diff --git a/test/Utils/update-verify-tests/diag-at-eof.swift b/test/Utils/update-verify-tests/diag-at-eof.swift
new file mode 100644
index 0000000000000..f0cf41c353e08
--- /dev/null
+++ b/test/Utils/update-verify-tests/diag-at-eof.swift
@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+
+//--- test.swift.expected
+// expected-note@+1{{to match this opening '{'}}
+func foo() {
+
+// expected-error@+1{{expected '}' at end of brace statement}}
diff --git a/test/Utils/update-verify-tests/duplicate-diag.swift b/test/Utils/update-verify-tests/duplicate-diag.swift
new file mode 100644
index 0000000000000..02ac1beaab443
--- /dev/null
+++ b/test/Utils/update-verify-tests/duplicate-diag.swift
@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  // expected-error@+1{{cannot find 'a' in scope}}
+  a = 2; a = 2;
+  b = 2; b = 2;
+
+  // expected-error@+1 3{{cannot find 'c' in scope}}
+  c = 2; c = 2;
+  // expected-error 3{{asdf}}
+}
+//--- test.swift.expected
+func foo() {
+  // expected-error@+1 2{{cannot find 'a' in scope}}
+  a = 2; a = 2;
+  // expected-error@+1 2{{cannot find 'b' in scope}}
+  b = 2; b = 2;
+
+  // expected-error@+1 2{{cannot find 'c' in scope}}
+  c = 2; c = 2;
+}
diff --git a/test/Utils/update-verify-tests/infer-indentation.swift b/test/Utils/update-verify-tests/infer-indentation.swift
new file mode 100644
index 0000000000000..0fecc83e27872
--- /dev/null
+++ b/test/Utils/update-verify-tests/infer-indentation.swift
@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+         //     expected-error@+1    2{{cannot find 'a' in scope}}
+    a = 2; a = 2; b = 2; b = 2; c = 2;
+         //     expected-error@+1    2{{asdf}}
+    d = 2;
+    e = 2; f = 2;                 //     expected-error    2{{cannot find 'e' in scope}}
+}
+
+//--- test.swift.expected
+func foo() {
+         //     expected-error@+3    {{cannot find 'c' in scope}}
+         //     expected-error@+2    2{{cannot find 'b' in scope}}
+         //     expected-error@+1    2{{cannot find 'a' in scope}}
+    a = 2; a = 2; b = 2; b = 2; c = 2;
+         //     expected-error@+1    {{cannot find 'd' in scope}}
+    d = 2;
+    //     expected-error@+1    {{cannot find 'f' in scope}}
+    e = 2; f = 2;                 //     expected-error    {{cannot find 'e' in scope}}
+}
+
diff --git a/test/Utils/update-verify-tests/leave-existing-diags.swift b/test/Utils/update-verify-tests/leave-existing-diags.swift
new file mode 100644
index 0000000000000..4cb3e12c87932
--- /dev/null
+++ b/test/Utils/update-verify-tests/leave-existing-diags.swift
@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+    a = 2;
+    // expected-error@-1{{cannot find 'a' in scope}}
+    b = 2;// expected-error{{cannot find 'b' in scope}}
+    c = 2;
+    // expected-error@5{{cannot find 'c' in scope}}
+    d = 2; // expected-error{{'d' in scope}}
+
+    e = 2; // error to trigger mismatch
+}
+
+//--- test.swift.expected
+func foo() {
+    a = 2;
+    // expected-error@-1{{cannot find 'a' in scope}}
+    b = 2;// expected-error{{cannot find 'b' in scope}}
+    c = 2;
+    // expected-error@5{{cannot find 'c' in scope}}
+    d = 2; // expected-error{{'d' in scope}}
+
+    // expected-error@+1{{cannot find 'e' in scope}}
+    e = 2; // error to trigger mismatch
+}
+
diff --git a/test/Utils/update-verify-tests/multiple-errors.swift b/test/Utils/update-verify-tests/multiple-errors.swift
new file mode 100644
index 0000000000000..dbc6b72b36d53
--- /dev/null
+++ b/test/Utils/update-verify-tests/multiple-errors.swift
@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  a = 2
+  b = 2
+
+  c = 2
+}
+//--- test.swift.expected
+func foo() {
+  // expected-error@+1{{cannot find 'a' in scope}}
+  a = 2
+  // expected-error@+1{{cannot find 'b' in scope}}
+  b = 2
+
+  // expected-error@+1{{cannot find 'c' in scope}}
+  c = 2
+}
