x/tools/go/analysis/nilness: heuristics for flagging conversions from nil *T to interface

[adonovan]

We have forever been discussing ways to make the nilness analyzer report conversions from a definitely-nil pointer of type *T to an interface type. The challenge is how to distinguish the legitimate uses (when a nil *T pointer is valid) from the mistakes. This issue is a place to publicly record some of the raw data we got from running a simple heuristic across the Go module mirror corpus and discuss potential improvements.

The analysis is just this patch to the existing code:

diff --git a/go/analysis/passes/nilness/nilness.go b/go/analysis/passes/nilness/nilness.go
index faaf12a93..8b049a4ea 100644
--- a/go/analysis/passes/nilness/nilness.go
+++ b/go/analysis/passes/nilness/nilness.go
@@ -121,6 +121,15 @@ func runFunc(pass *analysis.Pass, fn *ssa.Function) {
                        case *ssa.Send:
                                // (Not a runtime error, but a likely mistake.)
                                notNil(stack, instr, instr.Chan, "send to nil channel")
+
+                       case *ssa.MakeInterface:
+                               switch instr.X.Type().Underlying().(type) {
+                               case *types.Slice, *types.Map:
+                                       // nils of these types are fine
+                               default:
+                                       notNil(stack, instr, instr.X,
+                                               fmt.Sprintf("converting nil %s to interface %s", instr.X.Type(), instr.Type()))
+                               }
                        }
                }

