tests/lint: explain difference between i386 and sparc for 259

Seen in usr.bin/make/cond.c 1.278 from 2021-09-21, line 800, the call to
is_token, where unsigned char gets converted to unsigned int or unsigned
long, depending on the platform.

tests/usr.bin/xlint/lint1/platform_int.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: platform_int.c,v 1.2 2021/09/26 14:28:22 rillig Exp $	*/
+/*	$NetBSD: platform_int.c,v 1.3 2021/09/26 14:52:37 rillig Exp $	*/
 # 3 "platform_int.c"
 
 /*
@@ -11,9 +11,16 @@
 
 void to_size(typeof(sizeof(int)));
 
+/* See should_warn_about_prototype_conversion. */
 void
 convert_unsigned_char_to_size(unsigned char uc)
 {
+	/*
+	 * In this function call, uc is first promoted to INT. It is then
+	 * converted to size_t, which is UINT. The portable bit size of INT
+	 * and UINT is the same, 32, but the signedness changes, therefore
+	 * the warning.
+	 */
 	/* expect+1: warning: argument #1 is converted from 'unsigned char' to 'unsigned int' due to prototype [259] */
 	to_size(uc);
 }

tests/usr.bin/xlint/lint1/platform_int.exp

@@ -1 +1 @@
-platform_int.c(18): warning: argument #1 is converted from 'unsigned char' to 'unsigned int' due to prototype [259]
+platform_int.c(25): warning: argument #1 is converted from 'unsigned char' to 'unsigned int' due to prototype [259]

tests/usr.bin/xlint/lint1/platform_long.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: platform_long.c,v 1.2 2021/09/26 14:28:22 rillig Exp $	*/
+/*	$NetBSD: platform_long.c,v 1.3 2021/09/26 14:52:37 rillig Exp $	*/
 # 3 "platform_long.c"
 
 /*
@@ -11,10 +11,22 @@
 
 void to_size(typeof(sizeof(int)));
 
+/* See should_warn_about_prototype_conversion. */
 void
 convert_unsigned_char_to_size(unsigned char uc)
 {
-	/* no warning, unlike in platform_int */
+	/*
+	 * In this function call, uc is first promoted to INT. It is then
+	 * converted to size_t, which is ULONG. The portable bit size of INT
+	 * is 24 (see INT_RSIZE in inittyp.c), which is less than the 32 of
+	 * ULONG. Since the portable bit size increases from 24 to 32, there
+	 * is no warning.
+	 *
+	 * XXX: Investigate whether this rule makes sense. Warning 259 is
+	 * about prototype mismatch, not about lossy integer conversions,
+	 * and there is a clear mismatch here between INT and LONG,
+	 * therefore a warning makes sense.
+	 */
 	to_size(uc);
 }
 

tests/usr.bin/xlint/lint1/platform_long.exp

@@ -1 +1 @@
-platform_long.c(22): warning: static variable unused_variable unused [226]
+platform_long.c(34): warning: static variable unused_variable unused [226]

usr.bin/xlint/lint1/tree.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: tree.c,v 1.382 2021/09/18 10:46:17 jmcneill Exp $	*/
+/*	$NetBSD: tree.c,v 1.383 2021/09/26 14:52:37 rillig Exp $	*/
 
 /*
  * Copyright (c) 1994, 1995 Jochen Pohl
@@ -37,7 +37,7 @@
 
 #include <sys/cdefs.h>
 #if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: tree.c,v 1.382 2021/09/18 10:46:17 jmcneill Exp $");
+__RCSID("$NetBSD: tree.c,v 1.383 2021/09/26 14:52:37 rillig Exp $");
 #endif
 
 #include <float.h>
@@ -2035,6 +2035,10 @@ should_warn_about_prototype_conversion(tspec_t nt,
 		/* representation and/or width change */
 		if (!is_integer(ot))
 			return true;
+		/*
+		 * XXX: Investigate whether this rule makes sense; see
+		 * tests/usr.bin/xlint/lint1/platform_long.c.
+		 */
 		return portable_size_in_bits(ot) > portable_size_in_bits(INT);
 	}