diff --git a/test/Utils/update-verify-tests/multiple-missing-errors-same-line.swift b/test/Utils/update-verify-tests/multiple-missing-errors-same-line.swift
new file mode 100644
index 0000000000000..894b941df2e95
--- /dev/null
+++ b/test/Utils/update-verify-tests/multiple-missing-errors-same-line.swift
@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  a = 2;  b = 2; c = 2;
+}
+
+func bar() {
+  x = 2;  y = 2; z = 2;
+  // expected-error@-1{{cannot find 'x' in scope}}
+}
+//--- test.swift.expected
+func foo() {
+  // expected-error@+3{{cannot find 'c' in scope}}
+  // expected-error@+2{{cannot find 'b' in scope}}
+  // expected-error@+1{{cannot find 'a' in scope}}
+  a = 2;  b = 2; c = 2;
+}
+
+func bar() {
+  x = 2;  y = 2; z = 2;
+  // expected-error@-1{{cannot find 'x' in scope}}
+  // expected-error@-2{{cannot find 'y' in scope}}
+  // expected-error@-3{{cannot find 'z' in scope}}
+}
diff --git a/test/Utils/update-verify-tests/no-checks.swift b/test/Utils/update-verify-tests/no-checks.swift
new file mode 100644
index 0000000000000..098b527753910
--- /dev/null
+++ b/test/Utils/update-verify-tests/no-checks.swift
@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  bar = 2
+}
+//--- test.swift.expected
+func foo() {
+  // expected-error@+1{{cannot find 'bar' in scope}}
+  bar = 2
+}
diff --git a/test/Utils/update-verify-tests/no-diags.swift b/test/Utils/update-verify-tests/no-diags.swift
new file mode 100644
index 0000000000000..663429207c61e
--- /dev/null
+++ b/test/Utils/update-verify-tests/no-diags.swift
@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  // expected-error@+1{{asdf}}
+  let _ = 2
+}
+//--- test.swift.expected
+func foo() {
+  let _ = 2
+}
diff --git a/test/Utils/update-verify-tests/non-default-prefix.swift b/test/Utils/update-verify-tests/non-default-prefix.swift
new file mode 100644
index 0000000000000..a8ba434adf87b
--- /dev/null
+++ b/test/Utils/update-verify-tests/non-default-prefix.swift
@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -verify-additional-prefix check- -typecheck %t/test.swift 2>&1 | %update-verify-tests --prefix check-
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift -verify-additional-prefix check- 
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  a = 2
+  // expected-check-error{{foo}}
+  // expected-error{{bar}}
+
+  // expected-error@+1{{baz}}
+  b = 3
+}
+//--- test.swift.expected
+func foo() {
+  // expected-check-error@+1{{cannot find 'a' in scope}}
+  a = 2
+
+  // expected-error@+1{{cannot find 'b' in scope}}
+  b = 3
+}
diff --git a/test/Utils/update-verify-tests/update-same-line.swift b/test/Utils/update-verify-tests/update-same-line.swift
new file mode 100644
index 0000000000000..74789b48567e5
--- /dev/null
+++ b/test/Utils/update-verify-tests/update-same-line.swift
@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  bar = 2     //   expected-error       {{asdf}}
+}
+
+//--- test.swift.expected
+func foo() {
+  bar = 2     //   expected-error       {{cannot find 'bar' in scope}}
+}
+
diff --git a/test/Utils/update-verify-tests/update-single-check.swift b/test/Utils/update-verify-tests/update-single-check.swift
new file mode 100644
index 0000000000000..f4fc631033846
--- /dev/null
+++ b/test/Utils/update-verify-tests/update-single-check.swift
@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  // expected-error@+1{{AAA}}
+  bar = 2
+}
+
+//--- test.swift.expected
+func foo() {
+  // expected-error@+1{{cannot find 'bar' in scope}}
+  bar = 2
+}
+
diff --git a/test/Utils/update-verify-tests/wrong-category.swift b/test/Utils/update-verify-tests/wrong-category.swift
new file mode 100644
index 0000000000000..c6803bf1fa6fc
--- /dev/null
+++ b/test/Utils/update-verify-tests/wrong-category.swift
@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: not %target-swift-frontend-verify -typecheck %t/test.swift 2>&1 | %update-verify-tests
