Add a known panic case

Readme.md

@@ -32,22 +32,40 @@ $ ./build.sh
 
 ## Not yet supported
 
-* [ ] Checked binops
-* [ ] Statics
-* [ ] Drop glue
+* Checked binops
+* Statics
+* Drop glue
 
-* [ ] Building libraries
-* [ ] Other call abi's
-* [ ] Unsized types
-* [ ] Slice indexing
-* [ ] Sub slice
-* [ ] Closures
-* [ ] Some rvalue's
+* Building libraries
+* Other call abi's
+* Unsized types
+* Slice indexing
+* Sub slice
+* Closures
+* Some rvalue's
 
-* [ ] Inline assembly
-* [ ] Custom sections
+* Inline assembly
+* Custom sections
 
 ## Known errors
 
-* [ ] cranelift-module api seems to be used wrong, thus causing panic for some consts
-* [ ] cranelift-codegen doesn't have encodings for some instructions for types smaller than I32
+* cranelift-module api seems to be used wrong, thus causing panic for some consts
+* cranelift-codegen doesn't have encodings for some instructions for types smaller than I32
+
+```
+[...]
+warning: DefId(0/0:128 ~ lib[8787]::f64[0]::{{impl}}[0]::classify[0]):
+fn f64::<impl at ./target/libcore/src/libcore/num/f64.rs:156:1: 490:2>::classify(_1: f64) -> num::FpCategory{
+[...]
+}
+warning: stmt _3 = _1
+warning: stmt _5 = BitAnd(move _6, const Unevaluated(DefId(0/0:130 ~ lib[8787]::f64[0]::{{impl}}[0]::classify[0]::MAN_MASK[0]), []):u64)
+thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:345:21
+stack backtrace:
+[...]
+  10: <core::option::Option<T>>::unwrap
+             at /checkout/src/libcore/macros.rs:20
+  11: rustc_codegen_cranelift::constant::trans_constant
+             at src/constant.rs:26
+[...]
+```