Resize capacity for fstring

When a string is #frozen, it's capacity is resized to fit (if it is much
larger), since we know it will no longer be mutated.

    > puts ObjectSpace.dump(String.new("a"*30, capacity: 1000))
    {"type":"STRING", "class":"0x7feaf00b7bf0", "bytesize":30, "capacity":1000, "value":"...
    > puts ObjectSpace.dump(String.new("a"*30, capacity: 1000).freeze)
    {"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "bytesize":30, "value":"...

(ObjectSpace.dump doesn't show capacity if capacity is equal to bytesize)

Previously, if we dedup into an fstring, using String#-@, capacity would
not be reduced.

    > puts ObjectSpace.dump(-String.new("a"*30, capacity: 1000))
    {"type":"STRING", "class":"0x7feaf00b7bf0", "frozen":true, "fstring":true, "bytesize":30, "capacity":1000, "value":"...

This commit makes rb_fstring call rb_str_resize, the same as
rb_str_freeze does.

Closes: https://github.com/ruby/ruby/pull/2256

1 files changed, 3 insertions, 0 deletions

diff --git a/string.c b/string.c
index 030465235a..8ac33e3e52 100644
--- a/string.c
+++ b/string.c
@@ -321,6 +321,9 @@ rb_fstring(VALUE str)
 	return str;
     }
 
+    if (!OBJ_FROZEN(str))
+        rb_str_resize(str, RSTRING_LEN(str));
+
     fstr = register_fstring(str);
 
     if (!bare) {