+// RUN: %target-swift-frontend-verify -typecheck %t/test.swift
+// RUN: %diff %t/test.swift %t/test.swift.expected
+
+//--- test.swift
+func foo() {
+  // expected-warning@+1{{cannot find 'bar' in scope}}
+  bar = 2
+}
+
+//--- test.swift.expected
+func foo() {
+  // expected-error@+1{{cannot find 'bar' in scope}}
+  bar = 2
+}
+
diff --git a/test/lit.cfg b/test/lit.cfg
index b4800b1e16507..65aa8d8cb8fc4 100644
--- a/test/lit.cfg
+++ b/test/lit.cfg
@@ -766,6 +766,9 @@ config.substitutions.append( ('%validate-json', f"{config.python} -m json.tool")
 config.clang_include_dir = make_path(config.llvm_obj_root, 'include')
 config.substitutions.append( ('%clang-include-dir', config.clang_include_dir) )
 
+config.update_verify_tests = make_path(config.swift_utils, "update-verify-tests.py")
+config.substitutions.append( ('%update-verify-tests', '%s %s' % (config.python, config.update_verify_tests)) )
+
 config.swift_include_dir = make_path(config.swift_obj_root, 'include')
 config.substitutions.append( ('%swift-include-dir', config.swift_include_dir) )
 
diff --git a/utils/update-verify-tests.py b/utils/update-verify-tests.py
new file mode 100644
index 0000000000000..de7c5fdeb60c5
--- /dev/null
+++ b/utils/update-verify-tests.py
@@ -0,0 +1,44 @@
+import sys
+import argparse
+from update_verify_tests.core import check_expectations
+
+"""
+ Pipe output from swift-frontend's -verify into this script to have the test case updated to expect the actual diagnostic output.
+ When inserting new expected-* checks it will place them on the line before the location of the diagnostic, with an @+1,
+ or @+N for some N if there are multiple diagnostics emitted on the same line. If the current checks are using @-N for
+ this line, the new check will follow that convention also.
+ Existing checks will be left untouched as much as possible, including their location and whitespace content, to minimize
+ diffs. If inaccurate their count will be updated, or the check removed entirely.
+
+ Missing features:
+  - multiple prefixes on the same line (-verify-additional-prefix my-prefix -verify-additional-prefix my-other-prefix)
+  - multiple prefixes on separate RUN lines (RUN: -verify-additional-prefix my-prefix\nRUN: -verify-additional-prefix my-other-prefix)
+  - regexes matchers
+  - multiple checks targeting the same line are supported, but a line may only contain one check
+  - if multiple checks targeting the same line are failing the script is not guaranteed to produce a minimal diff
+  - remarks
+  - expansions
+  - columns
+  - fix-its
+  - doc files
+
+Example usage:
+  swift-frontend -verify [file] | python3 update-verify-tests.py
+  swift-frontend -verify -verify-additional-prefix check- [file] | python3 update-verify-tests.py --prefix check-
+"""
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--prefix", default="", help="The prefix passed to -verify"
+    )
+    args = parser.parse_args()
+    (ret_code, output) = check_expectations(sys.stdin.readlines(), args.prefix)
+    print(output)
+    sys.exit(ret_code)
+
+
+if __name__ == "__main__":
+    main()
+
diff --git a/utils/update_verify_tests/core.py b/utils/update_verify_tests/core.py
new file mode 100644
index 0000000000000..20ad70d393b9b
--- /dev/null
+++ b/utils/update_verify_tests/core.py
@@ -0,0 +1,509 @@
+import sys
+import re
+
+DEBUG = False
+
+
+def dprint(*args):
+    if DEBUG:
+        print(*args, file=sys.stderr)
+
+
+class KnownException(Exception):
+    pass
+
+
+def parse_error_category(s, prefix):
+    if "no expected directives found" in s:
+        return None
+    parts = s.split("diagnostics")
+    diag_category = parts[0]
+    category_parts = parts[0].strip().strip("'").split("-")
+    expected = category_parts[0]
+    if expected != prefix:
+        raise Exception(
+            f"expected prefix '{prefix}', but found '{expected}'. Multiple verify prefixes are not supported."
