Include the context name for errors that happen in the non-default context #10378
Description
Consider this example (it uses Melange but that's only for demo purposes, could happen in any multi-context setup):
$ cat > dune-project << EOF
> (lang dune 3.13)
> (using melange 0.1)
> (package (name bar) (allow_empty))
> (package (name baz) (allow_empty))
> EOF
$ cat > dune-workspace << EOF
> (lang dune 3.13)
>
> (context default)
>
> (context
> (default
> (name melange)))
> EOF
$ cat > dune << EOF
> (library
> (name foo)
> (public_name bar.foo)
> (enabled_if (= %{context_name} "default")))
> (library
> (name foo)
> (public_name baz.foo)
> (modes melange)
> (enabled_if (= %{context_name} "melange")))
> EOF
$ cat > foo.ml <<EOF
> let t = Str.regexp
> EOF
$ dune build
File "foo.ml", line 1, characters 8-18:
1 | let t = Str.regexp
^^^^^^^^^^
Error: Unbound module Str
[1]
The error message does not allow the user to know under which context the error is happening. This can be quite confusing when using more than one context at scale.
I wonder if Dune could include the context name for "alt contexts" (non-default), similarly to what is done currently for display=short, e.g.:
$ dune build
[melange]
File "foo.ml", line 1, characters 8-18:
1 | let t = Str.regexp
^^^^^^^^^^
Error: Unbound module Str
[1]
cc @davesnx