introduce operating system version ranges as part of the target

 * re-introduce `std.build.Target` which is distinct from `std.Target`.
   `std.build.Target` wraps `std.Target` so that it can be annotated as
   "the native target" or an explicitly specified target.
 * `std.Target.Os` is moved to `std.Target.Os.Tag`. The former is now a
   struct which has the tag as well as version range information.
 * `std.elf` gains some more ELF header constants.
 * `std.Target.parse` gains the ability to parse operating system
   version ranges as well as glibc version.
 * Added `std.Target.isGnuLibC()`.
 * self-hosted dynamic linker detection and glibc version detection.
   This also adds the improved logic using `/usr/bin/env` rather than
   invoking the system C compiler to find the dynamic linker when zig
   is statically linked. Related: #2084
   Note: this `/usr/bin/env` code is work-in-progress.
 * `-target-glibc` CLI option is removed in favor of the new `-target`
   syntax. Example: `-target x86_64-linux-gnu.2.27`

closes #1907

1 files changed, 8 insertions, 8 deletions

diff --git a/lib/std/start.zig b/lib/std/start.zig
index b58b6e8144..b8e3e97f94 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -12,7 +12,7 @@ const start_sym_name = if (builtin.arch.isMIPS()) "__start" else "_start";
 
 comptime {
     if (builtin.output_mode == .Lib and builtin.link_mode == .Dynamic) {
-        if (builtin.os == .windows and !@hasDecl(root, "_DllMainCRTStartup")) {
+        if (builtin.os.tag == .windows and !@hasDecl(root, "_DllMainCRTStartup")) {
             @export(_DllMainCRTStartup, .{ .name = "_DllMainCRTStartup" });
         }
     } else if (builtin.output_mode == .Exe or @hasDecl(root, "main")) {
@@ -20,17 +20,17 @@ comptime {
             if (@typeInfo(@TypeOf(root.main)).Fn.calling_convention != .C) {
                 @export(main, .{ .name = "main", .linkage = .Weak });
             }
-        } else if (builtin.os == .windows) {
+        } else if (builtin.os.tag == .windows) {
             if (!@hasDecl(root, "WinMain") and !@hasDecl(root, "WinMainCRTStartup") and
                 !@hasDecl(root, "wWinMain") and !@hasDecl(root, "wWinMainCRTStartup"))
             {
                 @export(WinMainCRTStartup, .{ .name = "WinMainCRTStartup" });
             }
-        } else if (builtin.os == .uefi) {
+        } else if (builtin.os.tag == .uefi) {
             if (!@hasDecl(root, "EfiMain")) @export(EfiMain, .{ .name = "EfiMain" });
-        } else if (builtin.arch.isWasm() and builtin.os == .freestanding) {
+        } else if (builtin.arch.isWasm() and builtin.os.tag == .freestanding) {
             if (!@hasDecl(root, start_sym_name)) @export(wasm_freestanding_start, .{ .name = start_sym_name });
-        } else if (builtin.os != .other and builtin.os != .freestanding) {
+        } else if (builtin.os.tag != .other and builtin.os.tag != .freestanding) {
             if (!@hasDecl(root, start_sym_name)) @export(_start, .{ .name = start_sym_name });
         }
     }
@@ -78,7 +78,7 @@ fn EfiMain(handle: uefi.Handle, system_table: *uefi.tables.SystemTable) callconv
 }
 
 fn _start() callconv(.Naked) noreturn {
-    if (builtin.os == builtin.Os.wasi) {
+    if (builtin.os.tag == .wasi) {
         // This is marked inline because for some reason LLVM in release mode fails to inline it,
         // and we want fewer call frames in stack traces.
         std.os.wasi.proc_exit(@call(.{ .modifier = .always_inline }, callMain, .{}));
@@ -133,7 +133,7 @@ fn WinMainCRTStartup() callconv(.Stdcall) noreturn {
 
 // TODO https://github.com/ziglang/zig/issues/265
 fn posixCallMainAndExit() noreturn {
-    if (builtin.os == builtin.Os.freebsd) {
+    if (builtin.os.tag == .freebsd) {
         @setAlignStack(16);
     }
     const argc = starting_stack_ptr[0];
@@ -144,7 +144,7 @@ fn posixCallMainAndExit() noreturn {
     while (envp_optional[envp_count]) |_| : (envp_count += 1) {}
     const envp = @ptrCast([*][*:0]u8, envp_optional)[0..envp_count];
 
-    if (builtin.os == .linux) {
+    if (builtin.os.tag == .linux) {
         // Find the beginning of the auxiliary vector
         const auxv = @ptrCast([*]std.elf.Auxv, @alignCast(@alignOf(usize), envp.ptr + envp_count + 1));
         std.os.linux.elf_aux_maybe = auxv;