Pass positions when evaluating

[9999years]

Motivation

This includes position information in more places, making debugging easier. This removes «none»:0: (source not available) from error messages in many cases.

Context

Final fix for #561 (comment).

Before:

error:
       … while evaluating an attribute name

         at «none»:0: (source not available)

       error: value is a set while a string was expected

After:

error:
       … while evaluating an attribute name

         at /Users/wiggles/nix/bad.nix:4:11:

            3|   key = {};
            4| in attr.${key}
             |           ^
            5|

       error: value is a set while a string was expected

Before:

$ nix-instantiate --show-trace --eval tests/functional/lang/eval-fail-call-primop.nix
error:
       … while calling the 'length' builtin

         at /Users/wiggles/nix/tests/functional/lang/eval-fail-call-primop.nix:1:1:

            1| builtins.length 1
             | ^
            2|

       … while evaluating the first argument passed to builtins.length

         at «none»:0: (source not available)

       error: value is an integer while a list was expected

After:

error:
       … while calling the 'length' builtin

         at /Users/wiggles/nix/tests/functional/lang/eval-fail-call-primop.nix:1:1:

            1| builtins.length 1
             | ^
            2|

       … while evaluating the first argument passed to builtins.length

       error: value is an integer while a list was expected

Before:

$ nix-instantiate --show-trace --eval tests/functional/lang/eval-fail-not-throws.nix
error:
       … in the argument of the not operator

         at «none»:0: (source not available)

       … while calling the 'throw' builtin

         at /Users/wiggles/nix/tests/functional/lang/eval-fail-not-throws.nix:1:4:

            1| ! (throw "uh oh!")
             |    ^
            2|

       error: uh oh!

After:

error:
       … in the argument of the not operator

         at /Users/wiggles/nix/tests/functional/lang/eval-fail-not-throws.nix:1:4:

            1| ! (throw "uh oh!")
             |    ^
            2|

       … while calling the 'throw' builtin

         at /Users/wiggles/nix/tests/functional/lang/eval-fail-not-throws.nix:1:4:

            1| ! (throw "uh oh!")
             |    ^
            2|

       error: uh oh!

Priorities

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[roberth]

Could you turn the example into a test case?
You could add it as tests/functional/lang/eval-fail-<name>.nix and then when you run make tests/functional/lang.sh.test the error message will help you along.

[9999years]

Could you turn the example into a test case?

Done! (I finished this at the end of the day and didn't have time to write tests.)

[roberth]

Suggested change

	at «none»:0: (source not available)

It didn't do that anymore since 2.19.

• Don't print unknown locations unless requested for dev purposes #8905

You may set _NIX_DEVELOPER_SHOW_UNKNOWN_LOCATIONS=1 to show the missing positions once more.

[roberth]

Awesome, thanks!