diff --git a/spec/manual/string_normalize_spec.cr b/spec/manual/string_normalize_spec.cr
index 96d58c946894..9a67a891b702 100644
--- a/spec/manual/string_normalize_spec.cr
+++ b/spec/manual/string_normalize_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "http/client"
-require "../support/string"
+require "spec/helpers/string"
 
 UCD_ROOT = "http://www.unicode.org/Public/#{Unicode::VERSION}/ucd/"
 
@@ -27,31 +27,9 @@ private struct CodepointsEqualExpectation
   end
 end
 
-# same as `assert_prints`, but uses `CodepointsEqualExpectation` instead of `eq`
 private macro assert_prints_codepoints(call, str, desc, *, file = __FILE__, line = __LINE__)
   %expectation = CodepointsEqualExpectation.new(({{ str }}).as(String), {{ desc }})
-
-  %result = {{ call }}
-  %result.should be_a(String), file: {{ file }}, line: {{ line }}
-  %result.should %expectation, file: {{ file }}, line: {{ line }}
-
-  String.build do |io|
-    {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
-      io,
-      {% for arg in call.args %} {{ arg }}, {% end %}
-      {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
-    ) {{ call.block }}
-  end.should %expectation, file: {{ file }}, line: {{ line }}
-
-  {% unless flag?(:without_iconv) %}
-    string_build_via_utf16 do |io|
-      {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
-        io,
-        {% for arg in call.args %} {{ arg }}, {% end %}
-        {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
-      ) {{ call.block }}
-    end.should %expectation, file: {{ file }}, line: {{ line }}
-  {% end %}
+  assert_prints({{ call }}, should: %expectation, file: {{ file }}, line: {{ line }})
 end
 
 private def assert_normalized(source, target, form : Unicode::NormalizationForm, *, file = __FILE__, line = __LINE__)
diff --git a/spec/std/base64_spec.cr b/spec/std/base64_spec.cr
index 89e5d863825a..567340d6fa38 100644
--- a/spec/std/base64_spec.cr
+++ b/spec/std/base64_spec.cr
@@ -1,7 +1,7 @@
 require "spec"
 require "base64"
 require "crystal/digest/md5"
-require "../support/string"
+require "spec/helpers/string"
 
 # rearrange parameters for `assert_prints`
 {% for method in %w(encode strict_encode urlsafe_encode) %}
diff --git a/spec/std/big/big_decimal_spec.cr b/spec/std/big/big_decimal_spec.cr
index 085736de5294..81fd2fb82b36 100644
--- a/spec/std/big/big_decimal_spec.cr
+++ b/spec/std/big/big_decimal_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "big"
-require "../../support/string"
+require "spec/helpers/string"
 
 describe BigDecimal do
   it "initializes from valid input" do
diff --git a/spec/std/big/big_float_spec.cr b/spec/std/big/big_float_spec.cr
index 69322ef86c1a..beea465df8c3 100644
--- a/spec/std/big/big_float_spec.cr
+++ b/spec/std/big/big_float_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "big"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def it_converts_to_s(value : BigFloat, str, *, file = __FILE__, line = __LINE__)
   it "converts to #{str}", file: file, line: line do
diff --git a/spec/std/char_spec.cr b/spec/std/char_spec.cr
index 2c5ea0345252..e947fc17d296 100644
--- a/spec/std/char_spec.cr
+++ b/spec/std/char_spec.cr
@@ -1,7 +1,7 @@
 require "spec"
 require "unicode"
 require "spec/helpers/iterate"
-require "../support/string"
+require "spec/helpers/string"
 
 describe "Char" do
   describe "#upcase" do
diff --git a/spec/std/csv/csv_build_spec.cr b/spec/std/csv/csv_build_spec.cr
index 7cbb91f7f746..b222df22ab93 100644
--- a/spec/std/csv/csv_build_spec.cr
+++ b/spec/std/csv/csv_build_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "csv"
-require "../../support/string"
+require "spec/helpers/string"
 
