gen: add callconv attribute for fn and type (#14027)

doc/docs.md

@@ -5978,10 +5978,15 @@ fn custom_allocations() {
 struct C.Foo {
 }
 
-// Used in Win32 API code when you need to pass callback function
-[windows_stdcall]
+// Used to add a custom calling convention to a function, available calling convention: stdcall, fastcall and cdecl.
+// This list aslo apply for type aliases (see below).
+[callconv: "stdcall"]
 fn C.DefWindowProc(hwnd int, msg int, lparam int, wparam int)
 
+// Used to add a custom calling convention to a function type aliases.
+[callconv: "fastcall"]
+type FastFn = fn (int) bool
+
 // Windows only:
 // If a default graphics library is imported (ex. gg, ui), then the graphical window takes
 // priority and no console window is created, effectively disabling println() statements.

vlib/v/ast/ast.v

@@ -1152,6 +1152,7 @@ pub:
 	pos      token.Pos
 	type_pos token.Pos
 	comments []Comment
+	attrs    []Attr // attributes of type declaration
 }
 
 // TODO: handle this differently

vlib/v/fmt/fmt.v

@@ -1290,6 +1290,7 @@ pub fn (mut f Fmt) alias_type_decl(node ast.AliasTypeDecl) {
 }
 
 pub fn (mut f Fmt) fn_type_decl(node ast.FnTypeDecl) {
+	f.attrs(node.attrs)
 	if node.is_pub {
 		f.write('pub ')
 	}

vlib/v/fmt/tests/fn_type_attributes_and_calling_convention_keep.vv

@@ -0,0 +1,10 @@
+[callconv: 'stdcall']
+fn C.DefWindowProc(hwnd int, msg int, lparam int, wparam int)
+
+[callconv: 'stdcall']
+type FastFn = fn (int) bool
+
+[callconv: 'fastcall']
+fn zzz(x int, y int) int {
+	return x + y * 2
+}

vlib/v/gen/c/cgen.v

@@ -96,6 +96,7 @@ mut:
 	is_json_fn                bool // inside json.encode()
 	is_js_call                bool // for handling a special type arg #1 `json.decode(User, ...)`
 	is_fn_index_call          bool
+	is_cc_msvc                bool   // g.pref.ccompiler == 'msvc'
 	vlines_path               string // set to the proper path for generating #line directives
 	optionals                 map[string]string // to avoid duplicates
 	done_optionals            shared []string   // to avoid duplicates
@@ -258,6 +259,7 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 		inner_loop: &ast.EmptyStmt{}
 		field_data_type: ast.Type(table.find_type_idx('FieldData'))
 		init: strings.new_builder(100)
+		is_cc_msvc: pref.ccompiler == 'msvc'
 	}
 	// anon fn may include assert and thus this needs
 	// to be included before any test contents are written
@@ -557,6 +559,7 @@ fn cgen_process_one_file_cb(p &pool.PoolProcessor, idx int, wid int) &Gen {
 		done_optionals: global_g.done_optionals
 		is_autofree: global_g.pref.autofree
 		referenced_fns: global_g.referenced_fns
+		is_cc_msvc: global_g.is_cc_msvc
 	}
 	g.gen_file()
 	return g
@@ -1343,14 +1346,35 @@ pub fn (mut g Gen) write_fn_typesymbol_declaration(sym ast.TypeSymbol) {
 	if !info.has_decl && (not_anon || is_fn_sig) && !func.return_type.has_flag(.generic)
 		&& !has_generic_arg {
 		fn_name := sym.cname
-		g.type_definitions.write_string('typedef ${g.typ(func.return_type)} (*$fn_name)(')
+
+		mut call_conv := ''
+		mut msvc_call_conv := ''
+		for attr in func.attrs {
+			match attr.name {
+				'callconv' {
+					if g.is_cc_msvc {
+						msvc_call_conv = '__$attr.arg '
+					} else {
+						call_conv = '$attr.arg'
+					}
+				}
+				else {}
+			}
+		}
+		call_conv_attribute_suffix := if call_conv.len != 0 {
+			'__attribute__(($call_conv))'
+		} else {
+			''
+		}
+
+		g.type_definitions.write_string('typedef ${g.typ(func.return_type)} ($msvc_call_conv*$fn_name)(')
 		for i, param in func.params {
 			g.type_definitions.write_string(g.typ(param.typ))
 			if i < func.params.len - 1 {
 				g.type_definitions.write_string(',')
 			}
 		}
-		g.type_definitions.writeln(');')
+		g.type_definitions.writeln(')$call_conv_attribute_suffix;')
 	}
 }
 

