[clang][Parse] Diagnose useless null statements / empty init-statements

Summary:
clang has `-Wextra-semi` (D43162), which is not dictated by the currently selected standard.
While that is great, there is at least one more source of need-less semis - 'null statements'.
Sometimes, they are needed:
```
for(int x = 0; continueToDoWork(x); x++)
  ; // Ugly code, but the semi is needed here.
```

But sometimes they are just there for no reason:
```
switch(X) {
case 0:
  return -2345;
case 5:
  return 0;
default:
  return 42;
}; // <- oops

;;;;;;;;;;; <- OOOOPS, still not diagnosed. Clearly this is junk.
```

Additionally:
```
if(; // <- empty init-statement
   true)
  ;

switch (; // empty init-statement
        x) {
  ...
}

for (; // <- empty init-statement
     int y : S())
  ;
}

As usual, things may or may not go sideways in the presence of macros.
While evaluating this diag on my codebase of interest, it was unsurprisingly
discovered that Google Test macros are *very* prone to this.
And it seems many issues are deep within the GTest itself, not
in the snippets passed from the codebase that uses GTest.

So after some thought, i decided not do issue a diagnostic if the semi
is within *any* macro, be it either from the normal header, or system header.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=39111 | PR39111 ]]

Reviewers: rsmith, aaron.ballman, efriedma

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52695

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@347339 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 57 insertions, 0 deletions

diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 5e6dfdf1c1..ca95b7c780 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -55,6 +55,63 @@ Major New Features
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- ``-Wextra-semi-stmt`` is a new diagnostic that diagnoses extra semicolons,
+  much like ``-Wextra-semi``. This new diagnostic diagnoses all *unnecessary*
+  null statements (expression statements without an expression), unless: the
+  semicolon directly follows a macro that was expanded to nothing or if the
+  semicolon is within the macro itself. This applies to macros defined in system
+  headers as well as user-defined macros.
+
+  .. code-block:: c++
+
+      #define MACRO(x) int x;
+      #define NULLMACRO(varname)
+
+      void test() {
+        ; // <- warning: ';' with no preceding expression is a null statement
+
+        while (true)
+          ; // OK, it is needed.
+
+        switch (my_enum) {
+        case E1:
+          // stuff
+          break;
+        case E2:
+          ; // OK, it is needed.
+        }
+
+        MACRO(v0;) // Extra semicolon, but within macro, so ignored.
+
+        MACRO(v1); // <- warning: ';' with no preceding expression is a null statement
+
+        NULLMACRO(v2); // ignored, NULLMACRO expanded to nothing.
+      }
+
+- ``-Wempty-init-stmt`` is a new diagnostic that diagnoses empty init-statements
+  of ``if``, ``switch``, ``range-based for``, unless: the semicolon directly
+  follows a macro that was expanded to nothing or if the semicolon is within the
+  macro itself (both macros from system headers, and normal macros). This
+  diagnostic is in the ``-Wextra-semi-stmt`` group and is enabled in
+  ``-Wextra``.
+
+  .. code-block:: c++
+
+      void test() {
+        if(; // <- warning: init-statement of 'if' is a null statement
+           true)
+          ;
+
+        switch (; // <- warning: init-statement of 'switch' is a null statement
+                x) {
+          ...
+        }
+
+        for (; // <- warning: init-statement of 'range-based for' is a null statement
+             int y : S())
+          ;
+      }
+
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------