treewide: Remove toView() because it leads to segfaults when compiled with newer nixpkgs

Firstly, this is now available on darwin where the default in llvm 19.
Secondly, this leads to very weird segfaults when building with newer nixpkgs for some reason.
(It's UB after all).

This appears when building with the following:

mesonComponentOverrides = finalAttrs: prevAttrs: {
mesonBuildType = "debugoptimized";
dontStrip = true;
doCheck = false;
separateDebugInfo = false;
preConfigure = (prevAttrs.preConfigure or "") + ''
  case "$mesonBuildType" in
  release|minsize|debugoptimized) appendToVar mesonFlags "-Db_lto=true"  ;;
  *)                              appendToVar mesonFlags "-Db_lto=false" ;;
  esac
'';
};

And with the following nixpkgs input:

nix build ".#nix-cli" -L --override-input nixpkgs "https://releases.nixos.org/nixos/unstable/nixos-25.11pre870157.7df7ff7d8e00/nixexprs.tar.xz"

Stacktrace:

 #0  0x00000000006afdc0 in ?? ()
 #1  0x00007ffff71cebb6 in _Unwind_ForcedUnwind_Phase2 () from /nix/store/41ym1jm1b7j3rhglk82gwg9jml26z1km-gcc-14.3.0-lib/lib/libgcc_s.so.1
 #2  0x00007ffff71cf5b5 in _Unwind_Resume () from /nix/store/41ym1jm1b7j3rhglk82gwg9jml26z1km-gcc-14.3.0-lib/lib/libgcc_s.so.1
 #3  0x00007ffff7eac7d8 in std::basic_ios<char, std::char_traits<char> >::~basic_ios (this=<optimized out>, this=<optimized out>)
     at /nix/store/82kmz7r96navanrc2fgckh2bamiqrgsw-gcc-14.3.0/include/c++/14.3.0/bits/basic_ios.h:286
 #4  std::__cxx11::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >::basic_ostringstream (this=<optimized out>, this=<optimized out>)
     at /nix/store/82kmz7r96navanrc2fgckh2bamiqrgsw-gcc-14.3.0/include/c++/14.3.0/sstream:806
 #5  nix::SimpleLogger::logEI (this=<optimized out>, ei=...) at ../logging.cc:121
 #6  0x00007ffff7515794 in nix::Logger::logEI (this=0x675450, lvl=nix::lvlError, ei=...) at /nix/store/bkshji3nnxmrmgwa4n2kaxadajkwvn65-nix-util-2.32.0pre-dev/include/nix/util/logging.hh:144
 #7  nix::handleExceptions (programName=..., fun=...) at ../shared.cc:336
 #8  0x000000000047b76b in main (argc=<optimized out>, argv=<optimized out>) at /nix/store/82kmz7r96navanrc2fgckh2bamiqrgsw-gcc-14.3.0/include/c++/14.3.0/bits/new_allocator.h:88

src/libcmd/repl.cc

@@ -669,7 +669,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 ss << "No documentation found.\n\n";
             }
 
-            auto markdown = toView(ss);
+            auto markdown = ss.view();
             logger->cout(trim(renderMarkdownToTerminal(markdown)));
 
         } else

src/libexpr/eval.cc

@@ -591,7 +591,7 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
             .name = name,
             .arity = 0, // FIXME: figure out how deep by syntax only? It's not semantically useful though...
             .args = {},
-            .doc = makeImmutableString(toView(s)), // NOTE: memory leak when compiled without GC
+            .doc = makeImmutableString(s.view()), // NOTE: memory leak when compiled without GC
         };
     }
     if (isFunctor(v)) {
@@ -1811,7 +1811,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        auto exprStr = toView(out);
+        auto exprStr = out.view();
 
         if (auto eq = dynamic_cast<ExprOpEq *>(cond)) {
             try {

src/libexpr/primops.cc

@@ -2412,7 +2412,7 @@ static void prim_toXML(EvalState & state, const PosIdx pos, Value ** args, Value
     std::ostringstream out;
     NixStringContext context;
     printValueAsXML(state, true, false, *args[0], out, context, pos);
-    v.mkString(toView(out), context);
+    v.mkString(out.view(), context);
 }
 
 static RegisterPrimOp primop_toXML({
@@ -2520,7 +2520,7 @@ static void prim_toJSON(EvalState & state, const PosIdx pos, Value ** args, Valu
     std::ostringstream out;
     NixStringContext context;
     printValueAsJSON(state, true, *args[0], pos, out, context);
-    v.mkString(toView(out), context);
+    v.mkString(out.view(), context);
 }
 
 static RegisterPrimOp primop_toJSON({

src/libexpr/primops/fromTOML.cc

@@ -139,7 +139,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value ** args, Va
                 attrs.alloc("_type").mkStringNoCopy("timestamp");
                 std::ostringstream s;
                 s << t;
-                auto str = toView(s);
+                auto str = s.view();
                 forceNoNullByte(str);
                 attrs.alloc("value").mkString(str);
                 v.mkAttrs(attrs);

src/libexpr/print.cc

@@ -461,7 +461,7 @@ private:
 
                 std::ostringstream s;
                 s << state.positions[v.lambda().fun->pos];
-                output << " @ " << filterANSIEscapes(toView(s));
+                output << " @ " << filterANSIEscapes(s.view());
             }
         } else if (v.isPrimOp()) {
             if (v.primOp())

src/libmain/progress-bar.cc

@@ -183,7 +183,7 @@ public:
         std::ostringstream oss;
         showErrorInfo(oss, ei, loggerSettings.showTrace.get());
 
