Add ``redundant-u-string-prefix`` checker (#4804)

* Add ``redundant-u-string-prefix`` checker
This adds a checker for u-prefixes for strings, as used in Python 2. It only work in python 3.8 and above.
Closes #4102

16 files changed, 52 insertions, 26 deletions

diff --git a/tests/functional/a/anomalous_unicode_escape_py3.py b/tests/functional/a/anomalous_unicode_escape_py3.py
index 18ebdf5f6..7c863097b 100644
--- a/tests/functional/a/anomalous_unicode_escape_py3.py
+++ b/tests/functional/a/anomalous_unicode_escape_py3.py
@@ -1,4 +1,4 @@
-# pylint:disable=pointless-string-statement
+# pylint:disable=pointless-string-statement, redundant-u-string-prefix
 """Test for backslash escapes in byte vs unicode strings"""
 
 # Would be valid in Unicode, but probably not what you want otherwise
diff --git a/tests/functional/a/assign/assigning_non_slot.py b/tests/functional/a/assign/assigning_non_slot.py
index 600a2d5a2..4789014ff 100644
--- a/tests/functional/a/assign/assigning_non_slot.py
+++ b/tests/functional/a/assign/assigning_non_slot.py
@@ -1,7 +1,7 @@
 """ Checks assigning attributes not found in class slots
 will trigger assigning-non-slot warning.
 """
-# pylint: disable=too-few-public-methods, no-init, missing-docstring, no-absolute-import, import-error, useless-object-inheritance
+# pylint: disable=too-few-public-methods, no-init, missing-docstring, no-absolute-import, import-error, useless-object-inheritance, redundant-u-string-prefix
 from collections import deque
 
 from missing import Unknown
diff --git a/tests/functional/d/duplicate_dict_literal_key.py b/tests/functional/d/duplicate_dict_literal_key.py
index 67eab280f..6d359ff0a 100644
--- a/tests/functional/d/duplicate_dict_literal_key.py
+++ b/tests/functional/d/duplicate_dict_literal_key.py
@@ -1,5 +1,5 @@
 """Check multiple key definition"""
-# pylint: disable=pointless-statement
+# pylint: disable=pointless-statement, redundant-u-string-prefix
 
 correct_dict = {
     'tea': 'for two',
diff --git a/tests/functional/e/excess_escapes.py b/tests/functional/e/excess_escapes.py
index 04fa4db31..19fd5cb4d 100644
--- a/tests/functional/e/excess_escapes.py
+++ b/tests/functional/e/excess_escapes.py
@@ -1,5 +1,6 @@
 # pylint:disable=pointless-string-statement, fixme, misplaced-comparison-constant, comparison-with-itself
 """Stray backslash escapes may be missing a raw-string prefix."""
+# pylint: disable=redundant-u-string-prefix
 
 __revision__ = '$Id$'
 
diff --git a/tests/functional/e/excess_escapes.txt b/tests/functional/e/excess_escapes.txt
index 20fa55243..4082e4709 100644
--- a/tests/functional/e/excess_escapes.txt
+++ b/tests/functional/e/excess_escapes.txt
@@ -1,9 +1,9 @@
-anomalous-backslash-in-string:7:5::"Anomalous backslash in string: '\['. String constant might be missing an r prefix."
-anomalous-backslash-in-string:7:7::"Anomalous backslash in string: '\]'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:8:8::"Anomalous backslash in string: '\/'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:9:20::"Anomalous backslash in string: '\`'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:16:15::"Anomalous backslash in string: '\o'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:16:20::"Anomalous backslash in string: '\o'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:18:13::"Anomalous backslash in string: '\8'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:18:17::"Anomalous backslash in string: '\9'. String constant might be missing an r prefix."
-anomalous-backslash-in-string:31:42::"Anomalous backslash in string: '\P'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:8:5::"Anomalous backslash in string: '\['. String constant might be missing an r prefix."
+anomalous-backslash-in-string:8:7::"Anomalous backslash in string: '\]'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:9:8::"Anomalous backslash in string: '\/'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:10:20::"Anomalous backslash in string: '\`'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:17:15::"Anomalous backslash in string: '\o'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:17:20::"Anomalous backslash in string: '\o'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:19:13::"Anomalous backslash in string: '\8'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:19:17::"Anomalous backslash in string: '\9'. String constant might be missing an r prefix."
+anomalous-backslash-in-string:32:42::"Anomalous backslash in string: '\P'. String constant might be missing an r prefix."
diff --git a/tests/functional/i/implicit/implicit_str_concat.py b/tests/functional/i/implicit/implicit_str_concat.py
index 707fa75bf..1ad9aed8c 100644
--- a/tests/functional/i/implicit/implicit_str_concat.py
+++ b/tests/functional/i/implicit/implicit_str_concat.py
@@ -1,4 +1,4 @@
-#pylint: disable=invalid-name,missing-docstring
+#pylint: disable=invalid-name,missing-docstring,redundant-u-string-prefix
 
