go/parser: ParseExprFrom and ParseExpr might return ast.Expr(nil) when AllErrors is unset

[mateusz834]

ParseExprFrom is documented as such:

go/src/go/parser/interface.go

Lines 193 to 198 in c8fb6ae

	// If the source couldn't be read, the returned AST is nil and the error
	// indicates the specific failure. If the source was read but syntax
	// errors were found, the result is a partial AST (with [ast.Bad]* nodes
	// representing the fragments of erroneous source code). Multiple errors
	// are returned via a scanner.ErrorList which is sorted by source position.
	func ParseExprFrom(fset *token.FileSet, filename string, src any, mode Mode) (expr ast.Expr, err error) {

From the docs it is clear that, if reading the source succeeds, then the returned Expr shoudn't be nil, but it is not actually true (in case of a bailout, also see #70725).

Reproducer:

func TestEarlyBailout2(t *testing.T) {
	const src = `func() {
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
		func () { var a }
}()
`

	fset := token.NewFileSet()
	expr, err := ParseExprFrom(fset, "test.go", src, SkipObjectResolution|ParseComments)
	t.Log(expr, err) // <nil> test.go:2:19: expected type, found '}' (and 10 more errors)
}

Same with ParseExpr:

go/src/go/parser/interface.go

Lines 243 to 248 in c8fb6ae

	// If syntax errors were found, the result is a partial AST (with [ast.Bad]* nodes
	// representing the fragments of erroneous source code). Multiple errors are
	// returned via a scanner.ErrorList which is sorted by source position.
	func ParseExpr(x string) (ast.Expr, error) {
	return ParseExprFrom(token.NewFileSet(), "", []byte(x), 0)
	}

Per docs it should not return a nil ast.Expr.

CC @griesemer @alandonovan

[gabyhelp]

Related Issues

• go/parser: ParseExprFrom doesn't appear to handle bailouts with errors correctly #34241 (closed)
• go/parser: (*ast.File).Decls is empty after bailout #70725
• go/parser: ParseExpr should return (partial) result even in case of an error #34211 (closed)
• go/parser: ParseExprFrom doesn't return errors if there were too many errors #34274 (closed)

Related Code Changes

• go/parser: fix ignored errors in ParseExprFrom

Related Documentation

• Package parser > func ParseExprFrom ¶ 1.5

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[mateusz834]

Also it is worth nothing that there is no way to set AllErrors for the ParseExpr.