[clang-tidy] Add a check to find unintended semicolons that changes the semantics.

Reviewers: hokein, alexfh

Differential Revision: http://reviews.llvm.org/D16535

llvm-svn: 260503

2 files changed, 73 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index 5bdc7163d1a..fe1a0225e34 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -61,6 +61,7 @@ Clang-Tidy Checks
    misc-sizeof-container
    misc-static-assert
    misc-string-integer-assignment
+   misc-suspicious-semicolon
    misc-swapped-arguments
    misc-throw-by-value-catch-by-reference
    misc-undelegated-constructor
diff --git a/clang-tools-extra/docs/clang-tidy/checks/misc-suspicious-semicolon.rst b/clang-tools-extra/docs/clang-tidy/checks/misc-suspicious-semicolon.rst
new file mode 100644
index 00000000000..b58fed68d1e
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/misc-suspicious-semicolon.rst
@@ -0,0 +1,72 @@
+.. title:: clang-tidy - misc-suspicious-semicolon
+
+misc-suspicious-semicolon
+=========================
+
+Finds most instances of stray semicolons that unexpectedly alter the meaning of
+the code. More specifically, it looks for `if`, `while`, `for` and `for-range`
+statements whose body is a single semicolon, and then analyzes the context of
+the code (e.g. indentation) in an attempt to determine whether that is
+intentional.
+
+  .. code-block:: c++
+
+    if(x < y);
+    {
+      x++;
+    }
+
+Here the body of the `if` statement consists of only the semicolon at the end of
+the first line, and `x` will be incremented regardless of the condition.
+
+
+  .. code-block:: c++
+
+    while((line = readLine(file)) != NULL);
+      processLine(line);
+
+As a result of this code, `processLine()` will only be called once, when the
+`while` loop with the empty body exits with `line == NULL`. The indentation of
+the code indicates the intention of the programmer.
+
+
+  .. code-block:: c++
+
+    if(x >= y);
+    x -= y;
+
+While the indentation does not imply any nesting, there is simply no valid
+reason to have an `if` statement with an empty body (but it can make sense for
+a loop). So this check issues a warning for the code above.
+
+To solve the issue remove the stray semicolon or in case the empty body is
+intentional, reflect this using code indentation or put the semicolon in a new
+line. For example:
+
+  .. code-block:: c++
+
+    while(readWhitespace());
+      Token t = readNextToken();
+
+Here the second line is indented in a way that suggests that it is meant to be
+the body of the `while` loop - whose body is in fact empty, becasue of the
+semicolon at the end of the first line.
+
+Either remove the indentation from the second line:
+
+  .. code-block:: c++
+
+    while(readWhitespace());
+    Token t = readNextToken();
+
+... or move the semicolon from the end of the first line to a new line:
+
+  .. code-block:: c++
+
+    while(readWhitespace())
+      ;
+
+      Token t = readNextToken();
+
+In this case the check will assume that you know what you are doing, and will
+not raise a warning.