Auto merge of #1158 - glyn:large-bitfield-units, r=emilio

Support bitfield allocation units larger than 64 bits

Individual bitfields are still limited to at most 64 bits, but this restriction can be weakened when Rust supports `u128`.

This implements issue #816.

Usage notes:

* Since common code is added to each generated binding, a program which uses
  more than one binding may need to work around the duplication by including
  each binding in its own module.
* The values created by bitfield allocation unit constructors can be assigned
  directly to the corresponding struct fields with no need for transmutation.

Implementation notes:

`__BindgenBitfieldUnit` represents a bitfield allocation unit using a `Storage`
type accessible as a slice of `u8`. The alignment of the unit is inherited from
an `Align` type by virtue of the field:
```
align: [Align; 0],
```
The position of this field in the struct is irrelevant.

It is assumed that the alignment of the `Storage` type is no larger than the
alignment of the `Align` type, which will be true if the `Storage` type is, for
example, an array of `u8`. This assumption is checked in a debug assertion.

Although the double underscore (__) prefix is reserved for implementations of
C++, there are precedents for this convention elsewhere in bindgen and so the
convention is adopted here too.

Acknowledgement:

Thanks to @fitzgen for an initial implementation of `__BindgenBitfieldUnit` and
code to integrate it into bindgen.

r? @emilio

2 files changed, 49 insertions, 14 deletions

diff --git a/bindgen-integration/cpp/Test.cc b/bindgen-integration/cpp/Test.cc
index f125109a..57c2186b 100644
--- a/bindgen-integration/cpp/Test.cc
+++ b/bindgen-integration/cpp/Test.cc
@@ -69,11 +69,11 @@ Date2::assert(unsigned short nWeekDay,
               unsigned short nYear,
               unsigned short byte)
 {
-  return this->nWeekDay == nWeekDay &&
-      this->nMonthDay == nMonthDay &&
-      this->nMonth == nMonth &&
-      this->nYear == nYear &&
-      this->byte == byte;
+    return this->nWeekDay == nWeekDay &&
+        this->nMonthDay == nMonthDay &&
+        this->nMonth == nMonth &&
+        this->nYear == nYear &&
+        this->byte == byte;
 }
 
 bool
@@ -83,11 +83,11 @@ Fifth::assert(unsigned short nWeekDay,
               unsigned short nYear,
               unsigned char byte)
 {
-  return this->nWeekDay == nWeekDay &&
-      this->nMonthDay == nMonthDay &&
-      this->nMonth == nMonth &&
-      this->nYear == nYear &&
-      this->byte == byte;
+    return this->nWeekDay == nWeekDay &&
+        this->nMonthDay == nMonthDay &&
+        this->nMonth == nMonth &&
+        this->nYear == nYear &&
+        this->byte == byte;
 }
 
 bool
@@ -95,10 +95,27 @@ Sixth::assert(unsigned char byte,
               unsigned char nWeekDay,
               unsigned char nMonth,
               unsigned char nMonthDay) {
-  return this->nWeekDay == nWeekDay &&
-      this->nMonthDay == nMonthDay &&
-      this->nMonth == nMonth &&
-      this->byte == byte;
+    return this->nWeekDay == nWeekDay &&
+        this->nMonthDay == nMonthDay &&
+        this->nMonth == nMonth &&
+        this->byte == byte;
+};
+
+bool
+Seventh::assert(bool first,
+                int second,
+                unsigned short third,
+                unsigned int fourth,
+                unsigned short fifth,
+                bool sixth,
+                int seventh) {
+  return this->first_one_bit == first &&
+      this->second_thirty_bits == second &&
+      this->third_two_bits == third &&
+      this->fourth_thirty_bits == fourth &&
+      this->fifth_two_bits == fifth &&
+      this->sixth_one_bit == sixth &&
+      this->seventh_thirty_bits == seventh;
 };
 
 } // namespace bitfields
diff --git a/bindgen-integration/cpp/Test.h b/bindgen-integration/cpp/Test.h
index e23a32e6..7ddb98bd 100644
--- a/bindgen-integration/cpp/Test.h
+++ b/bindgen-integration/cpp/Test.h
@@ -121,6 +121,24 @@ struct Sixth {
                 unsigned char nMonthDay);
 };
 
+struct Seventh {
+    bool first_one_bit : 1;
+    unsigned int second_thirty_bits : 30;
+    unsigned short third_two_bits : 2;
+    unsigned int fourth_thirty_bits : 30;
+    unsigned short fifth_two_bits : 2;
+    bool sixth_one_bit : 1;
+    unsigned int seventh_thirty_bits : 30;
+
+    /// Returns true if the bitfields match the arguments, false otherwise.
+    bool assert(bool first,
+                int second,
+                unsigned short third,
+                unsigned int fourth,
+                unsigned short fifth,
+                bool sixth,
+                int seventh);
+};
 
 } // namespace bitfields