refactor(3p/nix): Make all single-argument constructors explicit

Implicit constructors can be confusing, especially in a codebase that
is already as unintentionally obfuscated as this one.

https://google.github.io/styleguide/cppguide.html#Explicit_Constructors

third_party/nix/src/libstore/build.cc

@@ -133,7 +133,7 @@ class Goal : public std::enable_shared_from_this<Goal> {
   /* Whether the goal is finished. */
   ExitCode exitCode;
 
-  Goal(Worker& worker) : worker(worker) {
+  explicit Goal(Worker& worker) : worker(worker) {
     nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
     exitCode = ecBusy;
   }
@@ -264,7 +264,7 @@ class Worker {
      it answers with "decline-permanently", we don't try again. */
   bool tryBuildHook = true;
 
-  Worker(LocalStore& store);
+  explicit Worker(LocalStore& store);
   ~Worker();
 
   /* Make a goal (with caching). */
@@ -823,7 +823,7 @@ class DerivationGoal : public Goal {
   struct ChrootPath {
     Path source;
     bool optional;
-    ChrootPath(Path source = "", bool optional = false)
+    explicit ChrootPath(Path source = "", bool optional = false)
         : source(source), optional(optional) {}
   };
   typedef map<Path, ChrootPath>
@@ -2037,12 +2037,12 @@ void DerivationGoal::startBuilder() {
       }
       size_t p = i.find('=');
       if (p == string::npos) {
-        dirsInChroot[i] = {i, optional};
+        dirsInChroot[i] = ChrootPath(i, optional);
       } else {
-        dirsInChroot[string(i, 0, p)] = {string(i, p + 1), optional};
+        dirsInChroot[string(i, 0, p)] = ChrootPath(string(i, p + 1), optional);
       }
     }
-    dirsInChroot[tmpDirInSandbox] = tmpDir;
+    dirsInChroot[tmpDirInSandbox] = ChrootPath(tmpDir);
 
     /* Add the closure of store paths to the chroot. */
     PathSet closure;
@@ -2058,7 +2058,7 @@ void DerivationGoal::startBuilder() {
       }
     }
     for (auto& i : closure) {
-      dirsInChroot[i] = i;
+      dirsInChroot[i] = ChrootPath(i);
     }
 
     PathSet allowedPaths = settings.allowedImpureHostPrefixes;
@@ -2088,7 +2088,7 @@ void DerivationGoal::startBuilder() {
                     drvPath % i);
       }
 
-      dirsInChroot[i] = i;
+      dirsInChroot[i] = ChrootPath(i);
     }
 
 #if __linux__
@@ -2170,7 +2170,7 @@ void DerivationGoal::startBuilder() {
         throw SysError(format("getting attributes of path '%1%'") % i);
       }
       if (S_ISDIR(st.st_mode)) {
-        dirsInChroot[i] = r;
+        dirsInChroot[i] = ChrootPath(r);
       } else {
         Path p = chrootRootDir + i;
         DLOG(INFO) << "linking '" << p << "' to '" << r << "'";
@@ -2264,9 +2264,9 @@ void DerivationGoal::startBuilder() {
         } else {
           auto p = line.find('=');
           if (p == string::npos) {
-            dirsInChroot[line] = line;
+            dirsInChroot[line] = ChrootPath(line);
           } else {
-            dirsInChroot[string(line, 0, p)] = string(line, p + 1);
+            dirsInChroot[string(line, 0, p)] = ChrootPath(string(line, p + 1));
           }
         }
       }

third_party/nix/src/libstore/export-import.cc

@@ -9,7 +9,7 @@ namespace nix {
 struct HashAndWriteSink : Sink {
   Sink& writeSink;
   HashSink hashSink;
-  HashAndWriteSink(Sink& writeSink)
+  explicit HashAndWriteSink(Sink& writeSink)
       : writeSink(writeSink), hashSink(htSHA256) {}
   virtual void operator()(const unsigned char* data, size_t len) {
     writeSink(data, len);

third_party/nix/src/libstore/gc.cc

@@ -505,7 +505,7 @@ struct LocalStore::GCState {
   unsigned long long bytesInvalidated;
   bool moveToTrash = true;
   bool shouldDelete;
-  GCState(GCResults& results_) : results(results_), bytesInvalidated(0) {}
+  explicit GCState(GCResults& results_) : results(results_), bytesInvalidated(0) {}
 };
 
 bool LocalStore::isActiveTempFile(const GCState& state, const Path& path,

third_party/nix/src/libstore/local-fs-store.cc

@@ -12,7 +12,7 @@ LocalFSStore::LocalFSStore(const Params& params) : Store(params) {}
 struct LocalStoreAccessor : public FSAccessor {
   ref<LocalFSStore> store;
 
-  LocalStoreAccessor(ref<LocalFSStore> store) : store(store) {}
+  explicit LocalStoreAccessor(ref<LocalFSStore> store) : store(store) {}
 
   Path toRealPath(const Path& path) {
     Path storePath = store->toStorePath(path);

third_party/nix/src/libstore/misc.cc

@@ -151,7 +151,7 @@ void Store::queryMissing(const PathSet& targets, PathSet& willBuild_,
     size_t left;
     bool done = false;
     PathSet outPaths;
-    DrvState(size_t left) : left(left) {}
+    explicit DrvState(size_t left) : left(left) {}
   };
 
   Sync<State> state_(State{PathSet(), unknown_, willSubstitute_, willBuild_,

third_party/nix/src/libstore/nar-accessor.cc

@@ -93,7 +93,7 @@ struct NarAccessor : public FSAccessor {
     }
   };
 
-  NarAccessor(ref<const std::string> nar) : nar(nar) {
+  explicit NarAccessor(ref<const std::string> nar) : nar(nar) {
     NarIndexer indexer(*this, *nar);
     parseDump(indexer, indexer);
   }

third_party/nix/src/libstore/optimise-store.cc

@@ -27,7 +27,7 @@ static void makeWritable(const Path& path) {
 
 struct MakeReadOnly {
   Path path;
-  MakeReadOnly(const Path& path) : path(path) {}
+  explicit MakeReadOnly(const Path& path) : path(path) {}
   ~MakeReadOnly() {
     try {
       /* This will make the path read-only. */

third_party/nix/src/libstore/remote-store.cc

@@ -215,7 +215,7 @@ struct ConnectionHandle {
   Pool<RemoteStore::Connection>::Handle handle;
   bool daemonException = false;
 
-  ConnectionHandle(Pool<RemoteStore::Connection>::Handle&& handle)
+  explicit ConnectionHandle(Pool<RemoteStore::Connection>::Handle&& handle)
       : handle(std::move(handle)) {}
 
   ConnectionHandle(ConnectionHandle&& h) : handle(std::move(h.handle)) {}