Use a struct not std::pair for SearchPathElem

I got very confused trying to keep all the `first` and `second` straight
reading the code, *especially* as there is also another `(boolean,
string)` pair type also being used.

Named fields is much better.

There are other cleanups that we can do (for example, the existing
TODO), but we can do them later. Doing them now would just make this
harder to review.

src/libexpr/eval.hh

@@ -65,8 +65,12 @@ std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
+struct SearchPathElem
+{
+    std::string prefix;
     // FIXME: maybe change this to an std::variant<SourcePath, URL>.
-typedef std::pair<std::string, std::string> SearchPathElem;
+    std::string path;
+};
 typedef std::list<SearchPathElem> SearchPath;
 
 

src/libexpr/parser.y

@@ -741,7 +741,10 @@ void EvalState::addToSearchPath(const std::string & s)
         path = std::string(s, pos + 1);
     }
 
-    searchPath.emplace_back(prefix, path);
+    searchPath.emplace_back(SearchPathElem {
+        .prefix = prefix,
+        .path = path,
+    });
 }
 
 
@@ -755,11 +758,11 @@ SourcePath EvalState::findFile(SearchPath & searchPath, const std::string_view p
 {
     for (auto & i : searchPath) {
         std::string suffix;
-        if (i.first.empty())
+        if (i.prefix.empty())
             suffix = concatStrings("/", path);
         else {
-            auto s = i.first.size();
+            auto s = i.prefix.size();
-            if (path.compare(0, s, i.first) != 0 ||
+            if (path.compare(0, s, i.prefix) != 0 ||
                 (path.size() > s && path[s] != '/'))
                 continue;
             suffix = path.size() == s ? "" : concatStrings("/", path.substr(s));
@@ -785,47 +788,47 @@ SourcePath EvalState::findFile(SearchPath & searchPath, const std::string_view p
 
 std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathElem & elem)
 {
-    auto i = searchPathResolved.find(elem.second);
+    auto i = searchPathResolved.find(elem.path);
     if (i != searchPathResolved.end()) return i->second;
 
     std::pair<bool, std::string> res;
 
-    if (EvalSettings::isPseudoUrl(elem.second)) {
+    if (EvalSettings::isPseudoUrl(elem.path)) {
         try {
             auto storePath = fetchers::downloadTarball(
-                store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).tree.storePath;
+                store, EvalSettings::resolvePseudoUrl(elem.path), "source", false).tree.storePath;
             res = { true, store->toRealPath(storePath) };
         } catch (FileTransferError & e) {
             logWarning({
-                .msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
+                .msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.path)
             });
             res = { false, "" };
         }
     }
 
-    else if (hasPrefix(elem.second, "flake:")) {
+    else if (hasPrefix(elem.path, "flake:")) {
         experimentalFeatureSettings.require(Xp::Flakes);
-        auto flakeRef = parseFlakeRef(elem.second.substr(6), {}, true, false);
+        auto flakeRef = parseFlakeRef(elem.path.substr(6), {}, true, false);
-        debug("fetching flake search path element '%s''", elem.second);
+        debug("fetching flake search path element '%s''", elem.path);
         auto storePath = flakeRef.resolve(store).fetchTree(store).first.storePath;
         res = { true, store->toRealPath(storePath) };
     }
 
     else {
-        auto path = absPath(elem.second);
+        auto path = absPath(elem.path);
         if (pathExists(path))
             res = { true, path };
         else {
             logWarning({
-                .msg = hintfmt("Nix search path entry '%1%' does not exist, ignoring", elem.second)
+                .msg = hintfmt("Nix search path entry '%1%' does not exist, ignoring", elem.path)
             });
             res = { false, "" };
         }
     }
 
-    debug("resolved search path element '%s' to '%s'", elem.second, res.second);
+    debug("resolved search path element '%s' to '%s'", elem.path, res.second);
 
-    searchPathResolved[elem.second] = res;
+    searchPathResolved[elem.path] = res;
     return res;
 }
 

src/libexpr/primops.cc

@@ -1656,7 +1656,10 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
             }));
         }
 
-        searchPath.emplace_back(prefix, path);
+        searchPath.emplace_back(SearchPathElem {
+            .prefix = prefix,
+            .path = path,
+        });
     }
 
     auto path = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.findFile");
@@ -4129,8 +4132,8 @@ void EvalState::createBaseEnv()
     int n = 0;
     for (auto & i : searchPath) {
         auto attrs = buildBindings(2);
-        attrs.alloc("path").mkString(i.second);
+        attrs.alloc("path").mkString(i.path);
-        attrs.alloc("prefix").mkString(i.first);
+        attrs.alloc("prefix").mkString(i.prefix);
         (v.listElems()[n++] = allocValue())->mkAttrs(attrs);
     }
     addConstant("__nixPath", v);