 describe CSV do
   describe "build" do
diff --git a/spec/std/enum_spec.cr b/spec/std/enum_spec.cr
index 0b7c846138c2..437ba42e1c1f 100644
--- a/spec/std/enum_spec.cr
+++ b/spec/std/enum_spec.cr
@@ -1,5 +1,5 @@
 require "spec"
-require "../support/string"
+require "spec/helpers/string"
 
 enum SpecEnum : Int8
   One
diff --git a/spec/std/float_printer_spec.cr b/spec/std/float_printer_spec.cr
index 94c2409c71a9..3db7b344b170 100644
--- a/spec/std/float_printer_spec.cr
+++ b/spec/std/float_printer_spec.cr
@@ -35,7 +35,7 @@
 
 require "spec"
 require "./spec_helper"
-require "../support/string"
+require "spec/helpers/string"
 require "../support/number"
 
 # Tests that `v.to_s` is the same as the *v* literal is written in the source
diff --git a/spec/std/float_spec.cr b/spec/std/float_spec.cr
index b93597c389c6..4ddfd194313d 100644
--- a/spec/std/float_spec.cr
+++ b/spec/std/float_spec.cr
@@ -1,5 +1,5 @@
 require "spec"
-require "../support/string"
+require "spec/helpers/string"
 
 describe "Float" do
   describe "**" do
diff --git a/spec/std/http/http_spec.cr b/spec/std/http/http_spec.cr
index 6d23b9356cd6..6159cdc9dc5e 100644
--- a/spec/std/http/http_spec.cr
+++ b/spec/std/http/http_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "http"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def http_quote_string(io : IO, string)
   HTTP.quote_string(string, io)
diff --git a/spec/std/humanize_spec.cr b/spec/std/humanize_spec.cr
index a7342b495947..482be02b77eb 100644
--- a/spec/std/humanize_spec.cr
+++ b/spec/std/humanize_spec.cr
@@ -1,5 +1,5 @@
 require "spec"
-require "../support/string"
+require "spec/helpers/string"
 
 private LENGTH_UNITS = ->(magnitude : Int32, number : Float64) do
   case magnitude
diff --git a/spec/std/json/builder_spec.cr b/spec/std/json/builder_spec.cr
index 66f9be33b741..2bbd9c49a5ef 100644
--- a/spec/std/json/builder_spec.cr
+++ b/spec/std/json/builder_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "json"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def assert_built(expected, *, file = __FILE__, line = __LINE__, &)
   assert_prints JSON.build { |json| with json yield json }, expected, file: file, line: line
diff --git a/spec/std/mime/media_type_spec.cr b/spec/std/mime/media_type_spec.cr
index 5ed73256ecb4..383e62c5ea42 100644
--- a/spec/std/mime/media_type_spec.cr
+++ b/spec/std/mime/media_type_spec.cr
@@ -1,6 +1,6 @@
 require "../spec_helper"
 require "mime/media_type"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def parse(string)
   type = MIME::MediaType.parse(string)
diff --git a/spec/std/process/status_spec.cr b/spec/std/process/status_spec.cr
index 871e2bd837df..45f60aba4c06 100644
--- a/spec/std/process/status_spec.cr
+++ b/spec/std/process/status_spec.cr
@@ -1,5 +1,5 @@
 require "spec"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def exit_status(status)
   {% if flag?(:unix) %}
diff --git a/spec/std/slice_spec.cr b/spec/std/slice_spec.cr
index 72803e0b661b..4a2469f69aa7 100644
--- a/spec/std/slice_spec.cr
+++ b/spec/std/slice_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "spec/helpers/iterate"
-require "../support/string"
+require "spec/helpers/string"
 
 private class BadSortingClass
   include Comparable(self)
diff --git a/spec/std/socket/address_spec.cr b/spec/std/socket/address_spec.cr
index 6835b8e518d1..d2e4768db987 100644
--- a/spec/std/socket/address_spec.cr
+++ b/spec/std/socket/address_spec.cr
@@ -1,7 +1,7 @@
 require "spec"
 require "socket"
 require "../../support/win32"
-require "../../support/string"
+require "spec/helpers/string"
 
 describe Socket::Address do
   describe ".parse" do
diff --git a/spec/std/sprintf_spec.cr b/spec/std/sprintf_spec.cr
index bd72c962bf35..ad6ceda4809d 100644
--- a/spec/std/sprintf_spec.cr
+++ b/spec/std/sprintf_spec.cr
@@ -1,5 +1,5 @@
 require "./spec_helper"
-require "../support/string"
+require "spec/helpers/string"
 require "big"
 
