std.Build: Add a finalize function to Build.Steps

It is called after the user has fully configured the steps in the build() function.

lib/compiler/build_runner.zig

@@ -335,6 +335,10 @@ pub fn main() !void {
         try builder.runBuild(root);
     }
 
+    for (builder.top_level_steps.values()) |s| {
+        try finalizeSteps(&s.step);
+    }
+
     if (graph.needed_lazy_dependencies.entries.len != 0) {
         var buffer: std.ArrayListUnmanaged(u8) = .{};
         for (graph.needed_lazy_dependencies.keys()) |k| {
@@ -635,6 +639,7 @@ fn runStepNames(
         test_count += s.test_results.test_count;
 
         switch (s.state) {
+            .unfinalized => unreachable,
             .precheck_unstarted => unreachable,
             .precheck_started => unreachable,
             .running => unreachable,
@@ -780,6 +785,7 @@ fn printStepStatus(
     run: *const Run,
 ) !void {
     switch (s.state) {
+        .unfinalized => unreachable,
         .precheck_unstarted => unreachable,
         .precheck_started => unreachable,
         .precheck_done => unreachable,
@@ -977,6 +983,34 @@ fn printTreeStep(
     }
 }
 
+/// Traverse the dependency graph after the user build() call,
+/// this allows for checks and postprocessing after the steps are fully configured by the user.
+fn finalizeSteps(
+    s: *Step,
+) !void {
+    switch (s.state) {
+        .unfinalized => {
+            try s.finalize();
+            s.state = .precheck_unstarted;
+
+            for (s.dependencies.items) |dep| {
+                try finalizeSteps(dep);
+            }
+        },
+
+        .precheck_unstarted => {},
+
+        .precheck_started => unreachable,
+        .precheck_done => unreachable,
+        .dependency_failure => unreachable,
+        .running => unreachable,
+        .success => unreachable,
+        .failure => unreachable,
+        .skipped => unreachable,
+        .skipped_oom => unreachable,
+    }
+}
+
 /// Traverse the dependency graph depth-first and make it undirected by having
 /// steps know their dependants (they only know dependencies at start).
 /// Along the way, check that there is no dependency loop, and record the steps
@@ -995,6 +1029,7 @@ fn constructGraphAndCheckForDependencyLoop(
     rand: std.Random,
 ) !void {
     switch (s.state) {
+        .unfinalized => unreachable,
         .precheck_started => {
             std.debug.print("dependency loop detected:\n  {s}\n", .{s.name});
             return error.DependencyLoopDetected;
@@ -1057,6 +1092,7 @@ fn workerMakeOneStep(
                 // dependency is not finished yet.
                 return;
             },
+            .unfinalized => unreachable,
             .precheck_unstarted => unreachable,
             .precheck_started => unreachable,
         }

lib/std/Build/Step.zig

@@ -1,6 +1,7 @@
 id: Id,
 name: []const u8,
 owner: *Build,
+finalizeFn: FinalizeFn,
 makeFn: MakeFn,
 
 dependencies: std.ArrayList(*Step),
@@ -68,6 +69,8 @@ pub const TestResults = struct {
     }
 };
 
+pub const FinalizeFn = *const fn (step: *Step) anyerror!void;
+
 pub const MakeOptions = struct {
     progress_node: std.Progress.Node,
     thread_pool: *std.Thread.Pool,
@@ -77,6 +80,7 @@ pub const MakeOptions = struct {
 pub const MakeFn = *const fn (step: *Step, options: MakeOptions) anyerror!void;
 
 pub const State = enum {
+    unfinalized,
     precheck_unstarted,
     precheck_started,
     /// This is also used to indicate "dirty" steps that have been modified
@@ -183,6 +187,7 @@ pub const StepOptions = struct {
     id: Id,
     name: []const u8,
     owner: *Build,
+    finalizeFn: FinalizeFn = finalizeNoOp,
     makeFn: MakeFn = makeNoOp,
     first_ret_addr: ?usize = null,
     max_rss: usize = 0,
@@ -195,11 +200,12 @@ pub fn init(options: StepOptions) Step {
         .id = options.id,
         .name = arena.dupe(u8, options.name) catch @panic("OOM"),
         .owner = options.owner,
+        .finalizeFn = options.finalizeFn,
         .makeFn = options.makeFn,
         .dependencies = std.ArrayList(*Step).init(arena),
         .dependants = .{},
         .inputs = Inputs.init,
-        .state = .precheck_unstarted,
+        .state = .unfinalized,
         .max_rss = options.max_rss,
         .debug_stack_trace = blk: {
             const addresses = arena.alloc(usize, options.owner.debug_stack_frames_count) catch @panic("OOM");
@@ -222,6 +228,11 @@ pub fn init(options: StepOptions) Step {
     };
 }
 
