`MemoryRef` bounds errors can be confusing

[LilithHafner]

Because of how MemoryRefs can represent views and because the MemoryRef itself is not captured in BoundsErrors, we can get confusing errors like this:

julia> x = Memory{Int}([1,2,3,4])
4-element Memory{Int64}:
 1
 2
 3
 4

julia> copyto!(x, 3, x, 3, 3)
ERROR: BoundsError: attempt to access MemoryRef{Int64} at index [3]
Stacktrace:
 [1] memoryref
   @ ./boot.jl:524 [inlined]
 [2] unsafe_copyto!
   @ ./genericmemory.jl:115 [inlined]
 [3] unsafe_copyto!
   @ ./genericmemory.jl:139 [inlined]
 [4] copyto!(dest::Memory{Int64}, doffs::Int64, src::Memory{Int64}, soffs::Int64, n::Int64)
   @ Base ./genericmemory.jl:175
 [5] top-level scope
   @ REPL[1]:1

This example is confusing because one could think that the memory has length 4 so accessing it at index 3 should be fine. What's happening is that a MemoryRef is constructed at an offset and then indexed out of bounds at index 3 when the new MemoryRef only has two indices.

[vtjnash]

It is captured, it is just that people decided it wasn't helpful to display the object that errored, and changed it to only print the type as a summary of it (although also that the show of a MemoryRef is not very good):

julia> err.stack[1].exception
BoundsError(MemoryRef{Int64}(Ptr{Nothing}(0x0000000103f3e8b0), [1, 2, 3, 4]), 3)

[LilithHafner]

Oh, delightful! This issue is very actionable with a better special case for displaying bounds errors on MemoryRef objects.

[KristofferC]

with a better special case for displaying bounds errors on MemoryRef objects

Or improve summary ofMemoryRef (like how it is done for String)?

[Omar-Elrefaei]

I was attempting to take a shot at this, but I'm not sure how to safely figure out how many "remaining" elements in the MemoryRef without doing pointer gymnastics.

I say this because I don't imagine replacing the error like this example will be helpful at all, since it is won't be clear what the problem is.

julia> x = Memory{Int}([1,2,3,4])
julia> copyto!(x, 1, x, 4, 2)
ERROR: BoundsError: attempt to access MemoryRef{Int64} at index [2]

# replace with this
ERROR: BoundsError: attempt to access a 4-element MemoryRef{Int64} at index [2]

# compared with
julia> SubString("1234", 4)[2]
ERROR: BoundsError: attempt to access 1-codeunit SubString{String} at index [2]

and so I was looking into trying to "count" the remaining elements that can be safely accessed such that:

mref = memoryref(x, 3)
remaining_length(mref) == 2

Note that in my example, the error is thrown from a call to memoryref(memoryref(mref, 4), 2) (with mref not x) from /base/genericmemory.jl:115

[oscardssmith]

The way to do this safely is to use memoryrefoffset combined with the length of the underlying memory

[Omar-Elrefaei]

thanks @oscardssmith! its a bummer that this was not discoverable using methodswith(GenericMemoryRef)

maybe it'd be a good idea to leave some "see also" hints in memoryref's docstring?

[oscardssmith]

seems like a good doc improvement.