Always resize input to zero length in String(::Vector{UInt8})

This makes the behavior more predictable than only resizing Vector{UInt8}
inputs when they have been allocated via StringVector, as the caller may have
obtained them from other code without knowing how they were created. This ensures
code will not rely on the fact that a copy is made in many common cases.
The behavior is also simpler to document.

NEWS.md

@@ -434,6 +434,9 @@ Library improvements
   * The function `thisind(s::AbstractString, i::Integer)` returns the largest valid index
     less or equal than `i` in the string `s` or `0` if no such index exists ([#24414]).
 
+  * `String(array)` now accepts an arbitrary `AbstractVector{UInt8}`, and resizes
+    `Vector{UInt8}` inputs to a zero length to prevent incorrect modification ([#26093]).
+
   * `Irrational` is now a subtype of `AbstractIrrational` ([#24245]).
 
   * Introduced the `empty` function, the functional pair to `empty!` which returns a new,
@@ -1324,3 +1327,4 @@ Command-line option changes
 [#25745]: https://github.com/JuliaLang/julia/issues/25745
 [#25896]: https://github.com/JuliaLang/julia/issues/25896
 [#25998]: https://github.com/JuliaLang/julia/issues/25998
+[#26093]: https://github.com/JuliaLang/julia/issues/26093

base/strings/string.jl

@@ -16,19 +16,18 @@ const ByteArray = Union{Vector{UInt8},Vector{Int8}}
 # String constructor docstring from boot.jl, workaround for #16730
 # and the unavailability of @doc in boot.jl context.
 """
-    String(v::Vector{UInt8})
+    String(v::AbstractVector{UInt8})
 
 Create a new `String` from a vector `v` of bytes containing
-UTF-8 encoded characters.   This function takes "ownership" of
-the array, which means that you should not subsequently modify
-`v` (since strings are supposed to be immutable in Julia) for
-as long as the string exists.
+UTF-8 encoded characters.
 
+This function takes "ownership" of the array: implementations may choose
+to resize `v` to a zero length to ensure its contents will not be modified.
+In particular, this is the case when `v` is a `Vector`.
 If you need to subsequently modify `v`, use `String(copy(v))` instead.
 """
-function String(v::Array{UInt8,1})
-    ccall(:jl_array_to_string, Ref{String}, (Any,), v)
-end
+String(v::AbstractVector{UInt8}) = String(copyto!(StringVector(length(v)), v))
+String(v::Vector{UInt8}) = ccall(:jl_array_to_string, Ref{String}, (Any,), v)
 
 """
     unsafe_string(p::Ptr{UInt8}, [length::Integer])
@@ -64,8 +63,6 @@ unsafe_wrap(::Type{Vector{UInt8}}, s::String) = ccall(:jl_string_to_array, Ref{V
 
 (::Type{Vector{UInt8}})(s::CodeUnits{UInt8,String}) = copyto!(Vector{UInt8}(uninitialized, length(s)), s)
 
-String(a::AbstractVector{UInt8}) = String(copyto!(StringVector(length(a)), a))
-
 String(s::CodeUnits{UInt8,String}) = s.s
 
 ## low-level functions ##

src/array.c

@@ -434,13 +434,14 @@ JL_DLLEXPORT jl_array_t *jl_pchar_to_array(const char *str, size_t len)
 
 JL_DLLEXPORT jl_value_t *jl_array_to_string(jl_array_t *a)
 {
+    size_t len = jl_array_len(a);
     if (a->flags.how == 3 && a->offset == 0 && a->elsize == 1 &&
         (jl_array_ndims(a) != 1 ||
-         ((a->maxsize + sizeof(void*) + 1 <= GC_MAX_SZCLASS) == (jl_array_len(a) + sizeof(void*) + 1 <= GC_MAX_SZCLASS)))) {
+         ((a->maxsize + sizeof(void*) + 1 <= GC_MAX_SZCLASS) == (len + sizeof(void*) + 1 <= GC_MAX_SZCLASS)))) {
         jl_value_t *o = jl_array_data_owner(a);
         if (jl_is_string(o)) {
             a->flags.isshared = 1;
-            *(size_t*)o = jl_array_len(a);
+            *(size_t*)o = len;
             a->nrows = 0;
 #ifdef STORE_ARRAY_LEN
             a->length = 0;
@@ -449,7 +450,12 @@ JL_DLLEXPORT jl_value_t *jl_array_to_string(jl_array_t *a)
             return o;
         }
     }
-    return jl_pchar_to_string((const char*)jl_array_data(a), jl_array_len(a));
+    a->nrows = 0;
+#ifdef STORE_ARRAY_LEN
+    a->length = 0;
+#endif
+    a->maxsize = 0;
+    return jl_pchar_to_string((const char*)jl_array_data(a), len);
 }
 
 JL_DLLEXPORT jl_value_t *jl_pchar_to_string(const char *str, size_t len)

test/arrayops.jl

@@ -2342,4 +2342,4 @@ Base.view(::T25958, args...) = args
     @test t[end,1,end]   == @view(t[end,1,end])   == @views t[end,1,end]
     @test t[end,end,1]   == @view(t[end,end,1])   == @views t[end,end,1]
     @test t[end,end,end] == @view(t[end,end,end]) == @views t[end,end,end]
-end
+end

test/strings/basic.jl

@@ -3,8 +3,10 @@
 using Random
 
 @testset "constructors" begin
-    @test String([0x61,0x62,0x63,0x21]) == "abc!"
+    v = [0x61,0x62,0x63,0x21]
+    @test String(v) == "abc!" && isempty(v)
     @test String("abc!") == "abc!"
+    @test String(0x61:0x63) == "abc"
 
     @test isempty(string())
     @test eltype(GenericString) == Char