Check switches on syntax.Expr for exhaustiveness. (#11113)

**What this PR does / why we need it**:
In preparation to serlialize the LogQL AST as JSON we are declaring the
`syntax.Expr` as a "sum type" and test all switch statements on it to be
exhaustive.

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](0d4416a4b0)

.golangci.yml

@@ -79,6 +79,7 @@ linters:
     - goimports
     - gosimple
     - staticcheck
+    - gochecksumtype
   disable:
     - unused
     - unparam

pkg/logql/engine.go

@@ -430,7 +430,7 @@ func (q *query) evalSample(ctx context.Context, expr syntax.SampleExpr) (promql_
 
 func (q *query) checkIntervalLimit(expr syntax.SampleExpr, limit time.Duration) error {
 	var err error
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch e := e.(type) {
 		case *syntax.RangeAggregationExpr:
 			if e.Left == nil || e.Left.Interval <= limit {

pkg/logql/evaluator.go

@@ -112,7 +112,7 @@ func Sortable(q Params) (bool, error) {
 	if err != nil {
 		return false, err
 	}
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		rangeExpr, ok := e.(*syntax.VectorAggregationExpr)
 		if !ok {
 			return

pkg/logql/optimize.go

@@ -6,7 +6,7 @@ import "github.com/grafana/loki/pkg/logql/syntax"
 func optimizeSampleExpr(expr syntax.SampleExpr) (syntax.SampleExpr, error) {
 	var skip bool
 	// we skip sharding AST for now, it's not easy to clone them since they are not part of the language.
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch e.(type) {
 		case *ConcatSampleExpr, *DownstreamSampleExpr:
 			skip = true
@@ -28,7 +28,7 @@ func optimizeSampleExpr(expr syntax.SampleExpr) (syntax.SampleExpr, error) {
 
 // removeLineformat removes unnecessary line_format within a SampleExpr.
 func removeLineformat(expr syntax.SampleExpr) {
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		rangeExpr, ok := e.(*syntax.RangeAggregationExpr)
 		if !ok {
 			return

pkg/logql/rangemapper.go

@@ -177,7 +177,7 @@ func (m RangeMapper) Map(expr syntax.SampleExpr, vectorAggrPushdown *syntax.Vect
 // Example: expression `count_over_time({app="foo"}[10m])` returns 10m
 func getRangeInterval(expr syntax.SampleExpr) time.Duration {
 	var rangeInterval time.Duration
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.RangeAggregationExpr:
 			rangeInterval = concrete.Left.Interval
@@ -190,7 +190,7 @@ func getRangeInterval(expr syntax.SampleExpr) time.Duration {
 // such as `| json` or `| logfmt`, that would result in an exploding amount of series in downstream queries.
 func hasLabelExtractionStage(expr syntax.SampleExpr) bool {
 	found := false
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.LogfmtParserExpr:
 			found = true
@@ -278,7 +278,7 @@ func (m RangeMapper) vectorAggrWithRangeDownstreams(expr *syntax.RangeAggregatio
 // Returns the updated downstream ConcatSampleExpr.
 func appendDownstream(downstreams *ConcatSampleExpr, expr syntax.SampleExpr, interval time.Duration, offset time.Duration) *ConcatSampleExpr {
 	sampleExpr := clone(expr)
-	sampleExpr.Walk(func(e interface{}) {
+	sampleExpr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.RangeAggregationExpr:
 			concrete.Left.Interval = interval
@@ -300,7 +300,7 @@ func getOffsets(expr syntax.SampleExpr) []time.Duration {
 	// Expect to always find at most 1 offset, so preallocate it accordingly
 	offsets := make([]time.Duration, 0, 1)
 
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.RangeAggregationExpr:
 			offsets = append(offsets, concrete.Left.Offset)

pkg/logql/syntax/ast.go

@@ -22,6 +22,8 @@ import (
 )
 
 // Expr is the root expression which can be a SampleExpr or LogSelectorExpr
+//
+//sumtype:decl
 type Expr interface {
 	logQLExpr()      // ensure it's not implemented accidentally
 	Shardable() bool // A recursive check on the AST to see if it's shardable.
@@ -2106,7 +2108,7 @@ func (e *VectorExpr) MatcherGroups() ([]MatcherRange, error)  { return nil, e.er
 func (e *VectorExpr) Extractor() (log.SampleExtractor, error) { return nil, nil }
 
 func ReducesLabels(e Expr) (conflict bool) {
-	e.Walk(func(e interface{}) {
+	e.Walk(func(e Expr) {
 		switch expr := e.(type) {
 		case *RangeAggregationExpr:
 			if groupingReducesLabels(expr.Grouping) {
@@ -2135,6 +2137,8 @@ func ReducesLabels(e Expr) (conflict bool) {
 					break
 				}
 			}
+		default:
+			return
 		}
 	})
 	return

pkg/logql/syntax/walk.go

@@ -1,6 +1,6 @@
 package syntax
 
-type WalkFn = func(e interface{})
+type WalkFn = func(e Expr)
 
 func walkAll(f WalkFn, xs ...Walkable) {
 	for _, x := range xs {

pkg/logql/syntax/walk_test.go

@@ -32,7 +32,7 @@ func Test_Walkable(t *testing.T) {
 			require.Nil(t, err)
 
 			var cnt int
-			expr.Walk(func(_ interface{}) { cnt++ })
+			expr.Walk(func(_ Expr) { cnt++ })
 			require.Equal(t, test.want, cnt)
 		})
 	}
@@ -77,7 +77,7 @@ func Test_AppendMatchers(t *testing.T) {
 			expr, err := ParseExpr(test.expr)
 			require.NoError(t, err)
 
-			expr.Walk(func(e interface{}) {
+			expr.Walk(func(e Expr) {
 				switch me := e.(type) {
 				case *MatchersExpr:
 					me.AppendMatchers(test.matchers)

pkg/querier/multi_tenant_querier.go

@@ -227,7 +227,7 @@ func removeTenantSelector(params logql.SelectSampleParams, tenantIDs []string) (
 // replaceMatchers traverses the passed expression and replaces all matchers.
 func replaceMatchers(expr syntax.Expr, matchers []*labels.Matcher) syntax.Expr {
 	expr, _ = syntax.Clone(expr)
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		switch concrete := e.(type) {
 		case *syntax.MatchersExpr:
 			concrete.Mts = matchers

pkg/querier/queryrange/split_by_interval.go

@@ -322,7 +322,7 @@ func maxRangeVectorAndOffsetDuration(q string) (time.Duration, time.Duration, er
 	}
 
 	var maxRVDuration, maxOffset time.Duration
-	expr.Walk(func(e interface{}) {
+	expr.Walk(func(e syntax.Expr) {
 		if r, ok := e.(*syntax.LogRange); ok {
 			if r.Interval > maxRVDuration {
 				maxRVDuration = r.Interval