+        )
+    diag_category = category_parts[1]
+    if "seen but not expected" in parts[1]:
+        seen = True
+    elif "expected but not seen" in parts[1]:
+        seen = False
+    else:
+        raise KnownException(f"unexpected category '{parts[1]}'")
+    return (diag_category, seen)
+
+
+class Line:
+    def __init__(self, content, line_n):
+        self.content = content
+        self.diag = None
+        self.line_n = line_n
+        self.targeting_diags = []
+
+    def update_line_n(self, n):
+        self.line_n = n
+
+    def render(self):
+        if not self.diag:
+            return self.content
+        assert "{{DIAG}}" in self.content
+        res = self.content.replace("{{DIAG}}", self.diag.render())
+        if not res.strip():
+            return ""
+        return res
+
+
+class Diag:
+    def __init__(
+        self,
+        prefix,
+        diag_content,
+        category,
+        parsed_target_line_n,
+        line_is_absolute,
+        count,
+        line,
+        is_re,
+        whitespace_strings,
+        is_from_source_file,
+    ):
+        self.prefix = prefix
+        self.diag_content = diag_content
+        self.category = category
+        self.parsed_target_line_n = parsed_target_line_n
+        self.line_is_absolute = line_is_absolute
+        self.count = count
+        self.line = line
+        self.target = None
+        self.is_re = is_re
+        self.absolute_target()
+        self.whitespace_strings = whitespace_strings
+        self.is_from_source_file = is_from_source_file
+
+    def decrement_count(self):
+        self.count -= 1
+        assert self.count >= 0
+
+    def increment_count(self):
+        assert self.count >= 0
+        self.count += 1
+
+    def unset_target(self):
+        assert self.target is not None
+        self.target.targeting_diags.remove(self)
+        self.target = None
+
+    def set_target(self, target):
+        if self.target:
+            self.unset_target()
+        self.target = target
+        self.target.targeting_diags.append(self)
+
+    def absolute_target(self):
+        if self.target:
+            return self.target.line_n
+        if self.line_is_absolute:
+            return self.parsed_target_line_n
+        return self.line.line_n + self.parsed_target_line_n
+
+    def relative_target(self):
+        return self.absolute_target() - self.line.line_n
+
+    def take(self, other_diag):
+        assert self.count == 0
+        assert other_diag.count > 0
+        assert other_diag.target == self.target
+        assert not other_diag.line_is_absolute
+        assert not other_diag.is_re and not self.is_re
+        self.line_is_absolute = False
+        self.diag_content = other_diag.diag_content
+        self.count = other_diag.count
+        self.category = other_diag.category
+        self.count = other_diag.count
+        other_diag.count = 0
+
+    def render(self):
+        assert self.count >= 0
+        if self.count == 0:
+            return ""
+        line_location_s = ""
+        if self.relative_target() != 0:
+            if self.line_is_absolute:
+                line_location_s = f"@{self.absolute_target()}"
+            elif self.relative_target() > 0:
+                line_location_s = f"@+{self.relative_target()}"
+            else:
+                line_location_s = (
+                    f"@{self.relative_target()}"  # the minus sign is implicit
+                )
+        count_s = "" if self.count == 1 else f"{self.count}"
+        re_s = "-re" if self.is_re else ""
+        if self.whitespace_strings:
+            whitespace1_s = self.whitespace_strings[0]
+            whitespace2_s = self.whitespace_strings[1]
+            whitespace3_s = self.whitespace_strings[2]
+        else:
+            whitespace1_s = " "
+            whitespace2_s = ""
+            whitespace3_s = ""
+        if count_s and not whitespace2_s:
+            whitespace2_s = " "  # required to parse correctly
+        elif not count_s and whitespace2_s == " ":
+            """Don't emit a weird extra space.
+            However if the whitespace is something other than the
+            standard single space, let it be to avoid disrupting manual formatting.
+            The existence of a non-empty whitespace2_s implies this was parsed with
+            a count > 1 and then decremented, otherwise this whitespace would have
+            been parsed as whitespace3_s.
