Use C11 static assert mechanism by default (#761, #766)

Hopefully this works better by default and is less confusing
when it doesn't. Added notes to migration document on how
to restore previous behavior.

2 files changed, 21 insertions, 0 deletions

diff --git a/docs/migration.md b/docs/migration.md
index 52711b5..f781276 100644
--- a/docs/migration.md
+++ b/docs/migration.md
@@ -31,6 +31,25 @@ as `:`.
 
 **Error indications:** "`protoc: Unknown flag: --nanopb_opt`"
 
+### `pb.h` uses C11 `_Static_assert` keyword by default
+
+**Rationale:** The nanopb generated headers use static assertions to catch
+errors at compile time. There are several mechanisms to implement this.
+The most widely supported is C11 `_Static_assert` keyword.
+Previously the code used negative size array definition trick, which is
+supported already in C99 but does not work with every compiler and can
+produce confusing error messages.
+
+**Changes:** Now `_Static_assert` is used by default. If preferred, the
+old default behavior can be restored by defining `PB_C99_STATIC_ASSERT`
+in `pb.h` or on compiler command line.
+
+**Required actions:** If the keyword is not recognized, set the compiler to
+C11 standard mode if available. If it is not available, define either `PB_C99_STATIC_ASSERT`
+or `PB_NO_STATIC_ASSERT`.
+
+**Error indications:** "`Undefined identifier _Static_assert`
+
 Nanopb-0.4.4 (2020-11-25)
 -------------------------
 
diff --git a/docs/reference.md b/docs/reference.md
index e378881..2d591ae 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -20,6 +20,8 @@ includes nanopb headers.
 * `PB_ENCODE_ARRAYS_UNPACKED`: Encode scalar arrays in the unpacked format, which takes up more space. Only to be used when the decoder on the receiving side cannot process packed arrays, such as [protobuf.js versions before 2020](https://github.com/protocolbuffers/protobuf/issues/1701).
 * `PB_CONVERT_DOUBLE_FLOAT`: Convert doubles to floats for platforms that do not support 64-bit `double` datatype. Mainly `AVR` processors.
 * `PB_VALIDATE_UTF8`: Check whether incoming strings are valid UTF-8 sequences. Adds a small performance and code size penalty.
+* `PB_C99_STATIC_ASSERT`: Use C99 style negative array trick for static assertions. For compilers that do not support C11 standard.
+* `PB_NO_STATIC_ASSERT`: Disable static assertions at compile time. Only for compilers with limited support of C standards.
 
 The `PB_MAX_REQUIRED_FIELDS` and `PB_FIELD_32BIT` settings allow
 raising some datatype limits to suit larger messages. Their need is