translate-c: packed struct implies align(1) on every field

Superceeds PR ziglang#12735 (now supporting all packed structs in GNU C)
Fixes issue ziglang#12733

This stops translating C packed struct as a Zig packed struct.
Instead use a regular `extern struct` with `align(1)`.

This is because (as @Vexu explained) Zig packed structs are really just integers (not structs).

Alignment issue is more complicated. I think @ifreund was the
first to notice it in his comment on PR ziglang#12735

Justification of my interpretion of the C(lang) behavior
comes from a careful reading of the GCC docs for type & variable attributes:

(clang emulates gnu's packed attribute here)

The final line of the documentation for __attribute__ ((aligned)) [on types] says:

> When used on a struct, or struct member, *the aligned attribute can only increase the alignment*; in order to decrease it, the packed attribute must be specified as well.

This implies that GCC uses the `packed` attribute for alignment purposes
in addition to eliminating padding.

The documentation for __attribute__((packed)) [on types], states:

> This attribute, attached to a struct, union, or C++ class type definition, specifies that each of its members (other than zero-width bit-fields) is placed to minimize the memory required. **This is equivalent to specifying the packed attribute on each of the members**.

The key is resolving this indirection, and looking at the documentation
for __attribute__((packed)) [on fields (wierdly under "variables" section)]:

> The packed attribute specifies that a **structure member should have the smallest possible alignment** — one bit for a bit-field and one byte otherwise, unless a larger value is specified with the aligned attribute. The attribute does not apply to non-member objects.

Furthermore, alignment is the only effect of the packed attribute mentioned in the GCC docs (for "common" architecture).
Based on this, it seems safe to completely substitute C 'packed' with Zig 'align(1)'.
Target-specific or undocumented behavior potentially changes this.

Unfortunately, the current implementation of `translate-c` translates as
`packed struct` without alignment info.

Because Zig packed structs are really integers (as mentioned above),
they are the wrong interpretation and we should be using 'extern struct'.

Running `translate-c` on the following code:

```c
struct foo {
    char a;
    int b;
} __attribute__((packed));

struct bar {
    char a;
    int b;
    short c;
    __attribute__((aligned(8))) long d;
} __attribute__((packed));
```

Previously used a 'packed struct' (which was not FFI-safe on stage1).

After applying this change, the translated structures have align(1)
explicitly applied to all of their fields AS EXPECTED (unless explicitly overriden).
This makes Zig behavior for `tranlsate-c` consistent with clang/GCC.

Here is the newly produced (correct) output for the above example:

```zig
pub const struct_foo = extern struct {
    a: u8 align(1),
    b: c_int align(1),
};
pub const struct_bar = extern struct {
    a: u8 align(1),
    b: c_int align(1),
    c: c_short align(1),
    d: c_long align(8),
};
```

Also note for reference: Since the last stable release (0.9.1),
there was a change in the language spec
related to the alignment of packed structures.

The docs for Zig 0.9.1 read:
> Packed structs have 1-byte alignment.

So the old behavior of translate-c (not specifying any alignment) was possibly correct back then.

However the current docs read:

> Packed structs have the same alignment as their backing integer

Suggsestive both to the change to an integer-backed representation
which is incompatible with C's notation.

src/clang.zig

@@ -470,6 +470,9 @@ pub const FieldDecl = opaque {
     pub const getAlignedAttribute = ZigClangFieldDecl_getAlignedAttribute;
     extern fn ZigClangFieldDecl_getAlignedAttribute(*const FieldDecl, *const ASTContext) c_uint;
 
+    pub const getPackedAttribute = ZigClangFieldDecl_getPackedAttribute;
+    extern fn ZigClangFieldDecl_getPackedAttribute(*const FieldDecl) bool;
+
     pub const isAnonymousStructOrUnion = ZigClangFieldDecl_isAnonymousStructOrUnion;
     extern fn ZigClangFieldDecl_isAnonymousStructOrUnion(*const FieldDecl) bool;
 
@@ -1015,6 +1018,9 @@ pub const VarDecl = opaque {
     pub const getAlignedAttribute = ZigClangVarDecl_getAlignedAttribute;
     extern fn ZigClangVarDecl_getAlignedAttribute(*const VarDecl, *const ASTContext) c_uint;
 
