Default::default() has bad optimization for String since 1.52.0

[tuplecats]

Since 1.52.0 Rust copy byte from static memory for creating String by Default::default(). But when uses "".to_string() it doesn't happen.

Additional instruction appears when use Default::default() (godbolt)

pub fn string_default() -> String {
    Default::default()
}

pub fn string_to_string() -> String {
    "".to_string()
}

example::string_default:
        mov     rax, rdi
        mov     rcx, qword ptr [rip + .L__unnamed_1]
        xorps   xmm0, xmm0
        movups  xmmword ptr [rdi + 8], xmm0
        mov     qword ptr [rdi], rcx
        ret

example::string_to_string:
        mov     rax, rdi
        xorps   xmm0, xmm0
        movups  xmmword ptr [rdi + 8], xmm0
        mov     qword ptr [rdi], 1
        ret

.L__unnamed_1:
        .asciz  "\001\000\000\000\000\000\000\000\000\000\000\000\000\000\000"

[hkratz]

String::default() just calls String::new(), so this is a duplicate of #86106.