fix(compiler): char_from_int(0) NUL string (B1) + top-level float-const binop (B2)

tools/compile.rail

@@ -975,7 +975,7 @@ is_float node env =
   let tg = head node
   if tg == "FL" then true
   else if tg == "V" then
-    efind (cat ["__float_", head (tail node)]) env != (0 - 1)
+    (efind (cat ["__float_", head (tail node)]) env != (0 - 1)) || (efind (cat ["__float_ret_", head (tail node)]) env != (0 - 1))
   else if tg == "O" then
     let op = head (tail node)
     if op == "+." || op == "-." || op == "*." || op == "/." then true
@@ -1144,10 +1144,16 @@ cg_bi2 fname args env ar lc sl tp fs =
   else if fname == "char_from_int" then
     let (aa, lc1, sl1) = cg (head args) env ar lc sl false fs
     -- Inverse of char_to_int: take a tagged int (ASCII code), return a
-    -- single-char Rail string. Allocates 2 bytes (char + null terminator),
-    -- writes the byte, wraps via _rail_wrap_str. Needed for buffer→string
-    -- reconstruction in pure-Rail socket I/O.
-    (cat [aa, "    asr x0, x0, #1\n    stp x29, x30, [sp, #-32]!\n    mov x29, sp\n    str x0, [x29, #16]\n    mov x0, #2\n    bl _rail_chained_malloc\n    ldr x1, [x29, #16]\n    strb w1, [x0]\n    mov w1, #0\n    strb w1, [x0, #1]\n    bl _rail_wrap_str\n    ldp x29, x30, [sp], #32\n"], lc1, sl1)
+    -- single-char Rail string. Builds a tagged string [tag=9, len=1, byte,
+    -- NUL] DIRECTLY with len hardcoded to 1, instead of going through
+    -- _rail_wrap_str (which recomputes length via _strlen and so truncates
+    -- to len=0 for a NUL byte). With this, `char_from_int 0` yields a true
+    -- 1-byte NUL string (length 1), and char_from_int 7/13/65/etc. are
+    -- byte-identical in behavior to before. NOTE: this fixes a SINGLE
+    -- char_from_int; embedded NULs still truncate through _strlen-based
+    -- concat (cat/join) — out of scope. Needed for buffer→string
+    -- reconstruction in pure-Rail socket I/O (CRLF uses 13/10, never 0).
+    (cat [aa, "    asr x0, x0, #1\n    stp x29, x30, [sp, #-32]!\n    mov x29, sp\n    str x0, [x29, #16]\n    mov x0, #18\n    bl _rail_alloc\n    mov x1, #9\n    str x1, [x0]\n    mov x1, #1\n    str x1, [x0, #8]\n    ldr x1, [x29, #16]\n    strb w1, [x0, #16]\n    mov w1, #0\n    strb w1, [x0, #17]\n    ldp x29, x30, [sp], #32\n"], lc1, sl1)
   else if fname == "bit_and" then
     -- (int, int) -> int. ARM64 and on raw 63-bit values. Strip both tags,
     -- AND, re-tag. Operates on the full 63-bit int width; if the caller
@@ -6243,9 +6249,15 @@ run_tests _ =
   let t168 = run_test "value_big_literal" "f x = if x == 0 then 16777216 else x\nmain =\n  let _ = print (show (f 0))\n  0" "16777216"
   let t169 = run_test "float_modulo" "main =\n  let a = 5.5\n  let b = 2.0\n  let _ = print (show_float (a % b))\n  0" "1.5"
   let s12 = (if t165 then 1 else 0) + (if t166 then 1 else 0) + (if t167 then 1 else 0) + (if t168 then 1 else 0) + (if t169 then 1 else 0)
-  let total = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10 + s11 + s12 + s_inf
-  let _ = print (cat [show total, "/175 tests passed"])
-  if total == 175 then 0 else 1
+  -- t171 (B1): char_from_int 0 must yield a 1-byte NUL string (length 1), not "".
+  let t171 = run_test "char_from_int_nul" "main =\n  let s = char_from_int 0\n  let _ = print (show (length s))\n  0" "1"
+  -- t172 (B2): a bare zero-arg top-level float constant in a binary op must be
+  -- classified as float by is_float (via __float_ret_) instead of segfaulting.
+  let t172 = run_test "top_float_const_binop" "l0 = 50.0\nl2 = 100.0\nmain =\n  let _ = print (show_float (l0 + l2))\n  0" "150"
+  let s_bx = (if t171 then 1 else 0) + (if t172 then 1 else 0)
+  let total = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s8 + s9 + s10 + s11 + s12 + s_inf + s_bx
+  let _ = print (cat [show total, "/177 tests passed"])
+  if total == 177 then 0 else 1
 
 self_compile _ =
   let _ = print "\n=== Self-compilation ==="