https://go-mod-viewer.appspot.com/github.com/cilium/ebpf@v0.16.0/linker.go#L319: converting nil *github.com/cilium/ebpf/btf.Func to interface github.com/cilium/ebpf/btf.Type
https://go-mod-viewer.appspot.com/github.com/cilium/ebpf@v0.16.0/linker.go#L319: converting nil *github.com/cilium/ebpf/btf.Func to interface github.com/cilium/ebpf/btf.Type
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_delegator_test.go#L207: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddDelegatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_permissionless_delegator_tx_test.go#L1861: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddPermissionlessDelegatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_permissionless_validator_tx_test.go#L1834: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddPermissionlessValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_subnet_validator_test.go#L215: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_subnet_validator_test.go#L221: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_subnet_validator_test.go#L227: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/ava-labs/avalanchego@v1.11.11/vms/platformvm/txs/add_validator_test.go#L224: converting nil *github.com/ava-labs/avalanchego/vms/platformvm/txs.AddValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L28: converting nil *github.com/go-spring/spring-base/log.AcceptAllFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L29: converting nil *github.com/go-spring/spring-base/log.DenyAllFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L30: converting nil *github.com/go-spring/spring-base/log.LevelFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L31: converting nil *github.com/go-spring/spring-base/log.LevelMatchFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L32: converting nil *github.com/go-spring/spring-base/log.LevelRangeFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L33: converting nil *github.com/go-spring/spring-base/log.TimeFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L34: converting nil *github.com/go-spring/spring-base/log.TagFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/go-spring/spring-base@v1.1.3/log/plugin_filter.go#L35: converting nil *github.com/go-spring/spring-base/log.CompositeFilter to interface github.com/go-spring/spring-base/log.Filter
https://go-mod-viewer.appspot.com/github.com/andersfylling/snowflake/v5@v5.0.1/snowflake_test.go#L66: converting nil *github.com/andersfylling/snowflake/v5.Snowflake to interface interface{}
https://go-mod-viewer.appspot.com/github.com/andersfylling/snowflake/v5@v5.0.1/snowflake_test.go#L88: converting nil *github.com/andersfylling/snowflake/v5.Snowflake to interface interface{}
https://go-mod-viewer.appspot.com/github.com/andersfylling/snowflake/v5@v5.0.1/snowflake_test.go#L111: converting nil *github.com/andersfylling/snowflake/v5.Snowflake to interface interface{}
https://go-mod-viewer.appspot.com/k8s.io/client-go@v0.31.1/rest/request_test.go#L1968: converting nil *k8s.io/apimachinery/pkg/apis/meta/v1.DeleteOptions to interface interface{}
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_delegator_test.go#L207: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddDelegatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_permissionless_delegator_tx_test.go#L1860: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddPermissionlessDelegatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_permissionless_validator_tx_test.go#L1833: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddPermissionlessValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_subnet_validator_test.go#L215: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_subnet_validator_test.go#L221: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_subnet_validator_test.go#L227: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddSubnetValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/MetalBlockchain/metalgo@v1.11.9/vms/platformvm/txs/add_validator_test.go#L224: converting nil *github.com/MetalBlockchain/metalgo/vms/platformvm/txs.AddValidatorTx to interface any
https://go-mod-viewer.appspot.com/github.com/enbility/spine-go@v0.7.0/util/type.go#L11: converting nil *T to interface any
https://go-mod-viewer.appspot.com/github.com/cilium/cilium@v1.16.2/pkg/types/portmap_test.go#L163: converting nil *github.com/cilium/cilium/pkg/types.namedPortMultiMap to interface github.com/cilium/cilium/pkg/types.NamedPortMultiMap
https://go-mod-viewer.appspot.com/github.com/vmware/govmomi@v0.43.0/vim25/types/helpers_test.go#L352: converting nil *T to interface any
https://go-mod-viewer.appspot.com/github.com/timandy/routine@v1.1.4/thread_local_map_entry_test.go#L68: converting nil *github.com/timandy/routine.personCloneable to interface github.com/timandy/routine.entry
https://go-mod-viewer.appspot.com/github.com/timandy/routine@v1.1.4/thread_local_map_entry_test.go#L137: converting nil *github.com/timandy/routine.personCloneable to interface github.com/timandy/routine.entry
https://go-mod-viewer.appspot.com/github.com/openimsdk/tools@v0.0.49/mw/replace_nil_test.go#L42: converting nil *github.com/openimsdk/tools/mw.A to interface any
https://go-mod-viewer.appspot.com/github.com/expr-lang/expr@v1.16.9/test/deref/deref_test.go#L188: converting nil *int32 to interface any
https://go-mod-viewer.appspot.com/goki.dev/laser@v0.1.34/basic_test.go#L26: converting nil *goki.dev/laser.A to interface any
https://go-mod-viewer.appspot.com/github.com/goki/ki@v1.1.17/kit/convert_test.go#L24: converting nil *github.com/goki/ki/kit.A to interface any
https://go-mod-viewer.appspot.com/github.com/davecgh/go-xdr@v0.0.0-20161123171359-e6a2ba005892/xdr2/decode_test.go#L699: converting nil *int32 to interface interface{}
https://go-mod-viewer.appspot.com/k8s.io/apiserver@v0.31.1/pkg/admission/plugin/cel/filter_test.go#L475: converting nil *k8s.io/apimachinery/pkg/apis/meta/v1/unstructured.Unstructured to interface k8s.io/apimachinery/pkg/runtime.Object
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L3317: converting nil *k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.valueStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L3322: converting nil **k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.valueStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L3333: converting nil *k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.pointerStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L3338: converting nil **k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.pointerStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L7131: converting nil *k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.valueStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L7137: converting nil **k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.valueStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L7151: converting nil *k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.pointerStringer to interface any
https://go-mod-viewer.appspot.com/k8s.io/kube-openapi@v0.0.0-20240826222958-65a50c78dec5/pkg/internal/third_party/go-json-experiment/json/arshal_test.go#L7157: converting nil **k8s.io/kube-openapi/pkg/internal/third_party/go-json-experiment/json.pointerStringer to interface any
https://go-mod-viewer.appspot.com/github.com/stellar/go-xdr@v0.0.0-20231122183749-b53fb00bcac2/xdr2/decode_test.go#L630: converting nil *int32 to interface interface{}
https://go-mod-viewer.appspot.com/github.com/stellar/go-xdr@v0.0.0-20231122183749-b53fb00bcac2/xdr3/decode_test.go#L755: converting nil *int32 to interface interface{}
https://go-mod-viewer.appspot.com/open-match.dev/open-match@v1.8.1/examples/demo/updater/updater_test.go#L47: converting nil *int to interface interface{}
https://go-mod-viewer.appspot.com/github.com/wI2L/jettison@v0.7.4/json_1.14_test.go#L96: converting nil *github.com/wI2L/jettison.niljsonm to interface encoding/json.Marshaler
https://go-mod-viewer.appspot.com/github.com/wI2L/jettison@v0.7.4/json_1.14_test.go#L104: converting nil *github.com/wI2L/jettison.niltextm to interface encoding.TextMarshaler
https://go-mod-viewer.appspot.com/github.com/wI2L/jettison@v0.7.4/json_1.14_test.go#L112: converting nil *github.com/wI2L/jettison.niljetim to interface github.com/wI2L/jettison.comboMarshaler
https://go-mod-viewer.appspot.com/github.com/wI2L/jettison@v0.7.4/json_1.14_test.go#L120: converting nil *github.com/wI2L/jettison.nilmjctx to interface github.com/wI2L/jettison.comboMarshalerCtx
https://go-mod-viewer.appspot.com/github.com/hashicorp/go-metrics@v0.5.3/circonus/circonus_test.go#L188: converting nil *github.com/hashicorp/go-metrics/circonus.CirconusSink to interface github.com/hashicorp/go-metrics.MetricSink
https://go-mod-viewer.appspot.com/github.com/hashicorp/go-metrics@v0.5.3/datadog/dogstatsd_test.go#L165: converting nil *github.com/hashicorp/go-metrics/datadog.DogStatsdSink to interface github.com/hashicorp/go-metrics.MetricSink
https://go-mod-viewer.appspot.com/github.com/hashicorp/go-metrics@v0.5.3/prometheus/prometheus_test.go#L395: converting nil *github.com/hashicorp/go-metrics/prometheus.PrometheusSink to interface github.com/hashicorp/go-metrics.MetricSink
https://go-mod-viewer.appspot.com/github.com/hashicorp/go-metrics@v0.5.3/prometheus/prometheus_test.go#L397: converting nil *github.com/hashicorp/go-metrics/prometheus.PrometheusPushSink to interface github.com/hashicorp/go-metrics.MetricSink
https://go-mod-viewer.appspot.com/bitbucket.org/ai69/amoy@v0.2.3/shell_test.go#L106: converting nil *bitbucket.org/ai69/amoy.customStruct to interface bitbucket.org/ai69/amoy.customInterface
https://go-mod-viewer.appspot.com/github.com/blend/go-sdk@v1.20240719.1/ex/ex_test.go#L343: converting nil *github.com/blend/go-sdk/ex.Ex to interface any
https://go-mod-viewer.appspot.com/github.com/blend/go-sdk@v1.20240719.1/ex/ex_test.go#L374: converting nil *github.com/blend/go-sdk/ex.Ex to interface any

