build: many more lazy dependencies, defer deps add unless needed

This makes more dependencies lazy. This has a practical effect of
reducing the number of dependencies that need to be downloaded when
running certain zig build steps.

This is all limited because we're blocked on an upstream Zig issue:
https://github.com/ziglang/zig/issues/21525 This prevents us from
fully avoiding downloading many dependencies, but at least they're
relatively small.

One major improvement here is the usage of `lazyImport` for
`zig-wayland` that prevents downloading `zig_wayland` unconditionally on
all platforms. On macOS, we don't download this at all anymore.

Another, weirder change is that all our transitive dependencies are now
marked lazy (e.g. glslang's upstream source) even if the glslang build
always requires it. This was necessary because without this, even if we
simply referenced glslang in the root build.zig, it would force the
source package to download unconditionally. This no longer happens.

10 files changed, 155 insertions, 140 deletions

diff --git a/pkg/cimgui/build.zig b/pkg/cimgui/build.zig
index 83d60737e..f14bc1242 100644
--- a/pkg/cimgui/build.zig
+++ b/pkg/cimgui/build.zig
@@ -11,7 +11,7 @@ pub fn build(b: *std.Build) !void {
         .optimize = optimize,
     });
 
-    const imgui = b.dependency("imgui", .{});
+    const imgui_ = b.lazyDependency("imgui", .{});
     const lib = b.addLibrary(.{
         .name = "cimgui",
         .root_module = b.createModule(.{
@@ -52,7 +52,7 @@ pub fn build(b: *std.Build) !void {
         }
     }
 
-    lib.addIncludePath(imgui.path(""));
+    if (imgui_) |imgui| lib.addIncludePath(imgui.path(""));
     module.addIncludePath(b.path("vendor"));
 
     var flags = std.ArrayList([]const u8).init(b.allocator);
@@ -72,32 +72,33 @@ pub fn build(b: *std.Build) !void {
         });
     }
 
-    lib.addCSourceFile(.{ .file = b.path("vendor/cimgui.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("imgui.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("imgui_draw.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("imgui_demo.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("imgui_widgets.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("imgui_tables.cpp"), .flags = flags.items });
-    lib.addCSourceFile(.{ .file = imgui.path("misc/freetype/imgui_freetype.cpp"), .flags = flags.items });
-
-    lib.addCSourceFile(.{
-        .file = imgui.path("backends/imgui_impl_opengl3.cpp"),
-        .flags = flags.items,
-    });
-
-    if (target.result.os.tag.isDarwin()) {
-        if (!target.query.isNative()) {
-            try @import("apple_sdk").addPaths(b, lib);
-        }
+    if (imgui_) |imgui| {
+        lib.addCSourceFile(.{ .file = b.path("vendor/cimgui.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("imgui.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("imgui_draw.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("imgui_demo.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("imgui_widgets.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("imgui_tables.cpp"), .flags = flags.items });
+        lib.addCSourceFile(.{ .file = imgui.path("misc/freetype/imgui_freetype.cpp"), .flags = flags.items });
         lib.addCSourceFile(.{
-            .file = imgui.path("backends/imgui_impl_metal.mm"),
+            .file = imgui.path("backends/imgui_impl_opengl3.cpp"),
             .flags = flags.items,
         });
-        if (target.result.os.tag == .macos) {
+
+        if (target.result.os.tag.isDarwin()) {
+            if (!target.query.isNative()) {
+                try @import("apple_sdk").addPaths(b, lib);
+            }
             lib.addCSourceFile(.{
-                .file = imgui.path("backends/imgui_impl_osx.mm"),
+                .file = imgui.path("backends/imgui_impl_metal.mm"),
                 .flags = flags.items,
             });
+            if (target.result.os.tag == .macos) {
+                lib.addCSourceFile(.{
+                    .file = imgui.path("backends/imgui_impl_osx.mm"),
+                    .flags = flags.items,
+                });
+            }
         }
     }
 
diff --git a/pkg/cimgui/build.zig.zon b/pkg/cimgui/build.zig.zon
index 2d62f6e84..f539a8fd6 100644
--- a/pkg/cimgui/build.zig.zon
+++ b/pkg/cimgui/build.zig.zon
@@ -10,6 +10,7 @@
             // ocornut/imgui
             .url = "https://deps.files.ghostty.org/imgui-1220bc6b9daceaf7c8c60f3c3998058045ba0c5c5f48ae255ff97776d9cd8bfc6402.tar.gz",
             .hash = "N-V-__8AAH0GaQC8a52s6vfIxg88OZgFgEW6DFxfSK4lX_l3",
+            .lazy = true,
         },
 
