libexpr: quote reserved keys when printing

This fixes a bug in commands like `nix eval' which would emit invalid attribute
sets if they contained reserved keywords such as "assert", "let", etc.

These keywords will not be quoted when printed, making them valid expressions.
All keywords recognized by the lexer are quoted except "or", which does not
require quotation.

3 files changed, 31 insertions, 4 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index e2b455b91..af9d037ec 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -94,7 +94,6 @@ RootValue allocRootValue(Value * v)
 #endif
 }
 
-
 void Value::print(const SymbolTable & symbols, std::ostream & str,
     std::set<const void *> * seen) const
 {
@@ -122,7 +121,13 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
         else {
             str << "{ ";
             for (auto & i : attrs->lexicographicOrder(symbols)) {
-                str << symbols[i->name] << " = ";
+                // Quote reserved keywords so that the output can be
+                // re-evaluated later without upsetting the lexer.
+                if (isReservedKeyword(symbols[i->name])) {
+                    str << "\"" << symbols[i->name] << "\" = ";
+                } else {
+                    str << symbols[i->name] << " = ";
+                }
                 i->value->print(symbols, str, seen);
                 str << "; ";
             }
diff --git a/src/libexpr/print.cc b/src/libexpr/print.cc
index d08672cfc..53ba70bdd 100644
--- a/src/libexpr/print.cc
+++ b/src/libexpr/print.cc
@@ -1,4 +1,5 @@
 #include "print.hh"
+#include <unordered_set>
 
 namespace nix {
 
@@ -25,11 +26,26 @@ printLiteralBool(std::ostream & str, bool boolean)
     return str;
 }
 
+// Returns `true' is a string is a reserved keyword which requires quotation
+// when printing attribute set field names.
+//
+// This list should generally be kept in sync with `./lexer.l'.
+// You can test if a keyword needs to be added by running:
+//   $ nix eval --expr '{ <KEYWORD> = 1; }'
+// For example `or' doesn't need to be quoted.
+bool isReservedKeyword(const std::string_view str)
+{
+    static const std::unordered_set<std::string_view> reservedKeywords = {
+        "if", "then", "else", "assert", "with", "let", "in", "rec", "inherit"
+    };
+    return reservedKeywords.contains(str);
+}
+
 std::ostream &
 printIdentifier(std::ostream & str, std::string_view s) {
     if (s.empty())
         str << "\"\"";
-    else if (s == "if") // FIXME: handle other keywords
+    else if (isReservedKeyword(s))
         str << '"' << s << '"';
     else {
         char c = s[0];
@@ -50,10 +66,10 @@ printIdentifier(std::ostream & str, std::string_view s) {
     return str;
 }
 
-// FIXME: keywords
 static bool isVarName(std::string_view s)
 {
     if (s.size() == 0) return false;
+    if (isReservedKeyword(s)) return false;
     char c = s[0];
     if ((c >= '0' && c <= '9') || c == '-' || c == '\'') return false;
     for (auto & i : s)
diff --git a/src/libexpr/print.hh b/src/libexpr/print.hh
index f9cfc3964..3b72ae201 100644
--- a/src/libexpr/print.hh
+++ b/src/libexpr/print.hh
@@ -36,6 +36,12 @@ namespace nix {
     std::ostream & printAttributeName(std::ostream & o, std::string_view s);
 
     /**
+     * Returns `true' is a string is a reserved keyword which requires quotation
+     * when printing attribute set field names.
+     */
+    bool isReservedKeyword(const std::string_view str);
+
+    /**
      * Print a string as an identifier in the Nix expression language syntax.
      *
      * FIXME: "identifier" is ambiguous. Identifiers do not have a single