+            """
+            whitespace2_s = ""
+        return f"//{whitespace1_s}expected-{self.prefix}{self.category}{re_s}{line_location_s}{whitespace2_s}{count_s}{whitespace3_s}{{{{{self.diag_content}}}}}"
+
+
+expected_diag_re = re.compile(
+    r"//(\s*)expected-([a-zA-Z-]*)(note|warning|error)(-re)?(@[+-]?\d+)?(?:(\s*)(\d+))?(\s*)\{\{(.*)\}\}"
+)
+
+
+def parse_diag(line, filename, prefix):
+    s = line.content
+    ms = expected_diag_re.findall(s)
+    if not ms:
+        return None
+    if len(ms) > 1:
+        raise KnownException(
+            f"multiple diags on line {filename}:{line.line_n}. Aborting due to missing implementation."
+        )
+    [
+        whitespace1_s,
+        check_prefix,
+        category_s,
+        re_s,
+        target_line_s,
+        whitespace2_s,
+        count_s,
+        whitespace3_s,
+        diag_s,
+    ] = ms[0]
+    if check_prefix != prefix and check_prefix != "":
+        return None
+    if not target_line_s:
+        target_line_n = 0
+        is_absolute = False
+    elif target_line_s.startswith("@+"):
+        target_line_n = int(target_line_s[2:])
+        is_absolute = False
+    elif target_line_s.startswith("@-"):
+        target_line_n = int(target_line_s[1:])
+        is_absolute = False
+    else:
+        target_line_n = int(target_line_s[1:])
+        is_absolute = True
+    count = int(count_s) if count_s else 1
+    line.content = expected_diag_re.sub("{{DIAG}}", s)
+
+    return Diag(
+        check_prefix,
+        diag_s,
+        category_s,
+        target_line_n,
+        is_absolute,
+        count,
+        line,
+        bool(re_s),
+        [whitespace1_s, whitespace2_s, whitespace3_s],
+        True,
+    )
+
+
+def add_line(new_line, lines):
+    lines.insert(new_line.line_n - 1, new_line)
+    for i in range(new_line.line_n, len(lines)):
+        line = lines[i]
+        assert line.line_n == i
+        line.update_line_n(i + 1)
+    assert all(line.line_n == i + 1 for i, line in enumerate(lines))
+
+
+def remove_line(old_line, lines):
+    lines.remove(old_line)
+    for i in range(old_line.line_n - 1, len(lines)):
+        line = lines[i]
+        assert line.line_n == i + 2
+        line.update_line_n(i + 1)
+    assert all(line.line_n == i + 1 for i, line in enumerate(lines))
+
+
+indent_re = re.compile(r"\s*")
+
+
+def get_indent(s):
+    return indent_re.match(s).group(0)
+
+
+def orig_line_n_to_new_line_n(line_n, orig_lines):
+    return orig_lines[line_n - 1].line_n
+
+
+def add_diag(orig_line_n, diag_s, diag_category, lines, orig_lines, prefix):
+    line_n = orig_line_n_to_new_line_n(orig_line_n, orig_lines)
+    target = lines[line_n - 1]
+    for other in target.targeting_diags:
+        if other.is_re:
+            raise KnownException(
+                "mismatching diag on line with regex matcher. Skipping due to missing implementation"
+            )
+    reverse = (
+        True
+        if [other for other in target.targeting_diags if other.relative_target() < 0]
+        else False
+    )
+
+    targeting = [
+        other for other in target.targeting_diags if not other.line_is_absolute
+    ]
+    targeting.sort(reverse=reverse, key=lambda d: d.relative_target())
+    prev_offset = 0
+    prev_line = target
+    direction = -1 if reverse else 1
+    for d in targeting:
+        if d.relative_target() != prev_offset + direction:
+            break
+        prev_offset = d.relative_target()
+        prev_line = d.line
+    total_offset = prev_offset - 1 if reverse else prev_offset + 1
+    if reverse:
+        new_line_n = prev_line.line_n + 1
+    else:
+        new_line_n = prev_line.line_n
+    assert new_line_n == line_n + (not reverse) - total_offset
+
+    new_line = Line(get_indent(prev_line.content) + "{{DIAG}}\n", new_line_n)
+    add_line(new_line, lines)
+
+    whitespace_strings = prev_line.diag.whitespace_strings if prev_line.diag else None
+    new_diag = Diag(
+        prefix,
+        diag_s,
+        diag_category,
+        total_offset,
+        False,
+        1,
+        new_line,
+        False,
+        whitespace_strings,
+        False,
+    )
+    new_line.diag = new_diag
