diff --git a/lib/compiler_rt/multi3.zig b/lib/compiler_rt/multi3.zig
index 42994a81bda2..9502dc5677ce 100644
--- a/lib/compiler_rt/multi3.zig
+++ b/lib/compiler_rt/multi3.zig
@@ -59,12 +59,12 @@ const twords = extern union {
     s: S,
 
     const S = if (native_endian == .Little)
-        struct {
+        extern struct {
             low: u64,
             high: u64,
         }
     else
-        struct {
+        extern struct {
             high: u64,
             low: u64,
         };
diff --git a/lib/compiler_rt/shift.zig b/lib/compiler_rt/shift.zig
index ee8b634fbb2e..e38c3973bc2b 100644
--- a/lib/compiler_rt/shift.zig
+++ b/lib/compiler_rt/shift.zig
@@ -31,9 +31,9 @@ fn Dwords(comptime T: type, comptime signed_half: bool) type {
 
         all: T,
         s: if (native_endian == .Little)
-            struct { low: HalfT, high: HalfT }
+            extern struct { low: HalfT, high: HalfT }
         else
-            struct { high: HalfT, low: HalfT },
+            extern struct { high: HalfT, low: HalfT },
     };
 }
 
diff --git a/lib/std/c/haiku.zig b/lib/std/c/haiku.zig
index 0d238d833edc..672f4fa4baa8 100644
--- a/lib/std/c/haiku.zig
+++ b/lib/std/c/haiku.zig
@@ -30,8 +30,8 @@ pub extern "c" fn _kern_get_current_team() i32;
 pub const sem_t = extern struct {
     type: i32,
     u: extern union {
-        named_sem_id: ?i32,
-        unnamed_sem: ?i32,
+        named_sem_id: i32,
+        unnamed_sem: i32,
     },
     padding: [2]i32,
 };
diff --git a/lib/std/start.zig b/lib/std/start.zig
index cf079863873b..788c979d4826 100644
--- a/lib/std/start.zig
+++ b/lib/std/start.zig
@@ -108,7 +108,7 @@ fn callMain2() noreturn {
     exit2(0);
 }
 
-fn wasiMain2() noreturn {
+fn wasiMain2() callconv(.C) noreturn {
     switch (@typeInfo(@typeInfo(@TypeOf(root.main)).Fn.return_type.?)) {
         .Void => {
             root.main();
diff --git a/src/Sema.zig b/src/Sema.zig
index 0e7c1c5937f8..d66c447e7c57 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -4740,12 +4740,19 @@ pub fn analyzeExport(
 
     try mod.ensureDeclAnalyzed(exported_decl_index);
     const exported_decl = mod.declPtr(exported_decl_index);
-    // TODO run the same checks as we do for C ABI struct fields
-    switch (exported_decl.ty.zigTypeTag()) {
-        .Fn, .Int, .Enum, .Struct, .Union, .Array, .Float, .Pointer, .Optional => {},
-        else => return sema.fail(block, src, "unable to export type '{}'", .{
-            exported_decl.ty.fmt(sema.mod),
-        }),
+
+    if (!(try sema.validateExternType(exported_decl.ty, .other))) {
+        const msg = msg: {
+            const msg = try sema.errMsg(block, src, "unable to export type '{}'", .{exported_decl.ty.fmt(sema.mod)});
+            errdefer msg.destroy(sema.gpa);
+
+            const src_decl = sema.mod.declPtr(block.src_decl);
+            try sema.explainWhyTypeIsNotExtern(block, src, msg, src.toSrcLoc(src_decl), exported_decl.ty, .other);
+
+            try sema.addDeclaredHereNote(msg, exported_decl.ty);
+            break :msg msg;
+        };
+        return sema.failWithOwnedErrorMsg(block, msg);
     }
 
     const gpa = mod.gpa;
@@ -13799,7 +13806,20 @@ fn validatePtrTy(sema: *Sema, block: *Block, elem_src: LazySrcLoc, ty: Type) Com
     } else if (ptr_info.size == .Many and pointee_tag == .Opaque) {
         return sema.fail(block, elem_src, "unknown-length pointer to opaque not allowed", .{});
     } else if (ptr_info.size == .C) {
-        // TODO check extern type
+        const elem_ty = ptr_info.pointee_type;
+        if (!(try sema.validateExternType(elem_ty, .other))) {
+            const msg = msg: {
+                const msg = try sema.errMsg(block, elem_src, "C pointers cannot point to non-C-ABI-compatible type '{}'", .{elem_ty.fmt(sema.mod)});
+                errdefer msg.destroy(sema.gpa);
+
+                const src_decl = sema.mod.declPtr(block.src_decl);
+                try sema.explainWhyTypeIsNotExtern(block, elem_src, msg, elem_src.toSrcLoc(src_decl), elem_ty, .other);
+
+                try sema.addDeclaredHereNote(msg, elem_ty);
+                break :msg msg;
+            };
+            return sema.failWithOwnedErrorMsg(block, msg);
+        }
         if (pointee_tag == .Opaque) {
             return sema.fail(block, elem_src, "C pointers cannot point to opaque types", .{});
         }
@@ -18098,10 +18118,12 @@ fn explainWhyTypeIsComptime(
         .NoReturn,
         .Undefined,
         .Null,
-        .Opaque,
-        .Optional,
         => return,
 