 # use same name for `sprintf` and `IO#printf` so that `assert_prints` can be leveraged
diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 7a9f587126f3..905930463cfc 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -1,6 +1,6 @@
 require "./spec_helper"
 require "spec/helpers/iterate"
-require "../support/string"
+require "spec/helpers/string"
 
 describe "String" do
   describe "[]" do
@@ -692,7 +692,8 @@ describe "String" do
     end
 
     it "does not touch invalid code units in an otherwise ascii string" do
-      assert_prints "\xB5!\xE0\xC1\xB5?".capitalize, "\xB5!\xE0\xC1\xB5?"
+      "\xB5!\xE0\xC1\xB5?".capitalize.should eq("\xB5!\xE0\xC1\xB5?")
+      String.build { |io| "\xB5!\xE0\xC1\xB5?".capitalize(io) }.should eq("\xB5!\xE0\xC1\xB5?".scrub)
     end
   end
 
@@ -710,8 +711,10 @@ describe "String" do
     end
 
     it "does not touch invalid code units in an otherwise ascii string" do
-      assert_prints "\xB5!\xE0\xC1\xB5?".titleize, "\xB5!\xE0\xC1\xB5?"
-      assert_prints "a\xA0b".titleize, "A\xA0b"
+      "\xB5!\xE0\xC1\xB5?".titleize.should eq("\xB5!\xE0\xC1\xB5?")
+      "a\xA0b".titleize.should eq("A\xA0b")
+      String.build { |io| "\xB5!\xE0\xC1\xB5?".titleize(io) }.should eq("\xB5!\xE0\xC1\xB5?".scrub)
+      String.build { |io| "a\xA0b".titleize(io) }.should eq("A\xA0b".scrub)
     end
   end
 
diff --git a/spec/std/time/format_spec.cr b/spec/std/time/format_spec.cr
index 08baa2e910fd..8e7cb27b6bd7 100644
--- a/spec/std/time/format_spec.cr
+++ b/spec/std/time/format_spec.cr
@@ -1,5 +1,5 @@
 require "./spec_helper"
-require "../../support/string"
+require "spec/helpers/string"
 
 def parse_time(format, string)
   Time.parse_utc(format, string)
diff --git a/spec/std/uri_spec.cr b/spec/std/uri_spec.cr
index 402263742a96..9f0e69458629 100644
--- a/spec/std/uri_spec.cr
+++ b/spec/std/uri_spec.cr
@@ -2,7 +2,7 @@ require "spec"
 require "uri"
 require "uri/json"
 require "uri/yaml"
-require "../support/string"
+require "spec/helpers/string"
 
 private def assert_uri(string, file = __FILE__, line = __LINE__, **args)
   it "`#{string}`", file, line do
diff --git a/spec/std/uuid_spec.cr b/spec/std/uuid_spec.cr
index 5086b7964f95..0992c92095d1 100644
--- a/spec/std/uuid_spec.cr
+++ b/spec/std/uuid_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "uuid"
-require "../support/string"
+require "spec/helpers/string"
 
 describe "UUID" do
   describe "#==" do
diff --git a/spec/std/xml/builder_spec.cr b/spec/std/xml/builder_spec.cr
index 0fd7450b0104..3aaaa9f503d0 100644
--- a/spec/std/xml/builder_spec.cr
+++ b/spec/std/xml/builder_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "xml"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def assert_built(expected, quote_char = nil, *, file = __FILE__, line = __LINE__, &)
   assert_prints XML.build(quote_char: quote_char) { |xml| with xml yield xml }, expected, file: file, line: line
diff --git a/spec/std/xml/xml_spec.cr b/spec/std/xml/xml_spec.cr
index 3c695f517497..efbb79e6e226 100644
--- a/spec/std/xml/xml_spec.cr
+++ b/spec/std/xml/xml_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "xml"
-require "../../support/string"
+require "spec/helpers/string"
 
