[clang-tidy] Add the abseil-duration-subtraction check

Summary:
This check uses the context of a subtraction expression as well as knowledge
about the Abseil Time types, to infer the type of the second operand of some
subtraction expressions in Duration conversions. For example:

   absl::ToDoubleSeconds(duration) - foo

can become
   absl::ToDoubleSeconds(duration - absl::Seconds(foo))

This ensures that time calculations are done in the proper domain, and also
makes it easier to further deduce the types of the second operands to these
expressions.

Reviewed By: JonasToth

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D55245

3 files changed, 43 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 9527cd49b1d..1976f788360 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -93,6 +93,12 @@ Improvements to clang-tidy
   Checks for cases where arguments to ``absl::Duration`` factory functions are
   scaled internally and could be changed to a different factory function.
 
+- New :doc:`abseil-duration-subtraction
+  <clang-tidy/checks/abseil-duration-subtraction>` check.
+
+  Checks for cases where subtraction should be performed in the
+  ``absl::Duration`` domain.
+
 - New :doc:`abseil-faster-strsplit-delimiter
   <clang-tidy/checks/abseil-faster-strsplit-delimiter>` check.
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/abseil-duration-subtraction.rst b/clang-tools-extra/docs/clang-tidy/checks/abseil-duration-subtraction.rst
new file mode 100644
index 00000000000..884eeb2cbed
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/abseil-duration-subtraction.rst
@@ -0,0 +1,36 @@
+.. title:: clang-tidy - abseil-duration-subtraction
+
+abseil-duration-subtraction
+===========================
+
+Checks for cases where subtraction should be performed in the
+``absl::Duration`` domain. When subtracting two values, and the first one is
+known to be a conversion from ``absl::Duration``, we can infer that the second
+should also be interpreted as an ``absl::Duration``, and make that inference
+explicit.
+
+Examples:
+
+.. code-block:: c++
+
+  // Original - Subtraction in the double domain
+  double x;
+  absl::Duration d;
+  double result = absl::ToDoubleSeconds(d) - x;
+
+  // Suggestion - Subtraction in the absl::Duration domain instead
+  double result = absl::ToDoubleSeconds(d - absl::Seconds(x));
+
+
+  // Original - Subtraction of two Durations in the double domain
+  absl::Duration d1, d2;
+  double result = absl::ToDoubleSeconds(d1) - absl::ToDoubleSeconds(d2);
+
+  // Suggestion - Subtraction in the absl::Duration domain instead
+  double result = absl::ToDoubleSeconds(d1 - d2);
+
+Note: As with other ``clang-tidy`` checks, it is possible that multiple fixes
+may overlap (as in the case of nested expressions), so not all occurences can
+be transformed in one run. In particular, this may occur for nested subtraction
+expressions. Running ``clang-tidy`` multiple times will find and fix these
+overlaps.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index 7febb37720a..e45420823a0 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -8,6 +8,7 @@ Clang-Tidy Checks
    abseil-duration-division
    abseil-duration-factory-float
    abseil-duration-factory-scale
+   abseil-duration-subtraction
    abseil-faster-strsplit-delimiter
    abseil-no-internal-dependencies
    abseil-no-namespace