feat: skipLibCheck take 2 (#350)

* feat: skipLibCheck take 2

Fixes #348

* refactor: use bool flags, finish error msg

* refactor: use string_flag instead of pair of bool_flag

* Update ts/defs.bzl

Co-authored-by: Sahin Yort <[email protected]>

* Update defs.bzl

---------

Co-authored-by: Sahin Yort <[email protected]>

ts/BUILD.bazel

@@ -1,10 +1,36 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
 
 exports_files(
     glob(["*.bzl"]),
     visibility = ["//docs:__pkg__"],
 )
 
+# Users can enable with --@aspect_rules_ts//ts:skipLibCheck=always
+string_flag(
+    # Note: this name is chosen to be searchable when users look for "skipLibCheck" in their repo,
+    # as that's the way it appears in tsconfig.json or tsc command line.
+    name = "skipLibCheck",
+    # TODO(2.0): change default to "unspecified"
+    build_setting_default = "honor_tsconfig",
+    values = [
+        "always",
+        "honor_tsconfig",
+        "unspecified",
+    ],
+)
+
+# Note, users could use a Transition to make a subgraph of their depgraph opt-in to skipLibCheck.
+config_setting(
+    name = "skip_lib_check.always",
+    flag_values = {":skipLibCheck": "always"},
+)
+
+config_setting(
+    name = "skip_lib_check.honor_tsconfig",
+    flag_values = {":skipLibCheck": "honor_tsconfig"},
+)
+
 bzl_library(
     name = "defs",
     srcs = ["defs.bzl"],

ts/defs.bzl

@@ -16,6 +16,37 @@ load("//ts/private:ts_lib.bzl", _lib = "lib")
 ts_config = _ts_config
 TsConfigInfo = _TsConfigInfo
 
+_skip_lib_check_selection_required = """
+
+######## Required Typecheck Performance Selection ########
+
+TypeScript's type-checking exposes a flag `--skipLibCheck`:
+https://www.typescriptlang.org/tsconfig#skipLibCheck
+
+Using this flag saves substantial time during type-checking.
+Rather than doing a full check of all d.ts files, TypeScript will only type-check the code you
+specifically refer to in your app's source code.
+We recommend this for most rules_ts users.
+
+HOWEVER this performance improvement comes at the expense of type-system accuracy. 
+For example, two packages could define two copies of the same type in an inconsistent way.
+If you publish a library from your repository, your incorrect types may result in errors for your users.
+
+You must choose exactly one of the following flags:
+
+1. To choose the faster performance, put this in /.bazelrc:
+
+    # passes an argument `--skipLibCheck` to *every* spawn of tsc
+    build --@aspect_rules_ts//ts:skipLibCheck=always
+
+2. To choose more correct typechecks, put this in /.bazelrc:
+
+    # honor the setting of `skipLibCheck` in the tsconfig.json file
+    build --@aspect_rules_ts//ts:skipLibCheck=honor_tsconfig
+
+##########################################################
+"""
+
 validate_options = rule(
     doc = """Validates that some tsconfig.json properties match attributes on ts_project.
     See the documentation of [`ts_project`](#ts_project) for more information.""",
@@ -400,7 +431,13 @@ def ts_project(
     ts_project_rule(
         name = tsc_target_name,
         srcs = srcs,
-        args = args,
+        args = args + select(
+            {
+                "@aspect_rules_ts//ts:skip_lib_check.always": ["--skipLibCheck"],
+                "@aspect_rules_ts//ts:skip_lib_check.honor_tsconfig": [],
+            },
+            no_match_error = _skip_lib_check_selection_required,
+        ),
         assets = assets,
         data = data,
         deps = tsc_deps,