x/tools/gopls: nil (or invalid) *ExprStmt pointer in AST

[adonovan]

#!stacks
"sigpanic" && "unusedresult.run.func1:+1"

Issue created by stacks.

func run(pass *analysis.Pass) (any, error) {
...
	inspect.Preorder(nodeFilter, func(n ast.Node) {
		call, ok := ast.Unparen(n.(*ast.ExprStmt).X).(*ast.CallExpr) // panics loading .X field

TEXT golang.org/x/tools/go/analysis/passes/unusedresult.run.func1(SB) go/analysis/passes/unusedresult/unusedresult.go
...
  unusedresult.go:142   0x9a9f8f                55                      PUSHQ BP                                                                                                                      
  unusedresult.go:142   0x9a9f90                4889e5                  MOVQ SP, BP                                                                                                                   
  unusedresult.go:142   0x9a9f93                4881ecd8000000          SUBQ $0xd8, SP                                                                                                                
  unusedresult.go:142   0x9a9f9a                48898424e8000000        MOVQ AX, 0xe8(SP)                                                                                                             
  unusedresult.go:142   0x9a9fa2                48899c24f0000000        MOVQ BX, 0xf0(SP)                                                                                                             
  unusedresult.go:142   0x9a9faa                488b7210                MOVQ 0x10(DX), SI                                                                                                             
  unusedresult.go:143   0x9a9fae                488d3d8b7ac700          LEAQ go:itab.*go/ast.ExprStmt,go/ast.Node(SB), DI                                                                             
  unusedresult.go:143   0x9a9fb5                4839f8                  CMPQ AX, DI                                                                                                                   
  unusedresult.go:143   0x9a9fb8                0f856d030000            JNE 0x9aa32b                                                                                                                  
  unusedresult.go:143   0x9a9fbe                488b0b                  MOVQ 0(BX), CX            // SEGV loading first word of ExprStmt.X
  unusedresult.go:143   0x9a9fc1                488b7b08                MOVQ 0x8(BX), DI                                                                                                              

If the ExprStmt pointer is nil, that means the parser has a bug or the AST has become corrupted. If it is non-nil, then we have memory corruption.

This stack NVOHJQ was reported by telemetry:

• crash/crash
• runtime.throw:+9,+0x47
• runtime.sigpanic:+33,+0x26b
• golang.org/x/tools/go/analysis/passes/unusedresult.run.func1:+1,+0x3e
• golang.org/x/tools/go/ast/inspector.(*Inspector).Preorder:+20,+0x90
• golang.org/x/tools/go/analysis/passes/unusedresult.run:+14,+0x233
• golang.org/x/tools/gopls/internal/cache.(*action).exec.func6:+31,+0x96
• golang.org/x/tools/gopls/internal/cache.(*action).exec:+199,+0xab8
• golang.org/x/tools/gopls/internal/cache.execActions.func1.1:+2,+0x51
• sync.(*Once).doSlow:+5,+0xab
• sync.(*Once).Do:=69,+0x84
• golang.org/x/tools/gopls/internal/cache.execActions.func1:+2,+0x4e
• runtime.goexit:+0,+0x0

golang.org/x/tools/gopls@v0.20.0 go1.25.2 linux/amd64 gopls.client=vscode (1)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=linux GOARCH=amd64 GOTOOLCHAIN=go1.25.2 go install golang.org/x/tools/gopls@v0.20.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less