Allow creating uninitialized C structs

This adds support for creating uninitialized C structures by
leaving out _all_ the field assignments when creating an instance. This
makes dealing with large structs that are initialized by a separate
function less painful, as one doesn't have to explicitly assign all
fields to some value (along with the necessary type casts).

Changelog: added

compiler/src/type_check/expressions.rs

@@ -3849,7 +3849,8 @@ impl<'a> CheckMethodBody<'a> {
             );
         }
 
-        let require_send = class.kind(self.db()).is_async();
+        let kind = class.kind(self.db());
+        let require_send = kind.is_async();
         let ins = ClassInstance::empty(self.db_mut(), class);
         let mut assigned = HashSet::new();
         let mut fields = Vec::new();
@@ -3984,17 +3985,27 @@ impl<'a> CheckMethodBody<'a> {
             fields.push((field, expected));
         }
 
-        for field in class.field_names(self.db()) {
-            if assigned.contains(&field) {
-                continue;
-            }
+        // For extern classes we allow either all fields to be specified, or all
+        // fields to be left out. The latter is useful when dealing with C
+        // structures that start on the stack as uninitialized data and are
+        // initialized using a dedicated function.
+        //
+        // If an extern class has one or more fields specifid, then we require
+        // _all_ fields to be specified, as leaving out fields in this case is
+        // likely the result of a mistake.
+        if !kind.is_extern() || !assigned.is_empty() {
+            for field in class.field_names(self.db()) {
+                if assigned.contains(&field) {
+                    continue;
+                }
 
-            self.state.diagnostics.error(
-                DiagnosticId::MissingField,
-                format!("the field '{}' must be assigned a value", field),
-                self.file(),
-                node.location.clone(),
-            );
+                self.state.diagnostics.error(
+                    DiagnosticId::MissingField,
+                    format!("the field '{}' must be assigned a value", field),
+                    self.file(),
+                    node.location.clone(),
+                );
+            }
         }
 
         let resolved_type = TypeRef::Owned(TypeId::ClassInstance(ins));

