eliminate the BoundFn type from the language and the compiler implementations

[andrewrk]

Currently the Zig type system has a "BoundFn" which can be observed thusly:

test "example" {
    var a: A = .{};
    @compileLog(@TypeOf(a.b));
}

const A = struct {
    fn b() void {}
};

This type is to be removed from the language. Instead, method call syntax will be explicitly recognized in the parser. Here's an example which currently passes but will become a compile error:

test "example" {
    var a1: A = .{};
    var a2: A = .{};
    (if (true) a1.b else a2.b)();
}

const A = struct {
    fn b(a: A) void {
        _ = a;
    }
};

After this change, this will give a compile error: struct A has no field named b.

Related: #7942

Here's a diff to get you started:

diff --git a/lib/std/builtin.zig b/lib/std/builtin.zig
index 9d432a3a0..f4daf5354 100644
--- a/lib/std/builtin.zig
+++ b/lib/std/builtin.zig
@@ -204,7 +204,6 @@ pub const TypeInfo = union(enum) {
     Enum: Enum,
     Union: Union,
     Fn: Fn,
-    BoundFn: Fn,
     Opaque: Opaque,
     Frame: Frame,
     AnyFrame: AnyFrame,
diff --git a/src/stage1/all_types.hpp b/src/stage1/all_types.hpp
index dfebc66cf..d36396c68 100644
--- a/src/stage1/all_types.hpp
+++ b/src/stage1/all_types.hpp
@@ -310,12 +310,6 @@ struct ConstErrValue {
     ZigValue *payload;
 };
 
-struct ConstBoundFnValue {
-    ZigFn *fn;
-    Stage1AirInst *first_arg;
-    AstNode *first_arg_src;
-};
-
 struct ConstArgTuple {
     size_t start_index;
     size_t end_index;
@@ -510,7 +504,6 @@ struct ZigValue {
         double x_f64;
         float128_t x_f128;
         bool x_bool;
-        ConstBoundFnValue x_bound_fn;
         ZigType *x_type;
         ZigValue *x_optional;
         ConstErrValue x_err_union;
@@ -1492,10 +1485,6 @@ struct ZigTypeFn {
     ZigType *bound_fn_parent;
 };
 
-struct ZigTypeBoundFn {
-    ZigType *fn_type;
-};
-
 // Needs to have the same memory layout as ZigTypeArray
 struct ZigTypeVector {
     // The type must be a pointer, integer, bool, or float
@@ -1528,7 +1517,6 @@ enum ZigTypeId {
     ZigTypeIdEnum,
     ZigTypeIdUnion,
     ZigTypeIdFn,
-    ZigTypeIdBoundFn,
     ZigTypeIdOpaque,
     ZigTypeIdFnFrame,
     ZigTypeIdAnyFrame,
@@ -1587,7 +1575,6 @@ struct ZigType {
         ZigTypeEnum enumeration;
         ZigTypeUnion unionation;
         ZigTypeFn fn;
-        ZigTypeBoundFn bound_fn;
         ZigTypeVector vector;
         ZigTypeOpaque opaque;
         ZigTypeFnFrame frame;

[zigazeljko]

What is the expected implementation here?

1. Add a new node type to parser to be used instead of the current NodeTypeFieldAccessExpr + NodeTypeFnCallExpr combo.
2. Keep the parser unchanged and handle the NodeTypeFieldAccessExpr + NodeTypeFnCallExpr combo inside astgen.

[andrewrk]

@zigazeljko apologies for not seeing your question earlier.

You'll need to change the parser as noted here:

Instead, method call syntax will be explicitly recognized in the parser

[edt-xx]

Probably a stupid question, but how will zig handle a 'class fn' in a struct? eg usually you want:

fn aaa(...) type {
     var bbb:u64 = ....
     return struct {
         ....
         ST = @This();
           fn ccc(self: *ST, ... ) ...  { ... }    // the common case
           
           fn ddd(eee: ... ) u64 { ... some code depending on bbb & eee  returning a u64 ... )  // how will we express this
           ....      
 }
 
 AaaType = aaa(...);
 AaaType.ddd(..);

[dacjames]

Is anyone working on this? I've been looking for something for a first time contribution that touches the language itself (since that's my interest area).

Aside from overall simplification, what is the motivation for removing BoundFn, @andrewrk ?

[fogti]

it is normal that I arrive at the following when applying the patch in the first comment?

$ zig build --zig-lib-dir lib --verbose
Semantic Analysis [440/860] Assertion failed at /build/source/src/stage1/ir.cpp:19106 in type_info_to_type. This is a bug in the Zig compiler.thread 303768 panic: 
Unable to dump stack trace: debug info stripped
Aborted (core dumped)

compiled with zig version 0.10.0 fbe5336
with first-comment patch applied on top 0e6d218
code snippet:

        case ZigTypeIdVector: {
            // one of the following asserts fails...
            assert(payload->special == ConstValSpecialStatic);
            assert(payload->type == ir_type_info_get_type(ira, "Vector", nullptr));
            BigInt *len = get_const_field_lit_int(ira, source_node, payload, "len", 0);
            if (len == nullptr)
                return ira->codegen->invalid_inst_gen->value->type;

            ZigType *child_type = get_const_field_meta_type(ira, source_node, payload, "child", 1);
            if ((err = ir_validate_vector_elem_type(ira, source_node, child_type))) {
                return ira->codegen->invalid_inst_gen->value->type;
            }
            return get_vector_type(ira->codegen, bigint_as_u32(len), child_type);
        }