non-function type error, with argument details.

[bburdette]

Motivation

Add some extra context when someone mistakenly uses something that isn't a function where a function should be.

Context

Issue #7405 provides an example where part of the nix expression is evaluated and assumed to be a function, since arguments follow. In this case, which part of the expression evaluates to the function is arguably unclear to the user.

Current error:

nix-repl> {a = 3;}.a or throw "what"  
error: attempt to call something which is not a function but an integer

       at «string»:1:1:

            1| {a = 3;}.a or throw "what"
             | ^

Error with extra context:

nix-repl> {a = 3;}.a or throw "what"
error: attempt to call something which is not a function but an integer: 3
       function arguments:
       a string: "what"

       at «string»:1:1:

            1| {a = 3;}.a or throw "what"
             | ^

To hopefully make cases like this more clear, I print the type and value of the non-function, followed by the type and value of its arguments. Because printing sets and lists can result in enormous walls of text, I only print the expr types for sets and lists.

nix-repl> np = import <nixos-22.11> {}

nix-repl> np
{ AAAAAASomeThingsFailToEvaluate = «error: error:
       … while calling the 'throw' builtin

         at /nix/store/ ^Cerror: interrupted by the user
«derivation
nix-repl> {a = 3;}.a or throw "what" 12 21 34 np
error: attempt to call something which is not a function but an integer: 3
       function arguments:
       a string: "what"
       an integer: 12
       an integer: 21
       an integer: 34
       a set: <set>

       at «string»:1:1:

            1| {a = 3;}.a or throw "what" 12 21 34 np
             | ^

Implementation notes

Because I wanted to make an error message with a dynamic number of arguments, I needed to create a new constructor for ErrorBuilder that takes a hintformat. Then I added a function to create this hintformat. Since allocating hintformats on the stack is discouraged, the build function and etc use a unique_ptr.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes