Avoid writing to const storage

[paulocsanz]

Fixes #8462

This avoids the null termination requirement of both String::substring and String::lastIndexOf by using APIs that don't require it. So we can stop writing to the buffer inside of const functions.

There might be a off by one error there, I need to take a better look at the code and would love if someone could review.

[earlephilhower]

Seems to have broken some host tests:

-------------------------------------------------------------------------------
String SSO works
-------------------------------------------------------------------------------
core/test_string.cpp:322
...............................................................................

core/test_string.cpp:420: FAILED:
  REQUIRE( s.length() == 5 )
with expansion:
  15 == 5

[earlephilhower]

Arduino/tests/host/core/test_string.cpp

Lines 417 to 420 in 30b0450

	s = "0123456789abcde";
	s = s.substring(s.indexOf('a'));
	REQUIRE(s == "abcde");
	REQUIRE(s.length() == 5);

[paulocsanz]

Will fix it, thanks! How can I run the host tests locally so I don't clutter the project's CI and can iterate over the code faster?

[earlephilhower]

Will fix it, thanks! How can I run the host tests locally so I don't clutter the project's CI and can iterate over the code faster?

In tests/host just run make test. You'll probably need valgrind and a couple other dependencies which will show up.

[paulocsanz]

Hmm, I'm getting a bunch of compilation errors because of std::enable_if_t, std::is_same_v, std::decay_t and #include "../lib/littlefs/lfs.h, but it seems I fixed the test.

[mcspr]

Were you intending to fix wbuffer() as well? It seems the const_cast originates b/c of the const qualifier(s), where the compiler did warn about the const misnomer but it was circumvented

[paulocsanz]

Yeah, I would prefer to make wbuffer into non const, should I change it in this PR too then?

I will mess with this PR's code after work today.

[paulocsanz]

@d-a-v Done, your trick worked!

I also changed wbuffer to make it non const.

[d-a-v]

LGTM