+    pub const getPackedAttribute = ZigClangVarDecl_getPackedAttribute;
+    extern fn ZigClangVarDecl_getPackedAttribute(*const VarDecl) bool;
+
     pub const getCleanupAttribute = ZigClangVarDecl_getCleanupAttribute;
     extern fn ZigClangVarDecl_getCleanupAttribute(*const VarDecl) ?*const FunctionDecl;
 

src/translate_c.zig

@@ -878,7 +878,7 @@ fn visitVarDecl(c: *Context, var_decl: *const clang.VarDecl, mangled_name: ?[]co
         .is_export = is_export,
         .is_threadlocal = is_threadlocal,
         .linksection_string = linksection_string,
-        .alignment = zigAlignment(var_decl.getAlignedAttribute(c.clang_context)),
+        .alignment = ClangAlignment.forVar(c, var_decl).zigAlignment(),
         .name = var_name,
         .type = type_node,
         .init = init_node,
@@ -1153,7 +1153,7 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
             const alignment = if (has_flexible_array and field_decl.getFieldIndex() == 0)
                 @intCast(c_uint, record_alignment)
             else
-                zigAlignment(field_decl.getAlignedAttribute(c.clang_context));
+                ClangAlignment.forField(c, field_decl, record_def).zigAlignment();
 
             if (is_anon) {
                 try c.decl_table.putNoClobber(c.gpa, @ptrToInt(field_decl.getCanonicalDecl()), field_name);
@@ -1166,15 +1166,11 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
             });
         }
 
-        if (!c.zig_is_stage1 and is_packed) {
-            return failDecl(c, record_loc, name, "cannot translate packed record union", .{});
-        }
-
         const record_payload = try c.arena.create(ast.Payload.Record);
         record_payload.* = .{
             .base = .{ .tag = ([2]Tag{ .@"struct", .@"union" })[@boolToInt(is_union)] },
             .data = .{
-                .layout = if (is_packed) .@"packed" else .@"extern",
+                .layout = if (is_packed and c.zig_is_stage1) .@"packed" else .@"extern",
                 .fields = try c.arena.dupe(ast.Payload.Record.Field, fields.items),
                 .functions = try c.arena.dupe(Node, functions.items),
                 .variables = &.{},
@@ -1851,12 +1847,62 @@ fn transCStyleCastExprClass(
     return maybeSuppressResult(c, scope, result_used, cast_node);
 }
 
-/// Clang reports the alignment in bits, we use bytes
-/// Clang uses 0 for "no alignment specified", we use null
-fn zigAlignment(bit_alignment: c_uint) ?c_uint {
-    if (bit_alignment == 0) return null;
-    return bit_alignment / 8;
-}
+/// The alignment of a variable or field
+const ClangAlignment = struct {
+    /// Clang reports the alignment in bits, we use bytes
+    /// Clang uses 0 for "no alignment specified", we use null
+    bit_alignment: c_uint,
+    /// If the field or variable is marked as 'packed'
+    ///
+    /// According to the GCC variable attribute docs, this impacts alignment
+    /// https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
+    ///
+    /// > The packed attribute specifies that a structure member
+    /// > should have the smallest possible alignment
+    ///
+    /// Note also that specifying the 'packed' attribute on a structure
+    /// implicitly packs all its fields (making their alignment 1).
+    ///
+    /// This will be null if the AST node doesn't support packing (functions)
+    is_packed: ?bool,
+
+    /// Get the alignment for a field, optionally taking into account the parent record
+    pub fn forField(c: *const Context, field: *const clang.FieldDecl, parent: ?*const clang.RecordDecl) ClangAlignment {
+        const parent_packed = if (parent) |record| record.getPackedAttribute() else false;
+        // NOTE: According to GCC docs, parent attribute packed implies child attribute packed
+        return ClangAlignment{
+            .bit_alignment = field.getAlignedAttribute(c.clang_context),
+            .is_packed = field.getPackedAttribute() or parent_packed,
+        };
+    }
+
+    pub fn forVar(c: *const Context, var_decl: *const clang.VarDecl) ClangAlignment {
+        return ClangAlignment{
+            .bit_alignment = var_decl.getAlignedAttribute(c.clang_context),
+            .is_packed = var_decl.getPackedAttribute(),
+        };
+    }
+
+    pub fn forFunc(c: *const Context, fun: *const clang.FunctionDecl) ClangAlignment {
+        return ClangAlignment{
+            .bit_alignment = fun.getAlignedAttribute(c.clang_context),
+            .is_packed = null, // not supported by GCC/clang (or meaningful),
+        };
+    }
+
+    /// Translate the clang alignment info into a zig alignment
+    ///
+    /// Returns null if there is no special alignment info
+    pub fn zigAlignment(self: ClangAlignment) ?c_uint {
+        if (self.bit_alignment != 0) {
+            return self.bit_alignment / 8;
+        } else if (self.is_packed orelse false) {
+            return 1;
+        } else {
+            return null;
+        }
+    }
+};
 