+    new_diag.set_target(target)
+
+
+def remove_dead_diags(lines):
+    for line in lines:
+        if not line.diag or line.diag.count != 0:
+            continue
+        if line.render() == "":
+            remove_line(line, lines)
+        else:
+            assert line.diag.is_from_source_file
+            for other_diag in line.targeting_diags:
+                if (
+                    other_diag.is_from_source_file
+                    or other_diag.count == 0
+                    or other_diag.category != line.diag.category
+                ):
+                    continue
+                if other_diag.is_re or line.diag.is_re:
+                    continue
+                line.diag.take(other_diag)
+                remove_line(other_diag.line, lines)
+
+
+def update_test_file(filename, diag_errors, prefix, updated_test_files):
+    dprint(f"updating test file {filename}")
+    if filename in updated_test_files:
+        raise KnownException(f"{filename} already updated, but got new output")
+    else:
+        updated_test_files.add(filename)
+    with open(filename, "r") as f:
+        lines = [Line(line, i + 1) for i, line in enumerate(f.readlines() + [''])]
+    orig_lines = list(lines)
+
+    for line in lines:
+        diag = parse_diag(line, filename, prefix)
+        if diag:
+            line.diag = diag
+            diag.set_target(lines[diag.absolute_target() - 1])
+
+    for diag_error in diag_errors:
+        if not isinstance(diag_error, NotFoundDiag):
+            continue
+        # this is a diagnostic expected but not seen
+        line_n = diag_error.line
+        assert lines[line_n - 1].diag
+        if not lines[line_n - 1].diag or diag_error.content != lines[line_n - 1].diag.diag_content:
+            raise KnownException(
+                f"{filename}:{line_n} - found diag {lines[line_n - 1].diag.diag_content} but expected {diag_error.content}"
+            )
+        if diag_error.category != lines[line_n - 1].diag.category:
+            raise KnownException(
+                f"{filename}:{line_n} - found {lines[line_n - 1].diag.category} diag but expected {diag_error.category}"
+            )
+        lines[line_n - 1].diag.decrement_count()
+
+    diag_errors.sort(reverse=True, key=lambda t: t.line)
+    for diag_error in diag_errors:
+        if not isinstance(diag_error, ExtraDiag):
+            continue
+        line_n = diag_error.line
+        target = orig_lines[line_n - 1]
+        other_diags = [
+            d
+            for d in target.targeting_diags
+            if d.diag_content == diag_error.content and d.category == diag_error.category
+        ]
+        other_diag = other_diags[0] if other_diags else None
+        if other_diag:
+            other_diag.increment_count()
+        else:
+            add_diag(line_n, diag_error.content, diag_error.category, lines, orig_lines, diag_error.prefix)
+    remove_dead_diags(lines)
+    with open(filename, "w") as f:
+        for line in lines:
+            f.write(line.render())
+
+
+def update_test_files(errors, prefix):
+    errors_by_file = {}
+    for error in errors:
+        filename = error.file
+        if filename not in errors_by_file:
+            errors_by_file[filename] = []
+        errors_by_file[filename].append(error)
+    updated_test_files = set()
+    for filename, diag_errors in errors_by_file.items():
+        try:
+            update_test_file(filename, diag_errors, prefix, updated_test_files)
+        except KnownException as e:
+            return f"Error in update-verify-tests while updating {filename}: {e}"
+    updated_files = list(updated_test_files)
+    assert updated_files
+    if len(updated_files) == 1:
+        return f"updated file {updated_files[0]}"
+    updated_files_s = "\n\t".join(updated_files)
+    return "updated files:\n\t{updated_files_s}"
+
+
+"""
+ex:
+test.swift:2:6: error: expected error not produced
+  // expected-error@+1{{asdf}}
+~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
+"""
+diag_error_re = re.compile(r"(\S+):(\d+):(\d+): error: expected (\S+) not produced")
+
+
+"""
+ex:
+test.swift:2:3: error: unexpected error produced: cannot find 'a' in scope
+  a = 2
+  ^