[gabyhelp]

Related Issues and Documentation

• reflect: MakeFunc functions cannot use the nil error return idiom #6871 (closed)
• x/tools/go/analysis/passes/nilness: bogus "impossible condition: non-nil == nil" that is actually possible #66835 (closed)
• cmd/compile: typecheck panic, interface conversion: types.Object is nil, not *ir.Name #50552 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[findleyr]

Thanks for collecting this data! The first 10 links I clicked were false positives.

Arbitrary Examples:

• target := btf.Type((*btf.Func)(nil))
• RegisterPlugin("TagFilter", PluginTypeFilter, (Filter)((*TagFilter)(nil)))
• params: runtime.Object(nilUnstructured)
• var dd *DogStatsdSink; _ = metrics.MetricSink(dd)
• {"Custom Interface Struct Nil", customInterface(structNil), true} (in a function called TestIsInterfaceNil)
• {P{1, 4}, addr(any((**valueStringer)(nil)))}
• var i32p *int32 ... TstDecode(bytes.NewReader(buf))(reflect.ValueOf(i32p)) ... ii32p := interface{}(i32p)

This looks like various patterns of:

• Intentionally distinguished nils.
• Implements checks.
• Intentional tests for typed nil handling.
• Imprecise analysis due to use of reflect.

Probably some of these can be avoided, but the lack of clear true positives does not bode well.

[adonovan]

the lack of clear true positives does not bode well.

To be clear, no-one intends to merge this change; it was just a starting point for other experiments.

I was surprised the corpus contained so few matches. I suspect the much more common case involves maybe nil pointers, e.g.

func maybeOpen(filename string) io.Reader
   var f *os.FIle
   if maybe { f, _ = os.Open(filename) }
   return f // oops
}

[ianlancetaylor]

Since the goal here is to identify cases of https://go.dev/doc/faq#nil_error, we shouldn't even be looking at cases where the code explicitly converts to interface type. We should only be looking at cases where the conversion is implicit. Unfortunately the current list has too many irrelevant items to judge whether this is useful or not.