpp
+++ b/src/mongo/embedded/periodic_runner_embedded.cpp
@@ -111,11 +111,12 @@ bool PeriodicRunnerEmbedded::tryPump() {
         switch (jobExecStatus) {
             default:
                 invariant(false);
+                [[fallthrough]];  //  Placate clang
             case PeriodicJobImpl::ExecutionStatus::kPaused:
             case PeriodicJobImpl::ExecutionStatus::kNotScheduled:
                 // Paused jobs should be moved to the paused list and removed from the running heap
                 _Pausedjobs.push_back(std::move(_jobs.back()));
-            // fall through
+                [[fallthrough]];
             case PeriodicJobImpl::ExecutionStatus::kCanceled:
                 // Cancelled jobs should be removed
                 _jobs.pop_back();
diff --git a/src/mongo/s/service_entry_point_mongos.cpp b/src/mongo/s/service_entry_point_mongos.cpp
index 43e2d5213f0..f2d3cb2b0ad 100644
--- a/src/mongo/s/service_entry_point_mongos.cpp
+++ b/src/mongo/s/service_entry_point_mongos.cpp
@@ -148,7 +148,7 @@ Future<DbResponse> HandleRequest::handleRequest() {
                 return Future<DbResponse>::makeReady(
                     makeErrorResponseToDeprecatedOpQuery("OP_QUERY is no longer supported"));
             }
-        // FALLTHROUGH: it's a query containing a command
+            [[fallthrough]];  // It's a query containing a command
         case dbMsg:
             return std::make_unique<CommandOpRunner>(shared_from_this())->run();
         case dbGetMore: {
diff --git a/src/mongo/tools/bridge.cpp b/src/mongo/tools/bridge.cpp
index ba5fa67a856..6150bb2f77b 100644
--- a/src/mongo/tools/bridge.cpp
+++ b/src/mongo/tools/bridge.cpp
@@ -388,8 +388,9 @@ Future<DbResponse> ServiceEntryPointBridge::handleRequest(OperationContext* opCt
                 }
                 return Future<DbResponse>::makeReady({Message()});
             }
-        // Forward the message to 'dest' after waiting for 'hostSettings.delay'
-        // milliseconds.
+            // Forward the message to 'dest' after waiting for 'hostSettings.delay'
+            // milliseconds.
+            [[fallthrough]];
         case HostSettings::State::kForward:
             sleepmillis(durationCount<Milliseconds>(hostSettings.delay));
             break;
diff --git a/src/mongo/unittest/unittest.cpp b/src/mongo/unittest/unittest.cpp
index 94adc7f1046..219726d8b29 100644
--- a/src/mongo/unittest/unittest.cpp
+++ b/src/mongo/unittest/unittest.cpp
@@ -336,6 +336,12 @@ bool isSubset(BSONObj haystack, BSONObj needle) {
             case Array:
                 // not supported
                 invariant(false);
+                // This annotation shouldn't really be needed because
+                // `invariantFailed` is annotated to be `noreturn`,
+                // but clang 12 doesn't seem to be able to capitalize
+                // on that fact to see that we are not actually
+                // falling through.
+                [[fallthrough]];
             default:
                 if (SimpleBSONElementComparator::kInstance.compare(foundElement, element) != 0) {
                     return false;
diff --git a/src/mongo/util/net/ssl/detail/io.hpp b/src/mongo/util/net/ssl/detail/io.hpp
index 90453c3d8e8..b48f7dbc3ec 100644
--- a/src/mongo/util/net/ssl/detail/io.hpp
+++ b/src/mongo/util/net/ssl/detail/io.hpp
@@ -214,6 +214,7 @@ public:
                                 break;
                             }
                     }
+                    [[fallthrough]];
 
                     default:
                         if (bytes_transferred == ~std::size_t(0))
@@ -248,7 +249,7 @@ public:
                                 core_.pending_write_.expires_at(core_.neg_infin());
 
                                 // Fall through to call handler.
-
+                                [[fallthrough]];
                             default:
 
                                 // Pass the result to the handler.
diff --git a/src/third_party/boost/boost/date_time/time_facet.hpp b/src/third_party/boost/boost/date_time/time_facet.hpp
index eaa365913a3..0b740216957 100644
--- a/src/third_party/boost/boost/date_time/time_facet.hpp
+++ b/src/third_party/boost/boost/date_time/time_facet.hpp
@@ -1140,6 +1140,9 @@ namespace date_time {
                     // %s is the same as %S%f so we drop through into %f if we are
                     // not at the end of the stream
                   }
+                  // MONGODB MODIFICATION: Without this we can't use
+                  // the automated fallthrough detection.
+                  BOOST_FALLTHROUGH;
                   /* Falls through. */
                 case 'f':
                   {
diff --git a/src/third_party/boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp b/src/third_party/boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp
index 0ece2808910..2aadcdd74ab 100644
--- a/src/third_party/boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp
+++ b/src/third_party/boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp
@@ -119,6 +119,12 @@ namespace boost { namespace property_tree {
             case object:
             default:
                 BOOST_ASSERT(false); // must start with string, i.e. call new_value
+                // MONGODB MODIFICATION: The use of `BOOST_ASSERT(false)` above is probably a bug,
+                // because falling through into the `key` case doesn't look like what we ought to
+                // do, and that is what would happen if BOOST_ASSERT was disabled. However, we honor
+                // the behavior of the code as written, and add a fallthrough annotation so we can
+                // make use of it in our own code.
+                BOOST_FALLTHROUGH;
             case key: {
                 l.t->push_back(std::make_pair(key_buffer, Ptree()));
                 l.k = object;