 describe XML do
   it "parses" do
diff --git a/spec/std/yaml/builder_spec.cr b/spec/std/yaml/builder_spec.cr
index e225a745587d..342777445ca6 100644
--- a/spec/std/yaml/builder_spec.cr
+++ b/spec/std/yaml/builder_spec.cr
@@ -1,6 +1,6 @@
 require "spec"
 require "yaml"
-require "../../support/string"
+require "spec/helpers/string"
 
 private def assert_built(expected, expect_document_end = false, *, file = __FILE__, line = __LINE__, &)
   # libyaml 0.2.1 removed the erroneously written document end marker (`...`) after some scalars in root context (see https://github.com/yaml/libyaml/pull/18).
diff --git a/spec/support/string.cr b/spec/support/string.cr
deleted file mode 100644
index be2306bdd509..000000000000
--- a/spec/support/string.cr
+++ /dev/null
@@ -1,61 +0,0 @@
-# Builds a `String` through a UTF-16 `IO`.
-#
-# Similar to `String.build`, but the yielded `IO` is configured to use the
-# UTF-16 encoding, and the written contents are decoded back into a UTF-8
-# `String`. This method is mainly used by `assert_prints` to test the behaviour
-# of string-generating methods under different encodings.
-#
-# Raises if the `without_iconv` flag is set.
-def string_build_via_utf16(& : IO -> _)
-  {% if flag?(:without_iconv) %}
-    raise NotImplementedError.new("string_build_via_utf16")
-  {% else %}
-    io = IO::Memory.new
-    io.set_encoding(IO::ByteFormat::SystemEndian == IO::ByteFormat::LittleEndian ? "UTF-16LE" : "UTF-16BE")
-    yield io
-    byte_slice = io.to_slice
-    utf16_slice = byte_slice.unsafe_slice_of(UInt16)
-    String.from_utf16(utf16_slice)
-  {% end %}
-end
-
-# Asserts that the given *call* and its `IO`-accepting variants produce the
-# given string *str*.
-#
-# Given a call of the form `foo.bar(*args, **opts)`, tests the following cases:
-#
-# * This call itself should return a `String` equal to *str*.
-# * `String.build { |io| foo.bar(io, *args, **opts) }` should be equal to
-#   `str.scrub`; writing to a `String::Builder` must not produce any invalid
-#   UTF-8 byte sequences.
-# * `string_build_via_utf16 { |io| foo.bar(io, *args, **opts) }` should also be
-#   equal to `str.scrub`; that is, the `IO` overload should not fail when the
-#   `IO` argument uses a non-default encoding. This case is skipped if the
-#   `without_iconv` flag is set.
-macro assert_prints(call, str, *, file = __FILE__, line = __LINE__)
-  %str = ({{ str }}).as(String)
-  %file = {{ file }}
-  %line = {{ line }}
-
-  %result = {{ call }}
-  %result.should be_a(String), file: %file, line: %line
-  %result.should eq(%str), file: %file, line: %line
-
-  String.build do |io|
-    {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
-      io,
-      {% for arg in call.args %} {{ arg }}, {% end %}
-      {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
-    ) {{ call.block }}
-  end.should eq(%str.scrub), file: %file, line: %line
-
-  {% unless flag?(:without_iconv) %}
-    string_build_via_utf16 do |io|
-      {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
-        io,
-        {% for arg in call.args %} {{ arg }}, {% end %}
-        {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
-      ) {{ call.block }}
-    end.should eq(%str.scrub), file: %file, line: %line
-  {% end %}
-end
diff --git a/src/spec/helpers/string.cr b/src/spec/helpers/string.cr
new file mode 100644
index 000000000000..892d2012524e
--- /dev/null
+++ b/src/spec/helpers/string.cr
@@ -0,0 +1,92 @@
+module Spec::Methods
+  # Asserts that the given *call* and its `IO`-accepting variant both match the