         .apple_sdk = .{ .path = "../apple-sdk" },
diff --git a/pkg/glslang/build.zig b/pkg/glslang/build.zig
index c5e76008a..52993a662 100644
--- a/pkg/glslang/build.zig
+++ b/pkg/glslang/build.zig
@@ -10,11 +10,11 @@ pub fn build(b: *std.Build) !void {
         .optimize = optimize,
     });
 
-    const upstream = b.dependency("glslang", .{});
+    const upstream = b.lazyDependency("glslang", .{});
     const lib = try buildGlslang(b, upstream, target, optimize);
     b.installArtifact(lib);
 
-    module.addIncludePath(upstream.path(""));
+    if (upstream) |v| module.addIncludePath(v.path(""));
     module.addIncludePath(b.path("override"));
 
     if (target.query.isNative()) {
@@ -38,7 +38,7 @@ pub fn build(b: *std.Build) !void {
 
 fn buildGlslang(
     b: *std.Build,
-    upstream: *std.Build.Dependency,
+    upstream_: ?*std.Build.Dependency,
     target: std.Build.ResolvedTarget,
     optimize: std.builtin.OptimizeMode,
 ) !*std.Build.Step.Compile {
@@ -52,7 +52,7 @@ fn buildGlslang(
     });
     lib.linkLibC();
     lib.linkLibCpp();
-    lib.addIncludePath(upstream.path(""));
+    if (upstream_) |upstream| lib.addIncludePath(upstream.path(""));
     lib.addIncludePath(b.path("override"));
     if (target.result.os.tag.isDarwin()) {
         const apple_sdk = @import("apple_sdk");
@@ -66,87 +66,89 @@ fn buildGlslang(
         "-fno-sanitize-trap=undefined",
     });
 
