Go中具有巨大字符串长度的单元测试覆盖率？

In some code I have a test ensuring that a string length is smaller than 2^32. But it is difficult to test since generating a bigger string for testing would probably crash the test program with an out of memory error.

How may I achieve 100% test coverage but still testing for such case just to be safe?

Refactor your code, move the limit outside of your function which tests can change:

var limit = 1 << 32

var ErrTooLarge = errors.New("String is too large!")

func Process(s string) error {
    if len(s) > limit {
        return ErrTooLarge
    }
    // All OK
    return nil
}

Testing it:

func TestProcess(t *testing.T) {
    // Save limit and restore it at the end:
    old := limit
    defer func() { limit = old }()

    // Test success
    if err := Process("123"); err != nil {
        t.Errorf("Expected success, got: %v", err)
    }

    // Test failure (too large string)
    limit = 5
    if err := Process("123456"); err != ErrTooLarge {
        t.Errorf("Expected ErrTooLarge, got: %v", err)
    }
}

Running go test -cover:

PASS
coverage: 100.0% of statements
ok      play    0.001s