From 7a44c0430fcc825f73e7496f201279f7e67a705e Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Thu, 21 Dec 2017 19:54:09 -0500
Subject: [PATCH] deprecate Vector<->String conversion in favor of something
 safer

add `CodeUnits` and `codeunits`

fixes #24388
---
 base/c.jl                 | 15 +++++++-------
 base/deprecated.jl        | 15 ++++++++++++--
 base/exports.jl           |  1 +
 base/io.jl                |  2 +-
 base/iobuffer.jl          |  2 +-
 base/loading.jl           |  4 ++--
 base/repl/LineEdit.jl     |  2 +-
 base/replutil.jl          |  2 +-
 base/strings/basic.jl     | 42 ++++++++++++++++++++++++++++++++++++---
 base/strings/io.jl        |  9 ++++++---
 base/strings/search.jl    |  4 ++--
 base/strings/string.jl    |  6 +++++-
 base/strings/strings.jl   |  1 -
 base/strings/substring.jl |  2 ++
 base/sysimg.jl            |  7 ++++---
 doc/src/stdlib/strings.md |  1 +
 test/char.jl              |  2 +-
 test/core.jl              |  2 +-
 test/misc.jl              |  2 +-
 test/random.jl            |  2 +-
 test/read.jl              | 14 ++++++-------
 test/strings/basic.jl     | 20 +++++++++++++++++--
 22 files changed, 116 insertions(+), 41 deletions(-)

diff --git a/base/c.jl b/base/c.jl
index 61163a93fcbc4..6a7403678771c 100644
--- a/base/c.jl
+++ b/base/c.jl
@@ -127,7 +127,7 @@ cconvert(::Type{Cstring}, s::AbstractString) =
     cconvert(Cstring, String(s)::String)
 
 function cconvert(::Type{Cwstring}, s::AbstractString)
-    v = transcode(Cwchar_t, Vector{UInt8}(String(s)))
+    v = transcode(Cwchar_t, String(s))
     !isempty(v) && v[end] == 0 || push!(v, 0)
     return v
 end
@@ -140,7 +140,7 @@ containsnul(p::Ptr, len) =
 containsnul(s::String) = containsnul(unsafe_convert(Ptr{Cchar}, s), sizeof(s))
 containsnul(s::AbstractString) = '\0' in s
 
-function unsafe_convert(::Type{Cstring}, s::Union{String,Vector{UInt8}})
+function unsafe_convert(::Type{Cstring}, s::Union{String,AbstractVector{UInt8}})
     p = unsafe_convert(Ptr{Cchar}, s)
     containsnul(p, sizeof(s)) &&
         throw(ArgumentError("embedded NULs are not allowed in C strings: $(repr(s))"))
@@ -174,7 +174,7 @@ same argument.
 This is only available on Windows.
 """
 function cwstring(s::AbstractString)
-    bytes = Vector{UInt8}(String(s))
+    bytes = codeunits(String(s))
     0 in bytes && throw(ArgumentError("embedded NULs are not allowed in C strings: $(repr(s))"))
     return push!(transcode(UInt16, bytes), 0)
 end
@@ -202,19 +202,20 @@ Only conversion to/from UTF-8 is currently supported.
 """
 function transcode end
 
-transcode(::Type{T}, src::Vector{T}) where {T<:Union{UInt8,UInt16,UInt32,Int32}} = src
+transcode(::Type{T}, src::AbstractVector{T}) where {T<:Union{UInt8,UInt16,UInt32,Int32}} = src
 transcode(::Type{T}, src::String) where {T<:Union{Int32,UInt32}} = T[T(c) for c in src]
-transcode(::Type{T}, src::Vector{UInt8}) where {T<:Union{Int32,UInt32}} = transcode(T, String(src))
+transcode(::Type{T}, src::Union{Vector{UInt8},CodeUnits{UInt8,String}}) where {T<:Union{Int32,UInt32}} =
+    transcode(T, String(src))
 function transcode(::Type{UInt8}, src::Vector{<:Union{Int32,UInt32}})
     buf = IOBuffer()
     for c in src; print(buf, Char(c)); end
     take!(buf)
 end
 transcode(::Type{String}, src::String) = src
-transcode(T, src::String) = transcode(T, Vector{UInt8}(src))
+transcode(T, src::String) = transcode(T, codeunits(src))
 transcode(::Type{String}, src) = String(transcode(UInt8, src))
 
-function transcode(::Type{UInt16}, src::Vector{UInt8})
+function transcode(::Type{UInt16}, src::Union{Vector{UInt8},CodeUnits{UInt8,String}})
     dst = UInt16[]
     i, n = 1, length(src)
     n > 0 || return dst
diff --git a/base/deprecated.jl b/base/deprecated.jl
index d9b2eededd1f1..a7a8f78b3d862 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -1555,8 +1555,19 @@ export hex2num
 @deprecate ctranspose adjoint
 @deprecate ctranspose! adjoint!
 