-    lib.addCSourceFiles(.{
-        .root = upstream.path(""),
-        .flags = flags.items,
-        .files = &.{
-            // GenericCodeGen
-            "glslang/GenericCodeGen/CodeGen.cpp",
-            "glslang/GenericCodeGen/Link.cpp",
-
-            // MachineIndependent
-            //"MachineIndependent/glslang.y",
-            "glslang/MachineIndependent/glslang_tab.cpp",
-            "glslang/MachineIndependent/attribute.cpp",
-            "glslang/MachineIndependent/Constant.cpp",
-            "glslang/MachineIndependent/iomapper.cpp",
-            "glslang/MachineIndependent/InfoSink.cpp",
-            "glslang/MachineIndependent/Initialize.cpp",
-            "glslang/MachineIndependent/IntermTraverse.cpp",
-            "glslang/MachineIndependent/Intermediate.cpp",
-            "glslang/MachineIndependent/ParseContextBase.cpp",
-            "glslang/MachineIndependent/ParseHelper.cpp",
-            "glslang/MachineIndependent/PoolAlloc.cpp",
-            "glslang/MachineIndependent/RemoveTree.cpp",
-            "glslang/MachineIndependent/Scan.cpp",
-            "glslang/MachineIndependent/ShaderLang.cpp",
-            "glslang/MachineIndependent/SpirvIntrinsics.cpp",
-            "glslang/MachineIndependent/SymbolTable.cpp",
-            "glslang/MachineIndependent/Versions.cpp",
-            "glslang/MachineIndependent/intermOut.cpp",
-            "glslang/MachineIndependent/limits.cpp",
-            "glslang/MachineIndependent/linkValidate.cpp",
-            "glslang/MachineIndependent/parseConst.cpp",
-            "glslang/MachineIndependent/reflection.cpp",
-            "glslang/MachineIndependent/preprocessor/Pp.cpp",
-            "glslang/MachineIndependent/preprocessor/PpAtom.cpp",
-            "glslang/MachineIndependent/preprocessor/PpContext.cpp",
-            "glslang/MachineIndependent/preprocessor/PpScanner.cpp",
-            "glslang/MachineIndependent/preprocessor/PpTokens.cpp",
-            "glslang/MachineIndependent/propagateNoContraction.cpp",
-
-            // C Interface
-            "glslang/CInterface/glslang_c_interface.cpp",
-
-            // ResourceLimits
-            "glslang/ResourceLimits/ResourceLimits.cpp",
-            "glslang/ResourceLimits/resource_limits_c.cpp",
-
-            // SPIRV
-            "SPIRV/GlslangToSpv.cpp",
-            "SPIRV/InReadableOrder.cpp",
-            "SPIRV/Logger.cpp",
-            "SPIRV/SpvBuilder.cpp",
-            "SPIRV/SpvPostProcess.cpp",
-            "SPIRV/doc.cpp",
-            "SPIRV/disassemble.cpp",
-            "SPIRV/CInterface/spirv_c_interface.cpp",
-        },
-    });
-
-    if (target.result.os.tag != .windows) {
-        lib.addCSourceFiles(.{
-            .root = upstream.path(""),
-            .flags = flags.items,
-            .files = &.{
-                "glslang/OSDependent/Unix/ossource.cpp",
-            },
-        });
-    } else {
+    if (upstream_) |upstream| {
         lib.addCSourceFiles(.{
             .root = upstream.path(""),
             .flags = flags.items,
             .files = &.{
-                "glslang/OSDependent/Windows/ossource.cpp",
+                // GenericCodeGen
+                "glslang/GenericCodeGen/CodeGen.cpp",
+                "glslang/GenericCodeGen/Link.cpp",
+
+                // MachineIndependent
+                //"MachineIndependent/glslang.y",
+                "glslang/MachineIndependent/glslang_tab.cpp",
+                "glslang/MachineIndependent/attribute.cpp",
+                "glslang/MachineIndependent/Constant.cpp",
+                "glslang/MachineIndependent/iomapper.cpp",
+                "glslang/MachineIndependent/InfoSink.cpp",
+                "glslang/MachineIndependent/Initialize.cpp",
+                "glslang/MachineIndependent/IntermTraverse.cpp",
+                "glslang/MachineIndependent/Intermediate.cpp",
+                "glslang/MachineIndependent/ParseContextBase.cpp",
+                "glslang/MachineIndependent/ParseHelper.cpp",
+                "glslang/MachineIndependent/PoolAlloc.cpp",
+                "glslang/MachineIndependent/RemoveTree.cpp",
+                "glslang/MachineIndependent/Scan.cpp",
+                "glslang/MachineIndependent/ShaderLang.cpp",
+                "glslang/MachineIndependent/SpirvIntrinsics.cpp",
+                "glslang/MachineIndependent/SymbolTable.cpp",
+                "glslang/MachineIndependent/Versions.cpp",
+                "glslang/MachineIndependent/intermOut.cpp",
+                "glslang/MachineIndependent/limits.cpp",
+                "glslang/MachineIndependent/linkValidate.cpp",
+                "glslang/MachineIndependent/parseConst.cpp",
+                "glslang/MachineIndependent/reflection.cpp",
+                "glslang/MachineIndependent/preprocessor/Pp.cpp",
+                "glslang/MachineIndependent/preprocessor/PpAtom.cpp",
+                "glslang/MachineIndependent/preprocessor/PpContext.cpp",
+                "glslang/MachineIndependent/preprocessor/PpScanner.cpp",
+                "glslang/MachineIndependent/preprocessor/PpTokens.cpp",
+                "glslang/MachineIndependent/propagateNoContraction.cpp",
+
+                // C Interface
+                "glslang/CInterface/glslang_c_interface.cpp",
+
+                // ResourceLimits
+                "glslang/ResourceLimits/ResourceLimits.cpp",
+                "glslang/ResourceLimits/resource_limits_c.cpp",
+
+                // SPIRV
+                "SPIRV/GlslangToSpv.cpp",
+                "SPIRV/InReadableOrder.cpp",
+                "SPIRV/Logger.cpp",
+                "SPIRV/SpvBuilder.cpp",
+                "SPIRV/SpvPostProcess.cpp",
+                "SPIRV/doc.cpp",
+                "SPIRV/disassemble.cpp",
+                "SPIRV/CInterface/spirv_c_interface.cpp",
             },
         });
-    }
 
