warn about missing requires and imports

[borkdude]

Warn about the following:

(ns foo)
(clojure.string/starts-with? ...)

;;=> missing require (or import) for clojure.string

(ns bar)
(foo/x ...)

;;=> unknown alias foo

although we cannot know if foo is supposed to be an alias, a single-segmented namespace, or Java classname.

We would have to exclude known things like goog in CLJS.
Clj-kondo already have a list of all the by default available classes like System.

The thing that should be very well researched is false positives.
There's already a function for detecting Java class names:
https://github.com/borkdude/clj-kondo/blob/2b4229ea652021e0e5ec6c73082fcc4c7e35d5f2/src/clj_kondo/impl/namespace.clj#L152

TODO:

• exclude js/ and goog/ in CLJS
• lint foo/bar usage, not only in call position
• think more about project.clj defproject kind of macros. can fully qualified symbols occur in macros without proper requires / imports? in fact, user/defproject is the only macro that I need to include for now. Maybe special case it.

Note: the current workaround doesn't work in emacs, since the filename is not included

• change linter name to something more neutral like 'unresolved-namespace' which is more factual. The message could be something like 'Unresolved namespaced. Did you forget a require?`.

Maybe later:

• in comment sections, handle requires as if they are top-level

[borkdude]

From #clojure Slack:

borkdude [5:17 PM]
Hmm. In Clojure when you write:

bar/baz`

bar/baz could in theory be a public field in a class bar right? so it's hard to say this is an unresolved symbol by linting statically
bronsa [5:19 PM]
there's no ambiguity between Klass/field and ns/var
borkdude [5:19 PM]
so yes?
in CLJS-land there are also things like goog/isFunction
that are just "there" without requiring
bronsa [5:21 PM]
yes, w/o keeping a runtime mapping of the available classes it's not easy to distinguish the two
borkdude [5:21 PM]
currently clj-kondo doesn't report anything with a namespace as unresolved, but thinking about this more, I think it's the correct behavior because of the above
👍
(given that underreporting is better than overreporting)

[borkdude]

Related #515

[markus-wa]

Hi. I was wondering what the rationale is for expecting a require for all used namespaces.

I'm fairly new to Clojure and I couldn't find this being suggested elsewhere. The code seems to work with or without require.

PS: Thanks for the awesome tool!

[borkdude]

@markus-wa In general, one should not rely on other namespaces to load certain libraries for you. E.g.: it's an implementation detail in clojure.core whether some of the already required namespaces loads clojure.string. You should explicitly require it, if you want to use that namespace.

[markus-wa]

Ah ok, that makes sense, thanks!