std.Build: introduce `ConfigHeader.getOutputDir`, small refactor

`std.Build.Step.ConfigHeader` emits a *directory* containing a config
header under a given sub path, but there's no good way to actually
access that directory as a `LazyPath` in the configure phase. This is
silly; it's perfectly valid to refer to that directory, perhaps to
explicitly pass as a "-I" flag to a different toolchain invoked via a
`Step.Run`. So now, instead of the `GeneratedFile` being the actual
*file*, it should be that *directory*, i.e. `cache/o/<digest>`. We can
then easily get the *file* if needed just by using `LazyPath.path` to go
"deeper", which there is a helper function for.

The legacy `getOutput` function is now a deprecated alias for
`getOutputFile`, and `getOutputDir` is introduced.

`std.Build.Module.IncludeDir.appendZigProcessFlags` needed a fix after
this change, so I took the opportunity to refactor it a little. I was
looking at this function while working on ziglang/translate-c yesterday
and realised it could be expressed much more simply -- particularly
after the `ConfigHeader` change here.

I had to update the test `standalone/cmakedefine/` -- it turns out this
test was well and truly reaching into build system internals, and doing
horrible not-really-allowed stuff like overriding the `makeFn` of a
`TopLevelStep`. To top it all off, the test forgot to set
`b.default_step` to its "test" step, so the test never even ran. I've
refactored it to follow accepted practices and to actually, like, work.

1 files changed, 13 insertions, 8 deletions

diff --git a/lib/std/Build/Step/ConfigHeader.zig b/lib/std/Build/Step/ConfigHeader.zig
index 6bfda7667b..967e1edd05 100644
--- a/lib/std/Build/Step/ConfigHeader.zig
+++ b/lib/std/Build/Step/ConfigHeader.zig
@@ -39,7 +39,8 @@ pub const Value = union(enum) {
 
 step: Step,
 values: std.StringArrayHashMap(Value),
-output_file: std.Build.GeneratedFile,
+/// This directory contains the generated file under the name `include_path`.
+generated_dir: std.Build.GeneratedFile,
 
 style: Style,
 max_bytes: usize,
@@ -99,7 +100,7 @@ pub fn create(owner: *std.Build, options: Options) *ConfigHeader {
         .max_bytes = options.max_bytes,
         .include_path = include_path,
         .include_guard_override = options.include_guard_override,
-        .output_file = .{ .step = &config_header.step },
+        .generated_dir = .{ .step = &config_header.step },
     };
 
     return config_header;
@@ -115,9 +116,15 @@ pub fn addValues(config_header: *ConfigHeader, values: anytype) void {
     }
 }
 
-pub fn getOutput(config_header: *ConfigHeader) std.Build.LazyPath {
-    return .{ .generated = .{ .file = &config_header.output_file } };
+pub fn getOutputDir(ch: *ConfigHeader) std.Build.LazyPath {
+    return .{ .generated = .{ .file = &ch.generated_dir } };
 }
+pub fn getOutputFile(ch: *ConfigHeader) std.Build.LazyPath {
+    return ch.getOutputDir().path(ch.step.owner, ch.include_path);
+}
+
+/// Deprecated; use `getOutputFile`.
+pub const getOutput = getOutputFile;
 
 fn addValueInner(config_header: *ConfigHeader, name: []const u8, comptime T: type, value: T) !void {
     switch (@typeInfo(T)) {
@@ -234,9 +241,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
 
     if (try step.cacheHit(&man)) {
         const digest = man.final();
-        config_header.output_file.path = try b.cache_root.join(arena, &.{
-            "o", &digest, config_header.include_path,
-        });
+        config_header.generated_dir.path = try b.cache_root.join(arena, &.{ "o", &digest });
         return;
     }
 
@@ -262,7 +267,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
         });
     };
 
-    config_header.output_file.path = try b.cache_root.join(arena, &.{sub_path});
+    config_header.generated_dir.path = try b.cache_root.join(arena, &.{ "o", &digest });
     try man.writeManifest();
 }