-    lib.installHeadersDirectory(
-        upstream.path(""),
-        "",
-        .{ .include_extensions = &.{".h"} },
-    );
+        if (target.result.os.tag != .windows) {
+            lib.addCSourceFiles(.{
+                .root = upstream.path(""),
+                .flags = flags.items,
+                .files = &.{
+                    "glslang/OSDependent/Unix/ossource.cpp",
+                },
+            });
+        } else {
+            lib.addCSourceFiles(.{
+                .root = upstream.path(""),
+                .flags = flags.items,
+                .files = &.{
+                    "glslang/OSDependent/Windows/ossource.cpp",
+                },
+            });
+        }
+
+        lib.installHeadersDirectory(
+            upstream.path(""),
+            "",
+            .{ .include_extensions = &.{".h"} },
+        );
+    }
 
     return lib;
 }
diff --git a/pkg/glslang/build.zig.zon b/pkg/glslang/build.zig.zon
index cbcb08e5d..252237e58 100644
--- a/pkg/glslang/build.zig.zon
+++ b/pkg/glslang/build.zig.zon
@@ -8,6 +8,7 @@
         .glslang = .{
             .url = "https://deps.files.ghostty.org/glslang-12201278a1a05c0ce0b6eb6026c65cd3e9247aa041b1c260324bf29cee559dd23ba1.tar.gz",
             .hash = "N-V-__8AABzkUgISeKGgXAzgtutgJsZc0-kkeqBBscJgMkvy",
+            .lazy = true,
         },
 
         .apple_sdk = .{ .path = "../apple-sdk" },
diff --git a/pkg/highway/build.zig b/pkg/highway/build.zig
index c8ef2d18e..1013f1643 100644
--- a/pkg/highway/build.zig
+++ b/pkg/highway/build.zig
@@ -4,7 +4,7 @@ pub fn build(b: *std.Build) !void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const upstream = b.dependency("highway", .{});
+    const upstream_ = b.lazyDependency("highway", .{});
 
     const module = b.addModule("highway", .{
         .root_source_file = b.path("main.zig"),
@@ -21,8 +21,10 @@ pub fn build(b: *std.Build) !void {
         .linkage = .static,
     });
     lib.linkLibCpp();
-    lib.addIncludePath(upstream.path(""));
-    module.addIncludePath(upstream.path(""));
+    if (upstream_) |upstream| {
+        lib.addIncludePath(upstream.path(""));
+        module.addIncludePath(upstream.path(""));
+    }
 
     if (target.result.os.tag.isDarwin()) {
         const apple_sdk = @import("apple_sdk");
@@ -74,24 +76,26 @@ pub fn build(b: *std.Build) !void {
     }
 
     lib.addCSourceFiles(.{ .flags = flags.items, .files = &.{"bridge.cpp"} });
-    lib.addCSourceFiles(.{
-        .root = upstream.path(""),
-        .flags = flags.items,
-        .files = &.{
-            "hwy/abort.cc",
-            "hwy/aligned_allocator.cc",
-            "hwy/nanobenchmark.cc",
-            "hwy/per_target.cc",
-            "hwy/print.cc",
-            "hwy/targets.cc",
-            "hwy/timer.cc",
-        },
-    });
-    lib.installHeadersDirectory(
-        upstream.path("hwy"),
-        "hwy",
-        .{ .include_extensions = &.{".h"} },
-    );
+    if (upstream_) |upstream| {
+        lib.addCSourceFiles(.{
+            .root = upstream.path(""),
+            .flags = flags.items,
+            .files = &.{
+                "hwy/abort.cc",
+                "hwy/aligned_allocator.cc",
+                "hwy/nanobenchmark.cc",
+                "hwy/per_target.cc",
+                "hwy/print.cc",
+                "hwy/targets.cc",
+                "hwy/timer.cc",
+            },
+        });
+        lib.installHeadersDirectory(
+            upstream.path("hwy"),
+            "hwy",
+            .{ .include_extensions = &.{".h"} },
+        );
+    }
 
     b.installArtifact(lib);
 
diff --git a/pkg/highway/build.zig.zon b/pkg/highway/build.zig.zon
index 8fc05641e..0777fcb7a 100644
--- a/pkg/highway/build.zig.zon
+++ b/pkg/highway/build.zig.zon
@@ -8,6 +8,7 @@
         .highway = .{
             .url = "https://deps.files.ghostty.org/highway-66486a10623fa0d72fe91260f96c892e41aceb06.tar.gz",
             .hash = "N-V-__8AAGmZhABbsPJLfbqrh6JTHsXhY6qCaLAQyx25e0XE",
+            .lazy = true,
         },
 
         .apple_sdk = .{ .path = "../apple-sdk" },
diff --git a/pkg/libxml2/build.zig b/pkg/libxml2/build.zig
index 7c3c2b607..acebfaf63 100644
--- a/pkg/libxml2/build.zig
+++ b/pkg/libxml2/build.zig
@@ -4,7 +4,7 @@ pub fn build(b: *std.Build) !void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const upstream = b.dependency("libxml2", .{});
+    const upstream_ = b.lazyDependency("libxml2", .{});
 
     const lib = b.addLibrary(.{
         .name = "xml2",
@@ -16,7 +16,7 @@ pub fn build(b: *std.Build) !void {
     });
     lib.linkLibC();
 