-@deprecate convert(::Type{Vector{UInt8}}, s::AbstractString)  Vector{UInt8}(s)
-@deprecate convert(::Type{Array{UInt8}}, s::AbstractString)   Vector{UInt8}(s)
+function convert(::Union{Type{Vector{UInt8}}, Type{Array{UInt8}}}, s::AbstractString)
+    depwarn("Strings can no longer be `convert`ed to byte arrays. Use `unsafe_wrap` or `codeunits` instead.", :Type)
+    unsafe_wrap(Vector{UInt8}, String(s))
+end
+function (::Type{Vector{UInt8}})(s::String)
+    depwarn("Vector{UInt8}(s::String) will copy data in the future. To avoid copying, use `unsafe_wrap` or `codeunits` instead.", :Type)
+    unsafe_wrap(Vector{UInt8}, s)
+end
+function (::Type{Array{UInt8}})(s::String)
+    depwarn("Array{UInt8}(s::String) will copy data in the future. To avoid copying, use `unsafe_wrap` or `codeunits` instead.", :Type)
+    unsafe_wrap(Vector{UInt8}, s)
+end
+
 @deprecate convert(::Type{Vector{Char}}, s::AbstractString)   Vector{Char}(s)
 @deprecate convert(::Type{Symbol}, s::AbstractString)         Symbol(s)
 @deprecate convert(::Type{String}, s::Symbol)                 String(s)
diff --git a/base/exports.jl b/base/exports.jl
index 377461f00ef79..8079432be3db0 100644
--- a/base/exports.jl
+++ b/base/exports.jl
@@ -686,6 +686,7 @@ export
     chomp,
     chop,
     codeunit,
+    codeunits,
     dec,
     digits,
     digits!,
diff --git a/base/io.jl b/base/io.jl
index ae71210cb1336..c53f916c56ea8 100644
--- a/base/io.jl
+++ b/base/io.jl
@@ -706,7 +706,7 @@ function readuntil(io::IO, target::AbstractString)
     # decide how we can index target
     if target isa String
         # convert String to a utf8-byte-iterator
-        target = Vector{UInt8}(target)
+        target = codeunits(target)
     #elseif applicable(codeunit, target)
     #   TODO: a more general version of above optimization
     #         would be to permit accessing any string via codeunit
diff --git a/base/iobuffer.jl b/base/iobuffer.jl
index 00635942ff561..b94b1f045e895 100644
--- a/base/iobuffer.jl
+++ b/base/iobuffer.jl
@@ -27,7 +27,7 @@ function GenericIOBuffer(data::T, readable::Bool, writable::Bool, seekable::Bool
 end
 
 # allocate Vector{UInt8}s for IOBuffer storage that can efficiently become Strings
-StringVector(n::Integer) = Vector{UInt8}(_string_n(n))
+StringVector(n::Integer) = unsafe_wrap(Vector{UInt8}, _string_n(n))
 
 # IOBuffers behave like Files. They are typically readable and writable. They are seekable. (They can be appendable).
 
diff --git a/base/loading.jl b/base/loading.jl
index cf1472437d9d2..a7ff6211f9c62 100644
--- a/base/loading.jl
+++ b/base/loading.jl
@@ -64,12 +64,12 @@ elseif Sys.isapple()
             break
         end
         # Hack to compensate for inability to create a string from a subarray with no allocations.
-        Vector{UInt8}(path_basename) == casepreserved_basename && return true
+        codeunits(path_basename) == casepreserved_basename && return true
 