vlib/v/gen/c/fn.v

@@ -2148,10 +2148,13 @@ fn (mut g Gen) write_fn_attrs(attrs []ast.Attr) string {
 			'windows_stdcall' {
 				// windows attributes (msvc/mingw)
 				// prefixed by windows to indicate they're for advanced users only and not really supported by V.
-				fn_attrs += '__stdcall '
+				fn_attrs += call_convention_attribute('stdcall', g.is_cc_msvc)
 			}
 			'_fastcall' {
-				fn_attrs += '__fastcall '
+				fn_attrs += call_convention_attribute('fastcall', g.is_cc_msvc)
+			}
+			'callconv' {
+				fn_attrs += call_convention_attribute(attr.arg, g.is_cc_msvc)
 			}
 			'console' {
 				g.force_main_console = true
@@ -2163,3 +2166,7 @@ fn (mut g Gen) write_fn_attrs(attrs []ast.Attr) string {
 	}
 	return fn_attrs
 }
+
+fn call_convention_attribute(cconvention string, is_cc_msvc bool) string {
+	return if is_cc_msvc { '__$cconvention ' } else { '__attribute__(($cconvention)) ' }
+}

vlib/v/parser/fn.v

@@ -185,18 +185,60 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	mut comments := []ast.Comment{}
 	for fna in p.attrs {
 		match fna.name {
-			'noreturn' { is_noreturn = true }
-			'manualfree' { is_manualfree = true }
-			'deprecated' { is_deprecated = true }
-			'direct_array_access' { is_direct_arr = true }
-			'keep_args_alive' { is_keep_alive = true }
-			'export' { is_exported = true }
-			'wasm_export' { is_exported = true }
-			'unsafe' { is_unsafe = true }
-			'trusted' { is_trusted = true }
-			'c2v_variadic' { is_c2v_variadic = true }
-			'use_new' { is_ctor_new = true }
-			'markused' { is_markused = true }
+			'noreturn' {
+				is_noreturn = true
+			}
+			'manualfree' {
+				is_manualfree = true
+			}
+			'deprecated' {
+				is_deprecated = true
+			}
+			'direct_array_access' {
+				is_direct_arr = true
+			}
+			'keep_args_alive' {
+				is_keep_alive = true
+			}
+			'export' {
+				is_exported = true
+			}
+			'wasm_export' {
+				is_exported = true
+			}
+			'unsafe' {
+				is_unsafe = true
+			}
+			'trusted' {
+				is_trusted = true
+			}
+			'c2v_variadic' {
+				is_c2v_variadic = true
+			}
+			'use_new' {
+				is_ctor_new = true
+			}
+			'markused' {
+				is_markused = true
+			}
+			'windows_stdcall' {
+				// TODO: uncomment after bootstrapping and replacing usages in builtin
+				// p.note_with_pos('windows_stdcall has been deprecated, it will be an error soon', p.tok.pos())
+			}
+			'_fastcall' {
+				// TODO: uncomment after bootstrapping and replacing usages in builtin
+				// p.note_with_pos('_fastcall has been deprecated, it will be an error soon', p.tok.pos())
+			}
+			'callconv' {
+				if !fna.has_arg {
+					p.error_with_pos('callconv attribute is present but its value is missing',
+						p.prev_tok.pos())
+				}
+				if fna.arg !in ['stdcall', 'fastcall', 'cdecl'] {
+					p.error_with_pos('unsupported calling convention, supported are stdcall, fastcall and cdecl',
+						p.prev_tok.pos())
+				}
+			}
 			else {}
 		}
 	}

vlib/v/parser/parse_type.v

@@ -229,6 +229,23 @@ pub fn (mut p Parser) parse_multi_return_type() ast.Type {
 pub fn (mut p Parser) parse_fn_type(name string) ast.Type {
 	// p.warn('parse fn')
 	p.check(.key_fn)
+
+	for attr in p.attrs {
+		match attr.name {
+			'callconv' {
+				if !attr.has_arg {
+					p.error_with_pos('callconv attribute is present but its value is missing',
+						p.prev_tok.pos())
+				}
+				if attr.arg !in ['stdcall', 'fastcall', 'cdecl'] {
+					p.error_with_pos('unsupported calling convention, supported are stdcall, fastcall and cdecl',
+						p.prev_tok.pos())
+				}
+			}
+			else {}
+		}
+	}
+
 	mut has_generic := false
 	line_nr := p.tok.line_nr
 	args, _, is_variadic := p.fn_args()
@@ -255,6 +272,7 @@ pub fn (mut p Parser) parse_fn_type(name string) ast.Type {
 		return_type: return_type
 		return_type_pos: return_type_pos
 		is_method: false
+		attrs: p.attrs
 	}
 	// MapFooFn typedefs are manually added in cheaders.v
 	// because typedefs get generated after the map struct is generated

vlib/v/parser/parser.v

@@ -3579,6 +3579,8 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 		p.table.sym(fn_type).is_pub = is_pub
 		type_pos = type_pos.extend(p.tok.pos())
 		comments = p.eat_comments(same_line: true)
+		attrs := p.attrs
+		p.attrs = []
 		return ast.FnTypeDecl{
 			name: fn_name
 			is_pub: is_pub
@@ -3586,6 +3588,7 @@ fn (mut p Parser) type_decl() ast.TypeDecl {
 			pos: decl_pos
 			type_pos: type_pos
 			comments: comments
+			attrs: attrs
 		}
 	}
 	sum_variants << p.parse_sum_type_variants()