[native] Remove the libc++ dependencies from the CoreCLR host's startup path

[simonrozsival]

Part of #12533 (drop libc++ from the CoreCLR host). Stacked on top of #12568.

This clears host.cc entirely. The CoreCLR host goes from 21 undefined libc++ references to 12, and every one that remains is now in a single file, assembly-store.cc.

The three uses

1. The assembly store path

scan_filesystem_for_assemblies_and_libraries joined the native library directory and the store filename with a std::string. It now uses Util::join_paths, added earlier in this stack, which keeps the path in a stack buffer unless it does not fit and frees only when the returned pointer differs from that buffer.

2. APP_CONTEXT_BASE_DIRECTORY

static std::string app_context_base_directory;

The static is needed because the value has to outlive coreclr_initialize — CoreCLR keeps the pointer. But std::string has a non-trivial constructor, so a function-local static of that type forces a guard variable, and this was the only source of __cxa_guard_acquire/__cxa_guard_release in the file. A plain char* is constant-initialized, so the guard pair disappears along with the string.

3. The FastDev TPA list

static std::string fastdev_tpa_list;
static std::vector<const char*> fastdev_prop_names;
static std::vector<const char*> fastdev_prop_values;

This block is inside if constexpr (Constants::is_debug_build), so it contributes nothing to the Release numbers below — but it has to go before libc++ can be dropped from Debug builds, which is why it is included here rather than left for later. The two property arrays are a fixed prop_count + 1 entries, so they become calloced arrays.

build_tpa_list now returns the string

It previously filled a std::string& out-parameter and returned bool; it now returns a malloced string the caller owns, or nullptr.

The TPA list is the one place in this PR with no useful upper bound — it holds one absolute path per assembly in the override directory — so a fixed buffer is not appropriate. It is accumulated through a small growable buffer that doubles on demand. Allocation failure falls back to the probe-only path rather than aborting, since FastDev is a debug convenience and must not take the application down.

open_assembly

new uint8_t[]/delete[] become malloc/free. That buffer is handed to CoreCLR and never freed — see the existing TODO above it about ownership — so this is not a behavioural change.

It also gains a null check. That is a small bug fix: new uint8_t[] would have thrown std::bad_alloc and aborted, whereas the new path logs, closes the file descriptor and returns nullptr. The previous code would have leaked asm_fd had it ever returned rather than aborted.

Header cleanup

<string> and <vector> are no longer included by host.cc, and <string> is gone from fastdev-assemblies.hh — which is where host.cc was picking it up from in the first place.

Results

Undefined libc++ references in the CoreCLR host, Release:

object	before	after
host.cc.o	9	0
assembly-store.cc.o	12	12
total	21	12

The nine removed from host.cc.o: __cxa_guard_acquire, __cxa_guard_release, operator new, operator delete, std::string::append, std::string::push_back, std::string::__grow_by_and_replace, ~basic_string and __libcpp_verbose_abort.

	before	after	delta
libnet-android.release.so	523,224	520,112	−3,112 B

Verification

• CoreCLR, MonoVM and NativeAOT all build clean.
• Debug was verified explicitly. fastdev-assemblies.cc is only added to the build under if(DEBUG_BUILD) (src/native/clr/host/CMakeLists.txt), so a Release build never compiles it and would not have caught a mistake in the FastDev work. There is no Debug CoreCLR build configured locally, so both host.cc and fastdev-assemblies.cc were compiled standalone with -DDEBUG -DDEBUG_BUILD using the exact flags from compile_commands.json. Both compile clean and llvm-nm --undefined-only reports 0 libc++ references for each.

Not covered

Only android-arm64 was built locally; the other ABIs rely on CI. The FastDev path is Debug-only and was not exercised on a device as part of this change.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 2

New issues introduced by this change (2)

Severity	Finding
	src/​native/​clr/​host/​fastdev-assemblies.cc — ERROR: FastDevAssemblies::tpa_in_use is set to true on success but never reset on failure. If…
	src/​native/​clr/​host/​host.cc — WARNING: This Debug-only FastDev path aborts the process if allocating the extended runtime…

What changed in this PR

This PR refactors the CoreCLR native host FastDev/initialization code to remove dependencies on libc++/STL types (e.g., std::string, std::vector) by switching to C allocation patterns and existing in-tree path helpers.

Changes:

• Change FastDev TPA list construction to return a malloc-owned char* instead of filling a std::string.
• Replace STL-based path/property list handling in host.cc with Util::join_paths, malloc/calloc, and explicit lifetime management.
• Update FastDev assembly loading buffer allocation to use malloc/free instead of new[]/delete[].

File	Description
src/​native/​clr/​include/​host/​fastdev-assemblies.hh	Updates the FastDev API to return an owned char* TPA list (no STL types).
src/​native/​clr/​host/​host.cc	Reworks CoreCLR init property handling and path building to avoid STL and manage memory explicitly.
src/​native/​clr/​host/​fastdev-assemblies.cc	Implements a growable C buffer for TPA list building and switches FastDev file buffer allocation to malloc.