this slightly increases the amount of memory used for any given symbol, but this
increase is more than made up for if the symbol is referenced more than once in
the EvalState that holds it. on average every symbol should be referenced at
least twice (once to introduce a binding, once to use it), so we expect no
increase in memory on average.
symbol tables are limited to 2³² entries like position tables, and similar
arguments apply to why overflow is not likely: 2³² symbols would require as many
string instances (at 24 bytes each) and map entries (at 24 bytes or more each,
assuming that the map holds on average at most one item per bucket as the docs
say). a full symbol table would require at least 192GB of memory just for
symbols, which is well out of reach. (an ofborg eval of nixpks today creates
less than a million symbols!)
Pos objects are somewhat wasteful as they duplicate the origin file name and
input type for each object. on files that produce more than one Pos when parsed
this a sizeable waste of memory (one pointer per Pos). the same goes for
ptr<Pos> on 64 bit machines: parsing enough source to require 8 bytes to locate
a position would need at least 8GB of input and 64GB of expression memory. it's
not likely that we'll hit that any time soon, so we can use a uint32_t index to
locate positions instead.
This allows closures to be imported at evaluation time, without
requiring the user to configure substituters. E.g.
builtins.fetchClosure {
storePath = /nix/store/f89g6yi63m1ywfxj96whv5sxsm74w5ka-python3.9-sqlparse-0.4.2;
from = "https://cache.ngi0.nixos.org";
}
This was introduced in #6174. However fetch{url,Tarball} are legacy
and we shouldn't have an undocumented attribute that does the same
thing as one that already exists ('sha256').
we'll retain the old coerceToString interface that returns a string, but callers
that don't need the returned value to outlive the Value it came from can save
copies by using the new interface instead. for values that weren't stringy we'll
pass a new buffer argument that'll be used for storage and shouldn't be
inspected.
Previously you had to remember to call value->attrs->sort() after
populating value->attrs. Now there is a BindingsBuilder helper that
wraps Bindings and ensures that sort() is called before you can use
it.
Rather than having them plain strings scattered through the whole
codebase, create an enum containing all the known experimental features.
This means that
- Nix can now `warn` when an unkwown experimental feature is passed
(making it much nicer to spot typos and spot deprecated features)
- It’s now easy to remove a feature altogether (once the feature isn’t
experimental anymore or is dropped) by just removing the field for the
enum and letting the compiler point us to all the now invalid usages
of it.
- This way we improve error messages
on infinite recursion
- Demo:
```nix
let x = builtins.fetchTree x;
in x
```
- Before:
```bash
$ nix-instantiate --extra-experimental-features flakes --strict
error: infinite recursion encountered
```
- After:
```bash
$ nix-instantiate --extra-experimental-features flakes --strict
error: infinite recursion encountered
at /data/github/kamadorueda/nix/test.nix:1:9:
1| let x = builtins.fetchTree x;
| ^
2| in x
```
Mentions: #3505
