move types from builtin to std

 * All the data types from `@import("builtin")` are moved to
  `@import("std").builtin`. The target-related types are moved
  to `std.Target`. This allows the data types to have methods, such as
  `std.Target.current.isDarwin()`.
 * `std.os.windows.subsystem` is moved to
   `std.Target.current.subsystem`.
 * Remove the concept of the panic package from the compiler
   implementation. Instead, `std.builtin.panic` is always the panic
   function. It checks for `@hasDecl(@import("root"), "panic")`,
   or else provides a default implementation.

This is an important step for multibuilds (#3028). Without this change,
the types inside the builtin namespace look like different types, when
trying to merge builds with different target settings. With this change,
Zig can figure out that, e.g., `std.builtin.Os` (the enum type) from one
compilation and `std.builtin.Os` from another compilation are the same
type, even if the target OS value differs.

Author: andrewrk

lib/std/build.zig

@@ -23,10 +23,6 @@ const elf = std.elf;
 const dl = @import("dynamic_library.zig");
 const MAX_PATH_BYTES = std.fs.MAX_PATH_BYTES;
 
-comptime {
-    assert(@import("std") == std); // std lib tests require --override-lib-dir
-}
-
 pub const darwin = @import("os/darwin.zig");
 pub const freebsd = @import("os/freebsd.zig");
 pub const linux = @import("os/linux.zig");
@@ -36,6 +32,22 @@ pub const wasi = @import("os/wasi.zig");
 pub const windows = @import("os/windows.zig");
 pub const zen = @import("os/zen.zig");
 
+comptime {
+    assert(@import("std") == std); // std lib tests require --override-lib-dir
+    if (builtin.is_test) {
+        _ = darwin;
+        _ = freebsd;
+        _ = linux;
+        _ = netbsd;
+        _ = uefi;
+        _ = wasi;
+        _ = windows;
+        _ = zen;
+
+        _ = @import("os/test.zig");
+    }
+}
+
 /// When linking libc, this is the C API. Otherwise, it is the OS-specific system interface.
 pub const system = if (builtin.link_libc) std.c else switch (builtin.os) {
     .macosx, .ios, .watchos, .tvos => darwin,
@@ -1063,7 +1075,6 @@ pub fn unlinkatW(dirfd: fd_t, sub_path_w: [*]const u16, flags: u32) UnlinkatErro
         return error.FileBusy;
     }
 
-
     var attr = w.OBJECT_ATTRIBUTES{
         .Length = @sizeOf(w.OBJECT_ATTRIBUTES),
         .RootDirectory = dirfd,
@@ -2813,16 +2824,3 @@ pub fn gethostname(name_buffer: *[HOST_NAME_MAX]u8) GetHostNameError![]u8 {
 
     @compileError("TODO implement gethostname for this OS");
 }
-
-test "" {
-    _ = @import("os/darwin.zig");
-    _ = @import("os/freebsd.zig");
-    _ = @import("os/linux.zig");
-    _ = @import("os/netbsd.zig");
-    _ = @import("os/uefi.zig");
-    _ = @import("os/wasi.zig");
-    _ = @import("os/windows.zig");
-    _ = @import("os/zen.zig");
-
-    _ = @import("os/test.zig");
-}

lib/std/builtin.zig

@@ -22,32 +22,6 @@ pub usingnamespace @import("windows/bits.zig");
 
 pub const self_process_handle = @intToPtr(HANDLE, maxInt(usize));
 
-/// `builtin` is missing `subsystem` when the subsystem is automatically detected,
-/// so Zig standard library has the subsystem detection logic here. This should generally be
-/// used rather than `builtin.subsystem`.
-/// On non-windows targets, this is `null`.
-pub const subsystem: ?builtin.SubSystem = blk: {
-    if (@hasDecl(builtin, "subsystem")) break :blk builtin.subsystem;
-    switch (builtin.os) {
-        .windows => {
-            if (builtin.is_test) {
-                break :blk builtin.SubSystem.Console;
-            }
-            const root = @import("root");
-            if (@hasDecl(root, "WinMain") or
-                @hasDecl(root, "wWinMain") or
-                @hasDecl(root, "WinMainCRTStartup") or
-                @hasDecl(root, "wWinMainCRTStartup"))
-            {
-                break :blk builtin.SubSystem.Windows;
-            } else {
-                break :blk builtin.SubSystem.Console;
-            }
-        },
-        else => break :blk null,
-    }
-};
-
 pub const CreateFileError = error{
     SharingViolation,
     PathAlreadyExists,

lib/std/os.zig

@@ -22,11 +22,13 @@ pub const SpinLock = @import("spinlock.zig").SpinLock;
 pub const StaticallyInitializedMutex = @import("statically_initialized_mutex.zig").StaticallyInitializedMutex;
 pub const StringHashMap = @import("hash_map.zig").StringHashMap;
 pub const TailQueue = @import("linked_list.zig").TailQueue;
+pub const Target = @import("target.zig").Target;
 pub const Thread = @import("thread.zig").Thread;
 
 pub const atomic = @import("atomic.zig");
 pub const base64 = @import("base64.zig");
 pub const build = @import("build.zig");
+pub const builtin = @import("builtin.zig");
 pub const c = @import("c.zig");
 pub const coff = @import("coff.zig");
 pub const crypto = @import("crypto.zig");