         # If there is no match, it's possible that the file does exist but HFS+
         # performed unicode normalization. See  https://developer.apple.com/library/mac/qa/qa1235/_index.html.
         isascii(path_basename) && return false
-        Vector{UInt8}(Unicode.normalize(path_basename, :NFD)) == casepreserved_basename
+        codeunits(Unicode.normalize(path_basename, :NFD)) == casepreserved_basename
     end
 else
     # Generic fallback that performs a slow directory listing.
diff --git a/base/repl/LineEdit.jl b/base/repl/LineEdit.jl
index 35039f068edcd..a6a4ea1bd6165 100644
--- a/base/repl/LineEdit.jl
+++ b/base/repl/LineEdit.jl
@@ -657,7 +657,7 @@ function edit_splice!(s, r::Region=region(s), ins::AbstractString = ""; rigid_ma
     elseif buf.mark >= B
         buf.mark += sizeof(ins) - B + A
     end
-    ret = splice!(buf.data, A+1:B, Vector{UInt8}(ins)) # position(), etc, are 0-indexed
+    ret = splice!(buf.data, A+1:B, codeunits(String(ins))) # position(), etc, are 0-indexed
     buf.size = buf.size + sizeof(ins) - B + A
     adjust_pos && seek(buf, position(buf) + sizeof(ins))
     String(ret)
diff --git a/base/replutil.jl b/base/replutil.jl
index 8ad88cef24d0e..29d890db61b99 100644
--- a/base/replutil.jl
+++ b/base/replutil.jl
@@ -283,7 +283,7 @@ function showerror(io::IO, ex::ErrorException)
     print(io, ex.msg)
     if ex.msg == "type String has no field data"
         println(io)
-        print(io, "Use `Vector{UInt8}(str)` instead.")
+        print(io, "Use `codeunits(str)` instead.")
     end
 end
 showerror(io::IO, ex::KeyError) = print(io, "KeyError: key $(repr(ex.key)) not found")
diff --git a/base/strings/basic.jl b/base/strings/basic.jl
index 703e455a6306b..0fba9536b194e 100644
--- a/base/strings/basic.jl
+++ b/base/strings/basic.jl
@@ -152,7 +152,6 @@ end
 getindex(s::AbstractString, i::Colon) = s
 # TODO: handle other ranges with stride ±1 specially?
 # TODO: add more @propagate_inbounds annotations?
-getindex(s::AbstractString, r::UnitRange{<:Integer}) = SubString(s, r)
 getindex(s::AbstractString, v::AbstractVector{<:Integer}) =
     sprint(io->(for i in v; write(io, s[i]) end), sizehint=length(v))
 getindex(s::AbstractString, v::AbstractVector{Bool}) =
@@ -185,8 +184,8 @@ checkbounds(s::AbstractString, I::Union{Integer,AbstractArray}) =
 string() = ""
 string(s::AbstractString) = s
 
-(::Type{Vector{UInt8}})(s::AbstractString) = Vector{UInt8}(String(s))
-(::Type{Array{UInt8}})(s::AbstractString) = Vector{UInt8}(s)
+(::Type{Vector{UInt8}})(s::AbstractString) = unsafe_wrap(Vector{UInt8}, String(s))
+(::Type{Array{UInt8}})(s::AbstractString) = unsafe_wrap(Vector{UInt8}, String(s))
 (::Type{Vector{Char}})(s::AbstractString) = collect(s)
 