+        .Opaque => {
+            try mod.errNoteNonLazy(src_loc, msg, "opaque type '{}' has undefined size", .{ty.fmt(sema.mod)});
+        },
+
         .Array, .Vector => {
             try sema.explainWhyTypeIsComptime(block, src, msg, src_loc, ty.elemType());
         },
@@ -18124,6 +18146,10 @@ fn explainWhyTypeIsComptime(
             try sema.explainWhyTypeIsComptime(block, src, msg, src_loc, ty.elemType());
         },
 
+        .Optional => {
+            var buf: Type.Payload.ElemType = undefined;
+            try sema.explainWhyTypeIsComptime(block, src, msg, src_loc, ty.optionalChild(&buf));
+        },
         .ErrorUnion => {
             try sema.explainWhyTypeIsComptime(block, src, msg, src_loc, ty.errorUnionPayload());
         },
@@ -18163,6 +18189,120 @@ fn explainWhyTypeIsComptime(
     }
 }
 
+const ExternPosition = enum {
+    ret_ty,
+    param_ty,
+    union_field,
+    other,
+};
+
+fn validateExternType(sema: *Sema, ty: Type, position: ExternPosition) CompileError!bool {
+    switch (ty.zigTypeTag()) {
+        .Type,
+        .ComptimeFloat,
+        .ComptimeInt,
+        .EnumLiteral,
+        .Undefined,
+        .Null,
+        .ErrorUnion,
+        .ErrorSet,
+        .BoundFn,
+        .Frame,
+        => return false,
+        .Void => return position == .union_field,
+        .NoReturn => return position == .ret_ty,
+        .Opaque,
+        .Bool,
+        .Float,
+        .Pointer,
+        .AnyFrame,
+        => return true,
+        .Int => switch (ty.intInfo(sema.mod.getTarget()).bits) {
+            8, 16, 32, 64, 128 => return true,
+            else => return false,
+        },
+        .Fn => return !ty.fnCallingConventionAllowsZigTypes(),
+        .Enum => {
+            var buf: Type.Payload.Bits = undefined;
+            return sema.validateExternType(ty.intTagType(&buf), position);
+        },
+        .Struct, .Union => switch (ty.containerLayout()) {
+            .Extern, .Packed => return true,
+            else => return false,
+        },
+        .Array => {
+            if (position == .ret_ty or position == .param_ty) return false;
+            return sema.validateExternType(ty.elemType2(), .other);
+        },
+        .Vector => return sema.validateExternType(ty.elemType2(), .other),
+        .Optional => return ty.isPtrLikeOptional(),
+    }
+}
+
+fn explainWhyTypeIsNotExtern(
+    sema: *Sema,
+    block: *Block,
+    src: LazySrcLoc,
+    msg: *Module.ErrorMsg,
+    src_loc: Module.SrcLoc,
+    ty: Type,
+    position: ExternPosition,
+) CompileError!void {
+    const mod = sema.mod;
+    switch (ty.zigTypeTag()) {
+        .Opaque,
+        .Bool,
+        .Float,
+        .Pointer,
+        .AnyFrame,
+        => return,
+
+        .Type,
+        .ComptimeFloat,
+        .ComptimeInt,
+        .EnumLiteral,
+        .Undefined,
+        .Null,
+        .ErrorUnion,
+        .ErrorSet,
+        .BoundFn,
+        .Frame,
+        => return,
+
+        .Void => try mod.errNoteNonLazy(src_loc, msg, "'void' is a zero bit type; for C 'void' use 'anyopaque'", .{}),
+        .NoReturn => try mod.errNoteNonLazy(src_loc, msg, "'noreturn' is only allowed as a return type", .{}),
+        .Int => if (ty.intInfo(sema.mod.getTarget()).bits > 128) {
+            try mod.errNoteNonLazy(src_loc, msg, "only integers with less than 128 bits are extern compatible", .{});
+        } else {
+            try mod.errNoteNonLazy(src_loc, msg, "only integers with power of two bits are extern compatible", .{});
+        },
+        .Fn => switch (ty.fnCallingConvention()) {
+            .Unspecified => try mod.errNoteNonLazy(src_loc, msg, "extern function must specify calling convention", .{}),
+            .Async => try mod.errNoteNonLazy(src_loc, msg, "async function cannot be extern", .{}),
+            .Inline => try mod.errNoteNonLazy(src_loc, msg, "inline function cannot be extern", .{}),
+            else => return,
+        },
