export and document transcode from JuliaLang#16974, add transcode(Str…

…ing, x) and transcode(T, ::String) convenience methods

base/c.jl

@@ -121,8 +121,21 @@ end
 
 # transcoding between data in UTF-8 and UTF-16 for Windows APIs
 
+"""
+    transcode(T, src)
+
+Convert string data between Unicode encodings.   `src` is either a
+`String` or an `Vector{UIntXX}` of UTF-XX code units, where
+`XX` is 8 or 16. `T` indicates the encoding of the return value:
+`String` to return a (UTF-8 encoded) `String` or `UIntXX`
+to return a `Vector{UIntXX}` of the UTF-`XX` data.
+"""
+function transcode end
+
 transcode{T<:Union{UInt8,UInt16}}(::Type{T}, src::Vector{T}) = src
 transcode(::Type{Int32}, src::Vector{UInt32}) = reinterpret(Int32, src)
+transcode(T, src::String) = transcode(T, src.data)
+transcode(::Type{String}, src) = String(transcode(UInt8, src))
 
 function transcode(::Type{UInt16}, src::Vector{UInt8})
     dst = UInt16[]

base/env.jl

@@ -19,7 +19,7 @@ function access_env(onError::Function, str::AbstractString)
         error(string("getenv: ", str, ' ', len, "-1 != ", ret, ": ", Libc.FormatMessage()))
     end
     pop!(val) # NUL
-    return String(transcode(UInt8, val))
+    return transcode(String, val)
 end
 
 function _setenv(svar::AbstractString, sval::AbstractString, overwrite::Bool=true)
@@ -97,7 +97,7 @@ function next(hash::EnvHash, block::Tuple{Ptr{UInt16},Ptr{UInt16}})
     len = ccall(:wcslen, UInt, (Ptr{UInt16},), pos)
     buf = Array{UInt16}(len)
     unsafe_copy!(pointer(buf), pos, len)
-    env = String(transcode(UInt8, buf))
+    env = transcode(String, buf)
     m = match(r"^(=?[^=]+)=(.*)$"s, env)
     if m === nothing
         error("malformed environment entry: $env")

base/exports.jl

@@ -876,6 +876,7 @@ export
     strip,
     strwidth,
     summary,
+    transcode,
     ucfirst,
     unescape_string,
     uppercase,

base/file.jl

@@ -203,7 +203,7 @@ function tempdir()
         error("GetTempPath failed: $(Libc.FormatMessage())")
     end
     resize!(temppath,lentemppath)
-    return String(transcode(UInt8, temppath))
+    return transcode(String, temppath)
 end
 tempname(uunique::UInt32=UInt32(0)) = tempname(tempdir(), uunique)
 const temp_prefix = cwstring("jl_")
@@ -216,7 +216,7 @@ function tempname(temppath::AbstractString,uunique::UInt32)
         error("GetTempFileName failed: $(Libc.FormatMessage())")
     end
     resize!(tname,lentname)
-    return String(transcode(UInt8, tname))
+    return transcode(String, tname)
 end
 function mktemp(parent=tempdir())
     filename = tempname(parent, UInt32(0))

base/interactiveutil.jl

@@ -150,7 +150,7 @@ elseif is_windows()
         len = 0
         while unsafe_load(plock, len+1) != 0; len += 1; end
         # get Vector{UInt16}, transcode data to UTF-8, make a String of it
-        s = String(transcode(UInt8, unsafe_wrap(Array, plock, len)))
+        s = transcode(String, unsafe_wrap(Array, plock, len))
         systemerror(:GlobalUnlock, 0==ccall((:GlobalUnlock, "kernel32"), stdcall, Cint, (Ptr{UInt16},), plock))
         return s
     end

base/libc.jl

@@ -277,7 +277,7 @@ if is_windows()
         buf = Array{UInt16}(len)
         unsafe_copy!(pointer(buf), p, len)
         ccall(:LocalFree,stdcall,Ptr{Void},(Ptr{Void},),p)
-        return String(transcode(UInt8, buf))
+        return transcode(String, buf)
     end
 end
 

base/path.jl

@@ -136,7 +136,7 @@ function realpath(path::AbstractString)
         systemerror(:realpath, n == 0)
         x = n < length(buf) # is the buffer big enough?
         resize!(buf, n) # shrink if x, grow if !x
-        x && return String(transcode(UInt8, buf))
+        x && return transcode(String, buf)
     end
 end
 
@@ -150,7 +150,7 @@ function longpath(path::AbstractString)
         systemerror(:longpath, n == 0)
         x = n < length(buf) # is the buffer big enough?
         resize!(buf, n) # shrink if x, grow if !x
-        x && return String(transcode(UInt8, buf))
+        x && return transcode(String, buf)
     end
 end
 

test/misc.jl

@@ -209,7 +209,6 @@ whos(IOBuffer(), Tmp14173) # warm up
 @test @allocated(whos(IOBuffer(), Tmp14173)) < 10000
 
 ## test conversion from UTF-8 to UTF-16 (for Windows APIs)
-import Base.Libc: transcode
 
 # empty arrays
 @test transcode(UInt16, UInt8[]) == UInt16[]
@@ -376,6 +375,13 @@ for (X,Y,Z) in ((V16,V16,V16), (I16,V16,I16), (V16,I16,V16), (V16,V16,I16), (I16
     end
 end
 
+let s = "abcα🐨\0x\0"
+    for T in (UInt8, UInt16)
+        @test transcode(T, s) == transcode(T, s.data)
+        @test transcode(String, transcode(T, s)) == s
+    end
+end
+
 # clipboard functionality
 if is_windows()
     for str in ("Hello, world.", "∀ x ∃ y", "")