use known-folders for install dir by default

A new build option `default-dir` is provided to achieve the original
behaviour prior to this commit by building with

    zig build -Ddefault-dir=zig

build.zig

@@ -5,8 +5,27 @@ pub fn build(b: *std.Build) !void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
+    const default_dir = b.option(
+        []const u8,
+        "default-dir",
+        "Use a static default install directory",
+    );
+
+    const build_options = b.addOptions();
+    build_options.addOption(bool, "use_known_folders", default_dir == null);
+    if (default_dir) |dir| {
+        build_options.addOption([]const u8, "default_dir", dir);
+    }
+
+    const known_folders = if (default_dir != null)
+        null
+    else if (b.lazyDependency("known_folders", .{})) |pkg|
+        pkg.module("known-folders")
+    else
+        null;
+
     const zigup_exe_native = blk: {
-        const exe = addZigupExe(b, target, optimize);
+        const exe = addZigupExe(b, target, optimize, build_options, known_folders);
         b.installArtifact(exe);
         const run_cmd = b.addRunArtifact(exe);
         run_cmd.step.dependOn(b.getInstallStep());
@@ -74,13 +93,15 @@ pub fn build(b: *std.Build) !void {
     ci_step.dependOn(test_step);
     ci_step.dependOn(unzip_step);
     ci_step.dependOn(zip_step);
-    try ci(b, ci_step, test_step, host_zip_exe);
+    try ci(b, ci_step, test_step, host_zip_exe, build_options, known_folders);
 }
 
 fn addZigupExe(
     b: *std.Build,
     target: std.Build.ResolvedTarget,
     optimize: std.builtin.Mode,
+    build_options: *std.Build.Step.Options,
+    known_folders: ?*std.Build.Module,
 ) *std.Build.Step.Compile {
     const win32exelink_mod: ?*std.Build.Module = blk: {
         if (target.result.os.tag == .windows) {
@@ -104,6 +125,12 @@ fn addZigupExe(
         .optimize = optimize,
     });
 
+    exe.root_module.addOptions("build-options", build_options);
+
+    if (known_folders) |mod| {
+        exe.root_module.addImport("known-folders", mod);
+    }
+
     if (target.result.os.tag == .windows) {
         exe.root_module.addImport("win32exelink", win32exelink_mod.?);
     }
@@ -115,6 +142,8 @@ fn ci(
     ci_step: *std.Build.Step,
     test_step: *std.Build.Step,
     host_zip_exe: *std.Build.Step.Compile,
+    build_options: *std.Build.Step.Options,
+    known_folders: ?*std.Build.Module,
 ) !void {
     const ci_targets = [_][]const u8{
         "x86_64-linux",
@@ -140,7 +169,7 @@ fn ci(
         const optimize: std.builtin.OptimizeMode =
             // Compile in ReleaseSafe on Windows for faster extraction
             if (target.result.os.tag == .windows) .ReleaseSafe else .Debug;
-        const zigup_exe = addZigupExe(b, target, optimize);
+        const zigup_exe = addZigupExe(b, target, optimize, build_options, known_folders);
         const zigup_exe_install = b.addInstallArtifact(zigup_exe, .{
             .dest_dir = .{ .override = .{ .custom = ci_target_str } },
         });

build.zig.zon

@@ -1,6 +1,13 @@
 .{
     .name = "zigup",
     .version = "0.0.1",
+    .dependencies = .{
+        .known_folders = .{
+            .url = "https://github.com/ziglibs/known-folders/archive/47076c6b11214a218e9244471d8762310820911a.tar.gz",
+            .hash = "12209d2738a2e1dbd3781c2e5f01a2ea877dcfeea53efdfa1913247297d328e6b207",
+            .lazy = true,
+        },
+    },
 
     .paths = .{
         "LICENSE",

zigup.zig

@@ -2,6 +2,9 @@ const std = @import("std");
 const builtin = @import("builtin");
 const mem = std.mem;
 
+const build_options = @import("build-options");
+const known_folders = @import("known-folders");
+
 const ArrayList = std.ArrayList;
 const Allocator = mem.Allocator;
 
@@ -126,19 +129,28 @@ fn getHomeDir() ![]const u8 {
 }
 
 fn allocInstallDirString(allocator: Allocator) ![]const u8 {
+    if (build_options.use_known_folders) {
+        if (try known_folders.getPath(allocator, .data)) |path| {
+            defer allocator.free(path);
+            return std.fs.path.join(allocator, &.{ path, "zigup" });
+        } else {
+            std.log.err("Could not find install directory", .{});
+            return error.AlreadyReported;
+        }
+    }
     // TODO: maybe support ZIG_INSTALL_DIR environment variable?
     // TODO: maybe support a file on the filesystem to configure install dir?
     if (builtin.os.tag == .windows) {
         const self_exe_dir = try std.fs.selfExeDirPathAlloc(allocator);
         defer allocator.free(self_exe_dir);
-        return std.fs.path.join(allocator, &.{ self_exe_dir, "zig" });
+        return std.fs.path.join(allocator, &.{ self_exe_dir, build_options.default_dir });
     }
     const home = try getHomeDir();
     if (!std.fs.path.isAbsolute(home)) {
         std.log.err("$HOME environment variable '{s}' is not an absolute path", .{home});
         return error.BadHomeEnvironmentVariable;
     }
-    return std.fs.path.join(allocator, &[_][]const u8{ home, "zig" });
+    return std.fs.path.join(allocator, &[_][]const u8{ home, build_options.default_dir });
 }
 const GetInstallDirOptions = struct {
     create: bool,