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Issue Description:

See schemekind_rawbuffer_count.swift (Compiler explorer link), which contains part of a project I'm working on.

Looking at the generated assembly in Compiler Explorer (at -O), we can see that the "doIt" function checks to see whether the String's code-units are contiguous, and if they are, calls WebURL.SchemeKind.init(buffer: UnsafeRawBufferPointer). Here's what that function looks like:

function signature specialization <Arg[1] = Dead> of output.WebURL.SchemeKind.init(buffer: Swift.UnsafeRawBufferPointer) -> output.WebURL.SchemeKind:
        push    rbp
        mov     rbp, rsp
        mov     al, 6
        test    rdi, rdi
        je      .LBB154_10
        test    rsi, rsi
        je      .LBB154_12
        
...

.LBB154_12:
        ud2

...

Similarly, we can see a number of calls to WebURL.SchemeKind.init(buffer🙂 in the specialisation for String.UTF8View (i.e. when the code-units are not contiguous)

Now, let's change the function so that, rather than an UnsafeRawBufferPointer, it takes separate UnsafeRawPointer and count (Int) parameters. Attached as schemekind_rawptr_splitcount.swift (Compiler Explorer link)

The trap is now gone. Moreover, the function is now inlined - both in "doIt", and the String.UTF8View specialisation. My benchmarks show that this can have a substantial performance benefit.