Currently `readConfiguration` relies on the file system to perform disk
utilities needed to read determine a project configuration file and read
it. This poses a challenge for the language service, which would like to
use `readConfiguration` to watch and read configurations dependent on
extended tsconfigs (#39134). Challenges are at least twofold:
1. To test this, the langauge service would need to provide to the
compiler a mock file system.
2. The language service uses file system utilities primarily through
TypeScript's `Project` abstraction. In general this should correspond
to the underlying file system, but it may differ and it is better to
go through one channel when possible.
This patch alleviates the concern by directly providing to the compiler
a "ParseConfigurationHost" with read-only "file system"-like utilties.
For the language service, this host is derived from the project owned by
the language service.
For more discussion see
https://docs.google.com/document/d/1TrbT-m7bqyYZICmZYHjnJ7NG9Vzt5Rd967h43Qx8jw0/edit?usp=sharing
PR Close#39619
In preparation for in-memory testing infrastructure, the existing Ivy
language service tests are moved to a `legacy` directory. These existing
tests rely on a single integration project in `test/project/app`, which
presents a number of challenges:
* adding extra fields/properties to the integration project for one test
can cause others to fail/flake.
* it's especially difficult to test any cases that require introducing
intentional errors, as those tend to break other tests.
* tests load files from disk, which is slower.
* tests rely on the real built versions of @angular/core and
@angular/common, which makes them both slow to build and require rebuilds
on every compiler change.
* tests share a single tsconfig.json, making it extremely difficult to test
how the language service handles different configuration scenarios (e.g.
different type-checking flags).
PR Close#39594
ayazhafiz
Alex Rickabaugh