builtin: Add zig_version

This will enable code to perform version checks and make it easier to
support multiple versions of Zig.

Within the SemVer implementation, an intermediate value needed to be
coerced to a slice to workaround a comptime bug.

Closes ziglang#6466

lib/std/SemanticVersion.zig

@@ -102,7 +102,7 @@ pub fn parse(text: []const u8) !Version {
     if (extra_index == null) return ver;
 
     // Slice optional pre-release or build metadata components.
-    const extra = text[extra_index.?..text.len];
+    const extra: []const u8 = text[extra_index.?..text.len];
     if (extra[0] == '-') {
         const build_index = std.mem.indexOfScalar(u8, extra, '+');
         ver.pre = extra[1..(build_index orelse extra.len)];
@@ -293,3 +293,12 @@ fn testFmt(expected: []const u8, comptime template: []const u8, args: anytype) !
     std.debug.warn("\n======================================\n", .{});
     return error.TestFailed;
 }
+
+test "zig_version" {
+    // An approximate Zig build that predates this test.
+    const older_version = .{ .major = 0, .minor = 8, .patch = 0, .pre = "dev.874" };
+
+    // Simulated compatibility check using Zig version.
+    const compatible = comptime @import("builtin").zig_version.order(older_version) == .gt;
+    if (!compatible) @compileError("zig_version test failed");
+}

src/Compilation.zig

@@ -2703,6 +2703,11 @@ pub fn generateBuiltinZigSource(comp: *Compilation, allocator: *Allocator) ![]u8
         \\pub const arch = Target.current.cpu.arch;
         \\/// Deprecated
         \\pub const endian = Target.current.cpu.arch.endian();
+        \\
+        \\/// Zig version. When writing code that supports multiple versions of Zig, prefer
+        \\/// feature detection (i.e. with `@hasDecl` or `@hasField`) over version checks.
+        \\pub const zig_version = try @import("std").SemanticVersion.parse("{s}");
+        \\
         \\pub const output_mode = OutputMode.{};
         \\pub const link_mode = LinkMode.{};
         \\pub const is_test = {};
@@ -2714,6 +2719,7 @@ pub fn generateBuiltinZigSource(comp: *Compilation, allocator: *Allocator) ![]u8
         \\    .features = Target.{}.featureSet(&[_]Target.{}.Feature{{
         \\
     , .{
+        build_options.version,
         std.zig.fmtId(@tagName(comp.bin_file.options.output_mode)),
         std.zig.fmtId(@tagName(comp.bin_file.options.link_mode)),
         comp.bin_file.options.is_test,