add -Denable-tidy flag and use it in the CI

Eliminates a TODO from the CI scripts; makes it easier to check validity
of html when working on the langref locally.

build.zig

@@ -31,10 +31,18 @@ pub fn build(b: *std.Build) !void {
     const skip_install_langref = b.option(bool, "no-langref", "skip copying of langref to the installation prefix") orelse skip_install_lib_files;
     const std_docs = b.option(bool, "std-docs", "include standard library autodocs") orelse false;
     const no_bin = b.option(bool, "no-bin", "skip emitting compiler binary") orelse false;
+    const enable_tidy = b.option(bool, "enable-tidy", "Check langref output HTML validity") orelse false;
 
     const langref_file = generateLangRef(b);
     const install_langref = b.addInstallFileWithDir(langref_file, .prefix, "doc/langref.html");
+    const check_langref = tidyCheck(b, langref_file);
+    const check_autodocs = tidyCheck(b, b.path("lib/docs/index.html"));
+    if (enable_tidy) {
+        test_step.dependOn(check_langref);
+        test_step.dependOn(check_autodocs);
+    }
     if (!skip_install_langref) {
+        if (enable_tidy) install_langref.step.dependOn(check_langref);
         b.getInstallStep().dependOn(&install_langref.step);
     }
 
@@ -51,6 +59,7 @@ pub fn build(b: *std.Build) !void {
         .install_subdir = "doc/std",
     });
     if (std_docs) {
+        if (enable_tidy) install_std_docs.step.dependOn(check_autodocs);
         b.getInstallStep().dependOn(&install_std_docs.step);
     }
 
@@ -1308,3 +1317,12 @@ fn generateLangRef(b: *std.Build) std.Build.LazyPath {
     docgen_cmd.addFileArg(b.path("doc/langref.html.in"));
     return docgen_cmd.addOutputFileArg("langref.html");
 }
+
+fn tidyCheck(b: *std.Build, html_file: std.Build.LazyPath) *std.Build.Step {
+    const run_tidy = b.addSystemCommand(&.{
+        "tidy", "--drop-empty-elements", "no", "-qe",
+    });
+    run_tidy.addFileArg(html_file);
+    run_tidy.expectExitCode(0);
+    return &run_tidy.step;
+}

ci/aarch64-linux-debug.sh

@@ -61,11 +61,8 @@ stage3-debug/bin/zig build test docs \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \
   --search-prefix "$PREFIX" \
-  --zig-lib-dir "$PWD/../lib"
-
-# Look for HTML errors.
-# TODO: move this to a build.zig flag (-Denable-tidy)
-tidy --drop-empty-elements no -qe "../zig-out/doc/langref.html"
+  --zig-lib-dir "$PWD/../lib" \
+  -Denable-tidy
 
 # Ensure that updating the wasm binary from this commit will result in a viable build.
 stage3-debug/bin/zig build update-zig1

ci/aarch64-linux-release.sh

@@ -61,11 +61,8 @@ stage3-release/bin/zig build test docs \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \
   --search-prefix "$PREFIX" \
-  --zig-lib-dir "$PWD/../lib"
-
-# Look for HTML errors.
-# TODO: move this to a build.zig flag (-Denable-tidy)
-tidy --drop-empty-elements no -qe "../zig-out/doc/langref.html"
+  --zig-lib-dir "$PWD/../lib" \
+  -Denable-tidy
 
 # Ensure that updating the wasm binary from this commit will result in a viable build.
 stage3-release/bin/zig build update-zig1

ci/x86_64-linux-debug.sh

@@ -69,11 +69,8 @@ stage3-debug/bin/zig build test docs \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \
   --search-prefix "$PREFIX" \
-  --zig-lib-dir "$PWD/../lib"
-
-# Look for HTML errors.
-# TODO: move this to a build.zig flag (-Denable-tidy)
-tidy --drop-empty-elements no -qe "../zig-out/doc/langref.html"
+  --zig-lib-dir "$PWD/../lib" \
+  -Denable-tidy
 
 # Ensure that updating the wasm binary from this commit will result in a viable build.
 stage3-debug/bin/zig build update-zig1

ci/x86_64-linux-release.sh

@@ -69,11 +69,8 @@ stage3-release/bin/zig build test docs \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \
   --search-prefix "$PREFIX" \
-  --zig-lib-dir "$PWD/../lib"
-
-# Look for HTML errors.
-# TODO: move this to a build.zig flag (-Denable-tidy)
-tidy --drop-empty-elements no -qe "../zig-out/doc/langref.html"
+  --zig-lib-dir "$PWD/../lib" \
+  -Denable-tidy
 
 # Ensure that stage3 and stage4 are byte-for-byte identical.
 stage3-release/bin/zig build \