These are somewhat stop-gap solutions to address #16625
core: Separate failure formatting in str methods slice, slice_to, slice_from
Use a separate inline-never function to format failure message for
str::slice() errors.
Using strcat's idea, this makes sure no formatting code from failure is
inlined when str::slice() is inlined. The number of `unreachable` being
inlined when usingi `.slice()` drops from 5 to just 1.
The testcase:
```
#![crate_type = "lib"]
pub fn slice(x: &str, a: uint, b: uint) -> &str {
x.slice(a, b)
}
```
shrinks from 16.9 kB to 3.3 kB llvm IR, and the number of `unreachable` drops from 5 to 1.