cmd/go/internal/tool: include dynamically buildable tools

`go tool` only listed binaries already built into `pkg/tool`, so users
couldn't discover builtin tools that can be built on demand. Extend the
listing to include cmd/ tools that can be built with the current toolchain
while avoiding duplicates and keeping the output sorted.

For #75960

Change-Id: I32b0f5ae6cd2c67cf3af465ffb5b4ac3d446ded4

Author: Agent-Hellboy

src/cmd/go/internal/tool/tool.go

@@ -19,6 +19,7 @@ import (
 	"os/exec"
 	"os/signal"
 	"path"
+	"path/filepath"
 	"slices"
 	"sort"
 	"strings"
@@ -148,6 +149,16 @@ func listTools(loaderstate *modload.State, ctx context.Context) {
 		return
 	}
 
+	toolSet := make(map[string]bool)
+	var toolNames []string
+	addTool := func(name string) {
+		if toolSet[name] {
+			return
+		}
+		toolSet[name] = true
+		toolNames = append(toolNames, name)
+	}
+
 	sort.Strings(names)
 	for _, name := range names {
 		// Unify presentation by going to lower case.
@@ -159,7 +170,37 @@ func listTools(loaderstate *modload.State, ctx context.Context) {
 		if cfg.BuildToolchainName == "gccgo" && !isGccgoTool(name) {
 			continue
 		}
-		fmt.Println(name)
+		addTool(name)
+	}
+
+	// Also list builtin tools that can be built on demand.
+	// These are packages in cmd/ that would be installed to the tool directory.
+	cmdDir := filepath.Join(cfg.GOROOT, "src", "cmd")
+	entries, err := os.ReadDir(cmdDir)
+	if err == nil {
+		for _, entry := range entries {
+			if !entry.IsDir() {
+				continue
+			}
+			toolName := entry.Name()
+			// Skip packages that are not tools.
+			if toolName == "internal" || toolName == "vendor" {
+				continue
+			}
+			// Check if this tool is already in the tool directory.
+			if toolSet[toolName] {
+				continue
+			}
+			// Check if it's a valid builtin tool; add if so to keep deduped set.
+			if tool := loadBuiltinTool(toolName); tool != "" {
+				addTool(toolName)
+			}
+		}
+	}
+
+	sort.Strings(toolNames)
+	for _, tool := range toolNames {
+		fmt.Println(tool)
 	}
 
 	loaderstate.InitWorkfile()

src/cmd/go/internal/tool/tool_test.go

@@ -0,0 +1,124 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tool
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestListToolsBuiltinDiscovery(t *testing.T) {
+	// Test the directory scanning logic that was added to listTools
+	// This tests that we correctly identify directories and skip non-directories
+
+	// Create a temporary directory structure to simulate cmd/ directory
+	tempDir := t.TempDir()
+	cmdDir := filepath.Join(tempDir, "cmd")
+	if err := os.MkdirAll(cmdDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create some tool directories
+	tools := []string{"vet", "cgo", "cover", "fix", "godoc"}
+	for _, tool := range tools {
+		toolDir := filepath.Join(cmdDir, tool)
+		if err := os.MkdirAll(toolDir, 0755); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// Create some non-tool directories that should be skipped
+	nonTools := []string{"internal", "vendor"}
+	for _, nonTool := range nonTools {
+		nonToolDir := filepath.Join(cmdDir, nonTool)
+		if err := os.MkdirAll(nonToolDir, 0755); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// Create a regular file (should be skipped)
+	filePath := filepath.Join(cmdDir, "not-a-directory.txt")
+	if err := os.WriteFile(filePath, []byte("test"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Test directory reading logic (simulating the logic from listTools)
+	entries, err := os.ReadDir(cmdDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var foundTools []string
+	for _, entry := range entries {
+		// Skip non-directories (this is the logic we added)
+		if !entry.IsDir() {
+			continue
+		}
+
+		toolName := entry.Name()
+		// Skip packages that are not tools (this is the logic we added)
+		if toolName == "internal" || toolName == "vendor" {
+			continue
+		}
+
+		foundTools = append(foundTools, toolName)
+	}
+
+	// Sort for consistent comparison
+	// (In the real code, this happens via the toolSet map and final output)
+	for i := 0; i < len(foundTools)-1; i++ {
+		for j := i + 1; j < len(foundTools); j++ {
+			if foundTools[i] > foundTools[j] {
+				foundTools[i], foundTools[j] = foundTools[j], foundTools[i]
+			}
+		}
+	}
+
+	// Verify we found the expected tools
+	expectedTools := []string{"cgo", "cover", "fix", "godoc", "vet"}
+	if len(foundTools) != len(expectedTools) {
+		t.Errorf("Found %d tools, expected %d: %v", len(foundTools), len(expectedTools), foundTools)
+	}
+
+	for i, expected := range expectedTools {
+		if i >= len(foundTools) || foundTools[i] != expected {
+			t.Errorf("Expected tool %q at position %d, got %q", expected, i, foundTools[i])
+		}
+	}
+}
+
+func TestToolSetTracking(t *testing.T) {
+	// Test the toolSet map logic that prevents duplicates
+	// This tests part of the new functionality in listTools
+
+	// Simulate the toolSet map logic
+	toolSet := make(map[string]bool)
+
+	// Add some tools to the set (simulating tools found in tool directory)
+	existingTools := []string{"vet", "cgo"}
+	for _, tool := range existingTools {
+		toolSet[tool] = true
+	}
+
+	// Test that existing tools are marked as present
+	for _, tool := range existingTools {
+		if !toolSet[tool] {
+			t.Errorf("Expected tool %q to be in toolSet", tool)
+		}
+	}
+
+	// Test that new tools can be added and checked
+	newTools := []string{"cover", "fix"}
+	for _, tool := range newTools {
+		if toolSet[tool] {
+			t.Errorf("Expected new tool %q to not be in toolSet initially", tool)
+		}
+		toolSet[tool] = true
+		if !toolSet[tool] {
+			t.Errorf("Expected tool %q to be in toolSet after adding", tool)
+		}
+	}
+}