Commit graph

17086 commits

Author SHA1 Message Date
Vincent Ambo
96e9f750c5 test(tvix/eval): add identity tests for literal evaluation
Change-Id: Id3f37fbe341e15e9428ef1d579d61a514232c0e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6132
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 23:11:12 +00:00
Vincent Ambo
3671056640 fix(tvix/value): align Display representation with Nix
Displaying the optimised representation is not useful anymore.

Change-Id: Icb962ff8865ec4207c144fbcb1aae87483b0fb7c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6131
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 23:11:12 +00:00
Vincent Ambo
c75fa365cd docs(tvix/eval): add README note on running the Nix test suite
Change-Id: I792154d5fea05414f2b14367ea4087dc2a88758e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6130
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 23:11:12 +00:00
Vincent Ambo
7d5691f0e5 chore(tvix/tests): gate Nix test suite behind nix_tests feature
Once we have full coverage they should be enabled by default.

Change-Id: Iace9e1ae9a9f901a0979ad336434004b8028fe8a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6129
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 23:11:12 +00:00
Vincent Ambo
8235384d2d chore(tvix/eval): only print AST if requested
Change-Id: If32c7fd03458a27aecb8c473f2c48b12f8cb5175
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6128
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-24 21:25:41 +00:00
Vincent Ambo
5685f7c594 fix(tvix/value): add ident_str representation of strings
When printing strings as identifiers (in attribute sets), the string
should only be quoted and escaped if it contains escape characters.

Change-Id: If2bcfa1e93dc8f00be4d7a57ec1d82fc679103c3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6127
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-24 21:25:41 +00:00
Vincent Ambo
92c53fe982 feat(tvix/tests): check in Nix' language test suite
This adds scaffolding code for running the Nix language test suite.

The majority of eval-okay-* tests should eventually be runnable as-is
by Tvix, however the eval-fail-* tests might not as we intend to have
more useful error messages than upstream Nix.

Change-Id: I4f3227f0889c55e4274b804a3072850fb78dd1bd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6126
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 21:25:41 +00:00
Vincent Ambo
b28da8ad56 chore(tvix/eval): add 'test-generator' as dev-dependency
This crate makes it easy to generate test cases from files on disk,
which is used for running the Nix test suite against Tvix.

Change-Id: I60ef26484d05e723982679dd42958adf52735916
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6125
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 21:23:12 +00:00
Vincent Ambo
4c9aad17ad fix(tvix/value): add escaping logic for Nix strings
Nix strings displayed to users must be escaped the same way as they
are in C++ Nix. This adds the scaffolding for escapes, but is most
likely not yet complete.

Change-Id: Icfdcb2ac98d292c567ba894a92b6529a53e0cc17
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6124
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-24 21:23:12 +00:00
Vincent Ambo
a2b4b4a485 fix(tvix/value): implement PartialOrd/PartialEq for strings
Instead of comparing the enum variants (which does not yield useful
behaviour), compare &str representations of the string instead.

Change-Id: I5e94b5f6c91b4561e1bc7c36d586f3d23c243764
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6112
Tested-by: BuildkiteCI
Reviewed-by: eta <tvl@eta.st>
2022-08-24 18:19:52 +00:00
Vincent Ambo
6dc9ca5723 feat(tvix/value): introduce string representation with &'static str
For cases where the strings are statically known (such as the
oft-occuring name/value), this can be a useful optimisation.

It's also much more convenient in tests.

Change-Id: Ie462b684805bd4986ea5e85ca4bff663bc2d3c3c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6111
Tested-by: BuildkiteCI
Reviewed-by: eta <tvl@eta.st>
2022-08-24 18:19:52 +00:00
Vincent Ambo
c7ba2dec04 test(tvix/value): add simple attrset construction tests
These do not yet test nested attribute sets; we need to add some more
inspection primitives first.

Change-Id: Icfc99bf17c73ebefc0d882a84f0ca73ec688a54d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6110
Reviewed-by: eta <tvl@eta.st>
Tested-by: BuildkiteCI
2022-08-24 18:19:52 +00:00
Vincent Ambo
08b4d65fbd feat(tvix/value): implement nested attribute set literals
With this change, nested attribute sets can now be created from
literals.

This required some logic for dealing with cases where at a deeper
nesting point a literal attribute set was constructed from an
optimised representation.

