utils: Propagate IncludeHandler file loading failures to DXC instead …

…of a panic (#27)

Not all file loading should be fallible; DXC usually invokes the loader
with multiple paths depending on relative includes, and happily accepts
and expects us to return `-2_147_024_894 // ERROR_FILE_NOT_FOUND /
0x80070002` for some.  As such the precedent should be to not panic but
let DXC decide if file loading failures should be critical or simply
result in a different path being tested.

In my case though the error seems to originate from DXIL compilation
being largely untested when ran on Linux systems through the WinAadapter
codepath.  In this particular stacktrace below DXC seems to "stat" (get
file information) for the current working directory, and somehow ends up
trying to open the directory as file through our include handler:

    #11 dxcutil::DxcArgsFileSystemImpl::TryFindOrOpen (this=<optimized out>, this@entry=0x5570a3b0ae10, lpFileName=lpFileName@entry=0x5570a3b5d0d0 L".../hassle-rs", index=<optimized out>)
        at .../DirectXShaderCompiler/tools/clang/tools/dxcompiler/dxcfilesystem.cpp:327
    #12 0x00007f6e64a41a8d in dxcutil::DxcArgsFileSystemImpl::GetFileAttributesW (this=0x5570a3b0ae10, lpFileName=0x5570a3b5d0d0 L".../hassle-rs")
        at .../DirectXShaderCompiler/tools/clang/tools/dxcompiler/dxcfilesystem.cpp:570
    #13 0x00007f6e64a42048 in dxcutil::DxcArgsFileSystemImpl::Stat (this=0x5570a3b0ae10, lpFileName=<optimized out>, Status=0x7fff9faec1e8)
        at .../DirectXShaderCompiler/tools/clang/tools/dxcompiler/dxcfilesystem.cpp:807
    #14 0x00007f6e64debc91 in llvm::sys::fs::status (Path=..., Result=...) at .../DirectXShaderCompiler/lib/Support/Unix/Path.inc:385
    #15 0x00007f6e64dec708 in llvm::sys::fs::current_path (result=...) at .../DirectXShaderCompiler/lib/Support/Unix/Path.inc:195
    #16 0x00007f6e64e64535 in clang::CodeGen::CGDebugInfo::getCurrentDirname (this=this@entry=0x5570a3b598f0)
        at .../DirectXShaderCompiler/tools/clang/lib/CodeGen/CGDebugInfo.cpp:301
    #17 0x00007f6e64e6230e in clang::CodeGen::CGDebugInfo::CreateCompileUnit (this=this@entry=0x5570a3b598f0)
        at .../DirectXShaderCompiler/tools/clang/lib/CodeGen/CGDebugInfo.cpp:369

This stacktrace originates from the following panic:

    thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 21, kind: IsADirectory, message: "Is a directory" }', src/utils.rs:51:48

src/utils.rs

@@ -48,7 +48,7 @@ impl DxcIncludeHandler for DefaultIncludeHandler {
         match std::fs::File::open(filename) {
             Ok(mut f) => {
                 let mut content = String::new();
-                f.read_to_string(&mut content).unwrap();
+                f.read_to_string(&mut content).ok()?;
                 Some(content)
             }
             Err(_) => None,