error on manually implementing field traits

Author: BennoLossin

compiler/rustc_error_codes/src/error_codes/E0806.md

@@ -0,0 +1,23 @@
+`UnalignedField` or `Field` was manually implemented, which is not allowed.
+
+Erroneous code examples:
+
+```compile_fail,E0806
+use std::field::UnalignedField;
+
+pub struct MyStruct;
+
+unsafe impl UnalignedField for MyStruct {
+    type Base = ();
+    type Type = ();
+    const OFFSET: usize = 0;
+}
+```
+
+```compile_fail,E0806
+use std::field::{Field, field_of};
+
+pub struct MyStruct(usize);
+
+unsafe impl Field for field_of!(MyStruct, 0) {}
+```

compiler/rustc_error_codes/src/lib.rs

@@ -548,6 +548,7 @@ E0802: 0802,
 E0803: 0803,
 E0804: 0804,
 E0805: 0805,
+E0806: 0806,
         );
     )
 }

compiler/rustc_hir_analysis/messages.ftl

@@ -161,6 +161,9 @@ hir_analysis_drop_impl_on_wrong_item =
     the `{$trait_}` trait may only be implemented for local structs, enums, and unions
     .label = must be a struct, enum, or union in the current crate
 
+hir_analysis_field_trait_impl =
+    the `{$trait_}` trait may not be implemented manually
+
 hir_analysis_drop_impl_reservation = reservation `Drop` impls are not supported
 
 hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twice

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -52,6 +52,8 @@ pub(super) fn check_trait<'tcx>(
         lang_items.coerce_pointee_validated_trait(),
         visit_implementation_of_coerce_pointee_validity,
     )?;
+    checker.check(lang_items.unaligned_field_trait(), visit_implementation_of_field_trait)?;
+    checker.check(lang_items.field_trait(), visit_implementation_of_field_trait)?;
     Ok(())
 }
 
@@ -374,6 +376,18 @@ fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<()
     }
 }
 
+fn visit_implementation_of_field_trait(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
+    let tcx = checker.tcx;
+    let impl_did = checker.impl_def_id;
+
+    let span = tcx.span_of_impl(impl_did.into()).expect("impl_did is local");
+
+    Err(tcx.dcx().emit_err(errors::FieldTraitImpl {
+        span,
+        trait_: tcx.item_name(checker.impl_header.trait_ref.skip_binder().def_id),
+    }))
+}
+
 pub(crate) fn coerce_unsized_info<'tcx>(
     tcx: TyCtxt<'tcx>,
     impl_did: LocalDefId,

compiler/rustc_hir_analysis/src/errors.rs

@@ -208,6 +208,14 @@ pub(crate) struct DropImplOnWrongItem {
     pub trait_: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_field_trait_impl, code = E0806)]
+pub(crate) struct FieldTraitImpl {
+    #[primary_span]
+    pub span: Span,
+    pub trait_: Symbol,
+}
+
 #[derive(Diagnostic)]
 pub(crate) enum FieldAlreadyDeclared {
     #[diag(hir_analysis_field_already_declared, code = E0124)]

tests/ui/field_projections/deny-manual-impl.rs

@@ -0,0 +1,18 @@
+#![allow(incomplete_features)]
+#![feature(field_projections)]
+
+use std::field::{Field, UnalignedField, field_of};
+
+#[repr(packed)]
+pub struct MyStruct(usize);
+
+unsafe impl UnalignedField for MyStruct {
+    //~^ ERROR: the `UnalignedField` trait may not be implemented manually [E0806]
+    type Base = ();
+    type Type = ();
+    const OFFSET: usize = 0;
+}
+
+unsafe impl Field for field_of!(MyStruct, 0) {} //~ ERROR: the `Field` trait may not be implemented manually [E0806]
+
+fn main() {}

tests/ui/field_projections/deny-manual-impl.stderr

@@ -0,0 +1,15 @@
+error[E0806]: the `UnalignedField` trait may not be implemented manually
+  --> $DIR/deny-manual-impl.rs:9:1
+   |
+LL | unsafe impl UnalignedField for MyStruct {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0806]: the `Field` trait may not be implemented manually
+  --> $DIR/deny-manual-impl.rs:16:1
+   |
+LL | unsafe impl Field for field_of!(MyStruct, 0) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0806`.