 # Basic test with a list
 TEST_LIST1 = ['a' 'b']  # [implicit-str-concat]
diff --git a/tests/functional/i/iterable_context.py b/tests/functional/i/iterable_context.py
index 6efefd550..2754d179f 100644
--- a/tests/functional/i/iterable_context.py
+++ b/tests/functional/i/iterable_context.py
@@ -2,7 +2,7 @@
 Checks that primitive values are not used in an
 iterating/mapping context.
 """
-# pylint: disable=missing-docstring,invalid-name,too-few-public-methods,no-init,no-self-use,import-error,unused-argument,bad-mcs-method-argument,wrong-import-position,no-else-return, useless-object-inheritance, unnecessary-comprehension
+# pylint: disable=missing-docstring,invalid-name,too-few-public-methods,no-init,no-self-use,import-error,unused-argument,bad-mcs-method-argument,wrong-import-position,no-else-return, useless-object-inheritance, unnecessary-comprehension,redundant-u-string-prefix
 from __future__ import print_function
 
 # primitives
diff --git a/tests/functional/m/member/membership_protocol.py b/tests/functional/m/member/membership_protocol.py
index 3401f4bb4..e5efc8916 100644
--- a/tests/functional/m/member/membership_protocol.py
+++ b/tests/functional/m/member/membership_protocol.py
@@ -1,4 +1,4 @@
-# pylint: disable=missing-docstring,pointless-statement,expression-not-assigned,too-few-public-methods,import-error,no-init,wrong-import-position,no-else-return, comparison-with-itself, useless-object-inheritance
+# pylint: disable=missing-docstring,pointless-statement,expression-not-assigned,too-few-public-methods,import-error,no-init,wrong-import-position,no-else-return, comparison-with-itself, useless-object-inheritance, redundant-u-string-prefix
 
 # standard types
 1 in [1, 2, 3]
diff --git a/tests/functional/r/raise/raising_format_tuple.py b/tests/functional/r/raise/raising_format_tuple.py
index 0f3a3349a..12e763d47 100644
--- a/tests/functional/r/raise/raising_format_tuple.py
+++ b/tests/functional/r/raise/raising_format_tuple.py
@@ -4,6 +4,7 @@ contains a percent sign, thus suggesting a % string formatting was intended
 instead.  The same holds for a string containing {...} suggesting str.format()
 was intended.
 '''
+# pylint: disable=redundant-u-string-prefix
 
