The code that counts the number of elided attrs incorrectly used the
per-printer "global" attribute counter instead of a counter that
was relevant only to the current attribute set.
This bug flew under the radar because often the attribute sets aren't
nested, not big enough, or we wouldn't pay attention to the numbers.
I've noticed the issue because the difference underflowed.
Although this behavior is tested by the functional test
lang/eval-fail-bad-string-interpolation-4.nix, the underflow slipped
through review. A simpler reproducer would be as follows, but I
haven't added it to the test suite to keep it simple and marginally
faster.
```
$ nix run nix/2.23.1 -- eval --expr '"" + (let v = { a = { a = 1; b = 2; c = 1; d = 1; e = 1; f = 1; g = 1; h = 1; }; b = { a = 1; b = 1; c = 1; }; }; in builtins.deepSeq v v)'
error:
… while evaluating a path segment
at «string»:1:6:
1| "" + (let v = { a = { a = 1; b = 2; c = 1; d = 1; e = 1; f = 1; g = 1; h = 1; }; b = { a = 1; b = 1; c = 1; }; }; in builtins.deepSeq v v)
| ^
error: cannot coerce a set to a string: { a = { a = 1; b = 2; c = 1; d = 1; e = 1; f = 1; g = 1; h = 1; }; b = { a = 1; «4294967289 attributes elided» }; }
```
Pretty-print values in the REPL by printing each item in a list or
attrset on a separate line. When possible, single-item lists and
attrsets are printed on one line, as long as they don't contain a nested
list, attrset, or thunk.
Before:
```
{ attrs = { a = { b = { c = { }; }; }; }; list = [ 1 ]; list' = [ 1 2 3 ]; }
```
After:
```
{
attrs = {
a = {
b = {
c = { };
};
};
};
list = [ 1 ];
list' = [
1
2
3
];
}
```
This extends the `error: cannot coerce a TYPE to a string` message
to print the value that could not be coerced. This helps with debugging
by making it easier to track down where the value is being produced
from, especially in errors with deep or unhelpful stack traces.
This keeps hint messages, source location information, and source code
snippets grouped together, while making stack traces shorter (so that
more stack frames can be viewed on the same terminal).
Before:
error:
… while evaluating the attribute 'body'
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:4:3:
3|
4| body = x "x";
| ^
5| }
… from call site
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:4:10:
3|
4| body = x "x";
| ^
5| }
… while calling 'x'
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:2:7:
1| let {
2| x = arg: assert arg == "y"; 123;
| ^
3|
error: assertion '(arg == "y")' failed
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:2:12:
1| let {
2| x = arg: assert arg == "y"; 123;
| ^
3|
After:
error:
… while evaluating the attribute 'body'
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:4:3:
3|
4| body = x "x";
| ^
5| }
… from call site
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:4:10:
3|
4| body = x "x";
| ^
5| }
… while calling 'x'
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:2:7:
1| let {
2| x = arg: assert arg == "y"; 123;
| ^
3|
error: assertion '(arg == "y")' failed
at /Users/wiggles/nix/tests/functional/lang/eval-fail-assert.nix:2:12:
1| let {
2| x = arg: assert arg == "y"; 123;
| ^
3|
