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-rw-r--r--third_party/nix/src/libexpr/eval.cc25
-rw-r--r--third_party/nix/src/libexpr/nixexpr.cc22
-rw-r--r--third_party/nix/src/libexpr/nixexpr.hh8
-rw-r--r--third_party/nix/src/libexpr/parser.y57
-rw-r--r--third_party/nix/src/libexpr/symbol-table.hh2
5 files changed, 64 insertions, 50 deletions
diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index 6d77928d7d4f..6df7e66b6542 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -6,6 +6,7 @@
 #include <fstream>
 #include <iostream>
 #include <new>
+#include <variant>
 
 #include <absl/strings/match.h>
 #include <gc/gc.h>
@@ -24,6 +25,7 @@
 #include "libutil/hash.hh"
 #include "libutil/json.hh"
 #include "libutil/util.hh"
+#include "libutil/visitor.hh"
 
 namespace nix {
 
@@ -185,13 +187,15 @@ static void* oomHandler(size_t requested) {
 #endif
 
 static Symbol getName(const AttrName& name, EvalState& state, Env& env) {
-  if (name.symbol.set()) {
-    return name.symbol;
-  }
-  Value nameValue;
-  name.expr->eval(state, env, nameValue);
-  state.forceStringNoCtx(nameValue);
-  return state.symbols.Create(nameValue.string.s);
+  return std::visit(
+      util::overloaded{[&](const Symbol& name) -> Symbol { return name; },
+                       [&](Expr* expr) -> Symbol {
+                         Value nameValue;
+                         expr->eval(state, env, nameValue);
+                         state.forceStringNoCtx(nameValue);
+                         return state.symbols.Create(nameValue.string.s);
+                       }},
+      name);
 }
 
 static bool gcInitialised = false;
@@ -890,12 +894,7 @@ static std::string showAttrPath(EvalState& state, Env& env,
     } else {
       first = false;
     }
-    try {
-      out << getName(i, state, env);
-    } catch (Error& e) {
-      assert(!i.symbol.set());
-      out << "\"${" << *i.expr << "}\"";
-    }
+    out << getName(i, state, env);
   }
   return out.str();
 }
diff --git a/third_party/nix/src/libexpr/nixexpr.cc b/third_party/nix/src/libexpr/nixexpr.cc
index 28d8eee7a7f8..2de37f50e9b9 100644
--- a/third_party/nix/src/libexpr/nixexpr.cc
+++ b/third_party/nix/src/libexpr/nixexpr.cc
@@ -1,9 +1,11 @@
 #include "libexpr/nixexpr.hh"
 
 #include <cstdlib>
+#include <variant>
 
 #include "libstore/derivations.hh"
 #include "libutil/util.hh"
+#include "libutil/visitor.hh"
 
 namespace nix {
 
@@ -198,17 +200,17 @@ std::ostream& operator<<(std::ostream& str, const Pos& pos) {
 std::string showAttrPath(const AttrPath& attrPath) {
   std::ostringstream out;
   bool first = true;
-  for (auto& i : attrPath) {
+  for (auto& attr : attrPath) {
     if (!first) {
       out << '.';
     } else {
       first = false;
     }
-    if (i.symbol.set()) {
-      out << i.symbol;
-    } else {
-      out << "\"${" << *i.expr << "}\"";
-    }
+
+    std::visit(util::overloaded{
+                   [&](const Symbol& sym) { out << sym; },
+                   [&](const Expr* expr) { out << "\"${" << *expr << "}\""; }},
+               attr);
   }
   return out.str();
 }
@@ -268,8 +270,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
     def->bindVars(env);
   }
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }
@@ -277,8 +279,8 @@ void ExprSelect::bindVars(const StaticEnv& env) {
 void ExprOpHasAttr::bindVars(const StaticEnv& env) {
   e->bindVars(env);
   for (auto& i : attrPath) {
-    if (!i.symbol.set()) {
-      i.expr->bindVars(env);
+    if (auto* expr = std::get_if<Expr*>(&i)) {
+      (*expr)->bindVars(env);
     }
   }
 }
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;
 
diff --git a/third_party/nix/src/libexpr/parser.y b/third_party/nix/src/libexpr/parser.y
index fe8759f3c6b7..eb3f92db7844 100644
--- a/third_party/nix/src/libexpr/parser.y
+++ b/third_party/nix/src/libexpr/parser.y
@@ -16,6 +16,7 @@
 #ifndef BISON_HEADER
 #define BISON_HEADER
 
