Add unsafe_string for Cwstring (#36040)

Also adds some tests for Cwstring handling, fixes #28863.

Author: simonbyrne

base/c.jl

@@ -414,6 +414,18 @@ function transcode(::Type{UInt8}, src::AbstractVector{UInt16})
     return dst
 end
 
+function unsafe_string(p::Ptr{T}, length::Integer) where {T<:Union{UInt16,UInt32,Cwchar_t}}
+    transcode(String, unsafe_wrap(Array, p, length; own=false))
+end
+function unsafe_string(cw::Cwstring)
+    p = convert(Ptr{Cwchar_t}, cw)
+    n = 1
+    while unsafe_load(p, n) != 0
+        n += 1
+    end
+    return unsafe_string(p, n - 1)
+end
+
 # deferring (or un-deferring) ctrl-c handler for external C code that
 # is not interrupt safe (see also issue #2622).  The sigatomic_begin/end
 # functions should always be called in matched pairs, ideally via:

test/ccall.jl

@@ -1686,3 +1686,18 @@ end
     @ccall free(strp[]::Cstring)::Cvoid
     @test str == "hi+1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-1.1-2.2-3.3-4.4-5.5-6.6-7.7-8.8-9.9\n"
 end
+
+@testset "Cwstring" begin
+    n = 100
+    buffer = Array{Cwchar_t}(undef, n)
+    if Sys.iswindows()
+        # sprintf throws an error on Windows, see https://github.com/JuliaLang/julia/pull/36040#issuecomment-634774055
+        len = @ccall swprintf_s(buffer::Ptr{Cwchar_t}, n::Csize_t, "α+%ls=%hhd"::Cwstring; "β"::Cwstring, 0xf::UInt8)::Cint
+    else
+        len = @ccall swprintf(buffer::Ptr{Cwchar_t}, n::Csize_t, "α+%ls=%hhd"::Cwstring; "β"::Cwstring, 0xf::UInt8)::Cint
+    end
+    str = GC.@preserve buffer unsafe_string(pointer(buffer), len)
+    @test str == "α+β=15"
+    str = GC.@preserve buffer unsafe_string(Cwstring(pointer(buffer)))
+    @test str == "α+β=15"
+end