+        .Enum => {
+            var buf: Type.Payload.Bits = undefined;
+            const tag_ty = ty.intTagType(&buf);
+            try mod.errNoteNonLazy(src_loc, msg, "enum tag type '{}' is not extern compatible", .{tag_ty.fmt(sema.mod)});
+            try sema.explainWhyTypeIsNotExtern(block, src, msg, src_loc, tag_ty, position);
+        },
+        .Struct => try mod.errNoteNonLazy(src_loc, msg, "only structs with packed or extern layout are extern compatible", .{}),
+        .Union => try mod.errNoteNonLazy(src_loc, msg, "only unions with packed or extern layout are extern compatible", .{}),
+        .Array => {
+            if (position == .ret_ty) {
+                try mod.errNoteNonLazy(src_loc, msg, "arrays are not allowed as a return type", .{});
+            } else if (position == .param_ty) {
+                try mod.errNoteNonLazy(src_loc, msg, "arrays are not allowed as a parameter type", .{});
+            }
+            try sema.explainWhyTypeIsNotExtern(block, src, msg, src_loc, ty.elemType2(), position);
+        },
+        .Vector => try sema.explainWhyTypeIsNotExtern(block, src, msg, src_loc, ty.elemType2(), position),
+        .Optional => try mod.errNoteNonLazy(src_loc, msg, "only pointer like optionals are extern compatible", .{}),
+    }
+}
+
 pub const PanicId = enum {
     unreach,
     unwrap_null,
@@ -24006,6 +24146,20 @@ fn resolveStructFully(
         struct_obj.status = .fully_resolved_wip;
         for (struct_obj.fields.values()) |field| {
             try sema.resolveTypeFully(block, src, field.ty);
+
+            if (struct_obj.layout == .Extern and !(try sema.validateExternType(field.ty, .other))) {
+                const msg = msg: {
+                    const msg = try sema.errMsg(block, src, "extern structs cannot contain fields of type '{}'", .{field.ty.fmt(sema.mod)});
+                    errdefer msg.destroy(sema.gpa);
+
+                    const src_decl = sema.mod.declPtr(block.src_decl);
+                    try sema.explainWhyTypeIsNotExtern(block, src, msg, src.toSrcLoc(src_decl), field.ty, .other);
+
+                    try sema.addDeclaredHereNote(msg, field.ty);
+                    break :msg msg;
+                };
+                return sema.failWithOwnedErrorMsg(block, msg);
+            }
         }
         struct_obj.status = .fully_resolved;
     }
@@ -24039,6 +24193,20 @@ fn resolveUnionFully(
         union_obj.status = .fully_resolved_wip;
         for (union_obj.fields.values()) |field| {
             try sema.resolveTypeFully(block, src, field.ty);
+
+            if (union_obj.layout == .Extern and !(try sema.validateExternType(field.ty, .union_field))) {
+                const msg = msg: {
+                    const msg = try sema.errMsg(block, src, "extern unions cannot contain fields of type '{}'", .{field.ty.fmt(sema.mod)});
+                    errdefer msg.destroy(sema.gpa);
+
+                    const src_decl = sema.mod.declPtr(block.src_decl);
+                    try sema.explainWhyTypeIsNotExtern(block, src, msg, src.toSrcLoc(src_decl), field.ty, .union_field);
+
+                    try sema.addDeclaredHereNote(msg, field.ty);
+                    break :msg msg;
+                };
+                return sema.failWithOwnedErrorMsg(block, msg);
+            }
         }
         union_obj.status = .fully_resolved;
     }
diff --git a/src/type.zig b/src/type.zig
index f14ad3a85bf5..765f1da18c12 100644
--- a/src/type.zig
+++ b/src/type.zig
@@ -3935,7 +3935,6 @@ pub const Type = extern union {
 
     /// Returns true if the type is optional and would be lowered to a single pointer
     /// address value, using 0 for null. Note that this returns true for C pointers.
-    /// See also `hasOptionalRepr`.
     pub fn isPtrLikeOptional(self: Type) bool {
         switch (self.tag()) {
             .optional_single_const_pointer,
@@ -4630,6 +4629,14 @@ pub const Type = extern union {
         };
     }
 