+#include <variant>
 #include "libutil/util.hh"
 #include "libexpr/nixexpr.hh"
 #include "libexpr/eval.hh"
@@ -84,30 +85,39 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
     // Checking attrPath validity.
     // ===========================
     for (i = attrPath.begin(); i + 1 < attrPath.end(); i++) {
-        if (i->symbol.set()) {
-            ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
+        if (const auto* sym = std::get_if<Symbol>(&(*i)); sym && sym->set()) {
+            ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(*sym);
             if (j != attrs->attrs.end()) {
                 if (!j->second.inherited) {
                     ExprAttrs * attrs2 = dynamic_cast<ExprAttrs *>(j->second.e);
                     if (!attrs2) { dupAttr(attrPath, pos, j->second.pos); }
                     attrs = attrs2;
-                } else
+                } else {
                     dupAttr(attrPath, pos, j->second.pos);
+                }
             } else {
-                ExprAttrs * nested = new ExprAttrs;
-                attrs->attrs[i->symbol] = ExprAttrs::AttrDef(nested, pos);
+                ExprAttrs* nested = new ExprAttrs;
+                attrs->attrs[*sym] = ExprAttrs::AttrDef(nested, pos);
                 attrs = nested;
             }
         } else {
-            ExprAttrs *nested = new ExprAttrs;
-            attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(i->expr, nested, pos));
-            attrs = nested;
+          // Yes, this code does not handle all conditions
+          // exhaustively. We use std::get to throw if the condition
+          // that isn't covered happens, which is potentially a
+          // behaviour change from the previous default constructed
+          // Symbol. It should alert us about anything untoward going
+          // on here.
+          auto* expr = std::get<Expr*>(*i);
+
+          ExprAttrs *nested = new ExprAttrs;
+          attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(expr, nested, pos));
+          attrs = nested;
         }
     }
     // Expr insertion.
     // ==========================
-    if (i->symbol.set()) {
-        ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(i->symbol);
+    if (auto* sym = std::get_if<Symbol>(&(*i)); sym && sym->set()) {
+        ExprAttrs::AttrDefs::iterator j = attrs->attrs.find(*sym);
         if (j != attrs->attrs.end()) {
             // This attr path is already defined. However, if both
             // e and the expr pointed by the attr path are two attribute sets,
@@ -118,8 +128,9 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             if (jAttrs && ae) {
                 for (auto & ad : ae->attrs) {
                     auto j2 = jAttrs->attrs.find(ad.first);
-                    if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
+                    if (j2 != jAttrs->attrs.end()) {// Attr already defined in iAttrs, error.
                         dupAttr(ad.first, j2->second.pos, ad.second.pos);
+                    }
                     jAttrs->attrs[ad.first] = ad.second;
                 }
             } else {
@@ -127,11 +138,13 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
             }
         } else {
             // This attr path is not defined. Let's create it.
-            attrs->attrs[i->symbol] = ExprAttrs::AttrDef(e, pos);
-            e->setName(i->symbol);
+            attrs->attrs[*sym] = ExprAttrs::AttrDef(e, pos);
+            e->setName(*sym);
         }
     } else {
-        attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(i->expr, e, pos));
+        // Same caveat as the identical line above.
+        auto* expr = std::get<Expr*>(*i);
+        attrs->dynamicAttrs.push_back(ExprAttrs::DynamicAttrDef(expr, e, pos));
     }
 }
 
@@ -444,19 +457,23 @@ binds
   | binds INHERIT attrs ';'
     { $$ = $1;
       for (auto & i : *$3) {
-          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              dupAttr(i.symbol, makeCurPos(@3, data), $$->attrs[i.symbol].pos);
+          auto sym = std::get<Symbol>(i);
+          if ($$->attrs.find(sym) != $$->attrs.end()) {
+              dupAttr(sym, makeCurPos(@3, data), $$->attrs[sym].pos);
+          }
           Pos pos = makeCurPos(@3, data);
-          $$->attrs[i.symbol] = ExprAttrs::AttrDef(new ExprVar(CUR_POS, i.symbol), pos, true);
+          $$->attrs[sym] = ExprAttrs::AttrDef(new ExprVar(CUR_POS, sym), pos, true);
       }
     }
   | binds INHERIT '(' expr ')' attrs ';'
     { $$ = $1;
       /* !!! Should ensure sharing of the expression in $4. */
       for (auto & i : *$6) {
-          if ($$->attrs.find(i.symbol) != $$->attrs.end())
-              dupAttr(i.symbol, makeCurPos(@6, data), $$->attrs[i.symbol].pos);
-          $$->attrs[i.symbol] = ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, i.symbol), makeCurPos(@6, data));
+          auto sym = std::get<Symbol>(i);
+          if ($$->attrs.find(sym) != $$->attrs.end()) {
+            dupAttr(sym, makeCurPos(@6, data), $$->attrs[sym].pos);
+          }
+          $$->attrs[sym] = ExprAttrs::AttrDef(new ExprSelect(CUR_POS, $4, sym), makeCurPos(@6, data));
       }
     }
   | { $$ = new ExprAttrs; }
diff --git a/third_party/nix/src/libexpr/symbol-table.hh b/third_party/nix/src/libexpr/symbol-table.hh
index c4c1163bed15..efc2314b75e8 100644
--- a/third_party/nix/src/libexpr/symbol-table.hh
+++ b/third_party/nix/src/libexpr/symbol-table.hh
@@ -21,7 +21,7 @@ class Symbol {
 
   bool operator<(const Symbol& s2) const { return *s < *s2.s; }
 
-  operator const std::string&() const { return *s; }
+  operator const std::string &() const { return *s; }
 
   bool set() const { return s; }
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