lix/doc/manual/rl-next/with-error-reporting.md
pennae 1fe66852ff reduce the size of Env by one pointer
since `up` and `values` are both pointer-aligned the type field will
also be pointer-aligned, wasting 48 bits of space on most machines. we
can get away with removing the type field altogether by encoding some
information into the `with` expr that created the env to begin with,
reducing the GC load for the absolutely massive amount of single-entry
envs we create for lambdas. this reduces memory usage of system eval by
quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB)
and gives similar savings in eval time.

running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'`

before:

  Time (mean ± σ):      5.576 s ±  0.003 s    [User: 5.197 s, System: 0.378 s]
  Range (min … max):    5.572 s …  5.581 s    10 runs

after:

  Time (mean ± σ):      5.408 s ±  0.002 s    [User: 5.019 s, System: 0.388 s]
  Range (min … max):    5.405 s …  5.411 s    10 runs
2023-12-30 18:55:13 +01:00

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Markdown

---
synopsis: Better error reporting for `with` expressions
prs: 9658
---
`with` expressions using non-attrset values to resolve variables are now reported with proper positions.
Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:
```
nix-repl> with 1; a
error:
… <borked>
at «none»:0: (source not available)
error: value is an integer while a set was expected
```
Now position information is preserved and reported as with most other errors:
```
nix-repl> with 1; a
error:
… while evaluating the first subexpression of a with expression
at «string»:1:1:
1| with 1; a
| ^
error: value is an integer while a set was expected
```