+"""
+diag_error_re2 = re.compile(r"(\S+):(\d+):(\d+): error: unexpected (\S+) produced: (.*)")
+
+
+"""
+ex:
+test.swift:2:43: error: incorrect message found
+  bar = 2  // expected-error{{asdf}}
+                              ^~~~
+                              cannot find 'bar' in scope
+"""
+diag_error_re3 = re.compile(r"(\S+):(\d+):(\d+): error: incorrect message found")
+
+
+"""
+ex:
+test.swift:2:15: error: expected warning, not error
+  // expected-warning@+1{{cannot find 'bar' in scope}}
+              ^~~~~~~
+              error
+"""
+diag_error_re4 = re.compile(r"(\S+):(\d+):(\d+): error: expected (\S+), not (\S+)")
+
+
+class NotFoundDiag:
+    def __init__(self, file, line, col, category, content, prefix):
+        self.file = file
+        self.line = line
+        self.col = col
+        self.category = category
+        self.content = content
+        self.prefix = prefix
+
+    def __str__(self):
+        return f"{self.file}:{self.line}:{self.col}: error expected {self.category} not produced (expected {self.content})"
+
+
+class ExtraDiag:
+    def __init__(self, file, line, col, category, content, prefix):
+        self.file = file
+        self.line = line
+        self.col = col
+        self.category = category
+        self.content = content
+        self.prefix = prefix
+
+    def __str__(self):
+        return f"{self.file}:{self.line}:{self.col}: error unexpected {self.category} produced: {self.content}"
+
+
+def check_expectations(tool_output, prefix):
+    """
+    The entry point function.
+    Called by the stand-alone update-verify-tests.py as well as litplugin.py.
+    """
+    curr = []
+    try:
+        i = 0
+        while i < len(tool_output):
+            line = tool_output[i].strip()
+
+            if not "error:" in line:
+                pass
+            elif m := diag_error_re.match(line):
+                diag = parse_diag(Line(tool_output[i+1], int(m.group(2))), m.group(1), prefix)
+                i += 2
+                curr.append(NotFoundDiag(m.group(1), int(m.group(2)), int(m.group(3)), m.group(4), diag.diag_content, diag.prefix))
+            elif m := diag_error_re2.match(line):
+                curr.append(ExtraDiag(m.group(1), int(m.group(2)), int(m.group(3)), m.group(4), m.group(5), prefix))
+                i += 2
+            # Create two mirroring mismatches when the compiler reports that the category or diagnostic is incorrect.
+            # This makes it easier to handle cases where the same diagnostic is mentioned both in an incorrect message/category
+            # diagnostic, as well as in an error not produced diagnostic. This can happen for things like 'expected-error 2{{foo}}'
+            # if only one diagnostic is emitted on that line, and the content of that diagnostic is actually 'bar'.
+            elif m := diag_error_re3.match(line):
+                diag = parse_diag(Line(tool_output[i+1], int(m.group(2))), m.group(1), prefix)
+                curr.append(NotFoundDiag(m.group(1), int(m.group(2)), int(m.group(3)), diag.category, diag.diag_content, diag.prefix))
+                curr.append(ExtraDiag(m.group(1), diag.absolute_target(), int(m.group(3)), diag.category, tool_output[i+3].strip(), diag.prefix))
+                i += 3
+            elif m := diag_error_re4.match(line):
+                diag = parse_diag(Line(tool_output[i+1], int(m.group(2))), m.group(1), prefix)
+                assert diag.category == m.group(4)
+                assert tool_output[i+3].strip() == m.group(5)
+                curr.append(NotFoundDiag(m.group(1), int(m.group(2)), int(m.group(3)), diag.category, diag.diag_content, diag.prefix))
+                curr.append(ExtraDiag(m.group(1), diag.absolute_target(), int(m.group(3)), m.group(5), diag.diag_content, diag.prefix))
+                i += 3
+            else:
+                dprint("no match")
+                dprint(line.strip())
+            i += 1
+
+    except KnownException as e:
+        return (1, f"Error in update-verify-tests while parsing tool output: {e}")
+    if curr:
+        return (0, update_test_files(curr, prefix))
+    else:
+        return (1, "no mismatching diagnostics found")
+