Inline only-used-once closures in ExprConcatStrings::eval

Refactor `ExprConcatStrings::eval` by inlining two only-called-once
closures into the call-site, so that the code is easier to reason about
locally (especially since the variables that were closed over were
mutated all over the place within this function).

Also use curly braces with each branch for consistency in the the
resulting code.

This is a pure refactor, but also arguably causes us to depend less on
the optimizer; now, we don't have to make sure that this closure is
inlined.

Author: glittershark

src/libexpr/eval.cc

@@ -2029,27 +2029,6 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     bool first = !forceString;
     ValueType firstType = nString;
 
-    const auto str = [&] {
-        std::string result;
-        result.reserve(sSize);
-        for (const auto & part : strings)
-            result += *part;
-        return result;
-    };
-    /* c_str() is not str().c_str() because we want to create a string
-       Value. allocating a GC'd string directly and moving it into a
-       Value lets us avoid an allocation and copy. */
-    const auto c_str = [&] {
-        char * result = allocString(sSize + 1);
-        char * tmp = result;
-        for (const auto & part : strings) {
-            memcpy(tmp, part->data(), part->size());
-            tmp += part->size();
-        }
-        *tmp = 0;
-        return result;
-    };
-
     // List of returned strings. References to these Values must NOT be persisted.
     SmallTemporaryValueVector<conservativeStackReservation> values(es.size());
     Value * vTmpP = values.data();
@@ -2111,19 +2090,32 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         first = false;
     }
 
-    if (firstType == nInt)
+    if (firstType == nInt) {
         v.mkInt(n);
-    else if (firstType == nFloat)
+    } else if (firstType == nFloat) {
         v.mkFloat(nf);
-    else if (firstType == nPath) {
+    } else if (firstType == nPath) {
         if (!context.empty())
             state.error<EvalError>("a string that refers to a store path cannot be appended to a path")
                 .atPos(pos)
                 .withFrame(env, *this)
                 .debugThrow();
-        v.mkPath(state.rootPath(CanonPath(str())));
-    } else
-        v.mkStringMove(c_str(), context);
+        std::string result_str;
+        result_str.reserve(sSize);
+        for (const auto & part : strings) {
+            result_str += *part;
+        }
+        v.mkPath(state.rootPath(CanonPath(result_str)));
+    } else {
+        char * result_str = allocString(sSize + 1);
+        char * tmp = result_str;
+        for (const auto & part : strings) {
+            memcpy(tmp, part->data(), part->size());
+            tmp += part->size();
+        }
+        *tmp = 0;
+        v.mkStringMove(result_str, context);
+    }
 }
 
 void ExprPos::eval(EvalState & state, Env & env, Value & v)