+    /// Asserts the type is a function.
+    pub fn fnCallingConventionAllowsZigTypes(self: Type) bool {
+        return switch (self.fnCallingConvention()) {
+            .Unspecified, .Async, .Inline, .PtxKernel => true,
+            else => false,
+        };
+    }
+
     /// Asserts the type is a function.
     pub fn fnIsVarArgs(self: Type) bool {
         return switch (self.tag()) {
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index be01b3a048af..caa78be97792 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -84,18 +84,19 @@ test "comptime union field access" {
 
 const FooExtern = extern union {
     int: i32,
-    str: struct {
-        slice: []const u8,
+    str: extern struct {
+        slice: [*:0]const u8,
     },
 };
 
 test "basic extern unions" {
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
 
     var foo = FooExtern{ .int = 1 };
     try expect(foo.int == 1);
     foo.str.slice = "Well";
-    try expect(std.mem.eql(u8, foo.str.slice, "Well"));
+    try expect(std.mem.eql(u8, std.mem.sliceTo(foo.str.slice, 0), "Well"));
 }
 
 const ExternPtrOrInt = extern union {
diff --git a/test/cases/compile_errors/c_pointer_to_void.zig b/test/cases/compile_errors/c_pointer_to_void.zig
new file mode 100644
index 000000000000..aeb5f42dec8c
--- /dev/null
+++ b/test/cases/compile_errors/c_pointer_to_void.zig
@@ -0,0 +1,11 @@
+export fn entry() void {
+    var a: [*c]void = undefined;
+    _ = a;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:1: error: C pointers cannot point to non-C-ABI-compatible type 'void'
+// :1:1: note: 'void' is a zero bit type; for C 'void' use 'anyopaque'
diff --git a/test/cases/compile_errors/exported_enum_without_explicit_integer_tag_type.zig b/test/cases/compile_errors/exported_enum_without_explicit_integer_tag_type.zig
new file mode 100644
index 000000000000..606dda4b6e8b
--- /dev/null
+++ b/test/cases/compile_errors/exported_enum_without_explicit_integer_tag_type.zig
@@ -0,0 +1,18 @@
+const E = enum { one, two };
+comptime {
+    @export(E, .{ .name = "E" });
+}
+const e: E = .two;
+comptime {
+    @export(e, .{ .name = "e" });
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:5: error: unable to export type 'type'
+// :7:5: error: unable to export type 'tmp.E'
+// :7:5: note: enum tag type 'u1' is not extern compatible
+// :7:5: note: only integers with power of two bits are extern compatible
+// :1:11: note: enum declared here
diff --git a/test/cases/compile_errors/stage1/obj/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig b/test/cases/compile_errors/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig
similarity index 86%
rename from test/cases/compile_errors/stage1/obj/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig
rename to test/cases/compile_errors/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig
index 8f45a0e5dc2f..17405743895b 100644
--- a/test/cases/compile_errors/stage1/obj/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig
+++ b/test/cases/compile_errors/extern_struct_with_extern-compatible_but_inferred_integer_tag_type.zig
@@ -25,19 +25,21 @@ pub const E = enum {
 @"227",@"228",@"229",@"230",@"231",@"232",@"233",@"234",@"235",
 @"236",@"237",@"238",@"239",@"240",@"241",@"242",@"243",@"244",
 @"245",@"246",@"247",@"248",@"249",@"250",@"251",@"252",@"253",
-@"254",@"255"
+@"254",@"255", @"256"
 };
 pub const S = extern struct {
     e: E,
 };
 export fn entry() void {
-    if (@typeInfo(E).Enum.tag_type != u8) @compileError("did not infer u8 tag type");
     const s: S = undefined;
     _ = s;
 }
 