For example, this is valid Nix code:

```nix
{
  a = {};   # creates optimised empty representation
  a.b = 1;  # wants to add a `b = 1` to it

  b = { name = "foo"; value = "bar"; }; # creates optimised K/V repr
  b.foo = 42; # wants to add an additional `foo = 42`
}
```

In these cases, the attribute set must be coerced to a map
representation first which is achieved by the new internal
NixAttr::map_mut helper.

Change-Id: Ia61d3d9d14c4e0f5e207c00f6a2f4daa3265afb2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6109
Reviewed-by: eta <tvl@eta.st>
Tested-by: BuildkiteCI
2022-08-24 18:19:52 +00:00
Vincent Ambo
293fb0ef53 refactor(tvix/value): encapsulate attrset logic within value::attrs
The internal optimisations of the set representation were previously
leaking into the VM, which is highly undesirable.

Keeping it encapsulated allows us to do additional optimisations
within value::attrs without being concerned about its use in the VM.

Change-Id: I7e7020bb0983b9d355d3db747b049b2faa60131f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6108
Reviewed-by: eta <tvl@eta.st>
Tested-by: BuildkiteCI
2022-08-24 18:19:52 +00:00
Vincent Ambo
6edbfe3cba chore(paroxysm): bump cargo dependencies
Change-Id: Ibf82bf6b9a10274fe6928db7a6a2beb74da92b13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6122
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
2022-08-22 21:15:01 +00:00
Vincent Ambo
ba4e979d19 chore(web/atward): bump cargo dependencies
Change-Id: I691e87c6b596fcc7028717a5a22d5619bd87a089
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6121
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
2022-08-22 21:15:01 +00:00
Vincent Ambo
b66b1aee69 chore(tools/cheddar): bump cargo dependencies
Change-Id: I41e26046a67635ec3dba2ac955e31e6ca7451cc6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6120
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
2022-08-22 21:15:01 +00:00
Vincent Ambo
742e2a611b chore(tazjin/tgsa): bump cargo dependencies
Change-Id: Id52ab2937e32385324ba93f4df7e1da17a1f4eec
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6119
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
2022-08-22 14:12:00 +00:00
sterni
df7cd63fa1 chore(3p/sources): Bump channels & overlays
* //users/tazjin/predlozhnik: bump wasm-bindgen & run `cargo update`

* //users/Profpatsch/writers:
  use proper keyword syntax to make make flake8 happy.

* //third_party/nixpkgs: pick binaryen from stable channel

Change-Id: Ia711d8c396aa09b78413d80f35eedf63fa315596
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6118
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Profpatsch <mail@profpatsch.de>
2022-08-22 13:54:18 +00:00
Profpatsch
fdc5763f5b feat(users/Profpatsch): init haskell arglib-netencode
Mirrors the rust exec-helpers

Change-Id: I96f211d6545e228b8ae0c1dce4e60e6f6867690c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6117
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
2022-08-22 12:47:53 +00:00
Profpatsch
0fc1634255 feat(users/Profpatsch): init haskell exec-helpers
Mirrors the rust exec-helpers

Change-Id: I8fba49525949a8f38ca8c215e2ec7ba9d97d3bf9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6116
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
2022-08-22 12:47:53 +00:00
Profpatsch
240edcd289 fix(users/Profpatsch/my-prelude): use exactSource
Change-Id: I1b797fb69f69e09f2dda99fce2f12adef0ac2ad2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6115
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
2022-08-22 12:47:53 +00:00
Profpatsch
aed86ea4b3 feat(users/Profpatsch): init netencode-hs
Haskell implementation of the netncode parser & generators.

Change-Id: I904df752e78641110af2857e71d8206bf432d427
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6114
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
2022-08-22 12:47:53 +00:00
sterni
68f81e843f chore(sterni/emacs): weed out subscriptions
Change-Id: Ie5f57300c380874a83d1cbd25a61eb595760bbf4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6113
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
2022-08-16 14:27:33 +00:00
Vincent Ambo
52736a1dbc refactor(tvix/rm): introduce helper for AttrSet Entry API
There are multiple points where an insertion needs to be done into an
attribute set, but copying the key or checking for presence before
insertion should be avoided

As that is a little bit noisy, it's been factored out into a helper
function in this commit.