 Symbol(s::AbstractString) = Symbol(String(s))
@@ -629,3 +628,40 @@ next(r::Iterators.Reverse{<:AbstractString}, i) = (r.itr[i], prevind(r.itr, i))
 start(r::Iterators.Reverse{<:EachStringIndex}) = endof(r.itr.s)
 done(r::Iterators.Reverse{<:EachStringIndex}, i) = i < start(r.itr.s)
 next(r::Iterators.Reverse{<:EachStringIndex}, i) = (i, prevind(r.itr.s, i))
+
+## code unit access ##
+
+"""
+    CodeUnits(s::AbstractString)
+
+Wrap a string (without copying) in an immutable vector-like object that accesses the code units
+of the string's representation.
+"""
+struct CodeUnits{T,S<:AbstractString} <: DenseVector{T}
+    s::S
+    CodeUnits(s::S) where {S<:AbstractString} = new{codeunit(s),S}(s)
+end
+
+length(s::CodeUnits) = ncodeunits(s.s)
+sizeof(s::CodeUnits{T}) where {T} = ncodeunits(s.s) * sizeof(T)
+size(s::CodeUnits) = (length(s),)
+strides(s::CodeUnits) = (1,)
+@propagate_inbounds getindex(s::CodeUnits, i::Int) = codeunit(s.s, i)
+IndexStyle(::Type{<:CodeUnits}) = IndexLinear()
+start(s::CodeUnits) = 1
+next(s::CodeUnits, i) = (@_propagate_inbounds_meta; (s[i], i+1))
+done(s::CodeUnits, i) = (@_inline_meta; i == length(s)+1)
+
+write(io::IO, s::CodeUnits) = write(io, s.s)
+
+unsafe_convert(::Type{Ptr{T}},    s::CodeUnits{T}) where {T} = unsafe_convert(Ptr{T}, s.s)
+unsafe_convert(::Type{Ptr{Int8}}, s::CodeUnits{UInt8}) = unsafe_convert(Ptr{Int8}, s.s)
+
+"""
+    codeunits(s::AbstractString)
+
+Obtain a vector-like object containing the code units of a string.
+Returns a `CodeUnits` wrapper by default, but `codeunits` may optionally be defined
+for new string types if necessary.
+"""
+codeunits(s::AbstractString) = CodeUnits(s)
diff --git a/base/strings/io.jl b/base/strings/io.jl
index 538d7a55242e6..ca008fc61506f 100644
--- a/base/strings/io.jl
+++ b/base/strings/io.jl
@@ -188,8 +188,8 @@ julia> String(take!(io))
 "Haho"
 ```
 """
-IOBuffer(str::String) = IOBuffer(Vector{UInt8}(str))
-IOBuffer(s::SubString{String}) = IOBuffer(view(Vector{UInt8}(s.string), s.offset + 1 : s.offset + sizeof(s)))
+IOBuffer(str::String) = IOBuffer(unsafe_wrap(Vector{UInt8}, str))
+IOBuffer(s::SubString{String}) = IOBuffer(view(unsafe_wrap(Vector{UInt8}, s.string), s.offset + 1 : s.offset + sizeof(s)))
 
 # join is implemented using IO
 
@@ -373,7 +373,10 @@ function unescape_string(io, s::AbstractString)
     end
 end
 
-macro b_str(s); :(Vector{UInt8}($(unescape_string(s)))); end
+macro b_str(s)
+    v = Vector{UInt8}(codeunits(unescape_string(s)))
+    QuoteNode(v)
+end
 
 """
     @raw_str -> String
diff --git a/base/strings/search.jl b/base/strings/search.jl
index 9f895da99e066..f920b77520252 100644
--- a/base/strings/search.jl
+++ b/base/strings/search.jl
@@ -31,7 +31,7 @@ function search(a::ByteArray, b::Char, i::Integer = 1)
     if isascii(b)
         search(a,UInt8(b),i)
     else
-        search(a,Vector{UInt8}(string(b)),i).start
+        search(a,unsafe_wrap(Vector{UInt8},string(b)),i).start
     end
 end
 
@@ -62,7 +62,7 @@ function rsearch(a::ByteArray, b::Char, i::Integer = length(a))
     if isascii(b)
         rsearch(a,UInt8(b),i)
     else
-        rsearch(a,Vector{UInt8}(string(b)),i).start
+        rsearch(a,unsafe_wrap(Vector{UInt8},string(b)),i).start
     end
 end
 
diff --git a/base/strings/string.jl b/base/strings/string.jl
index 888cda75c6046..1d1a15199f25d 100644
--- a/base/strings/string.jl
+++ b/base/strings/string.jl
@@ -60,7 +60,11 @@ This representation is often appropriate for passing strings to C.
 String(s::AbstractString) = print_to_string(s)
 String(s::Symbol) = unsafe_string(unsafe_convert(Ptr{UInt8}, s))
 