 fn transDeclStmtOne(
     c: *Context,
@@ -1910,7 +1956,7 @@ fn transDeclStmtOne(
                 .is_export = false,
                 .is_threadlocal = var_decl.getTLSKind() != .None,
                 .linksection_string = null,
-                .alignment = zigAlignment(var_decl.getAlignedAttribute(c.clang_context)),
+                .alignment = ClangAlignment.forVar(c, var_decl).zigAlignment(),
                 .name = var_name,
                 .type = type_node,
                 .init = init_node,
@@ -5054,7 +5100,7 @@ fn finishTransFnProto(
         break :blk null;
     };
 
-    const alignment = if (fn_decl) |decl| zigAlignment(decl.getAlignedAttribute(c.clang_context)) else null;
+    const alignment = if (fn_decl) |decl| ClangAlignment.forFunc(c, decl).zigAlignment() else null;
 
     const explicit_callconv = if ((is_inline or is_export or is_extern) and cc == .C) null else cc;
 

src/zig_clang.cpp

@@ -1985,6 +1985,24 @@ unsigned ZigClangFunctionDecl_getAlignedAttribute(const struct ZigClangFunctionD
     return 0;
 }
 
+bool ZigClangVarDecl_getPackedAttribute(const struct ZigClangVarDecl *self) {
+    auto casted_self = reinterpret_cast<const clang::VarDecl *>(self);
+    if (const clang::PackedAttr *PA = casted_self->getAttr<clang::PackedAttr>()) {
+        return true;
+    } else {
+        return false;
+    }
+}
+
+bool ZigClangFieldDecl_getPackedAttribute(const struct ZigClangFieldDecl *self) {
+    auto casted_self = reinterpret_cast<const clang::FieldDecl *>(self);
+    if (const clang::PackedAttr *PA = casted_self->getAttr<clang::PackedAttr>()) {
+        return true;
+    } else {
+        return false;
+    }
+}
+
 ZigClangQualType ZigClangParmVarDecl_getOriginalType(const struct ZigClangParmVarDecl *self) {
     return bitcast(reinterpret_cast<const clang::ParmVarDecl *>(self)->getOriginalType());
 }

src/zig_clang.h

@@ -1101,6 +1101,8 @@ ZIG_EXTERN_C const struct ZigClangFunctionDecl *ZigClangVarDecl_getCleanupAttrib
 ZIG_EXTERN_C unsigned ZigClangVarDecl_getAlignedAttribute(const struct ZigClangVarDecl *self, const ZigClangASTContext* ctx);
 ZIG_EXTERN_C unsigned ZigClangFunctionDecl_getAlignedAttribute(const struct ZigClangFunctionDecl *self, const ZigClangASTContext* ctx);
 ZIG_EXTERN_C unsigned ZigClangFieldDecl_getAlignedAttribute(const struct ZigClangFieldDecl *self, const ZigClangASTContext* ctx);
+ZIG_EXTERN_C bool ZigClangVarDecl_getPackedAttribute(const struct ZigClangVarDecl *self);
+ZIG_EXTERN_C bool ZigClangFieldDecl_getPackedAttribute(const struct ZigClangFieldDecl *self);
 
 ZIG_EXTERN_C const struct ZigClangStringLiteral *ZigClangFileScopeAsmDecl_getAsmString(const struct ZigClangFileScopeAsmDecl *self);