String.new(capacity:) don't substract termlen (#11027)

[Bug #20585]

This was changed in 36a06efdd9f0604093dccbaf96d4e2cb17874dc8 because
`String.new(1024)` would end up allocating `1025` bytes, but the problem
with this change is that the caller may be trying to right size a String.

So instead, we should just better document the behavior of `capacity:`.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

1 files changed, 1 insertions, 6 deletions

diff --git a/string.c b/string.c
index 3d4650ff05..635be6988d 100644
--- a/string.c
+++ b/string.c
@@ -1970,12 +1970,7 @@ rb_str_s_new(int argc, VALUE *argv, VALUE klass)
         }
     }
 
-    long fake_len = capa - termlen;
-    if (fake_len < 0) {
-        fake_len = 0;
-    }
-
-    VALUE str = str_new0(klass, NULL, fake_len, termlen);
+    VALUE str = str_new0(klass, NULL, capa, termlen);
     STR_SET_LEN(str, 0);
     TERM_FILL(RSTRING_PTR(str), termlen);