v3/wasm: resolve import aliases, fix loop post scope

Address PR review:

- Import aliases: `import moda as m; m.answer()` keyed the call as
  m.answer, but candidates are keyed from the module declaration
  (moda.answer), so the call hit the i32.const 0 fallback. Collect
  `import mod as alias` mappings (import_decl.value/.typ) and map the
  selector base back to the real module when resolving call keys.

- C-style for post scope: the loop snapshot was only restored after the
  post statement, so a body-local that shadowed the loop variable was
  still bound when the post was emitted (e.g. `for ; i < 1; i++ { i := 10 }`
  incremented the inner local and never advanced the loop). Snapshot the
  bindings after the initializer and restore them before emitting the
  post so it rebinds to the loop variable.

Also handle blank assignments (`_ = expr`) by evaluating the rhs for
side effects and dropping it, instead of warning.

Adds regression tests for both review items.

Author: medvednikov

vlib/v3/gen/wasm/gen.v

@@ -77,9 +77,10 @@ mut:
 	cur_ret       WType
 	cur_fn_module string
 	// module-wide; keyed by qualified function name (see qualified_fn_key)
-	fn_index  map[string]int
-	fn_ret    map[string]WType
-	fn_params map[string][]WType
+	fn_index       map[string]int
+	fn_ret         map[string]WType
+	fn_params      map[string][]WType
+	module_aliases map[string]string // import alias -> real module name
 	data_pool []u8
 	data_off  map[string]int
 	uses_print bool
@@ -100,6 +101,7 @@ pub fn Gen.new(a &flat.FlatAst, tc &types.TypeChecker, used_fns map[string]bool)
 
 // gen builds the whole module from the flat AST.
 pub fn (mut g Gen) gen() {
+	g.collect_module_aliases()
 	user_fns := g.collect_user_fns()
 	g.uses_print = g.detect_print(user_fns)
 
@@ -505,7 +507,13 @@ fn (mut g Gen) gen_assign(node flat.Node) {
 	for i + 1 < node.children_count {
 		lhs := g.a.child_node(&node, i)
 		rhs_id := g.a.child(&node, i + 1)
-		if lhs.kind == .ident && lhs.value in g.var_index {
+		if lhs.kind == .ident && lhs.value == '_' {
+			// Blank assignment `_ = expr`: evaluate for side effects, discard.
+			w := g.gen_expr(rhs_id)
+			if w != .void {
+				g.cur.drop()
+			}
+		} else if lhs.kind == .ident && lhs.value in g.var_index {
 			idx := g.var_index[lhs.value]
 			w := g.var_wtype[lhs.value]
 			if node.op == .assign {
@@ -592,6 +600,9 @@ fn (mut g Gen) gen_for(node flat.Node) {
 	if init_node.kind != .empty {
 		g.gen_stmt(g.a.child(&node, 0))
 	}
+	// Bindings visible to the condition and post: the loop variable from the
+	// initializer, but no body-local shadows.
+	header_scope := g.snapshot_scope()
 	g.cur.block_void() // break target
 	g.frames << Frame{.brk}
 	g.cur.loop_void() // back-edge target
@@ -611,6 +622,8 @@ fn (mut g Gen) gen_for(node flat.Node) {
 	g.cur.end()
 	g.frames.pop()
 
+	// Drop body-local shadows so the post statement rebinds to the loop var.
+	g.restore_scope(header_scope)
 	if post_node.kind != .empty {
 		g.gen_stmt(g.a.child(&node, 2))
 	}
@@ -938,10 +951,21 @@ fn (mut g Gen) gen_call(node flat.Node) WType {
 	return .i32
 }
 
+// collect_module_aliases records `import mod as alias` mappings so aliased
+// calls (`m.answer()`) resolve to the real module's qualified key.
+fn (mut g Gen) collect_module_aliases() {
+	for node in g.a.nodes {
+		if node.kind == .import_decl && node.typ.len > 0 {
+			g.module_aliases[node.typ] = node.value
+		}
+	}
+}
+
 // resolve_call_keys returns the candidate fn_index keys for a callee, in
 // priority order. A bare ident inside an imported module resolves to the
 // same-module function first (`mod.name`) and falls back to a main-module name;
-// a `module.fn()` selector resolves to that module's qualified name.
+// a `module.fn()` selector resolves to that module's qualified name, with any
+// import alias mapped back to the real module.
 fn (g &Gen) resolve_call_keys(callee &flat.Node, cur_mod string) []string {
 	if callee.kind == .ident {
 		if cur_mod != '' && cur_mod != 'main' {
@@ -952,7 +976,8 @@ fn (g &Gen) resolve_call_keys(callee &flat.Node, cur_mod string) []string {
 	if callee.kind == .selector && callee.children_count > 0 {
 		base := g.a.child_node(callee, 0)
 		if base.kind == .ident {
-			return ['${base.value}.${callee.value}']
+			real := g.module_aliases[base.value] or { base.value }
+			return ['${real}.${callee.value}']
 		}
 	}
 	return []

vlib/v3/tests/wasm_codegen_test.v

@@ -268,6 +268,37 @@ fn test_wasm_imported_module_numeric_call() {
 	}
 }
 
+fn test_wasm_for_post_uses_loop_var_not_body_shadow() {
+	v3_bin := v3_binary()
+	// A body-local `i` must not rebind the name used by the post `i++`; the
+	// outer loop counter must still advance. The count/break bound keeps the
+	// test terminating even if the fix regresses (it would loop otherwise).
+	src := 'fn main() {\n\tmut i := 0\n\tmut count := 0\n\tfor ; i < 3; i++ {\n\t\ti := 10\n\t\t_ = i\n\t\tcount++\n\t\tif count > 100 {\n\t\t\tbreak\n\t\t}\n\t}\n\tprintln(i)\n\tprintln(count)\n}\n'
+	wasm := compile_to_wasm(v3_bin, src, 'wasm_loopshadow')
+	assert_valid_wasm(wasm)
+	run_wasi_expect(wasm, ['3', '3'])
+}
+
+fn test_wasm_imported_module_alias_call() {
+	v3_bin := v3_binary()
+	dir := os.join_path(os.vtmp_dir(), 'wasm_modalias')
+	os.rmdir_all(dir) or {}
+	os.mkdir_all(os.join_path(dir, 'moda')) or { panic(err) }
+	os.write_file(os.join_path(dir, 'main.v'), 'import moda as m\n\nfn main() {\n\tprintln(m.answer())\n\tprintln(m.add(3, 4))\n}\n') or {
+		panic(err)
+	}
+	os.write_file(os.join_path(dir, 'moda', 'moda.v'), 'module moda\n\npub fn answer() int {\n\treturn 42\n}\n\npub fn add(a int, b int) int {\n\treturn a + b\n}\n') or {
+		panic(err)
+	}
+	out_wasm := os.join_path(dir, 'main.wasm')
+	main_v := os.join_path(dir, 'main.v')
+	res := os.execute('${os.quoted_path(v3_bin)} -b wasm -o ${os.quoted_path(out_wasm)} ${os.quoted_path(main_v)}')
+	assert res.exit_code == 0, res.output
+	assert_valid_wasm(out_wasm)
+	assert !res.output.contains('unsupported call'), res.output
+	run_wasi_expect(out_wasm, ['42', '7'])
+}
+
 const wasi_runner_js = "import { WASI } from 'node:wasi';
 import { readFile } from 'node:fs/promises';
 const wasi = new WASI({ version: 'preview1', args: [], env: {} });