Add a string_output_stream_t to collect builtin output

This is used when creating a function; this breaks a dependency on the
more complicated buffered_output_stream_t to ease refactoring.

src/io.cpp

@@ -373,6 +373,8 @@ void fd_output_stream_t::append(const wchar_t *s, size_t amt) {
 
 void null_output_stream_t::append(const wchar_t *, size_t) {}
 
+void string_output_stream_t::append(const wchar_t *s, size_t amt) { contents_.append(s, amt); }
+
 void buffered_output_stream_t::append(const wchar_t *s, size_t amt) { buffer_.append(s, s + amt); }
 
 void buffered_output_stream_t::append_with_separation(const wchar_t *s, size_t len,

src/io.h

@@ -475,6 +475,19 @@ class fd_output_stream_t final : public output_stream_t {
     bool errored_{false};
 };
 
+/// A simple output stream which buffers into a wcstring.
+class string_output_stream_t final : public output_stream_t {
+   public:
+    string_output_stream_t() = default;
+    void append(const wchar_t *s, size_t amt) override;
+
+    /// \return the wcstring containing the output.
+    const wcstring &contents() const { return contents_; }
+
+   private:
+    wcstring contents_;
+};
+
 /// An output stream for builtins which buffers into a separated buffer.
 class buffered_output_stream_t final : public output_stream_t {
    public:

src/parse_execution.cpp

@@ -390,9 +390,8 @@ end_execution_reason_t parse_execution_context_t::run_function_statement(
         return result;
     }
     trace_if_enabled(*parser, L"function", arguments);
-    // no limit on the amount of output from builtin_function()
+    null_output_stream_t outs;
-    buffered_output_stream_t outs(0);
+    string_output_stream_t errs;
-    buffered_output_stream_t errs(0);
     io_streams_t streams(outs, errs);
     int err_code = 0;
     maybe_t<int> err = builtin_function(*parser, streams, arguments, pstree, statement);
@@ -402,7 +401,7 @@ end_execution_reason_t parse_execution_context_t::run_function_statement(
         parser->set_last_statuses(statuses_t::just(err_code));
     }
 
-    wcstring errtext = errs.contents();
+    const wcstring &errtext = errs.contents();
     if (!errtext.empty()) {
         return this->report_error(err_code, header, L"%ls", errtext.c_str());
     }