compiler: lock incremental dependency tracking behind --debug-incremental

This logic (currently) has a non-trivial cost (particularly in terms of
peak RSS) for tracking dependencies. Until incremental compilation is in
use in the wild, it doesn't make sense for users to pay that cost.

src/Compilation.zig

@@ -156,6 +156,7 @@ time_report: bool,
 stack_report: bool,
 debug_compiler_runtime_libs: bool,
 debug_compile_errors: bool,
+debug_incremental: bool,
 job_queued_compiler_rt_lib: bool = false,
 job_queued_compiler_rt_obj: bool = false,
 job_queued_update_builtin_zig: bool,
@@ -1079,6 +1080,7 @@ pub const CreateOptions = struct {
     verbose_llvm_cpu_features: bool = false,
     debug_compiler_runtime_libs: bool = false,
     debug_compile_errors: bool = false,
+    debug_incremental: bool = false,
     /// Normally when you create a `Compilation`, Zig will automatically build
     /// and link in required dependencies, such as compiler-rt and libc. When
     /// building such dependencies themselves, this flag must be set to avoid
@@ -1508,6 +1510,7 @@ pub fn create(gpa: Allocator, arena: Allocator, options: CreateOptions) !*Compil
             .test_name_prefix = options.test_name_prefix,
             .debug_compiler_runtime_libs = options.debug_compiler_runtime_libs,
             .debug_compile_errors = options.debug_compile_errors,
+            .debug_incremental = options.debug_incremental,
             .libcxx_abi_version = options.libcxx_abi_version,
             .root_name = root_name,
             .sysroot = sysroot,

src/Module.zig

@@ -3139,6 +3139,8 @@ fn markDeclDependenciesPotentiallyOutdated(zcu: *Zcu, decl_index: Decl.Index) !v
 }
 
 pub fn findOutdatedToAnalyze(zcu: *Zcu) Allocator.Error!?InternPool.Depender {
+    if (!zcu.comp.debug_incremental) return null;
+
     if (zcu.outdated.count() == 0 and zcu.potentially_outdated.count() == 0) {
         log.debug("findOutdatedToAnalyze: no outdated depender", .{});
         return null;

src/Sema.zig

@@ -2788,11 +2788,13 @@ fn zirStructDecl(
     new_decl.owns_tv = true;
     errdefer mod.abortAnonDecl(new_decl_index);
 
+    if (sema.mod.comp.debug_incremental) {
         try ip.addDependency(
             sema.gpa,
             InternPool.Depender.wrap(.{ .decl = new_decl_index }),
             .{ .src_hash = try ip.trackZir(sema.gpa, block.getFileScope(mod), inst) },
         );
+    }
 
     const new_namespace_index = try mod.createNamespace(.{
         .parent = block.namespace.toOptional(),
@@ -2978,11 +2980,13 @@ fn zirEnumDecl(
     new_decl.owns_tv = true;
     errdefer if (!done) mod.abortAnonDecl(new_decl_index);
 
+    if (sema.mod.comp.debug_incremental) {
         try mod.intern_pool.addDependency(
             sema.gpa,
             InternPool.Depender.wrap(.{ .decl = new_decl_index }),
             .{ .src_hash = try mod.intern_pool.trackZir(sema.gpa, block.getFileScope(mod), inst) },
         );
+    }
 
     const new_namespace_index = try mod.createNamespace(.{
         .parent = block.namespace.toOptional(),
@@ -3237,11 +3241,13 @@ fn zirUnionDecl(
     new_decl.owns_tv = true;
     errdefer mod.abortAnonDecl(new_decl_index);
 
+    if (sema.mod.comp.debug_incremental) {
         try mod.intern_pool.addDependency(
             sema.gpa,
             InternPool.Depender.wrap(.{ .decl = new_decl_index }),
             .{ .src_hash = try mod.intern_pool.trackZir(sema.gpa, block.getFileScope(mod), inst) },
         );
+    }
 
     const new_namespace_index = try mod.createNamespace(.{
         .parent = block.namespace.toOptional(),
@@ -3336,11 +3342,13 @@ fn zirOpaqueDecl(
     new_decl.owns_tv = true;
     errdefer mod.abortAnonDecl(new_decl_index);
 
+    if (sema.mod.comp.debug_incremental) {
         try mod.intern_pool.addDependency(
             sema.gpa,
             InternPool.Depender.wrap(.{ .decl = new_decl_index }),
             .{ .src_hash = try mod.intern_pool.trackZir(sema.gpa, block.getFileScope(mod), inst) },
         );
+    }
 
     const new_namespace_index = try mod.createNamespace(.{
         .parent = block.namespace.toOptional(),
@@ -32435,7 +32443,7 @@ fn analyzeDeclRefInner(sema: *Sema, decl_index: InternPool.DeclIndex, analyze_fn
 
     const decl = mod.declPtr(decl_index);
     const decl_tv = try decl.typedValue();
-    // TODO: if this is a `decl_ref`, only depend on decl type
+    // TODO: if this is a `decl_ref` of a non-variable decl, only depend on decl type
     try sema.declareDependency(.{ .decl_val = decl_index });
     const ptr_ty = try sema.ptrType(.{
         .child = decl_tv.ty.toIntern(),
@@ -38925,6 +38933,7 @@ fn ptrType(sema: *Sema, info: InternPool.Key.PtrType) CompileError!Type {
 }
 
 pub fn declareDependency(sema: *Sema, dependee: InternPool.Dependee) !void {
+    if (!sema.mod.comp.debug_incremental) return;
     const depender = InternPool.Depender.wrap(
         if (sema.owner_func_index != .none)
             .{ .func = sema.owner_func_index }

src/main.zig

@@ -3255,6 +3255,7 @@ fn buildOutputType(
         .cache_mode = cache_mode,
         .subsystem = subsystem,
         .debug_compile_errors = debug_compile_errors,
+        .debug_incremental = debug_incremental,
         .enable_link_snapshots = enable_link_snapshots,
         .install_name = install_name,
         .entitlements = entitlements,