refactor(3p/nix/libexpr): Make nix::AttrName a std::variant r/1278

nix:AttrName was one of the few classes that relied on the default
constructor of nix::Symbol (which I am trying to remove in a separate
change).

The class essentially represents the name of an attribute in a set,
which is either just a string expression or a dynamically evaluated
expression (e.g. string interpolation).

Previously it would be constructed by only setting one of the fields
and defaulting the other, now it is an explicit std::variant.

Note that there are several code paths where not all eventualities are
handled and this code is bug-for-bug compatible with those, except
that unknown conditions (which should never work) are now throwing
instead of silently doing ... something.

The language tests pass with this change, and the depot derivations
that I tested with evaluated successfully.

Change-Id: Icf1ee60a5f8308f4ab18a82749e00cf37a938a8f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1138
Reviewed-by: edef <edef@edef.eu>
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 2 insertions, 6 deletions

diff --git a/third_party/nix/src/libexpr/nixexpr.hh b/third_party/nix/src/libexpr/nixexpr.hh
index 0bf5245181ec..7ec6f7f17f39 100644
--- a/third_party/nix/src/libexpr/nixexpr.hh
+++ b/third_party/nix/src/libexpr/nixexpr.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <map>
+#include <variant>
 
 #include "libexpr/symbol-table.hh"
 #include "libexpr/value.hh"
@@ -60,12 +61,7 @@ class EvalState;
 struct StaticEnv;
 
 /* An attribute path is a sequence of attribute names. */
-struct AttrName {
-  Symbol symbol;
-  Expr* expr;
-  AttrName(const Symbol& s) : symbol(s){};
-  AttrName(Expr* e) : expr(e){};
-};
+using AttrName = std::variant<Symbol, Expr*>;
 
 typedef std::vector<AttrName> AttrPath;