docs/source/getting-started/ffi.md

@@ -309,6 +309,25 @@ class async Main {
 }
 ```
 
+When creating an instance of a struct we can leave out all the fields. This
+reserves the necessary stack space but doesn't initialize it. This is useful
+when dealing with large structs that are initialized by a C function:
+
+```inko
+class extern Timespec {
+  let @tv_sec: Int64
+  let @tv_nsec: Int64
+}
+
+class async Main {
+  fn async main {
+    let spec = Timespec()
+
+    hypothetical_initialization_function_that_sets_the_fields(mut spec)
+  }
+}
+```
+
 Structures are allocated on the stack and are value types, meaning a move
 results in a copy (unless this is optimised away). Reading and writing of
 structure fields uses the same syntax as regular Inko classes:

std/fixtures/diagnostics/class_extern_without_fields.inko

@@ -0,0 +1,19 @@
+class extern Foo {
+  let @a: Int32
+  let @b: Int32
+  let @c: Int32
+}
+
+fn example1 {
+  let foo = Foo()
+}
+
+fn example2 {
+  let foo = Foo(a: 0 as Int32, b: 1 as Int32, c: 2 as Int32)
+}
+
+fn example3 {
+  let foo = Foo(a: 0 as Int32, b: 1 as Int32)
+}
+
+# class_extern_without_fields.inko:16:13 error(missing-field): the field 'c' must be assigned a value

std/src/std/sys/freebsd/fs.inko

@@ -28,36 +28,7 @@ fn stat_to_metadata(buf: Pointer[libc.StatBuf]) -> Metadata {
 }
 
 fn file_metadata(fd: Int32) -> Result[Metadata, Error] {
-  let buf = libc.StatBuf(
-    st_dev: 0 as UInt64,
-    st_ino: 0 as UInt64,
-    st_nlink: 0 as UInt64,
-    st_mode: 0 as UInt16,
-    __pad0: 0 as Int16,
-    st_uid: 0 as UInt32,
-    st_gid: 0 as UInt32,
-    __pad1: 0 as Int32,
-    st_rdev: 0 as UInt64,
-    st_atim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_mtim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_ctim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_birthtim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_size: 0 as Int64,
-    st_blocks: 0 as Int64,
-    st_blksize: 0 as Int32,
-    st_flags: 0 as UInt32,
-    st_gen: 0 as UInt64,
-    st_spare0: 0 as Int64,
-    st_spare1: 0 as Int64,
-    st_spare2: 0 as Int64,
-    st_spare3: 0 as Int64,
-    st_spare4: 0 as Int64,
-    st_spare5: 0 as Int64,
-    st_spare6: 0 as Int64,
-    st_spare7: 0 as Int64,
-    st_spare8: 0 as Int64,
-    st_spare9: 0 as Int64,
-  )
+  let buf = libc.StatBuf()
 
   start_blocking
 
@@ -70,36 +41,7 @@ fn file_metadata(fd: Int32) -> Result[Metadata, Error] {
 }
 
 fn path_metadata(path: String) -> Result[Metadata, Error] {
-  let buf = libc.StatBuf(
-    st_dev: 0 as UInt64,
-    st_ino: 0 as UInt64,
-    st_nlink: 0 as UInt64,
-    st_mode: 0 as UInt16,
-    __pad0: 0 as Int16,
-    st_uid: 0 as UInt32,
-    st_gid: 0 as UInt32,
-    __pad1: 0 as Int32,
-    st_rdev: 0 as UInt64,
-    st_atim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_mtim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_ctim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_birthtim: libc.Timespec(tv_sec: 0 as Int64, tv_nsec: 0 as Int64),
-    st_size: 0 as Int64,
-    st_blocks: 0 as Int64,
-    st_blksize: 0 as Int32,
-    st_flags: 0 as UInt32,
-    st_gen: 0 as UInt64,
-    st_spare0: 0 as Int64,
-    st_spare1: 0 as Int64,
-    st_spare2: 0 as Int64,
-    st_spare3: 0 as Int64,
-    st_spare4: 0 as Int64,
-    st_spare5: 0 as Int64,
-    st_spare6: 0 as Int64,
-    st_spare7: 0 as Int64,
-    st_spare8: 0 as Int64,
-    st_spare9: 0 as Int64,
-  )
+  let buf = libc.StatBuf()
 