+pub fn finalize(s: *Step) !void {
+    assert(s.state == .unfinalized);
+    try s.finalizeFn(s);
+}
+
 /// If the Step's `make` function reports `error.MakeFailed`, it indicates they
 /// have already reported the error. Otherwise, we add a simple error report
 /// here.
@@ -266,6 +277,10 @@ pub fn getStackTrace(s: *Step) ?std.builtin.StackTrace {
     };
 }
 
+fn finalizeNoOp(step: *Step) anyerror!void {
+    _ = step;
+}
+
 fn makeNoOp(step: *Step, options: MakeOptions) anyerror!void {
     _ = options;
 

lib/std/Build/Step/Compile.zig

@@ -207,9 +207,9 @@ generated_h: ?*GeneratedFile,
 /// Defaults to `std.math.maxInt(u16)`
 error_limit: ?u32 = null,
 
-/// Computed during make().
+/// Computed during finalize().
 is_linking_libc: bool = false,
-/// Computed during make().
+/// Computed during finalize().
 is_linking_libcpp: bool = false,
 
 no_builtin: bool = false,
@@ -393,6 +393,7 @@ pub fn create(owner: *std.Build, options: Options) *Compile {
             .id = base_id,
             .name = step_name,
             .owner = owner,
+            .finalizeFn = finalize,
             .makeFn = make,
             .max_rss = options.max_rss,
         }),
@@ -1095,24 +1096,6 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
         // emitted if there is nothing to link.
         var total_linker_objects: usize = @intFromBool(compile.root_module.root_source_file != null);
 
-        {
-            // Fully recursive iteration including dynamic libraries to detect
-            // libc and libc++ linkage.
-            var dep_it = compile.root_module.iterateDependencies(compile, true);
-            while (dep_it.next()) |key| {
-                if (key.module.link_libc == true) compile.is_linking_libc = true;
-                if (key.module.link_libcpp == true) compile.is_linking_libcpp = true;
-            }
-        }
-
-        if (compile.kind == .pch) {
-            // precompiled headers must have a single input header file.
-            var it = compile.root_module.iterateDependencies(compile, false);
-            const link_objects = it.next().?.module.link_objects;
-            assert(link_objects.items.len == 1 and link_objects.items[0] == .c_source_file);
-            assert(it.next() == null);
-        }
-
         var cli_named_modules = try CliNamedModules.init(arena, &compile.root_module);
 
         // For this loop, don't chase dynamic libraries because their link
@@ -1839,6 +1822,28 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
     return try zig_args.toOwnedSlice();
 }
 
+fn finalize(step: *Step) !void {
+    const compile: *Compile = @fieldParentPtr("step", step);
+
+    {
+        // Fully recursive iteration including dynamic libraries to detect
+        // libc and libc++ linkage.
+        var dep_it = compile.root_module.iterateDependencies(compile, true);
+        while (dep_it.next()) |key| {
+            if (key.module.link_libc == true) compile.is_linking_libc = true;
+            if (key.module.link_libcpp == true) compile.is_linking_libcpp = true;
+        }
+    }
+
+    if (compile.kind == .pch) {
+        // precompiled headers must have a single input header file.
+        var it = compile.root_module.iterateDependencies(compile, false);
+        const link_objects = it.next().?.module.link_objects;
+        assert(link_objects.items.len == 1 and link_objects.items[0] == .c_source_file);
+        assert(it.next() == null);
+    }
+}
+
 fn make(step: *Step, options: Step.MakeOptions) !void {
     const b = step.owner;
     const compile: *Compile = @fieldParentPtr("step", step);