rb_str_buf_append: add a fast path for ENC_CODERANGE_VALID

If the RHS has valid encoding, and both strings have the same
encoding, we can use the fast path.

However we need to update the LHS coderange.

```
compare-ruby: ruby 3.2.0dev (2022-07-21T14:46:32Z master cdbb9b8555) [arm64-darwin21]
built-ruby: ruby 3.2.0dev (2022-07-25T07:25:41Z string-concat-vali.. 11a2772bdd) [arm64-darwin21]
warming up...

|                    |compare-ruby|built-ruby|
|:-------------------|-----------:|---------:|
|binary_concat_7bit  |    554.816k|  556.460k|
|                    |           -|     1.00x|
|utf8_concat_7bit    |    556.367k|  555.101k|
|                    |       1.00x|         -|
|utf8_concat_UTF8    |    412.555k|  556.824k|
|                    |           -|     1.35x|
```

1 files changed, 18 insertions, 3 deletions

diff --git a/string.c b/string.c
index f3f5ea92cc..f5e089aa21 100644
--- a/string.c
+++ b/string.c
@@ -3329,9 +3329,24 @@ VALUE
 rb_str_buf_append(VALUE str, VALUE str2)
 {
     int str2_cr = rb_enc_str_coderange(str2);
-    if (str2_cr == ENC_CODERANGE_7BIT && str_enc_fastpath(str)) {
-        str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
-        return str;
+
+    if (str_enc_fastpath(str)) {
+        switch (str2_cr) {
+          case ENC_CODERANGE_7BIT:
+            // If RHS is 7bit we can do simple concatenation
+            str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
+            return str;
+          case ENC_CODERANGE_VALID:
+            // If RHS is valid, we can do simple concatenation if encodings are the same
+            if (ENCODING_GET_INLINED(str) == ENCODING_GET_INLINED(str2)) {
+                str_buf_cat4(str, RSTRING_PTR(str2), RSTRING_LEN(str2), true);
+                int str_cr = ENC_CODERANGE(str);
+                if (UNLIKELY(str_cr != ENC_CODERANGE_VALID)) {
+                    ENC_CODERANGE_SET(str, RB_ENC_CODERANGE_AND(str_cr, str2_cr));
+                }
+                return str;
+            }
+        }
     }
 
     rb_enc_cr_str_buf_cat(str, RSTRING_PTR(str2), RSTRING_LEN(str2),