Unify UTF-8 handling using til::u8u16 & revise WriteConsoleAImpl (#4422)

Replace `utf8Parser` with `til::u8u16` in order to have the same
conversion algorithms used in terminal and conhost.

This PR addresses item 2 in this list:
1. ✉ Implement `til::u8u16` and `til::u16u8` (done in PR #4093)
2. ✔ **Unify UTF-8 handling using `til::u8u16` (this PR)**
    2.1. ✔ **Update VtInputThread::_HandleRunInput()**
    2.2. ✔ **Update ApiRoutines::WriteConsoleAImpl()**
    2.3. ❌ (optional / ask the core team) Remove Utf8ToWideCharParser from the code base to avoid further use
3. ❌ Enable BOM discarding (follow up)
    3.1. ❌ extend `til::u8u16` and `til::u16u8` with a 3rd parameter to enable discarding the BOM
    3.2. ❌ Make use of the 3rd parameter to discard the BOM in all current function callers, or (optional / ask the core team) make it the default for  `til::u8u16` and `til::u16u8` 
4. ❌ Find UTF-16 to UTF-8 conversions and examine if they can be unified, too (follow up)

Closes #4086
Closes #3378

src/host/VtInputThread.cpp

@@ -28,7 +28,7 @@ VtInputThread::VtInputThread(_In_ wil::unique_hfile hPipe,
                              const bool inheritCursor) :
     _hFile{ std::move(hPipe) },
     _hThread{},
-    _utf8Parser{ CP_UTF8 },
+    _u8State{},
     _dwThreadId{ 0 },
     _exitRequested{ false },
     _exitResult{ S_OK }
@@ -47,15 +47,14 @@ VtInputThread::VtInputThread(_In_ wil::unique_hfile hPipe,
 }
 
 // Method Description:
-// - Processes a buffer of input characters. The characters should be utf-8
-//      encoded, and will get converted to wchar_t's to be processed by the
+// - Processes a string of input characters. The characters should be UTF-8
+//      encoded, and will get converted to wstring to be processed by the
 //      input state machine.
 // Arguments:
-// - charBuffer - the UTF-8 characters recieved.
-// - cch - number of UTF-8 characters in charBuffer
+// - u8Str - the UTF-8 string received.
 // Return Value:
 // - S_OK on success, otherwise an appropriate failure.
-[[nodiscard]] HRESULT VtInputThread::_HandleRunInput(_In_reads_(cch) const byte* const charBuffer, const int cch)
+[[nodiscard]] HRESULT VtInputThread::_HandleRunInput(const std::string_view u8Str)
 {
     // Make sure to call the GLOBAL Lock/Unlock, not the gci's lock/unlock.
     // Only the global unlock attempts to dispatch ctrl events. If you use the
@@ -67,16 +66,14 @@ VtInputThread::VtInputThread(_In_ wil::unique_hfile hPipe,
 
     try
     {
-        std::unique_ptr<wchar_t[]> pwsSequence;
-        unsigned int cchConsumed;
-        unsigned int cchSequence;
-        auto hr = _utf8Parser.Parse(charBuffer, cch, cchConsumed, pwsSequence, cchSequence);
+        std::wstring wstr{};
+        auto hr = til::u8u16(u8Str, wstr, _u8State);
         // If we hit a parsing error, eat it. It's bad utf-8, we can't do anything with it.
         if (FAILED(hr))
         {
             return S_FALSE;
         }
-        _pInputStateMachine->ProcessString({ pwsSequence.get(), cchSequence });
+        _pInputStateMachine->ProcessString(wstr);
     }
     CATCH_RETURN();
 
@@ -100,12 +97,12 @@ DWORD WINAPI VtInputThread::StaticVtInputThreadProc(_In_ LPVOID lpParameter)
 //      failed, throw or log, depending on what the caller wants.
 // Arguments:
 // - throwOnFail: If true, throw an exception if there was an error processing
-//      the input recieved. Otherwise, log the error.
+//      the input received. Otherwise, log the error.
 // Return Value:
 // - <none>
 void VtInputThread::DoReadInput(const bool throwOnFail)
 {
-    byte buffer[256];
+    char buffer[256];
     DWORD dwRead = 0;
     bool fSuccess = !!ReadFile(_hFile.get(), buffer, ARRAYSIZE(buffer), &dwRead, nullptr);
 
@@ -120,7 +117,7 @@ void VtInputThread::DoReadInput(const bool throwOnFail)
         return;
     }
 
-    HRESULT hr = _HandleRunInput(buffer, dwRead);
+    HRESULT hr = _HandleRunInput({ buffer, gsl::narrow_cast<size_t>(dwRead) });
     if (FAILED(hr))
     {
         if (throwOnFail)

src/host/VtInputThread.hpp

@@ -15,7 +15,6 @@ Author(s):
 #pragma once
 
 #include "..\terminal\parser\StateMachine.hpp"
-#include "utf8ToWideCharParser.hpp"
 
 namespace Microsoft::Console
 {
@@ -29,7 +28,7 @@ namespace Microsoft::Console
         void DoReadInput(const bool throwOnFail);
 
     private:
-        [[nodiscard]] HRESULT _HandleRunInput(_In_reads_(cch) const byte* const charBuffer, const int cch);
+        [[nodiscard]] HRESULT _HandleRunInput(const std::string_view u8Str);
         DWORD _InputThread();
 
         wil::unique_hfile _hFile;
@@ -40,6 +39,6 @@ namespace Microsoft::Console
         HRESULT _exitResult;
 
         std::unique_ptr<Microsoft::Console::VirtualTerminal::StateMachine> _pInputStateMachine;
-        Utf8ToWideCharParser _utf8Parser;
+        til::u8state _u8State;
     };
 }