diff --git a/NEWS.md b/NEWS.md
index f2ffb103abb8b..d277817a98277 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -182,6 +182,8 @@ Deprecated or removed
 
   * The unexported type `AbstractIOBuffer` has been renamed to `GenericIOBuffer` ([#17360] [#22796]).
 
+  * The method `String(io::IOBuffer)` is deprecated to `String(take!(copy(io)))` ([#21438]).
+
 
 Julia v0.6.0 Release Notes
 ==========================
diff --git a/base/deprecated.jl b/base/deprecated.jl
index c42d669a8fcde..153d758d5b959 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -1589,6 +1589,8 @@ end
 
 @deprecate_binding AbstractIOBuffer GenericIOBuffer false
 
+@deprecate String(io::GenericIOBuffer) String(take!(copy(io)))
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations
diff --git a/base/iobuffer.jl b/base/iobuffer.jl
index 23b1de0281fb6..a379adfff0e25 100644
--- a/base/iobuffer.jl
+++ b/base/iobuffer.jl
@@ -257,12 +257,6 @@ end
 
 isopen(io::GenericIOBuffer) = io.readable || io.writable || io.seekable || nb_available(io) > 0
 
-function String(io::GenericIOBuffer)
-    io.readable || throw(ArgumentError("IOBuffer is not readable"))
-    io.seekable || throw(ArgumentError("IOBuffer is not seekable"))
-    return unsafe_string(pointer(io.data), io.size)
-end
-
 """
     take!(b::IOBuffer)
 
diff --git a/base/repl/LineEdit.jl b/base/repl/LineEdit.jl
index 941a184ce0743..05802bb6cc747 100644
--- a/base/repl/LineEdit.jl
+++ b/base/repl/LineEdit.jl
@@ -64,7 +64,7 @@ mutable struct PromptState <: ModeState
     indent::Int
 end
 
-input_string(s::PromptState) = String(s.input_buffer)
+input_string(s::PromptState) = String(take!(copy(s.input_buffer)))
 
 input_string_newlines(s::PromptState) = count(c->(c == '\n'), input_string(s))
 function input_string_newlines_aftercursor(s::PromptState)
@@ -1045,7 +1045,7 @@ refresh_multi_line(termbuf::TerminalBuffer, terminal::UnixTerminal,
     s::Union{PromptState,PrefixSearchState}) = s.ias =
     refresh_multi_line(termbuf, terminal, buffer(s), s.ias, s, indent = s.indent)
 
-input_string(s::PrefixSearchState) = String(s.response_buffer)
+input_string(s::PrefixSearchState) = String(take!(copy(s.response_buffer)))
 
 # a meta-prompt that presents itself as parent_prompt, but which has an independent keymap
 # for prefix searching
diff --git a/base/repl/REPL.jl b/base/repl/REPL.jl
index 76a8bca75189d..f6208db887883 100644
--- a/base/repl/REPL.jl
+++ b/base/repl/REPL.jl
@@ -392,7 +392,7 @@ function mode_idx(hist::REPLHistoryProvider, mode)
 end
 
 function add_history(hist::REPLHistoryProvider, s)
-    str = rstrip(String(s.input_buffer))
+    str = rstrip(String(take!(copy(s.input_buffer))))
     isempty(strip(str)) && return
     mode = mode_idx(hist, LineEdit.mode(s))
     !isempty(hist.history) &&
@@ -506,7 +506,7 @@ function history_move_prefix(s::LineEdit.PrefixSearchState,
                              prefix::AbstractString,
                              backwards::Bool,
                              cur_idx = hist.cur_idx)
-    cur_response = String(LineEdit.buffer(s))
+    cur_response = String(take!(copy(LineEdit.buffer(s))))
     # when searching forward, start at last_idx
     if !backwards && hist.last_idx > 0
         cur_idx = hist.last_idx
@@ -547,7 +547,7 @@ function history_search(hist::REPLHistoryProvider, query_buffer::IOBuffer, respo
     qpos = position(query_buffer)
     qpos > 0 || return true
     searchdata = beforecursor(query_buffer)
-    response_str = String(response_buffer)
+    response_str = String(take!(copy(response_buffer)))
 
     # Alright, first try to see if the current match still works
     a = position(response_buffer) + 1 # position is zero-indexed
@@ -596,7 +596,7 @@ LineEdit.reset_state(hist::REPLHistoryProvider) = history_reset_state(hist)
 
 function return_callback(s)
     ast = Base.syntax_deprecation_warnings(false) do
-        Base.parse_input_line(String(LineEdit.buffer(s)))
+        Base.parse_input_line(String(take!(copy(LineEdit.buffer(s)))))
     end
     if  !isa(ast, Expr) || (ast.head != :continue && ast.head != :incomplete)
         return true
diff --git a/test/dict.jl b/test/dict.jl
index d8001e27fc62e..c27af86751a5b 100644
--- a/test/dict.jl
+++ b/test/dict.jl
@@ -301,7 +301,7 @@ let d = Dict((1=>2) => (3=>45), (3=>10) => (10=>11))
 