-(::Type{Vector{UInt8}})(s::String) = ccall(:jl_string_to_array, Ref{Vector{UInt8}}, (Any,), s)
+unsafe_wrap(::Type{Vector{UInt8}}, s::String) = ccall(:jl_string_to_array, Ref{Vector{UInt8}}, (Any,), s)
+
+(::Type{Vector{UInt8}})(s::CodeUnits{UInt8,String}) = copyto!(Vector{UInt8}(uninitialized, length(s)), s)
+
+String(s::CodeUnits{UInt8,String}) = s.s
 
 ## low-level functions ##
 
diff --git a/base/strings/strings.jl b/base/strings/strings.jl
index 91f10436a4e73..0edc0c4e4bba6 100644
--- a/base/strings/strings.jl
+++ b/base/strings/strings.jl
@@ -1,7 +1,6 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 include("strings/substring.jl")
-include("strings/basic.jl")
 include("strings/search.jl")
 include("strings/unicode.jl")
 include("strings/util.jl")
diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index f7abe7dfb1e4f..f4cf3c9311537 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -155,3 +155,5 @@ function reverse(s::Union{String,SubString{String}})::String
         end
     end
 end
+
+getindex(s::AbstractString, r::UnitRange{<:Integer}) = SubString(s, r)
diff --git a/base/sysimg.jl b/base/sysimg.jl
index bb9851697d205..ab6906a7bd905 100644
--- a/base/sysimg.jl
+++ b/base/sysimg.jl
@@ -183,9 +183,6 @@ macro MIME_str(s)
     :(MIME{$(Expr(:quote, Symbol(s)))})
 end
 
-include("char.jl")
-include("strings/string.jl")
-
 # SIMD loops
 include("simdloop.jl")
 using .SimdLoop
@@ -212,6 +209,10 @@ include("iterators.jl")
 using .Iterators: zip, enumerate
 using .Iterators: Flatten, product  # for generators
 
+include("char.jl")
+include("strings/basic.jl")
+include("strings/string.jl")
+
 # Definition of StridedArray
 StridedReshapedArray{T,N,A<:Union{DenseArray,FastContiguousSubArray}} = ReshapedArray{T,N,A}
 StridedReinterpretArray{T,N,A<:Union{DenseArray,FastContiguousSubArray}} = ReinterpretArray{T,N,S,A} where S
diff --git a/doc/src/stdlib/strings.md b/doc/src/stdlib/strings.md
index 74ab2ca518f22..79e2b1b9fb448 100644
--- a/doc/src/stdlib/strings.md
+++ b/doc/src/stdlib/strings.md
@@ -15,6 +15,7 @@ Base.transcode
 Base.unsafe_string
 Base.ncodeunits(::AbstractString)
 Base.codeunit
+Base.codeunits
 Base.ascii
 Base.@r_str
 Base.@raw_str
diff --git a/test/char.jl b/test/char.jl
index 9163122c95659..c5e98d302e41b 100644
--- a/test/char.jl
+++ b/test/char.jl
@@ -201,7 +201,7 @@ end
 @testset "read incomplete character at end of stream or file" begin
     local file = tempname()
     local iob = IOBuffer([0xf0])
-    local bytes(c::Char) = Vector{UInt8}(string(c))
+    local bytes(c::Char) = codeunits(string(c))
     @test bytes(read(iob, Char)) == [0xf0]
     @test eof(iob)
     try
diff --git a/test/core.jl b/test/core.jl
index 407d6c137257f..68b00969a094f 100644
--- a/test/core.jl
+++ b/test/core.jl
@@ -4114,7 +4114,7 @@ b = "aaa"
 c = [0x2, 0x1, 0x3]
 
 @test check_nul(a)
-@test check_nul(Vector{UInt8}(b))
+@test check_nul(unsafe_wrap(Vector{UInt8},b))
 @test check_nul(c)
 d = [0x2, 0x1, 0x3]
 @test check_nul(d)
diff --git a/test/misc.jl b/test/misc.jl
index 12099f2aac4bf..9a1cd659988e5 100644
--- a/test/misc.jl
+++ b/test/misc.jl
@@ -394,7 +394,7 @@ end
 
 let s = "abcα🐨\0x\0"
     for T in (UInt8, UInt16, UInt32, Int32)
-        @test transcode(T, s) == transcode(T, Vector{UInt8}(s))
+        @test transcode(T, s) == transcode(T, codeunits(s))
         @test transcode(String, transcode(T, s)) == s
     end
 end
diff --git a/test/random.jl b/test/random.jl
index 2cbd423897b5d..614a6f36c8284 100644
--- a/test/random.jl
+++ b/test/random.jl
