golang.org/x/tools v0.1.1: cfg.Block

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify golang.org/x/tools/go/cfg.Block in pkg:golang/golang.org/x/tools@v0.1.1
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/tools@v0.1.1
Demonstrate these symbols/APIs:
  - golang.org/x/tools/go/cfg.Block

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:1527fa01316e8237c0cbc0896d88046c76e4279c201e95be769c65d3299d6ce7","contract":["cfg.Block represents basic blocks in a control-flow graph with Index, Live, Nodes, and Succs properties","cfg.Block.Return returns the terminating *ast.ReturnStmt for return blocks and nil for non-return blocks","cfg.Block.String formats the block index and comment description","cfg.Block marks unreachable basic blocks after unconditional return with Live == false","cfg.Block Succs links conditional branches to two successor blocks and linear flow to one successor block"],"goal":"verify golang.org/x/tools/go/cfg.Block in pkg:golang/golang.org/x/tools@v0.1.1","kind":"HOW","packages":["pkg:golang/golang.org/x/tools@v0.1.1"],"schemaVersion":1,"symbols":["golang.org/x/tools/go/cfg.Block"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/golang.org/x/tools@v0.1.1"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/tools@v0.1.1","symbols":["golang.org/x/tools/go/cfg.Block"],"verifierAdapter":"golang@1"}

module sample

go 1.21

require golang.org/x/tools v0.1.1

golang.org/x/tools v0.1.1 h1:wGiQel/hW0NnEkJUk8lbzkX2gFJU6PFxf1v5OlCfuOs=
golang.org/x/tools v0.1.1/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"

	"golang.org/x/tools/go/cfg"
)

func main() {
	src := `package p
func classify(n int) int {
	if n > 0 {
		return 1
	}
	return 0
}`
	fset := token.NewFileSet()
	file, err := parser.ParseFile(fset, "p.go", src, 0)
	if err != nil {
		fmt.Printf("parse error: %v\n", err)
		return
	}

	var fn *ast.FuncDecl
	for _, decl := range file.Decls {
		if fd, ok := decl.(*ast.FuncDecl); ok {
			fn = fd
			break
		}
	}

	if fn == nil || fn.Body == nil {
		fmt.Println("missing function body")
		return
	}

	graph := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
	for _, blk := range graph.Blocks {
		var b *cfg.Block = blk
		fmt.Printf("Block %d: live=%v succs=%d nodes=%d return=%v\n", b.Index, b.Live, len(b.Succs), len(b.Nodes), b.Return() != nil)
	}
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/tools/go/cfg.Block in pkg:golang/golang.org/x/tools@v0.1.1",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/tools@v0.1.1"
  ],
  "symbols": [
    "golang.org/x/tools/go/cfg.Block"
  ]
}

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"os"
	"strings"

	"golang.org/x/tools/go/cfg"
)

func mustParseFunc(src string) (*ast.FuncDecl, error) {
	fset := token.NewFileSet()
	file, err := parser.ParseFile(fset, "input.go", src, 0)
	if err != nil {
		return nil, fmt.Errorf("parse error: %w", err)
	}
	for _, decl := range file.Decls {
		if fd, ok := decl.(*ast.FuncDecl); ok {
			return fd, nil
		}
	}
	return nil, fmt.Errorf("no FuncDecl found")
}

func main() {
	// Assertion 1: cfg.Block represents basic blocks in a control-flow graph with Index, Live, Nodes, and Succs properties
	{
		src := `package p
func simple() {
	a := 1
	_ = a
}`
		fn, err := mustParseFunc(src)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: %v\n", err)
			os.Exit(1)
		}
		g := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
		if len(g.Blocks) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: expected non-empty Blocks\n")
			os.Exit(1)
		}
		var entry *cfg.Block = g.Blocks[0]
		if entry.Index != 0 || !entry.Live || len(entry.Nodes) == 0 {
			fmt.Fprintf(os.Stderr, "assertion 1 failed: unexpected entry block state: index=%d live=%v nodes=%d\n", entry.Index, entry.Live, len(entry.Nodes))
			os.Exit(1)
		}
	}

	// Assertion 2: cfg.Block.Return returns the terminating *ast.ReturnStmt for return blocks and nil for non-return blocks
	{
		src := `package p
func cond(v int) int {
	if v > 0 {
		return 10
	}
	return 20
}`
		fn, err := mustParseFunc(src)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: %v\n", err)
			os.Exit(1)
		}
		g := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
		var hasReturn, hasNonReturn bool
		for _, b := range g.Blocks {
			if b.Return() != nil {
				hasReturn = true
			} else {
				hasNonReturn = true
			}
		}
		if !hasReturn || !hasNonReturn {
			fmt.Fprintf(os.Stderr, "assertion 2 failed: hasReturn=%v hasNonReturn=%v\n", hasReturn, hasNonReturn)
			os.Exit(1)
		}
	}

	// Assertion 3: cfg.Block.String formats the block index and comment description
	{
		src := `package p
func ret() int {
	return 7
}`
		fn, err := mustParseFunc(src)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: %v\n", err)
			os.Exit(1)
		}
		g := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
		s := g.Blocks[0].String()
		if !strings.Contains(s, "block 0") {
			fmt.Fprintf(os.Stderr, "assertion 3 failed: unexpected String() output %q\n", s)
			os.Exit(1)
		}
	}

	// Assertion 4: cfg.Block marks unreachable basic blocks after unconditional return with Live == false
	{
		src := `package p
func dead() int {
	return 1
	y := 2
	return y
}`
		fn, err := mustParseFunc(src)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: %v\n", err)
			os.Exit(1)
		}
		g := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
		var sawDead bool
		for _, b := range g.Blocks {
			if !b.Live {
				sawDead = true
				break
			}
		}
		if !sawDead {
			fmt.Fprintf(os.Stderr, "assertion 4 failed: expected at least one dead block with Live == false\n")
			os.Exit(1)
		}
	}

	// Assertion 5: cfg.Block Succs links conditional branches to two successor blocks and linear flow to one successor block
	{
		src := `package p
func branch(flag bool) {
	if flag {
		_ = 1
	}
	_ = 2
}`
		fn, err := mustParseFunc(src)
		if err != nil {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: %v\n", err)
			os.Exit(1)
		}
		g := cfg.New(fn.Body, func(call *ast.CallExpr) bool { return true })
		var twoSuccs, oneSucc bool
		for _, b := range g.Blocks {
			switch len(b.Succs) {
			case 2:
				twoSuccs = true
			case 1:
				oneSucc = true
			}
		}
		if !twoSuccs || !oneSucc {
			fmt.Fprintf(os.Stderr, "assertion 5 failed: twoSuccs=%v oneSucc=%v\n", twoSuccs, oneSucc)
			os.Exit(1)
		}
	}

	fmt.Println("contract ok")
}