Preallocate dependency info on BuildTarget (#3023)

src/core/build_target.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"slices"
 	"sort"
 	"strings"
 	"sync"
@@ -1630,6 +1631,11 @@ func (target *BuildTarget) AddDependency(dep BuildLabel) {
 	target.AddMaybeExportedDependency(dep, false, false, false)
 }
 
+// HintDependencies allocates space for at least the given number of dependencies without reallocating.
+func (target *BuildTarget) HintDependencies(n int) {
+	target.dependencies = slices.Grow(target.dependencies, n)
+}
+
 // AddMaybeExportedDependency adds a dependency to this target which may be exported. It deduplicates against any existing deps.
 func (target *BuildTarget) AddMaybeExportedDependency(dep BuildLabel, exported, source, internal bool) {
 	if dep == target.Label {

src/parse/asp/targets.go

@@ -263,6 +263,7 @@ func populateTarget(s *scope, t *core.BuildTarget, args []pyObject) {
 	addMaybeNamed(s, "data", args[dataBuildRuleArgIdx], t.AddDatum, t.AddNamedDatum, false, false)
 	addMaybeNamedOutput(s, "outs", args[outsBuildRuleArgIdx], t.AddOutput, t.AddNamedOutput, t, false)
 	addMaybeNamedOutput(s, "optional_outs", args[optionalOutsBuildRuleArgIdx], t.AddOptionalOutput, nil, t, true)
+	t.HintDependencies(depLen(args[depsBuildRuleArgIdx]) + depLen(args[exportedDepsBuildRuleArgIdx]) + depLen(args[internalDepsBuildRuleArgIdx]))
 	addDependencies(s, "deps", args[depsBuildRuleArgIdx], t, false, false)
 	addDependencies(s, "exported_deps", args[exportedDepsBuildRuleArgIdx], t, true, false)
 	addDependencies(s, "internal_deps", args[internalDepsBuildRuleArgIdx], t, false, true)
@@ -301,6 +302,14 @@ func populateTarget(s *scope, t *core.BuildTarget, args []pyObject) {
 	}
 }
 
+// depLen returns the length of a (potential) list
+func depLen(obj pyObject) int {
+	if l, ok := asList(obj); ok {
+		return len(l)
+	}
+	return 0
+}
+
 // addEntryPoints adds entry points to a target
 func addEntryPoints(s *scope, arg pyObject, target *core.BuildTarget) {
 	entryPointsPy, ok := asDict(arg)