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Make more string values work as installables

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As discussed in #7417, it would be good to make more string values work
as installables. That is to say, if an installable refers to a value,
and the value is a string, it used to not work at all, since #7484, it
works somewhat, and this PR make it work some more.

The new cases that are added for `BuiltPath` contexts:

- Fixed input- or content-addressed derivation:

  ```
  nix-repl> hello.out.outPath
  "/nix/store/jppfl2bp1zhx8sgs2mgifmsx6dv16mv2-hello-2.12"

  nix-repl> :p builtins.getContext hello.out.outPath
  { "/nix/store/c7jrxqjhdda93lhbkanqfs07x2bzazbm-hello-2.12.drv" = { outputs = [ "out" ]; }; }

  The string matches the specified single output of that derivation, so
  it should also be valid.

- Floating content-addressed derivation:

  ```
  nix-repl> (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  "/1a08j26xqc0zm8agps8anxpjji410yvsx4pcgyn4bfan1ddkx2g0"

  nix-repl> :p builtins.getContext (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  { "/nix/store/qc645pyf9wl37c6qvqzaqkwsm1gp48al-hello-2.12.drv" = { outputs = [ "out" ]; }; }
  ```

  The string is not a path but a placeholder, however it also matches
  the context, and because it is a CA derivation we have no better
  option. This should also be valid.

We may also want to think about richer attrset based values (also
discussed in that issue and #6507), but this change "completes" our
string-based building blocks, from which the others can be desugared
into or at least described/document/taught in terms of.

Progress towards #7417

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
This commit is contained in:
John Ericson 2023-01-10 17:18:59 -05:00
parent 5a23b80b0a
commit d2162e7acd
6 changed files with 110 additions and 26 deletions
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18
src/libcmd/installable-value.hh
View file

@ -98,6 +98,24 @@ struct InstallableValue : Installable
static InstallableValue & require(Installable & installable);
static ref<InstallableValue> require(ref<Installable> installable);
protected:
/**
* Handles either a plain path, or a string with a single string
* context elem in the right format. The latter case is handled by
* `EvalState::coerceToDerivedPath()`; see it for details.
*
* @param v Value that is hopefully a string or path per the above.
*
* @param pos Position of value to aid with diagnostics.
*
* @param errorCtx Arbitrary message for use in potential error message when something is wrong with `v`.
*
* @result A derived path (with empty info, for now) if the value
* matched the above criteria.
*/
std::optional<DerivedPathWithInfo> trySinglePathToDerivedPaths(Value & v, const PosIdx pos, std::string_view errorCtx);
};
}