Re-introduce mkStringNoCopy (revised)

In b70d22b `mkStringNoCopy()` was renamed to
`mkString()`, but this is a bit risky since in code like

    vStringRegular.mkString("regular");

we want to be sure that the right overload is picked. (This is
especially problematic since the overload that takes an
`std::string_view` *does* allocate.)  So let's be explicit.

(Rebased from https://github.com/NixOS/nix/pull/11551)

src/libexpr/eval.cc

@@ -292,10 +292,10 @@ EvalState::EvalState(
     vNull.mkNull();
     vTrue.mkBool(true);
     vFalse.mkBool(false);
-    vStringRegular.mkString("regular");
-    vStringDirectory.mkString("directory");
-    vStringSymlink.mkString("symlink");
-    vStringUnknown.mkString("unknown");
+    vStringRegular.mkStringNoCopy("regular");
+    vStringDirectory.mkStringNoCopy("directory");
+    vStringSymlink.mkStringNoCopy("symlink");
+    vStringUnknown.mkStringNoCopy("unknown");
 
     /* Construct the Nix expression search path. */
     assert(lookupPath.elements.empty());
@@ -824,7 +824,7 @@ DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
 
 void Value::mkString(std::string_view s)
 {
-    mkString(makeImmutableString(s));
+    mkStringNoCopy(makeImmutableString(s));
 }
 
 static const char ** encodeContext(const NixStringContext & context)
@@ -843,12 +843,12 @@ static const char ** encodeContext(const NixStringContext & context)
 
 void Value::mkString(std::string_view s, const NixStringContext & context)
 {
-    mkString(makeImmutableString(s), encodeContext(context));
+    mkStringNoCopy(makeImmutableString(s), encodeContext(context));
 }
 
 void Value::mkStringMove(const char * s, const NixStringContext & context)
 {
-    mkString(s, encodeContext(context));
+    mkStringNoCopy(s, encodeContext(context));
 }
 
 void Value::mkPath(const SourcePath & path)

src/libexpr/include/nix/expr/nixexpr.hh

@@ -158,7 +158,7 @@ struct ExprString : Expr
     ExprString(std::string && s)
         : s(std::move(s))
     {
-        v.mkString(this->s.data());
+        v.mkStringNoCopy(this->s.data());
     };
 
     Value * maybeThunk(EvalState & state, Env & env) override;

src/libexpr/include/nix/expr/symbol-table.hh

@@ -113,12 +113,12 @@ public:
         // for multi-threaded implementations: lock store and allocator here
         const auto & [v, idx] = key.store.add(SymbolValue{});
         if (size == 0) {
-            v.mkString("", nullptr);
+            v.mkStringNoCopy("", nullptr);
         } else {
             auto s = key.alloc.allocate(size + 1);
             memcpy(s, key.s.data(), size);
             s[size] = '\0';
-            v.mkString(s, nullptr);
+            v.mkStringNoCopy(s, nullptr);
         }
         v.size_ = size;
         v.idx = idx;

src/libexpr/include/nix/expr/value.hh

@@ -960,7 +960,7 @@ public:
         setStorage(b);
     }
 
-    inline void mkString(const char * s, const char ** context = 0) noexcept
+    void mkStringNoCopy(const char * s, const char ** context = 0) noexcept
     {
         setStorage(StringWithContext{.c_str = s, .context = context});
     }

src/libexpr/primops.cc

@@ -4349,7 +4349,7 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value ** args, V
     if (len == 0) {
         state.forceValue(*args[2], pos);
         if (args[2]->type() == nString) {
-            v.mkString("", args[2]->context());
+            v.mkStringNoCopy("", args[2]->context());
             return;
         }
     }

src/libexpr/primops/fromTOML.cc

@@ -136,7 +136,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value ** args, Va
                 normalizeDatetimeFormat(t);
 #endif
                 auto attrs = state.buildBindings(2);
-                attrs.alloc("_type").mkString("timestamp");
+                attrs.alloc("_type").mkStringNoCopy("timestamp");
                 std::ostringstream s;
                 s << t;
                 auto str = toView(s);