   start_blocking
 

std/src/std/sys/linux/fs.inko

@@ -12,59 +12,7 @@ fn statx_time(time: Pointer[libc.StatxTimestamp]) -> Time {
 }
 
 fn statx(fd: Int32, path: String) -> Result[Metadata, Error] {
-  let buf = libc.StatxBuf(
-    stx_mask: 0 as UInt32,
-    stx_blksize: 0 as UInt32,
-    stx_attributes: 0 as UInt64,
-    stx_nlink: 0 as UInt32,
-    stx_uid: 0 as UInt32,
-    stx_gid: 0 as UInt32,
-    stx_mode: 0 as UInt16,
-    __pad0: 0 as UInt16,
-    stx_ino: 0 as UInt64,
-    stx_size: 0 as UInt64,
-    stx_blocks: 0 as UInt64,
-    stx_attributes_mask: 0 as UInt64,
-    stx_atime: libc.StatxTimestamp(
-      tv_sec: 0 as Int64,
-      tv_nsec: 0 as UInt32,
-      __pad0: 0 as Int32,
-    ),
-    stx_btime: libc.StatxTimestamp(
-      tv_sec: 0 as Int64,
-      tv_nsec: 0 as UInt32,
-      __pad0: 0 as Int32,
-    ),
-    stx_ctime: libc.StatxTimestamp(
-      tv_sec: 0 as Int64,
-      tv_nsec: 0 as UInt32,
-      __pad0: 0 as Int32,
-    ),
-    stx_mtime: libc.StatxTimestamp(
-      tv_sec: 0 as Int64,
-      tv_nsec: 0 as UInt32,
-      __pad0: 0 as Int32,
-    ),
-    stx_rdev_major: 0 as UInt32,
-    stx_rdev_minor: 0 as UInt32,
-    stx_dev_major: 0 as UInt32,
-    stx_dev_minor: 0 as UInt32,
-    stx_mnt_id: 0 as UInt64,
-    stx_dio_mem_align: 0 as UInt32,
-    stx_dio_offset_align: 0 as UInt32,
-    __pad1: 0 as UInt64,
-    __pad2: 0 as UInt64,
-    __pad3: 0 as UInt64,
-    __pad4: 0 as UInt64,
-    __pad5: 0 as UInt64,
-    __pad6: 0 as UInt64,
-    __pad7: 0 as UInt64,
-    __pad8: 0 as UInt64,
-    __pad9: 0 as UInt64,
-    __pad10: 0 as UInt64,
-    __pad11: 0 as UInt64,
-    __pad12: 0 as UInt64,
-  )
+  let buf = libc.StatxBuf()
 
   start_blocking
 

std/src/std/sys/mac/fs.inko

@@ -29,31 +29,7 @@ fn stat_to_metadata(buf: Pointer[libc.StatBuf]) -> Metadata {
 }
 
 fn file_metadata(fd: Int32) -> Result[Metadata, Error] {
-  let buf = libc.StatBuf(
-    st_dev: 0 as Int32,
-    st_mode: 0 as UInt16,
-    st_nlink: 0 as UInt16,
-    st_ino: 0 as UInt64,
-    st_uid: 0 as UInt32,
-    st_gid: 0 as UInt32,
-    st_rdev: 0 as Int32,
-    st_atime: 0 as Int64,
-    st_atime_nsec: 0 as Int64,
-    st_mtime: 0 as Int64,
-    st_mtime_nsec: 0 as Int64,
-    st_ctime: 0 as Int64,
-    st_ctime_nsec: 0 as Int64,
-    st_birthtime: 0 as Int64,
-    st_birthtime_nsec: 0 as Int64,
-    st_size: 0 as Int64,
-    st_blocks: 0 as Int64,
-    st_blksize: 0 as Int32,
-    st_flags: 0 as UInt32,
-    st_gen: 0 as UInt32,
-    st_lspare: 0 as Int32,
-    st_qspare0: 0 as Int64,
-    st_qspare1: 0 as Int64,
-  )
+  let buf = libc.StatBuf()
 
   start_blocking
 
@@ -66,31 +42,7 @@ fn file_metadata(fd: Int32) -> Result[Metadata, Error] {
 }
 
 fn path_metadata(path: String) -> Result[Metadata, Error] {
-  let buf = libc.StatBuf(
-    st_dev: 0 as Int32,
-    st_mode: 0 as UInt16,
-    st_nlink: 0 as UInt16,
-    st_ino: 0 as UInt64,
-    st_uid: 0 as UInt32,
-    st_gid: 0 as UInt32,
-    st_rdev: 0 as Int32,
-    st_atime: 0 as Int64,
-    st_atime_nsec: 0 as Int64,
-    st_mtime: 0 as Int64,
-    st_mtime_nsec: 0 as Int64,
-    st_ctime: 0 as Int64,
-    st_ctime_nsec: 0 as Int64,
-    st_birthtime: 0 as Int64,
-    st_birthtime_nsec: 0 as Int64,
-    st_size: 0 as Int64,
-    st_blocks: 0 as Int64,
-    st_blksize: 0 as Int32,
-    st_flags: 0 as UInt32,
-    st_gen: 0 as UInt32,
-    st_lspare: 0 as Int32,
-    st_qspare0: 0 as Int64,
-    st_qspare1: 0 as Int64,
-  )
+  let buf = libc.StatBuf()
 
   start_blocking