fix(php-src): assert php_gcvt's unported arguments up front

php_gcvt only ports the ndigit < 0 (dtoa mode 0) path and hardcodes the
'.'/'E' separators of its sole caller into the placement logic, but it
accepted any argument and silently produced a wrong string for the rest.
Assert the supported combination instead, and turn the mode 2 bail-out
into unreachable! now that the assert rules it out.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/crates/shirabe-php-src/src/main/snprintf.rs b/crates/shirabe-php-src/src/main/snprintf.rs
index f1411d6e..c0a3f23a 100644
--- a/crates/shirabe-php-src/src/main/snprintf.rs
+++ b/crates/shirabe-php-src/src/main/snprintf.rs
@@ -6,9 +6,13 @@
 /// produces the same digit string as `zend_dtoa` in mode 0 (both compute the unique shortest
 /// decimal that round-trips), so only the digit placement logic is ported here.
 pub fn php_gcvt(value: f64, ndigit: i32, dec_point: char, exponent: char) -> String {
+    assert!(ndigit == -1);
+    assert!(dec_point == '.');
+    assert!(exponent == 'E');
+
     let mode = if ndigit >= 0 { 2 } else { 0 };
     if mode != 0 {
-        todo!("php_gcvt is only ported for ndigit < 0 (serialize_precision=-1)");
+        unreachable!("php_gcvt is only ported for ndigit < 0 (serialize_precision=-1)");
     }
     let ndigit = 17i32;