Change-Id: Ibcb054ebeb25a1236c06c812f47c8e74180d4fc9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6107
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-14 00:24:33 +00:00
Vincent Ambo
295d6e1d59 feat(tvix/vm): implement first nested attribute set construction
This can construct non-overlapping nested attribute sets (i.e. `{ a.b
= 1; b.c = 2; }`, but not `{ a.b = 1; a.c = 2; }`).

In order to do the latter, it's necessary to gain the ability to
manipulate the in-progress attribute set construction. There's
multiple different options for this ...

Change-Id: If1a762a720b175e8eb4216cbf96a7434d22640fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6106
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-14 00:23:32 +00:00
Vincent Ambo
8c2bc683cd refactor(tvix/value): explicitly implement PartialEq for value
There are some notions of equality (due to e.g. different backing
variants for types, or Nix particularities) that don't work correctly
when deriving PartialEq.

Change-Id: Ide83ae67d051cc0b3ca89cefb283f17d0207acce
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6105
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 20:24:12 +00:00
Vincent Ambo
e15bd9aa63 fix(tvix/eval): Fail on duplicate attribute set keys
Change-Id: I57373ca76d0e25a5d08a8dfce9d5949099326fc0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6104
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 20:24:12 +00:00
Vincent Ambo
175eb97505 feat(tvix/eval): construct internal attribute path representation
This is required for constructing nested attribute sets at runtime.

There'll be quite a lot of optimisation potential with this solution
eventually, if it should turn out to be a bottleneck.

This introduces a conceptual change, in that the `Value` enum is now
an enum representing "all runtime values" instead of "all Nix language
types". This makes sense in general, as this type will also contain
Chunk representations etc. which are not exposed to users.

Change-Id: Ic5f72b2a0965b146c6a451efad34c6a81ca1aad8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6103
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
2022-08-13 20:24:12 +00:00
Vincent Ambo
ec1770f95a feat(tvix/vm): implement construction of optimised KV attrsets
For name/value pairs (which occur extremely often in Nix and make up a
significant chunk of the runtime cost of evaluating nixpkgs) we
substitute an optimised representation.

For now this will only be used if the name/value pair keys were
specified as literal identifiers or strings (i.e. if chunks are
encountered as keys they are not forced and a normal attribute set
backed by a map will be constructed).

Change-Id: Ic79746c323e627528bd58b1a6024ee8d0aff7858
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6102
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
2022-08-13 20:24:12 +00:00
Vincent Ambo
e876c3a41c fix(tvix/value): KV struct needs to carry name as Value, too
Users may construct a pair that falls into the name/value optimisation
but where `name` is not actually a string, as from the language
perspective there is nothing special about this attribute set.

We also can not conditionally apply this by forcing the key at this
point, as this would change the language semantics.

Therefore, the name in the optimised representation is also carried as
`Value`.

Change-Id: I5be8a4c98ba19ebdfb7203a929f714a04492512e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6101
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
2022-08-13 20:24:12 +00:00
Vincent Ambo
2dcbbb8c4a feat(tvix/value): implement Display properly for lists
Change-Id: I991d235cf52fbd42eb839b384f9c55ee64fa86c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6100
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 18:28:36 +00:00
Vincent Ambo
56caaf70ca fix(tvix/errors): display a useful intermediate error representation
Change-Id: If979a2aa21444320427f54e6530a55cab873856b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6099
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 18:28:36 +00:00
Vincent Ambo
a93933b487 feat(tvix): implement string interpolation
This adds a new instruction which assembles an interpolated string
from a specified number of fragments, which are already going to be
located on the stack in the right position.

This will raise a type error if any of the fragments do not evaluate
to a string.

Change-Id: I5756248fa3e9fcc3d063c14db40b332f7e20a588
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6098
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 18:28:36 +00:00
Vincent Ambo
3577841bde feat(tvix/compiler): compile non-interpolated string literals
This sets up the scaffolding for compiling interpolation, but those
instructions do not yet exist.

