Add accessors for Formatter flags

[sfackler]

cc #27726

r? @alexcrichton

[alexcrichton]

@bors: r+
Suppose a future version of the Rust standard library doesn't depend on any C runtime on Windows. If the user doesn't want a C runtime at all, how are they supposed to get definitions for types like wchar_t?
Assuming Rust is dependent on a particular version of the C runtime, exposing functions like malloc is a trap because other DLLs are likely to have access to a different version of malloc which isn't compatible. On the other hand, if you're statically linking against code written in C, you might end up needing whatever malloc happens to be linked in.
The current proposed version of libc exposes functions which take paths as *const c_char... but you never want to use those functions on Windows because of encoding problems.

There's probably an argument to be made that libc shouldn't expose any functions at all on Windows. (The crate wouldn't be completely empty because it would contain some types.)

It might be a good idea to explicitly exclude pure math functions from the scope of libc (sin(), abs(), etc.). They aren't necessary, and we'd end up either reimplementing a lot of them in Rust or randomly excluding them on various platforms.

[bors]

📌 Commit e5ee13f has been approved by alexcrichton

[alexcrichton]

Oops, that was the wrong thing to copy paste

At least bors got the right idea!

[bors]

⌛ Testing commit e5ee13f with merge 7ebfd85...

[bors]

💔 Test failed - auto-linux-32-opt

[sfackler]

@bors retry

[bors]

⚡ Previous build results for auto-mac-32-opt, auto-win-msvc-32-opt are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-64-opt...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt