packed structs support comptime bitcasting

 * `type_size_store` is no longer a thing. loading and storing a pointer
   to a value may dereference up to `@sizeOf(T)` bytes, even for
   integers such as `u24`.
 * fix `types_have_same_zig_comptime_repr` to not think that the
   same `ZigTypeId` means the `ConstExprValue` neccesarily has the
   same representation.
 * implement `buf_write_value_bytes` and `buf_read_value_bytes` for
   `ContainerLayoutPacked`

closes #1120

4 files changed, 58 insertions, 2 deletions

diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index a9c748bcda..07e677b8ed 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -318,12 +318,12 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         "reading past end of pointer casted array",
         \\comptime {
         \\    const array = "aoeu";
-        \\    const slice = array[2..];
+        \\    const slice = array[1..];
         \\    const int_ptr = @ptrCast(*const u24, slice.ptr);
         \\    const deref = int_ptr.*;
         \\}
     ,
-        ".tmp_source.zig:5:26: error: attempt to read 3 bytes from [4]u8 at index 2 which is 2 bytes",
+        ".tmp_source.zig:5:26: error: attempt to read 4 bytes from [4]u8 at index 1 which is 3 bytes",
     );
 
     cases.add(
diff --git a/test/stage1/behavior.zig b/test/stage1/behavior.zig
index df311637fa..708bb91ac0 100644
--- a/test/stage1/behavior.zig
+++ b/test/stage1/behavior.zig
@@ -11,6 +11,7 @@ comptime {
     _ = @import("behavior/bswap.zig");
     _ = @import("behavior/bugs/1076.zig");
     _ = @import("behavior/bugs/1111.zig");
+    _ = @import("behavior/bugs/1120.zig");
     _ = @import("behavior/bugs/1277.zig");
     _ = @import("behavior/bugs/1322.zig");
     _ = @import("behavior/bugs/1381.zig");
diff --git a/test/stage1/behavior/bitcast.zig b/test/stage1/behavior/bitcast.zig
index 9607d2e3ef..e498905f4e 100644
--- a/test/stage1/behavior/bitcast.zig
+++ b/test/stage1/behavior/bitcast.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const builtin = @import("builtin");
 const expect = std.testing.expect;
 const maxInt = std.math.maxInt;
 
@@ -34,3 +35,34 @@ test "@bitCast extern enum to its integer type" {
     SOCK.testBitCastExternEnum();
     comptime SOCK.testBitCastExternEnum();
 }
+
+test "@bitCast packed structs at runtime and comptime" {
+    const Full = packed struct {
+        number: u16,
+    };
+    const Divided = packed struct {
+        half1: u8,
+        quarter3: u4,
+        quarter4: u4,
+    };
+    const S = struct {
+        fn doTheTest() void {
+            var full = Full{ .number = 0x1234 };
+            var two_halves = @bitCast(Divided, full);
+            switch (builtin.endian) {
+                builtin.Endian.Big => {
+                    expect(two_halves.half1 == 0x12);
+                    expect(two_halves.quarter3 == 0x3);
+                    expect(two_halves.quarter4 == 0x4);
+                },
+                builtin.Endian.Little => {
+                    expect(two_halves.half1 == 0x34);
+                    expect(two_halves.quarter3 == 0x2);
+                    expect(two_halves.quarter4 == 0x1);
+                },
+            }
+        }
+    };
+    S.doTheTest();
+    comptime S.doTheTest();
+}
diff --git a/test/stage1/behavior/bugs/1120.zig b/test/stage1/behavior/bugs/1120.zig
new file mode 100644
index 0000000000..dda46e8e1c
--- /dev/null
+++ b/test/stage1/behavior/bugs/1120.zig
@@ -0,0 +1,23 @@
+const std = @import("std");
+const expect = std.testing.expect;
+
+const A = packed struct {
+    a: u2,
+    b: u6,
+};
+const B = packed struct {
+    q: u8,
+    a: u2,
+    b: u6,
+};
+test "bug 1120" {
+    var a = A{ .a = 2, .b = 2 };
+    var b = B{ .q = 22, .a = 3, .b = 2 };
+    var t: usize = 0;
+    const ptr = switch (t) {
+        0 => &a.a,
+        1 => &b.a,
+        else => unreachable,
+    };
+    expect(ptr.* == 2);
+}