 def bad_percent(arg):
     '''Raising a percent-formatted string and an argument'''
diff --git a/tests/functional/r/raise/raising_format_tuple.txt b/tests/functional/r/raise/raising_format_tuple.txt
index 756fd1092..d663ca454 100644
--- a/tests/functional/r/raise/raising_format_tuple.txt
+++ b/tests/functional/r/raise/raising_format_tuple.txt
@@ -1,7 +1,7 @@
-raising-format-tuple:10:4:bad_percent:Exception arguments suggest string formatting might be intended
-raising-format-tuple:18:4:bad_multiarg:Exception arguments suggest string formatting might be intended
-raising-format-tuple:26:4:bad_braces:Exception arguments suggest string formatting might be intended
-raising-format-tuple:34:4:bad_multistring:Exception arguments suggest string formatting might be intended
-raising-format-tuple:40:4:bad_triplequote:Exception arguments suggest string formatting might be intended
-raising-format-tuple:46:4:bad_unicode:Exception arguments suggest string formatting might be intended
-raising-format-tuple:51:4:raise_something_without_name:Exception arguments suggest string formatting might be intended
+raising-format-tuple:11:4:bad_percent:Exception arguments suggest string formatting might be intended
+raising-format-tuple:19:4:bad_multiarg:Exception arguments suggest string formatting might be intended
+raising-format-tuple:27:4:bad_braces:Exception arguments suggest string formatting might be intended
+raising-format-tuple:35:4:bad_multistring:Exception arguments suggest string formatting might be intended
+raising-format-tuple:41:4:bad_triplequote:Exception arguments suggest string formatting might be intended
+raising-format-tuple:47:4:bad_unicode:Exception arguments suggest string formatting might be intended
+raising-format-tuple:52:4:raise_something_without_name:Exception arguments suggest string formatting might be intended
diff --git a/tests/functional/r/redundant_u_string_prefix.py b/tests/functional/r/redundant_u_string_prefix.py
new file mode 100644
index 000000000..7ffbabd09
--- /dev/null
+++ b/tests/functional/r/redundant_u_string_prefix.py
@@ -0,0 +1,14 @@
+""""Checks for redundant u-prefixes for strings"""
+# pylint: disable=missing-function-docstring
+
+def print_good():
+    print("String")
+    print(f"String{1 + 1}")
+
+
+def print_bad():
+    print(u"String")  # [redundant-u-string-prefix]
+    print(u'String')  # [redundant-u-string-prefix]
+    print([u"String", u"String2"])  # [redundant-u-string-prefix, redundant-u-string-prefix]
+    print((u"String", u"String2"))  # [redundant-u-string-prefix, redundant-u-string-prefix]
+    print({1: u"String"})  # [redundant-u-string-prefix]
diff --git a/tests/functional/r/redundant_u_string_prefix.rc b/tests/functional/r/redundant_u_string_prefix.rc
new file mode 100644
index 000000000..85fc502b3
--- /dev/null
+++ b/tests/functional/r/redundant_u_string_prefix.rc
@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8
diff --git a/tests/functional/r/redundant_u_string_prefix.txt b/tests/functional/r/redundant_u_string_prefix.txt
new file mode 100644
index 000000000..f970ef8e8
--- /dev/null
+++ b/tests/functional/r/redundant_u_string_prefix.txt
@@ -0,0 +1,7 @@
+redundant-u-string-prefix:10:10::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:11:10::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:12:11::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:12:22::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:13:11::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:13:22::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
+redundant-u-string-prefix:14:14::The u prefix for strings is no longer necessary in Python >=3.0:HIGH
diff --git a/tests/functional/s/suspicious_str_strip_call_py3.py b/tests/functional/s/suspicious_str_strip_call_py3.py
index e859f25f8..40684cc1d 100644
--- a/tests/functional/s/suspicious_str_strip_call_py3.py
+++ b/tests/functional/s/suspicious_str_strip_call_py3.py
@@ -1,4 +1,5 @@
 """Suspicious str.strip calls."""
+# pylint: disable=redundant-u-string-prefix
 __revision__ = 1
 
 ''.strip('yo')
diff --git a/tests/functional/s/suspicious_str_strip_call_py3.txt b/tests/functional/s/suspicious_str_strip_call_py3.txt
index dc50d46c1..7e338386c 100644
--- a/tests/functional/s/suspicious_str_strip_call_py3.txt
+++ b/tests/functional/s/suspicious_str_strip_call_py3.txt
@@ -1,3 +1,3 @@
-bad-str-strip-call:7:0::Suspicious argument in str.strip call
-bad-str-strip-call:8:0::Suspicious argument in str.lstrip call
-bad-str-strip-call:9:0::Suspicious argument in bytes.rstrip call
+bad-str-strip-call:8:0::Suspicious argument in str.strip call
+bad-str-strip-call:9:0::Suspicious argument in str.lstrip call
+bad-str-strip-call:10:0::Suspicious argument in bytes.rstrip call
diff --git a/tests/functional/u/unsubscriptable_value.py b/tests/functional/u/unsubscriptable_value.py
index 73c3ac40e..6a97cc4c8 100644
--- a/tests/functional/u/unsubscriptable_value.py
+++ b/tests/functional/u/unsubscriptable_value.py
@@ -3,7 +3,7 @@ Checks that value used in a subscript supports subscription
 (i.e. defines __getitem__ method).
 """
 # pylint: disable=missing-docstring,pointless-statement,expression-not-assigned,wrong-import-position, unnecessary-comprehension
-# pylint: disable=too-few-public-methods,import-error,invalid-name,wrong-import-order, useless-object-inheritance
+# pylint: disable=too-few-public-methods,import-error,invalid-name,wrong-import-order, useless-object-inheritance, redundant-u-string-prefix
 import six
 
 # primitives