make SPIR-V use an independent definition of std.lang.ExternOptions #35973

Partial overlap with #30873.

Based these observations:

• The only fields of ExternOptions that are relevant to SPIR-V are name and decoration.
• decoration must be null on non-SPIR-V.

These should be different types.

--- a/lib/std/lang.zig
+++ b/lib/std/lang.zig
@@ -1160,35 +1160,45 @@ pub const ExportOptions = struct {
 
 /// This data structure is used by the Zig language code generation and
 /// therefore must be kept in sync with the compiler implementation.
-pub const ExternOptions = struct {
- name: []const u8,
- library_name: ?[]const u8 = null,
- linkage: GlobalLinkage = .strong,
- visibility: SymbolVisibility = .default,
- /// Setting this to `true` makes the `@extern` a runtime value.
- is_thread_local: bool = false,
- is_dll_import: bool = false,
- relocation: Relocation = .any,
- decoration: ?Decoration = null,
+pub const ExternOptions = switch (builtin.cpu.arch) {
+ .spirv32, .spirv64 => struct {
+ name: []const u8,
+ decoration: ?Decoration = null,
+
+ pub const Decoration = union(enum) {
+ storage: Storage,
+ descriptor: Descriptor,
+ };
 
- pub const Decoration = union(enum) {
- location: u32,
- flat: u32,
- descriptor: Descriptor,
+ pub const Storage = struct {
+ location: u32,
+ flat: bool = false,
+ name: ?[]const u8 = null,
+ };
 
 pub const Descriptor = struct {
 set: u32,
 binding: u32,
 };
- };
-
- pub const Relocation = enum(u1) {
- /// Any type of relocation is allowed.
- any,
- /// A program-counter-relative relocation is required.
- /// Using this value makes the `@extern` a runtime value.
- pcrel,
- };
+ },
+ else => struct {
+ name: []const u8,
+ library_name: ?[]const u8 = null,
+ linkage: GlobalLinkage = .strong,
+ visibility: SymbolVisibility = .default,
+ /// Setting this to `true` makes the `@extern` a runtime value.
+ is_thread_local: bool = false,
+ is_dll_import: bool = false,
+ relocation: Relocation = .any,
+
+ pub const Relocation = enum(u1) {
+ /// Any type of relocation is allowed.
+ any,
+ /// A program-counter-relative relocation is required.
+ /// Using this value makes the `@extern` a runtime value.
+ pcrel,
+ };
+ },
 };
 
 /// This data structure is used by the Zig language code generation and

Original version of this proposal was based on my incorrect understanding that name was unused on SPIR-V when in fact it appears to be always used, at least for debug information. Also incorrect understanding that decoration was mandatory, when in fact it can be null on SPIR-V too. Even so, this proposal is accepted.

suggestion:
maybe instead of

location: u32,
flat: u32,

have

storage: struct {
 location: u32,
 flat: bool = false,
},

?

I'd argue against keeping the name field. Right now it has two purposes:

• Reserved names for emitting built-in variables such as position, point_size, etc. which we can move into the new ExternOptions structure without bloating it for CPU targets.
• Debug info which i found very weird here because symbols in SPIR-V do not exist the same way as other formats. They are accessed by set, binding, locations and other decorations which are all numeric. it's also confusing when both source name and externed name const <var_name> = @extern(T, .{ .name = "<symbol_name>", ... }) exists.

pubconstExternOptions=switch(builtin.cpu.arch){.spirv32,.spirv64=>struct{interface:Interface,descriptor:Descriptor,builtin:BuiltIn,pubconstInterface=struct{location:u32,flat:bool=false,};pubconstDescriptor=struct{set:u32,binding:u32,};pubconstBuiltIn=enum(u32){position=0,point_size=1,clip_distance=3,cull_distance=4,vertex_id=5,instance_id=6,..._,},},else=>struct{name:[]constu8,library_name:?[]constu8=null,linkage:GlobalLinkage=.strong,visibility:SymbolVisibility=.default,/// Setting this to `true` makes the `@extern` a runtime value.is_thread_local:bool=false,is_dll_import:bool=false,relocation:ExternRelocation=.any,},};