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 +434,11 @@ 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 "steals" the
+    memory buffer of mutable arrays, leaving the byte vector with an empty buffer which
+    is guaranteed not to be shared with the `String` object; if the byte vector is
+    immutable, it simply shares memory with the string and is not truncated ([#26093]).
+
   * `Irrational` is now a subtype of `AbstractIrrational` ([#24245]).
 
   * Introduced the `empty` function, the functional pair to `empty!` which returns a new,
@@ -1324,3 +1329,4 @@ 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 +16,22 @@ 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})
-
-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.
-
-If you need to subsequently modify `v`, use `String(copy(v))` instead.
+    String(v::AbstractVector{UInt8})
+
+Create a new `String` object from a byte vector `v` containing UTF-8 encoded
+characters. If `v` is `Vector{UInt8}` it will be truncated to zero length and
+future modification of `v` cannot affect the contents of the resulting string.
+To avoid truncation use `String(copy(v))`.
+
+When possible, the memory of `v` will be used without copying when the `String`
+object is created. This is guaranteed to be the case for byte vectors returned
+by [`take!`](@ref) on a writable [`IOBuffer`](@ref) and by calls to
+[`read(io, nb)`](@ref). This allows zero-copy conversion of I/O data to strings.
+In other cases, `Vector{UInt8}` data may be copied, but `v` is truncated anyway
+to guarantee consistent behavior.
 """
-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 +67,6 @@ 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/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