Zir: represent declarations via an instruction

This commit changes how declarations (`const`, `fn`, `usingnamespace`,
etc) are represented in ZIR. Previously, these were represented in the
container type's extra data (e.g. as trailing data on a `struct_decl`).
However, this introduced the complexity of the ZIR mapping logic having
to also correlate some ZIR extra data indices. That isn't really a
problem today, but it's tricky for the introduction of `TrackedInst` in
the commit following this one. Instead, these type declarations now
simply contain a trailing list of ZIR indices to `declaration`
instructions, which directly encode all data related to the declaration
(including containing the declaration's body). Additionally, the ZIR for
`align` etc have been split out into their own bodies. This is not
strictly necessary, but it's much simpler to understand for an
insignificant cost in bytes, and will simplify the resolution of ziglang#131
(where we may need to evaluate the pointer type, including align etc,
without immediately evaluating the value body).

Author: mlugg

src/AstGen.zig

@@ -6186,8 +6186,6 @@ fn finishFuncInstance(
         .generation = generation,
         .is_pub = fn_owner_decl.is_pub,
         .is_exported = fn_owner_decl.is_exported,
-        .has_linksection_or_addrspace = fn_owner_decl.has_linksection_or_addrspace,
-        .has_align = fn_owner_decl.has_align,
         .alive = true,
         .kind = .anon,
     });

src/Autodoc.zig

@@ -1224,6 +1224,10 @@ fn analyzeBodyInner(
             .trap           => break sema.zirTrap(block, inst),
             // zig fmt: on
 
+            // This instruction never exists in an analyzed body. It exists only in the declaration
+            // list for a container type.
+            .declaration => unreachable,
+
             .extended => ext: {
                 const extended = datas[@intFromEnum(inst)].extended;
                 break :ext switch (extended.opcode) {
@@ -2736,7 +2740,9 @@ pub fn getStructType(
         }
     }
 
-    extra_index = try mod.scanNamespace(namespace, extra_index, decls_len, mod.declPtr(decl));
+    const decls = sema.code.bodySlice(extra_index, decls_len);
+    try mod.scanNamespace(namespace, decls, mod.declPtr(decl));
+    extra_index += decls_len;
 
     const ty = try ip.getStructType(gpa, .{
         .decl = decl,
@@ -2973,7 +2979,9 @@ fn zirEnumDecl(
     const new_namespace = mod.namespacePtr(new_namespace_index);
     errdefer if (!done) mod.destroyNamespace(new_namespace_index);
 
-    extra_index = try mod.scanNamespace(new_namespace_index, extra_index, decls_len, new_decl);
+    const decls = sema.code.bodySlice(extra_index, decls_len);
+    try mod.scanNamespace(new_namespace_index, decls, new_decl);
+    extra_index += decls_len;
 
     const body = sema.code.bodySlice(extra_index, body_len);
     extra_index += body.len;
@@ -3263,7 +3271,8 @@ fn zirUnionDecl(
     new_decl.val = Value.fromInterned(union_ty);
     new_namespace.ty = Type.fromInterned(union_ty);
 
-    _ = try mod.scanNamespace(new_namespace_index, extra_index, decls_len, new_decl);
+    const decls = sema.code.bodySlice(extra_index, decls_len);
+    try mod.scanNamespace(new_namespace_index, decls, new_decl);
 
     const decl_val = sema.analyzeDeclVal(block, src, new_decl_index);
     try mod.finalizeAnonDecl(new_decl_index);
@@ -3326,7 +3335,8 @@ fn zirOpaqueDecl(
     new_decl.val = Value.fromInterned(opaque_ty);
     new_namespace.ty = Type.fromInterned(opaque_ty);
 
-    extra_index = try mod.scanNamespace(new_namespace_index, extra_index, decls_len, new_decl);
+    const decls = sema.code.bodySlice(extra_index, decls_len);
+    try mod.scanNamespace(new_namespace_index, decls, new_decl);
 
     const decl_val = sema.analyzeDeclVal(block, src, new_decl_index);
     try mod.finalizeAnonDecl(new_decl_index);
@@ -36331,9 +36341,7 @@ fn structZirInfo(zir: Zir, zir_index: Zir.Inst.Index) struct {
     }
 
     // Skip over decls.
-    var decls_it = zir.declIteratorInner(extra_index, decls_len);
-    while (decls_it.next()) |_| {}
-    extra_index = decls_it.extra_index;
+    extra_index += decls_len;
 
     return .{ fields_len, small, extra_index };
 }
@@ -36802,9 +36810,7 @@ fn semaUnionFields(mod: *Module, arena: Allocator, union_type: InternPool.Key.Un
     } else 0;
 
     // Skip over decls.
-    var decls_it = zir.declIteratorInner(extra_index, decls_len);
-    while (decls_it.next()) |_| {}
-    extra_index = decls_it.extra_index;
+    extra_index += decls_len;
 
     const body = zir.bodySlice(extra_index, body_len);
     extra_index += body.len;
@@ -37801,10 +37807,12 @@ pub fn analyzeAddressSpace(
     ctx: AddressSpaceContext,
 ) !std.builtin.AddressSpace {
     const mod = sema.mod;
-    const addrspace_tv = try sema.resolveInstConst(block, src, zir_ref, .{
+    const air_ref = try sema.resolveInst(zir_ref);
+    const coerced = try sema.coerce(block, Type.fromInterned(.address_space_type), air_ref, src);
+    const addrspace_val = try sema.resolveConstDefinedValue(block, src, coerced, .{
         .needed_comptime_reason = "address space must be comptime-known",
     });
-    const address_space = mod.toEnum(std.builtin.AddressSpace, addrspace_tv.val);
+    const address_space = mod.toEnum(std.builtin.AddressSpace, addrspace_val);
     const target = sema.mod.getTarget();
     const arch = target.cpu.arch;