zig build: allow to choose "lazy mode" for fetching process

lib/compiler/build_runner.zig

@@ -1293,7 +1293,9 @@ fn usage(b: *std.Build, out_stream: anytype) !void {
         \\  -j<N>                        Limit concurrent jobs (default is to use all CPU cores)
         \\  --maxrss <bytes>             Limit memory usage (default is to use available memory)
         \\  --skip-oom-steps             Instead of failing, skip steps that would exceed --maxrss
-        \\  --fetch                      Exit after fetching dependency tree
+        \\  --fetch[=mode]               Fetch dependency tree (optionally choose laziness) and exit
+        \\    needed                     (Default) Lazy dependencies are fetched as needed
+        \\    all                        Lazy dependencies are always fetched
         \\  --watch                      Continuously rebuild when source files are modified
         \\  --fuzz                       Continuously search for unit test failures
         \\  --debounce <ms>              Delay before rebuilding after changed file detected

src/Package/Fetch.zig

@@ -114,10 +114,18 @@ pub const JobQueue = struct {
     /// If this is true, `recursive` must be false.
     debug_hash: bool,
     work_around_btrfs_bug: bool,
+    mode: Mode,
     /// Set of hashes that will be additionally fetched even if they are marked
     /// as lazy.
     unlazy_set: UnlazySet = .{},
 
+    pub const Mode = enum {
+        /// Non-lazy dependencies are always fetched.
+        /// Lazy dependencies are fetched only when needed.
+        needed,
+        /// Both non-lazy and lazy dependencies are always fetched.
+        all,
+    };
     pub const Table = std.AutoArrayHashMapUnmanaged(Package.Hash, *Fetch);
     pub const UnlazySet = std.AutoArrayHashMapUnmanaged(Package.Hash, void);
 
@@ -754,7 +762,10 @@ fn queueJobsForDeps(f: *Fetch) RunError!void {
                 .location_tok = dep.location_tok,
                 .hash_tok = dep.hash_tok,
                 .name_tok = dep.name_tok,
-                .lazy_status = if (dep.lazy) .available else .eager,
+                .lazy_status = switch (f.job_queue.mode) {
+                    .needed => if (dep.lazy) .available else .eager,
+                    .all => .eager,
+                },
                 .parent_package_root = f.package_root,
                 .parent_manifest_ast = &f.manifest_ast,
                 .prog_node = f.prog_node,
@@ -2325,6 +2336,7 @@ const TestFetchBuilder = struct {
             .read_only = false,
             .debug_hash = false,
             .work_around_btrfs_bug = false,
+            .mode = .needed,
         };
 
         self.fetch = .{

src/main.zig

@@ -4843,6 +4843,7 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
     var verbose_cimport = false;
     var verbose_llvm_cpu_features = false;
     var fetch_only = false;
+    var fetch_mode: Package.Fetch.JobQueue.Mode = .needed;
     var system_pkg_dir_path: ?[]const u8 = null;
     var debug_target: ?[]const u8 = null;
 
@@ -4924,6 +4925,13 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                     reference_trace = 256;
                 } else if (mem.eql(u8, arg, "--fetch")) {
                     fetch_only = true;
+                } else if (mem.startsWith(u8, arg, "--fetch=")) {
+                    fetch_only = true;
+                    const sub_arg = arg["--fetch=".len..];
+                    fetch_mode = std.meta.stringToEnum(Package.Fetch.JobQueue.Mode, sub_arg) orelse
+                        fatal("expected [needed|all] after '--fetch=', found '{s}'", .{
+                            sub_arg,
+                        });
                 } else if (mem.eql(u8, arg, "--system")) {
                     if (i + 1 >= args.len) fatal("expected argument after '{s}'", .{arg});
                     i += 1;
@@ -5208,6 +5216,7 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
                     .debug_hash = false,
                     .work_around_btrfs_bug = work_around_btrfs_bug,
                     .unlazy_set = unlazy_set,
+                    .mode = fetch_mode,
                 };
                 defer job_queue.deinit();
 
@@ -7130,6 +7139,7 @@ fn cmdFetch(
         .read_only = false,
         .debug_hash = debug_hash,
         .work_around_btrfs_bug = work_around_btrfs_bug,
+        .mode = .all,
     };
     defer job_queue.deinit();