don't use Symbols for strings

string expressions by and large do not need the benefits a Symbol gives us,
instead they pollute the symbol table and cause unnecessary overhead for almost
all strings. the one place we can think of that benefits from them (attrpaths
with expressions) extracts the benefit in the parser, which we'll have to touch
anyway when changing ExprString to hold strings.

this gives a sizeable improvement on of 3-5% on all benchmarks we've run.

 before

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.844 s ±  0.045 s    [User: 6.750 s, System: 1.663 s]
    Range (min … max):    8.758 s …  8.922 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     367.4 ms ±   3.3 ms    [User: 332.3 ms, System: 35.2 ms]
    Range (min … max):   364.0 ms … 375.2 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.810 s ±  0.030 s    [User: 2.517 s, System: 0.225 s]
    Range (min … max):    2.742 s …  2.854 s    20 runs

 after

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.533 s ±  0.068 s    [User: 6.485 s, System: 1.642 s]
    Range (min … max):    8.404 s …  8.657 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     347.6 ms ±   3.1 ms    [User: 313.1 ms, System: 34.5 ms]
    Range (min … max):   343.3 ms … 354.6 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.709 s ±  0.032 s    [User: 2.414 s, System: 0.232 s]
    Range (min … max):    2.655 s …  2.788 s    20 runs

src/libexpr/nixexpr.hh

@@ -110,9 +110,9 @@ struct ExprFloat : Expr
 
 struct ExprString : Expr
 {
-    Symbol s;
+    string s;
     Value v;
-    ExprString(const Symbol & s) : s(s) { v.mkString(s); };
+    ExprString(std::string s) : s(std::move(s)) { v.mkString(this->s.data()); };
     COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
 };

src/libexpr/parser.y

@@ -166,7 +166,7 @@ static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
 static Expr * stripIndentation(const Pos & pos, SymbolTable & symbols,
     vector<std::pair<Pos, std::variant<Expr *, StringToken> > > & es)
 {
-    if (es.empty()) return new ExprString(symbols.create(""));
+    if (es.empty()) return new ExprString("");
 
     /* Figure out the minimum indentation.  Note that by design
        whitespace-only final lines are not taken into account.  (So
@@ -244,7 +244,7 @@ static Expr * stripIndentation(const Pos & pos, SymbolTable & symbols,
                 s2 = string(s2, 0, p + 1);
         }
 
-        es2->emplace_back(i->first, new ExprString(symbols.create(s2)));
+        es2->emplace_back(i->first, new ExprString(s2));
     };
     for (; i != es.end(); ++i, --n) {
         std::visit(overloaded { trimExpr, trimString }, i->second);
@@ -434,7 +434,7 @@ expr_simple
       $$ = new ExprCall(CUR_POS,
           new ExprVar(data->symbols.create("__findFile")),
           {new ExprVar(data->symbols.create("__nixPath")),
-           new ExprString(data->symbols.create(path))});
+           new ExprString(path)});
   }
   | URI {
       static bool noURLLiterals = settings.isExperimentalFeatureEnabled(Xp::NoUrlLiterals);
@@ -443,7 +443,7 @@ expr_simple
               .msg = hintfmt("URL literals are disabled"),
               .errPos = CUR_POS
           });
-      $$ = new ExprString(data->symbols.create($1));
+      $$ = new ExprString(string($1));
   }
   | '(' expr ')' { $$ = $2; }
   /* Let expressions `let {..., body = ...}' are just desugared
@@ -458,19 +458,19 @@ expr_simple
   ;
 
 string_parts
-  : STR { $$ = new ExprString(data->symbols.create($1)); }
+  : STR { $$ = new ExprString(string($1)); }
   | string_parts_interpolated { $$ = new ExprConcatStrings(CUR_POS, true, $1); }
-  | { $$ = new ExprString(data->symbols.create("")); }
+  | { $$ = new ExprString(""); }
   ;
 
 string_parts_interpolated
   : string_parts_interpolated STR
-  { $$ = $1; $1->emplace_back(makeCurPos(@2, data), new ExprString(data->symbols.create($2))); }
+  { $$ = $1; $1->emplace_back(makeCurPos(@2, data), new ExprString(string($2))); }
   | string_parts_interpolated DOLLAR_CURLY expr '}' { $$ = $1; $1->emplace_back(makeCurPos(@2, data), $3); }
   | DOLLAR_CURLY expr '}' { $$ = new vector<std::pair<Pos, Expr *> >; $$->emplace_back(makeCurPos(@1, data), $2); }
   | STR DOLLAR_CURLY expr '}' {
       $$ = new vector<std::pair<Pos, Expr *> >;
-      $$->emplace_back(makeCurPos(@1, data), new ExprString(data->symbols.create($1)));
+      $$->emplace_back(makeCurPos(@1, data), new ExprString(string($1)));
       $$->emplace_back(makeCurPos(@2, data), $3);
     }
   ;
@@ -524,7 +524,7 @@ attrs
     { $$ = $1;
       ExprString * str = dynamic_cast<ExprString *>($2);
       if (str) {
-          $$->push_back(AttrName(str->s));
+          $$->push_back(AttrName(data->symbols.create(str->s)));
           delete str;
       } else
           throw ParseError({
@@ -541,7 +541,7 @@ attrpath
     { $$ = $1;
       ExprString * str = dynamic_cast<ExprString *>($3);
       if (str) {
-          $$->push_back(AttrName(str->s));
+          $$->push_back(AttrName(data->symbols.create(str->s)));
           delete str;
       } else
           $$->push_back(AttrName($3));
@@ -551,7 +551,7 @@ attrpath
     { $$ = new vector<AttrName>;
       ExprString *str = dynamic_cast<ExprString *>($1);
       if (str) {
-          $$->push_back(AttrName(str->s));
+          $$->push_back(AttrName(data->symbols.create(str->s)));
           delete str;
       } else
           $$->push_back(AttrName($1));