lots of miscellaneous things all in one big commit

 * add `@compileLog(...)` builtin function
   - Helps debug code running at compile time
   - See #240
 * fix crash when there is an error on the start value of a slice
 * add implicit cast from int and float types to int and float
   literals if the value is known at compile time
 * make array concatenation work with slices in addition to
   arrays and c string literals
 * fix compile error message for something not having field access
 * fix crash when `@setDebugSafety()` was called from a
   function being evaluated at compile-time
 * fix compile-time evaluation of overflow math builtins.
 * avoid debug safety panic handler in builtin.o and compiler_rt.o
   since we use no debug safety in these modules anyway
 * add compiler_rt functions for division on ARM
   - Closes #254
 * move default panic handler to std.debug so users can
   call it manually
 * std.io.printf supports a width in the format specifier

1 files changed, 16 insertions, 3 deletions

diff --git a/doc/langref.md b/doc/langref.md
index b8a6a5775a..0aace75996 100644
--- a/doc/langref.md
+++ b/doc/langref.md
@@ -639,10 +639,23 @@ const b: u8 = @truncate(u8, a);
 
 ### @compileError(comptime msg: []u8)
 
-This function, when semantically analyzed, causes a compile error with the message `msg`.
+This function, when semantically analyzed, causes a compile error with the
+message `msg`.
 
-There are several ways that code avoids being semantically checked, such as using `if`
-or `switch` with compile time constants, and comptime functions.
+There are several ways that code avoids being semantically checked, such as
+using `if` or `switch` with compile time constants, and comptime functions.
+
+### @compileLog(args: ...)
+
+This function, when semantically analyzed, causes a compile error, but it does
+not prevent compile-time code from continuing to run, and it otherwise does not
+interfere with analysis.
+
+Each of the arguments will be serialized to a printable debug value and output
+to stderr, and then a newline at the end.
+
+This function can be used to do "printf debugging" on compile-time executing
+code.
 
 ### @intType(comptime is_signed: bool, comptime bit_count: u8) -> type