Be more explicit about what dependencies are solved for and what is going to be built

[lspitzner]

[edit: This issue turns out to be mostly due to some UI confusion. I'll keep the original description below, but I think it boils down to one relatively simple suggestion:]

Currently the first line of output when (new-)building is the rather plain

Resolving dependencies..

To clarify things a bit, I propose to change this to something like

Resolving dependencies for all packages of the project (local_package_1, local_package_2, local_package_3)

or

Resolving dependencies for the implicit project containing just package_in_current_dir

Another option would be to keep the first line this brief, but to append all relevant information if a dependency-resolution error occured

Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: mypackage-1.0.0.0 (user goal)
[__1] next goal: base (dependency of mypackage)
[__1] rejecting: base-4.10.1.0/installed-4.1... (conflict: mypackage =>
base>=4.9 && <4.9)
[..snip..]
when resolving the dependencies for the packages in the current project
containing the packages: mypackage, local_package_1, local_package_2, local_package_3
from the project file cabal.project.mysetup
[or]
from an implicit project

---

(original issue description)

With a project setup like:

a/a.cabal
a/b/b.cabal
a/c/c.cabal
a/cabal.project

where cabal.project has

packages: . b c

I seem unable to select a specific target and not build the other local packages. For example, if I do this:

> cd a/b
> cabal new-build lib:b --constraint='a<0' --constraint='c<0'

I run into an error because cabal for some reason makes a, b, and c "user goals"/"targets". This seems to be reproducable on empty/default-init'ialized projects. This is on

> cabal --version
cabal-install version 2.2.0.0
compiled using version 2.2.0.1 of the Cabal library 

On a related note, I would like to stress (again) that I think it would be very useful if cabal would be more verbose/explicit about what it takes as input, how it interprets that input and why it does the things that it does. In this case I have to turn on -v3 to get this line "targets: aaa, bbb, ccc" which still does not explain at all where these targets come from.. and on -v3 this is hidden in quite some amount of other dumps.

[sebright]

Yes, the targets on the command line don't affect the targets that are passed to the dependency solver, so the constraints can't be satisfied. new-build always tries to create an install plan for all targets in order to avoid rerunning the solver or rebuilding packages only because more targets were requested. new-build only solves for the extra targets, it doesn't build them until they're needed.

[lspitzner]

@GrayJay thanks for the explanation. I have re-worded the initial description to something a bit more constructive.