The code at

produces a debug representation for MSVC, which could be improved. Quoting from@MaulingMonkey inhttps://github.com/rust-lang/rust/pull/114607/files#r1454683264:

Not sure if proper support is possible here without bugging Microsoft. Even if it is possible, Microsoft hasn't bothered for their own SDK types (e.g._FLOAT128 can be found in the windows SDK, but it's just a struct around a couple of u64s and has no useful debug visualizers.)

If itwere possible:

* Emit debug info for a `u16`/`u128`-wrapping structure (I'm fuzzy on the details here, I've only wrapped existing emitted types rather than emitting my own.  Merely defining an unused std type might not be enough as such types will get culled IME?)* Write/extend a `*.natvis` file (probably [rust/src/etc/natvis/intrinsics.natvis](https://github.com/rust-lang/rust/blob/6bf600bc98879bf1bf2ab0f3b3b37c965f5bdff6/src/etc/natvis/intrinsic.natvis)?) that unpacks the bits and reconstitutes them as an approximate double or float in it's `<DisplayString>`.    * How do you transmute/type-pun from `u64` → `f64` in the subset of C++ allowed by the debugger (no pointer-to-temporary shenannigans)?  _Maybe_ abuse a union somehow...?  * See [`natvis-pdbs` Recommended Reading](https://crates.io/crates/natvis-pdbs#recommended-reading)  * If you added a new `*.nativs` file for some reason, make sure it's referenced properly (see e.g. [rust/src/bootstrap/src/core/build_steps/dist.rs](https://github.com/rust-lang/rust/blob/master/src/bootstrap/src/core/build_steps/dist.rs#L535-L538)  * Consider unit tests like [tests/debuginfo/pretty-std-collections-hash.rs](https://github.com/rust-lang/rust/blob/6bf600bc98879bf1bf2ab0f3b3b37c965f5bdff6/tests/debuginfo/pretty-std-collections-hash.rs#L1)

(It'spossible to call_CrtCheckMemory() from a watch window. I forget if this works in a*.natvis visualizer. If so, you could theoretically authorF128toApproxF64(high: u64, low: u64) and stuff it in a DLL? Or better yet, let someone more foolish take the time to go downthat rabbit hole and shave that yak...)

@rustbot label +F-f16_and_f128