     # Check explicitly for the expected strings, since the CPU bitness effects
     # dictionary ordering.
-    result = String(buf)
+    result = String(take!(buf))
     @test contains(result, "Dict")
     @test contains(result, "(1=>2)=>(3=>45)")
     @test contains(result, "(3=>10)=>(10=>11)")
@@ -314,8 +314,9 @@ let sbuff = IOBuffer(),
     io = Base.IOContext(Base.IOContext(sbuff, :limit => true), :displaysize => (10, 20))
 
     Base.show(io, MIME("text/plain"), Dict(Alpha()=>1))
-    @test !contains(String(sbuff), "…")
-    @test endswith(String(sbuff), "α => 1")
+    local str = String(take!(sbuff))
+    @test !contains(str, "…")
+    @test endswith(str, "α => 1")
 end
 
 # issue #2540
diff --git a/test/iobuffer.jl b/test/iobuffer.jl
index d8b3a19d80c8f..875504432879d 100644
--- a/test/iobuffer.jl
+++ b/test/iobuffer.jl
@@ -2,6 +2,8 @@
 
 ioslength(io::IOBuffer) = (io.seekable ? io.size : nb_available(io))
 
+bufcontents(io::Base.GenericIOBuffer) = unsafe_string(pointer(io.data), io.size)
+
 let io = IOBuffer()
 @test eof(io)
 @test_throws EOFError read(io,UInt8)
@@ -17,7 +19,7 @@ seek(io, 0)
 a = Array{UInt8}(2)
 @test read!(io, a) == a
 @test a == UInt8['b','c']
-@test String(io) == "abc"
+@test bufcontents(io) == "abc"
 seek(io, 1)
 truncate(io, 2)
 @test position(io) == 1
@@ -178,7 +180,7 @@ let io=IOBuffer(SubString("***αhelloworldω***",4,16)), io2 = IOBuffer(b"goodni
     @test_throws EOFError read(io,UInt8)
     skip(io, -3)
     @test readstring(io) == "dω"
-    @test String(io) == "αhelloworldω"
+    @test bufcontents(io) == "αhelloworldω"
     @test_throws ArgumentError write(io,"!")
     @test take!(io) == b"αhelloworldω"
     seek(io, 2)
@@ -193,7 +195,7 @@ let io=IOBuffer(SubString("***αhelloworldω***",4,16)), io2 = IOBuffer(b"goodni
     seek(io2, 0)
     write(io2, io2)
     @test readstring(io2) == ""
-    @test String(io2) == "goodnightmoonhelloworld"
+    @test bufcontents(io2) == "goodnightmoonhelloworld"
 end
 
 # issue #11917
diff --git a/test/repl.jl b/test/repl.jl
index 1e33a5ec0221d..5fb4d2655dd9e 100644
--- a/test/repl.jl
+++ b/test/repl.jl
@@ -677,7 +677,7 @@ function history_move_prefix(s::LineEdit.MIState,
     buf = LineEdit.buffer(s)
     pos = position(buf)
     prefix = REPL.beforecursor(buf)
-    allbuf = String(buf)
+    allbuf = String(take!(copy(buf)))
     cur_idx = hist.cur_idx
     # when searching forward, start at last_idx
     if !backwards && hist.last_idx > 0
diff --git a/test/replutil.jl b/test/replutil.jl
index 72f43108c00b3..d0597d4d919f5 100644
--- a/test/replutil.jl
+++ b/test/replutil.jl
@@ -450,7 +450,7 @@ let d = Dict(1 => 2, 3 => 45)
     buf = IOBuffer()
     td = TextDisplay(buf)
     display(td, d)
-    result = String(td.io)
+    result = String(take!(td.io))
 
     @test contains(result, summary(d))
 
@@ -466,13 +466,13 @@ let err, buf = IOBuffer()
     try Array() catch err end
     Base.show_method_candidates(buf,err)
     @test isa(err, MethodError)
-    @test contains(String(buf), "Closest candidates are:")
+    @test contains(String(take!(buf)), "Closest candidates are:")
 end
 
 # Issue 20111
 let K20111(x) = y -> x, buf = IOBuffer()
     show(buf, methods(K20111(1)))
-    @test contains(String(buf), " 1 method for generic function")
+    @test contains(String(take!(buf)), " 1 method for generic function")
 end
 
 # @macroexpand tests