 // error
-// backend=stage1
+// backend=stage2
 // target=native
 //
-// tmp.zig:31:5: error: extern structs cannot contain fields of type 'E'
+// :33:8: error: extern structs cannot contain fields of type 'tmp.E'
+// :33:8: note: enum tag type 'u9' is not extern compatible
+// :33:8: note: only integers with power of two bits are extern compatible
+// :1:15: note: enum declared here
diff --git a/test/cases/compile_errors/extern_struct_with_non-extern-compatible_integer_tag_type.zig b/test/cases/compile_errors/extern_struct_with_non-extern-compatible_integer_tag_type.zig
new file mode 100644
index 000000000000..5fb36b707d4e
--- /dev/null
+++ b/test/cases/compile_errors/extern_struct_with_non-extern-compatible_integer_tag_type.zig
@@ -0,0 +1,17 @@
+pub const E = enum(u31) { A, B, C };
+pub const S = extern struct {
+    e: E,
+};
+export fn entry() void {
+    const s: S = undefined;
+    _ = s;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :5:8: error: extern structs cannot contain fields of type 'tmp.E'
+// :5:8: note: enum tag type 'u31' is not extern compatible
+// :5:8: note: only integers with power of two bits are extern compatible
+// :1:15: note: enum declared here
diff --git a/test/cases/compile_errors/invalid_optional_type_in_extern_struct.zig b/test/cases/compile_errors/invalid_optional_type_in_extern_struct.zig
new file mode 100644
index 000000000000..9a6c77363cc1
--- /dev/null
+++ b/test/cases/compile_errors/invalid_optional_type_in_extern_struct.zig
@@ -0,0 +1,11 @@
+const stroo = extern struct {
+    moo: ?[*c]u8,
+};
+export fn testf(fluff: *stroo) void { _ = fluff; }
+
+// error
+// backend=stage2
+// target=native
+//
+// :4:8: error: extern structs cannot contain fields of type '?[*c]u8'
+// :4:8: note: only pointer like optionals are extern compatible
diff --git a/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig b/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig
index c6713b755a9c..b6edbc265a5e 100644
--- a/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig
+++ b/test/cases/compile_errors/non-const_variables_of_things_that_require_const_variables.zig
@@ -40,5 +40,6 @@ const Opaque = opaque {};
 // :14:8: note: to modify this variable at runtime, it must be given an explicit fixed-size number type
 // :18:8: error: variable of type '@TypeOf(null)' must be const or comptime
 // :22:19: error: values of type 'tmp.Opaque' must be comptime known, but operand value is runtime known
+// :22:19: note: opaque type 'tmp.Opaque' has undefined size
 // :26:8: error: variable of type 'type' must be const or comptime
 // :26:8: note: types are not available at runtime
diff --git a/test/cases/compile_errors/stage1/obj/exported_enum_without_explicit_integer_tag_type.zig b/test/cases/compile_errors/stage1/obj/exported_enum_without_explicit_integer_tag_type.zig
deleted file mode 100644
index c432bedd6b71..000000000000
--- a/test/cases/compile_errors/stage1/obj/exported_enum_without_explicit_integer_tag_type.zig
+++ /dev/null
@@ -1,15 +0,0 @@
-const E = enum { one, two };
-comptime {
-    @export(E, .{ .name = "E" });
-}
-const e: E = .two;
-comptime {
-    @export(e, .{ .name = "e" });
-}
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:13: error: exported enum without explicit integer tag type
-// tmp.zig:7:13: error: exported enum value without explicit integer tag type
diff --git a/test/cases/compile_errors/stage1/obj/extern_struct_with_non-extern-compatible_integer_tag_type.zig b/test/cases/compile_errors/stage1/obj/extern_struct_with_non-extern-compatible_integer_tag_type.zig
deleted file mode 100644
index aa508056a192..000000000000
--- a/test/cases/compile_errors/stage1/obj/extern_struct_with_non-extern-compatible_integer_tag_type.zig
+++ /dev/null
@@ -1,14 +0,0 @@
-pub const E = enum(u31) { A, B, C };
-pub const S = extern struct {
-    e: E,
-};
-export fn entry() void {
-    const s: S = undefined;
-    _ = s;
-}
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:3:5: error: extern structs cannot contain fields of type 'E'
diff --git a/test/cases/compile_errors/stage1/obj/invalid_optional_type_in_extern_struct.zig b/test/cases/compile_errors/stage1/obj/invalid_optional_type_in_extern_struct.zig
deleted file mode 100644
index dc2e2690b8cf..000000000000
--- a/test/cases/compile_errors/stage1/obj/invalid_optional_type_in_extern_struct.zig
+++ /dev/null
@@ -1,10 +0,0 @@
-const stroo = extern struct {
-    moo: ?[*c]u8,
-};
-export fn testf(fluff: *stroo) void { _ = fluff; }
-
-// error
-// backend=stage1
-// target=native
-//
-// tmp.zig:2:5: error: extern structs cannot contain fields of type '?[*c]u8'
diff --git a/test/cases/compile_errors/stage1/obj/optional_pointer_to_void_in_extern_struct.zig b/test/cases/compile_errors/stage1/optional_pointer_to_void_in_extern_struct.zig
similarity index 100%
rename from test/cases/compile_errors/stage1/obj/optional_pointer_to_void_in_extern_struct.zig
rename to test/cases/compile_errors/stage1/optional_pointer_to_void_in_extern_struct.zig