Change-Id: Ife41bbbf432d9661abe566c92437409dd0da44e7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6097
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 18:28:36 +00:00
Vincent Ambo
72863d81fc style(tvix/eval): display Display representation of runtime values
Change-Id: Ifbe05c2da9493c9e132a2d2e94a451d6091542a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6096
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 18:28:36 +00:00
Vincent Ambo
910336c68c feat(tvix/compiler): compile list literals
Change-Id: I2c6fedc3dbb7d449d700f3972c3fbd4a7d147f6c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6095
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-13 15:39:26 +00:00
Vincent Ambo
5763b62172 feat(tvix/vm): implement list construction
Change-Id: Iec2b4910800ab29daae6d71b58a8acd14ccb1cc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6094
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-13 15:35:24 +00:00
Vincent Ambo
c67747cbe1 feat(tvix/value): add runtime representation of simple lists
There might be more logic in the future to encapsulate different
backing implementations of lists as well.

Change-Id: Ib7064fab48bf88b0c8913b0ecfa2108177c7c9fd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6093
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-13 15:35:24 +00:00
Vincent Ambo
6f13c16f28 docs(tvix/eval): add design documentation for attrset opcodes
Change-Id: I303b57e035543f4597c6247983d1d533e4014638
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6092
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 15:31:50 +00:00
Vincent Ambo
57a723aaa9 feat(tvix/eval): implement trivial attribute set literals
Implements attribute set literals without nesting. Technically this
already supports dynamic key fragments (evaluating to strings), though
the only way to create these (interpolation) is not yet implemented.

However, creating simple attribute sets like `{ }`, or `{ a = 15; }`
or `{ a = 10 * 2; }` works.

Recursive attribute sets are not yet implemented as we do not have any
kind of scope access yet anyways.

This is implemented using a new instruction that creates an attribute
set with a given number of elements by popping key/value pairs off the
stack.

Change-Id: I0f9aac7a131a112d3f66b131297686b38aaeddf2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6091
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 15:31:50 +00:00
Vincent Ambo
20fc7bc0b2 chore(3p/sources): Bump channels & overlays
* tvl-slapd: move database to subdirectory (somehow now required)

Change-Id: I1792b856cf68b11959c0cc9caab4135e556f8c58
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6090
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 14:43:05 +00:00
Vincent Ambo
e24248df3b feat(tvix/value): add some necessary helpers for strings
Deriving Ord/Eq is required for the ordered BTreeMaps. Once interning
is implemented this will require some extra magic for the sort order,
but that's fine.

Change-Id: I0c654648eb3609a4a01d84868c25f43a4d35bc2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6089
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 11:50:37 +00:00
Vincent Ambo
865717a8db fix(tvix/eval): print code even if runtime fails
Change-Id: I357c9adf939cb6001afa73ad02282d94ee22d0ba
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6088
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 11:50:37 +00:00
Vincent Ambo
2ed38a7cdb feat(tvix/eval): add Value variants for strings & attrsets
Change-Id: Idebf663ab7fde3955aae50f635320f7eb6c353e8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6087
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 11:50:37 +00:00
Vincent Ambo
ba03226e51 feat(tvix/eval): add module for attribute set implementations
Change-Id: I6002bd5e5596b93325dea6c862370ba5235c0f08
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6086
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 11:50:37 +00:00
Vincent Ambo
d26c097d1b feat(tvix/eval): add module for string type implementation
Change-Id: I5e4465acc4a676c10d7374b14f7a09240202b466
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6085
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-13 11:50:37 +00:00
Vincent Ambo
fab5d23f14 feat(tvl.el): add magit-gerrit-push-private
This adds a new function (intentionally bound to a rare key (Q)) in
the push menu which can push a *private* change to Gerrit.

A private change is one that, until submitted, is only visible to its
owner and all explicitly added people (reviewers, CC).

Change-Id: I6ee13dbbad099584475d3efac96e5d9b86efbc26
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6061
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-13 00:31:26 +00:00
Vincent Ambo
31443d21ee chore(tvix): move nix-store CLI scaffolding to subfolder
For some reason a top-level Rust project ended up in this location,
which is incompatible with the actual project structure that's being
prepared for merge right now.

Change-Id: I9d919ad72fc7e4e4d8cbb9899e7f8d90fa7ca87a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6060
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
2022-08-12 15:58:59 +00:00
Vincent Ambo
3ed45caad1 feat(tvix/eval): implement Display trait for Value enum
This representation should match what the Nix REPL shows for result
values.

Change-Id: If3143d969fcdc123a6029e2aeb7bbd6ae51aeb71
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6082
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: grfn <grfn@gws.fyi>
2022-08-12 14:12:07 +00:00