@@ -613,7 +613,7 @@ let b = ['0':'9';'A':'Z';'a':'z']
         @test length(randstring(rng...)) == 8
         @test length(randstring(rng..., 20)) == 20
         @test issubset(randstring(rng...), b)
-        for c = ['a':'z', "qwèrtï", Set(Vector{UInt8}("gcat"))],
+        for c = ['a':'z', "qwèrtï", Set(codeunits("gcat"))],
                 len = [8, 20]
             s = len == 8 ? randstring(rng..., c) : randstring(rng..., c, len)
             @test length(s) == len
diff --git a/test/read.jl b/test/read.jl
index d590a378687d8..b6b6fef9bebd2 100644
--- a/test/read.jl
+++ b/test/read.jl
@@ -165,7 +165,7 @@ for (name, f) in l
         @test readuntil(io(t), s) == m
         @test readuntil(io(t), SubString(s, start(s), endof(s))) == m
         @test readuntil(io(t), GenericString(s)) == m
-        @test readuntil(io(t), Vector{UInt8}(s)) == Vector{UInt8}(m)
+        @test readuntil(io(t), unsafe_wrap(Vector{UInt8},s)) == unsafe_wrap(Vector{UInt8},m)
         @test readuntil(io(t), collect(s)::Vector{Char}) == Vector{Char}(m)
     end
     cleanup()
@@ -223,7 +223,7 @@ for (name, f) in l
 
 
         verbose && println("$name read...")
-        @test read(io()) == Vector{UInt8}(text)
+        @test read(io()) == unsafe_wrap(Vector{UInt8},text)
 
         @test read(io()) == read(filename)
 
@@ -331,7 +331,7 @@ for (name, f) in l
     @test read("$filename.to", String) == text
 
     verbose && println("$name write(::IOBuffer, ...)")
-    to = IOBuffer(copy(Vector{UInt8}(text)), false, true)
+    to = IOBuffer(Vector{UInt8}(codeunits(text)), false, true)
     write(to, io())
     @test String(take!(to)) == text
 
@@ -400,14 +400,14 @@ test_read_nbyte()
 
 
 let s = "qwerty"
-    @test read(IOBuffer(s)) == Vector{UInt8}(s)
-    @test read(IOBuffer(s), 10) == Vector{UInt8}(s)
-    @test read(IOBuffer(s), 1) == Vector{UInt8}(s)[1:1]
+    @test read(IOBuffer(s)) == codeunits(s)
+    @test read(IOBuffer(s), 10) == codeunits(s)
+    @test read(IOBuffer(s), 1) == codeunits(s)[1:1]
 
     # Test growing output array
     x = UInt8[]
     n = readbytes!(IOBuffer(s), x, 10)
-    @test x == Vector{UInt8}(s)
+    @test x == codeunits(s)
     @test n == length(x)
 end
 
diff --git a/test/strings/basic.jl b/test/strings/basic.jl
index 092828c6c4c76..26fe5fb5214b5 100644
--- a/test/strings/basic.jl
+++ b/test/strings/basic.jl
@@ -206,7 +206,7 @@ struct tstStringType <: AbstractString
     data::Array{UInt8,1}
 end
 @testset "AbstractString functions" begin
-    tstr = tstStringType(Vector{UInt8}("12"))
+    tstr = tstStringType(unsafe_wrap(Vector{UInt8},"12"))
     @test_throws MethodError ncodeunits(tstr)
     @test_throws MethodError codeunit(tstr)
     @test_throws MethodError codeunit(tstr, 1)
@@ -697,7 +697,7 @@ end
     end
 end
 
-@test Vector{UInt8}("\xcc\xdd\xee\xff\x80") == [0xcc,0xdd,0xee,0xff,0x80]
+@test unsafe_wrap(Vector{UInt8},"\xcc\xdd\xee\xff\x80") == [0xcc,0xdd,0xee,0xff,0x80]
 
 @test next("a", 1)[2] == 2
 @test nextind("a", 1) == 2
@@ -794,3 +794,19 @@ end
 @test nextind("\xf8\x9f\x98\x84", 1) == 2
 @test next("\xf8\x9f\x98\x84z", 1)[2] == 2
 @test nextind("\xf8\x9f\x98\x84z", 1) == 2
+
+# codeunit vectors
+
+let s = "∀x∃y", u = codeunits(s)
+    @test u isa Base.CodeUnits{UInt8,String}
+    @test length(u) == ncodeunits(s) == 8
+    @test sizeof(u) == sizeof(s)
+    @test eltype(u) === UInt8
+    @test size(u) == (length(u),)
+    @test strides(u) == (1,)
+    @test u[1] == 0xe2
+    @test u[2] == 0x88
+    @test u[8] == 0x79
+    @test_throws ErrorException (u[1] = 0x00)
+    @test collect(u) == b"∀x∃y"
+end