+  # given *expectation*, used to test string printing.
+  #
+  # Given a call of the form `foo.bar(*args, **opts)`, this tests the following
+  # cases:
+  #
+  # * The call itself. Additionally this call must return a `String`.
+  # * `String.build { |io| foo.bar(io, *args, **opts) }`, which constructs a
+  #   `String` via an `IO` overload.
+  # * `io = ...; foo.bar(io, *args, **opts); io.to_s`, where `io` is an `IO`
+  #   configured to use the UTF-16 encoding, and contents written to it are
+  #   decoded back into a UTF-8 `String`. This case ensures that the `IO`
+  #   overload does not produce malformed UTF-8 byte sequences via a non-default
+  #   encoding. This case is skipped if the `without_iconv` flag is set.
+  #
+  # The overload that accepts a *str* argument is usually easier to work with.
+  macro assert_prints(call, *, should expectation, file = __FILE__, line = __LINE__)
+    %expectation = {{ expectation }}
+    %file = {{ file }}
+    %line = {{ line }}
+
+    %result = {{ call }}
+    %result.should be_a(::String), file: %file, line: %line
+    %result.should(%expectation, file: %file, line: %line)
+
+    ::String.build do |io|
+      {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
+        io,
+        {% for arg in call.args %} {{ arg }}, {% end %}
+        {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
+      ) {{ call.block }}
+    end.should(%expectation, file: %file, line: %line)
+
+    {% unless flag?(:without_iconv) %}
+      %utf16_io = ::IO::Memory.new
+      %utf16_io.set_encoding(::IO::ByteFormat::SystemEndian == ::IO::ByteFormat::LittleEndian ? "UTF-16LE" : "UTF-16BE")
+      {% if call.receiver %}{{ call.receiver }}.{% end %}{{ call.name }}(
+        %utf16_io,
+        {% for arg in call.args %} {{ arg }}, {% end %}
+        {% if call.named_args %} {% for narg in call.named_args %} {{ narg.name }}: {{ narg.value }}, {% end %} {% end %}
+      ) {{ call.block }}
+      %result = ::String.from_utf16(%utf16_io.to_slice.unsafe_slice_of(::UInt16))
+      %result.should(%expectation, file: %file, line: %line)
+    {% end %}
+  end
+
+  # Asserts that the given *call* and its `IO`-accepting variant both produce
+  # the given string *str*.
+  #
+  # Equivalent to `assert_prints call, should: eq(str)`. *str* must be validly
+  # encoded in UTF-8.
+  #
+  # ```
+  # require "spec"
+  # require "spec/helpers/string"
+  #
+  # it "prints integers with `Int#to_s`" do
+  #   assert_prints 123.to_s, "123"
+  #   assert_prints 123.to_s(16), "7b"
+  # end
+  # ```
+  #
+  # Methods that do not follow the convention of `IO`-accepting and
+  # `String`-returning overloads can also be tested as long as suitable wrapper
+  # methods are defined:
+  #
+  # ```
+  # require "spec"
+  # require "spec/helpers/string"
+  #
+  # private def fprintf(format, *args)
+  #   sprintf(format, *args)
+  # end
+  #
+  # private def fprintf(io : IO, format, *args)
+  #   io.printf(format, *args)
+  # end
+  #
+  # it "prints with `sprintf` and `IO#printf`" do
+  #   assert_prints fprintf("%d", 123), "123"
+  #   assert_prints fprintf("%x %b", 123, 6), "7b 110"
+  # end
+  # ```
+  macro assert_prints(call, str, *, file = __FILE__, line = __LINE__)
+    %str = ({{ str }}).as(::String)
+    unless %str.valid_encoding?
+      ::fail "`str` contains invalid UTF-8 byte sequences: #{%str.inspect}", file: {{ file }}, line: {{ line }}
+    end
+    assert_prints({{ call }}, should: eq(%str), file: {{ file }}, line: {{ line }})
+  end
+end