Improve the messaging around re-precompilation

[IanButterworth]

When a user activates a different environment in an existing session, packages already loaded from the previous environment can silently disagree with what the new environment specifies. The next using of a transitive dep often triggers a surprising round of precompilation (or, worse, the user keeps using a stale version without realizing it). The current messaging around this is scattered and reactive. This issue is to track ideas for making it clearer and more proactive.

Existing / proposed related work

• status: highlight when deps have different loaded versions #4109 Annotates Pkg.status output with [loaded: vX] when the loaded version differs from the manifest. Only visible if the user runs status.

• activate: warn if different versions are loaded #4679 Emits a single warning at activate time, listing each loaded dep that disagrees with the new manifest (version mismatch) or is loaded from a different path. Catches the problem at the moment the user can still react (re-launch, choose a different env) before the precompile cascade.

• WIP: Reland "Warn if an already loaded package is attempted to be loaded from a different path" julia#52798 (WIP reland of 44329 / 52778 / 52788 / 52789) Hooks Base.require to warn at load time using pkgorigins[uuidkey].path vs locate_package(uuidkey). Complementary: catches mismatches that arise from any source (REPL --project, manual LOAD_PATH edits, dev workflows) regardless of whether Pkg.activate was used. Has been merged and reverted; treehash check is still a TODO there.

• Prefer adding already loaded dep versions #4507 Different angle entirely: bias the resolver toward the versions already in the session so Pkg.add / instantiate is less likely to create a mismatch in the first place.

• Prompt the user on a manifest version mismatch #3775 Earlier attempt to interactively prompt on activate; closed without merge. Useful prior discussion of UX trade-offs (warn vs prompt vs error).

• instantiate: add update_on_mismatch kwarg to fall back to update on a stale manifest #4678 Adjacent: lets instantiate automatically update when the manifest is stale relative to the project, reducing one class of "why is this recompiling?" surprises.

[mchitre]

This is great. Certainly will help a lot to provide appropriate warnings and guidance. But we need some tooling along with this to make it easier to always start with the correct environment, both from command line and from IDE. That will reduce chance of tripping over this for new users and improve experience. For existing environments, it's mostly there, but when starting a new environment, currently the tooling feels poor. Vscode doesn't let you pick an empty folder as environment. REPL with —project in a folder without Project.toml silently picks a parent environment.

[IanButterworth]

Re. VSCode, the julia extension does this, according to @pfitzseb

Basically, the startup code does the following

julia --project=$envpath
Base.set_active_project($internal_env)
load VSCodeServer (which doesn't have any dependencies)
Base.set_active_project($previous_env)

The user can then switch environments manually (in the REPL or with the selector UI, which uses Pkg.activate)

So I think #4679 may suffice

[pfitzseb]

I've also opened julia-vscode/julia-vscode#4102 to track the env bootstrapping issue in VS Code.