v3: promote fastc parallel assignments

Author: medvednikov

vlib/v3/README.md

@@ -113,10 +113,10 @@ C-string and embedded-NUL string literals, assertions, `sizeof`, comparison/logi
 division, and modulo expressions, and functions with narrow integer signatures are promoted to
 the complete lane. Inferred declarations and C-style loop initializers using the minimum `int`
 literal are promoted as well, as are expressions containing hexadecimal literals above the signed
-32-bit range. Indexing expressions are promoted, preserving V layouts, inferred types, element
-types, and bounds checks. Together these promotions preserve V's formatting, byte-length,
-diagnostics, boolean typing, integer-wrapping, safe-shift, and zero-divisor behavior instead of
-relying on incompatible raw C semantics.
+32-bit range. Parallel assignments and indexing expressions are promoted, preserving simultaneous
+assignment, V layouts, inferred types, element types, and bounds checks. Together these promotions
+preserve V's formatting, byte-length, diagnostics, boolean typing, integer-wrapping, safe-shift,
+and zero-divisor behavior instead of relying on incompatible raw C semantics.
 
 The direct path is limited to host-target, non-production, non-test, non-shared single-file builds.
 Compiler/self-host, strict C, and other non-direct modes enter the complete lane before source

vlib/v3/gen/fastc/fastc.v

@@ -529,6 +529,11 @@ fn (mut g DirectGen) read_expression_with_prefix(prefix string, stops []token.To
 		if paren_depth == 0 && g.tok in stops {
 			break
 		}
+		if paren_depth == 0 && g.tok == .comma {
+			// V's top-level commas form simultaneous multi-target assignments.
+			// Copying them to C would instead emit comma operators.
+			return g.unsupported('parallel assignments')
+		}
 		if g.tok in [.eq, .ne, .gt, .lt, .ge, .le, .and, .logical_or, .not] {
 			// C represents comparison and logical results as int. Without V type
 			// information, accepting them here would make generic printing and

vlib/v3/gen/fastc/fastc_test.v

@@ -173,6 +173,7 @@ fn test_type_sensitive_expressions_request_checked_lane() {
 		'module main\nfn main() { mut x := -2_147_483_648; x--; println(x) }\n',
 		'module main\nfn main() { for i := -2_147_483_648; true; i-- { println(i); break } }\n',
 		'module main\nfn main() { x := 0xffff_ffff | 0; println(x) }\n',
+		'module main\nfn main() { mut a := 1; mut b := 2; a, b = b, a; println(a); println(b) }\n',
 	] {
 		mut failed := false
 		_ := generate(source, 'typed_expression.v', prefs) or {
@@ -189,4 +190,7 @@ fn test_type_sensitive_expressions_request_checked_lane() {
 	low_hex_c := generate('module main\nfn main() { x := 0x7fff_ffff | 0; println(x) }\n',
 		'low_hex_literal.v', prefs) or { panic(err) }
 	assert low_hex_c.contains('__typeof__((0x7fffffff|0)) x = (0x7fffffff|0);')
+	call_c := generate('module main\nfn sum(a int, b int) int { return a + b }\nfn main() { println(sum(1, 2)) }\n',
+		'call_comma.v', prefs) or { panic(err) }
+	assert call_c.contains('println(sum(1,2));')
 }

vlib/v3/tests/fastc_backend_test.v

@@ -278,6 +278,29 @@ fn main() {
 	assert high_hex_run.exit_code == 0, high_hex_run.output
 	assert high_hex_run.output.trim_space() == '-1'
 
+	parallel_assign_source := os.join_path(root, 'parallel_assign.v')
+	os.write_file(parallel_assign_source, 'module main
+
+fn main() {
+	mut a := 1
+	mut b := 2
+	a, b = b, a
+	println(a)
+	println(b)
+}
+') or {
+		panic(err)
+	}
+	parallel_assign_binary := os.join_path(root, 'parallel_assign')
+	parallel_assign_compile := cmdexec.run(v3_bin, ['-silent', '-b', 'fastc', '-o',
+		parallel_assign_binary, parallel_assign_source])
+	assert parallel_assign_compile.exit_code == 0, parallel_assign_compile.output
+	parallel_assign_c := os.read_file(parallel_assign_binary + '.c') or { panic(err) }
+	assert !parallel_assign_c.contains('V_FASTC_PRINT_SELECT')
+	parallel_assign_run := cmdexec.run(parallel_assign_binary, [])
+	assert parallel_assign_run.exit_code == 0, parallel_assign_run.output
+	assert parallel_assign_run.output.trim_space() == '2\n1'
+
 	shift_source := os.join_path(root, 'oversized_shift.v')
 	os.write_file(shift_source, 'module main