-        log(*state, ei.level, toView(oss));
+        log(*state, ei.level, oss.view());
     }
 
     void log(State & state, Verbosity lvl, std::string_view s)

src/libstore/daemon.cc

@@ -102,7 +102,7 @@ struct TunnelLogger : public Logger
         showErrorInfo(oss, ei, false);
 
         StringSink buf;
-        buf << STDERR_NEXT << toView(oss);
+        buf << STDERR_NEXT << oss.view();
         enqueueMsg(buf.s);
     }
 

src/libutil/include/nix/util/strings.hh

@@ -12,11 +12,6 @@
 
 namespace nix {
 
-/*
- * workaround for unavailable view() method (C++20) of std::ostringstream under MacOS with clang-16
- */
-std::string_view toView(const std::ostringstream & os);
-
 /**
  * String tokenizer.
  *

src/libutil/logging.cc

@@ -121,7 +121,7 @@ public:
         std::ostringstream oss;
         showErrorInfo(oss, ei, loggerSettings.showTrace.get());
 
-        log(ei.level, toView(oss));
+        log(ei.level, oss.view());
     }
 
     void startActivity(

src/libutil/strings.cc

@@ -8,23 +8,6 @@
 
 namespace nix {
 
-struct view_stringbuf : public std::stringbuf
-{
-    inline std::string_view toView()
-    {
-        auto begin = pbase();
-        return {begin, begin + pubseekoff(0, std::ios_base::cur, std::ios_base::out)};
-    }
-};
-
-__attribute__((no_sanitize("undefined"))) std::string_view toView(const std::ostringstream & os)
-{
-    /* Downcasting like this is very much undefined behavior, so we disable
-       UBSAN for this function. */
-    auto buf = static_cast<view_stringbuf *>(os.rdbuf());
-    return buf->toView();
-}
-
 template std::list<std::string> tokenizeString(std::string_view s, std::string_view separators);
 template StringSet tokenizeString(std::string_view s, std::string_view separators);
 template std::vector<std::string> tokenizeString(std::string_view s, std::string_view separators);

src/nix/config-check.cc

@@ -100,7 +100,7 @@ struct CmdConfigCheck : StoreCommand
             ss << "Multiple versions of nix found in PATH:\n";
             for (auto & dir : dirs)
                 ss << "  " << dir << "\n";
-            return checkFail(toView(ss));
+            return checkFail(ss.view());
         }
 
         return checkPass("PATH contains only one nix version.");
@@ -143,7 +143,7 @@ struct CmdConfigCheck : StoreCommand
             for (auto & dir : dirs)
                 ss << "  " << dir << "\n";
             ss << "\n";
-            return checkFail(toView(ss));
+            return checkFail(ss.view());
         }
 
         return checkPass("All profiles are gcroots.");
@@ -162,7 +162,7 @@ struct CmdConfigCheck : StoreCommand
                << "sync with the daemon.\n\n"
                << "Client protocol: " << formatProtocol(clientProto) << "\n"
                << "Store protocol: " << formatProtocol(storeProto) << "\n\n";
-            return checkFail(toView(ss));
+            return checkFail(ss.view());
         }
 
         return checkPass("Client protocol matches store protocol.");

src/nix/nix-build/nix-build.cc

@@ -285,10 +285,10 @@ static void main_nix_build(int argc, char ** argv)
                         execArgs,
                         interpreter,
                         escapeShellArgAlways(script),
-                        toView(joined));
+                        joined.view());
             } else {
                 envCommand =
-                    fmt("exec %1% %2% %3% %4%", execArgs, interpreter, escapeShellArgAlways(script), toView(joined));
+                    fmt("exec %1% %2% %3% %4%", execArgs, interpreter, escapeShellArgAlways(script), joined.view());
             }
         }
 

src/nix/nix-env/user-env.cc

@@ -108,7 +108,7 @@ bool createUserEnv(
     auto manifestFile = ({
         std::ostringstream str;
         printAmbiguous(manifest, state.symbols, str, nullptr, std::numeric_limits<int>::max());
-        StringSource source{toView(str)};
+        StringSource source{str.view()};
         state.store->addToStoreFromDump(
             source,
             "env-manifest.nix",