There's no "safe" way to use Freetype

[ruby3141]

This issue contains long trivia about how I found the reason why it didn't work.
I was dead confused while solving this issue, so I elongated the explanation hoping to save some tortured souls.

If you just want to know how to enable Freetype right now, go to the end of problem description.

Is your feature request related to a problem? Please describe.

I tried using a new font (NanumSquare Neo, for those who wants to know), but it looks blurish and ugly.
So I enabled Freetype feature of imgui (and imgui-sys manually to use vcpkg),

imgui-sys = { version = "0.11.0", features = ["freetype", "use-vcpkg"] }
imgui = { version = "0.11.0", features = ["freetype"] }

but found no difference. The two result was pixel perfect identical.

Found this line on Imgui Freetype readme, so I confired that Freetype isn't activated, by changing oversampling settings.

imgui-rs/imgui-sys/third-party/imgui-master-freetype/imgui/misc/freetype/README.md

Lines 23 to 25 in cd97e17

	### Known issues
	- Oversampling settins are ignored but also not so much necessary with the higher quality rendering.

After many many tries of changing, rebuilding, trying, I digged into imgui-sys, insert some test codes on cpp files,
and finally found out IMGUI_ENABLE_FREETYPE is not defined when imgui main parts are compiling.

build.rs said IMGUI_ENABLE_FREETYPE sets with CIMGUI_FREETYPE, and it was.

imgui-rs/imgui-sys/build.rs

Lines 84 to 86 in cd97e17

	// Set flag for dear imgui
	build.define("CIMGUI_FREETYPE", None); // Sets IMGUI_ENABLE_FREETYPE
	println!("cargo:DEFINE_IMGUI_ENABLE_FREETYPE=");

imgui-rs/imgui-sys/third-party/imgui-master-freetype/cimgui.cpp

Lines 4 to 17 in cd97e17

	#define IMGUI_ENABLE_FREETYPE
	#ifdef IMGUI_ENABLE_FREETYPE
	#ifndef CIMGUI_FREETYPE
	#error "IMGUI_FREETYPE should be defined for Freetype linking"
	#endif
	#else
	#ifdef CIMGUI_FREETYPE
	#error "IMGUI_FREETYPE should not be defined without freetype generated cimgui"
	#endif
	#endif
	#include "./imgui/imgui.h"
	#ifdef IMGUI_ENABLE_FREETYPE
	#include "./imgui/misc/freetype/imgui_freetype.h"
	#endif

But cimgui.cpp compiles after Imgui things. It has no effect while building Imgui main parts.

imgui-rs/imgui-sys/include_imgui_master_freetype.cpp

Lines 1 to 12 in cd97e17

	// This improves build speed by only compiling a single file, and performance by
	// allowing the optimizer to inline across separate object files (note that even
	// when rust is built with LTO, unless the steps are taken to allow cross-lang
	// LTO (tricky), the C/C++ code won't be LTOed).
	#include "./third-party/imgui-master-freetype/imgui/imgui.cpp"
	#include "./third-party/imgui-master-freetype/imgui/imgui_demo.cpp"
	#include "./third-party/imgui-master-freetype/imgui/imgui_draw.cpp"
	#include "./third-party/imgui-master-freetype/imgui/imgui_widgets.cpp"
	#include "./third-party/imgui-master-freetype/imgui/imgui_tables.cpp"
	#include "./third-party/imgui-master-freetype/cimgui.cpp"
	#include "./third-party/imgui-master-freetype/imgui/misc/freetype/imgui_freetype.cpp"

If IMGUI_ENABLE_FREETYPE is not set during compilation of Imgui, imstb is included and used as default.

imgui-rs/imgui-sys/third-party/imgui-master-freetype/imgui/imgui_internal.h

Lines 106 to 108 in cd97e17

	#ifndef IMGUI_ENABLE_FREETYPE
	#define IMGUI_ENABLE_STB_TRUETYPE
	#endif

imgui-rs/imgui-sys/third-party/imgui-master-freetype/imgui/imgui_draw.cpp

Lines 2268 to 2271 in cd97e17

	#ifdef IMGUI_ENABLE_FREETYPE
	builder_io = ImGuiFreeType::GetBuilderForFreeType();
	#elif defined(IMGUI_ENABLE_STB_TRUETYPE)
	builder_io = ImFontAtlasGetBuilderForStbTruetype();

Since Freetype builder is built and included but not set as default,
it needs to be exchanged at runtime, before building and loading font atlas textures.
And I think, with current release of imgui-sys, that there's no other way than unsafely doing it.

unsafe {
    ctx.fonts().raw_mut().FontBuilderIO = ImGuiFreeType_GetBuilderForFreeType();
}

TL;DR

1. Imgui-rs doesn't set Freetype atlas builder as default even if the feature is enabled.
   (And I think it's misleading.)
2. For now, you can use Freetype by changing FontBuilderIO at runtime.
3. But you should do it unsafely.

Describe the solution you'd like

1. Make Freetype as default when the feature is enabled.

Describe alternatives you've considered

1. Make way to exchange font builder not unsafely using imgui-sys

Additional context

Maybe having example program about using Freetype would be very helpful IMO.