ngtsc will avoid emitting generated imports that would create an import
cycle in the user's program. The main way such imports can arise is when
a component would ordinarily reference its dependencies in its component
definition `directiveDefs` and `pipeDefs`. This requires adding imports,
which run the risk of creating a cycle.
When ngtsc detects that adding such an import would cause this to occur, it
instead falls back on a strategy called "remote scoping", where a side-
effectful call to `setComponentScope` in the component's NgModule file is
used to patch `directiveDefs` and `pipeDefs` onto the component. Since the
NgModule file already imports all of the component's dependencies (to
declare them in the NgModule), this approach does not risk adding a cycle.
It has several large downsides, however:
1. it breaks under `sideEffects: false` logic in bundlers including the CLI
2. it breaks tree-shaking for the given component and its dependencies
See this doc for further details: https://hackmd.io/Odw80D0pR6yfsOjg_7XCJg?view
In particular, the impact on tree-shaking was exacerbated by the naive logic
ngtsc used to employ here. When this feature was implemented, at the time of
generating the side-effectful `setComponentScope` call, the compiler did not
know which of the component's declared dependencies were actually used in
its template. This meant that unlike the generation of `directiveDefs` in
the component definition itself, `setComponentScope` calls had to list the
_entire_ compilation scope of the component's NgModule, including directives
and pipes which were not actually used in the template. This made the tree-
shaking impact much worse, since if the component's NgModule made use of any
shared NgModules (e.g. `CommonModule`), every declaration therein would
become un-treeshakable.
Today, ngtsc does have the information on which directives/pipes are
actually used in the template, but this was not being used during the remote
scoping operation. This commit modifies remote scoping to take advantage of
the extra context and only list used dependencies in `setComponentScope`
calls, which should ameliorate the tree-shaking impact somewhat.
PR Close#39662
c59f401f9a