-    lib.addIncludePath(upstream.path("include"));
+    if (upstream_) |upstream| lib.addIncludePath(upstream.path("include"));
     lib.addIncludePath(b.path("override/include"));
     if (target.result.os.tag == .windows) {
         lib.addIncludePath(b.path("override/config/win32"));
@@ -97,21 +97,23 @@ pub fn build(b: *std.Build) !void {
         }
     }
 
-    lib.addCSourceFiles(.{
-        .root = upstream.path(""),
-        .files = srcs,
-        .flags = flags.items,
-    });
+    if (upstream_) |upstream| {
+        lib.addCSourceFiles(.{
+            .root = upstream.path(""),
+            .files = srcs,
+            .flags = flags.items,
+        });
 
-    lib.installHeader(
-        b.path("override/include/libxml/xmlversion.h"),
-        "libxml/xmlversion.h",
-    );
-    lib.installHeadersDirectory(
-        upstream.path("include"),
-        "",
-        .{ .include_extensions = &.{".h"} },
-    );
+        lib.installHeader(
+            b.path("override/include/libxml/xmlversion.h"),
+            "libxml/xmlversion.h",
+        );
+        lib.installHeadersDirectory(
+            upstream.path("include"),
+            "",
+            .{ .include_extensions = &.{".h"} },
+        );
+    }
 
     b.installArtifact(lib);
 }
diff --git a/pkg/libxml2/build.zig.zon b/pkg/libxml2/build.zig.zon
index 3f6906a50..f932835aa 100644
--- a/pkg/libxml2/build.zig.zon
+++ b/pkg/libxml2/build.zig.zon
@@ -7,6 +7,7 @@
         .libxml2 = .{
             .url = "https://deps.files.ghostty.org/libxml2-2.11.5.tar.gz",
             .hash = "N-V-__8AAG3RoQEyRC2Vw7Qoro5SYBf62IHn3HjqtNVY6aWK",
+            .lazy = true,
         },
     },
 }
diff --git a/pkg/spirv-cross/build.zig b/pkg/spirv-cross/build.zig
index b359ccd9c..9e84689ca 100644
--- a/pkg/spirv-cross/build.zig
+++ b/pkg/spirv-cross/build.zig
@@ -4,10 +4,10 @@ pub fn build(b: *std.Build) !void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const upstream = b.dependency("spirv_cross", .{});
+    const upstream = b.lazyDependency("spirv_cross", .{});
 
     const module = b.addModule("spirv_cross", .{ .root_source_file = b.path("main.zig") });
-    module.addIncludePath(upstream.path(""));
+    if (upstream) |v| module.addIncludePath(v.path(""));
 
     const lib = try buildSpirvCross(b, upstream, target, optimize);
     b.installArtifact(lib);
@@ -33,7 +33,7 @@ pub fn build(b: *std.Build) !void {
 
 fn buildSpirvCross(
     b: *std.Build,
-    upstream: *std.Build.Dependency,
+    upstream_: ?*std.Build.Dependency,
     target: std.Build.ResolvedTarget,
     optimize: std.builtin.OptimizeMode,
 ) !*std.Build.Step.Compile {
@@ -62,6 +62,7 @@ fn buildSpirvCross(
         "-fno-sanitize-trap=undefined",
     });
 
+    const upstream = upstream_ orelse return lib;
     lib.addCSourceFiles(.{
         .root = upstream.path(""),
         .flags = flags.items,
diff --git a/pkg/spirv-cross/build.zig.zon b/pkg/spirv-cross/build.zig.zon
index a7ad7133d..30eea9501 100644
--- a/pkg/spirv-cross/build.zig.zon
+++ b/pkg/spirv-cross/build.zig.zon
@@ -8,6 +8,7 @@
         .spirv_cross = .{
             .url = "https://deps.files.ghostty.org/spirv_cross-1220fb3b5586e8be67bc3feb34cbe749cf42a60d628d2953632c2f8141302748c8da.tar.gz",
             .hash = "N-V-__8AANb6pwD7O1WG6L5nvD_rNMvnSc9Cpg1ijSlTYywv",
+            .lazy = true,
         },
 
         .apple_sdk = .{ .path = "../apple-sdk" },