EII new error code

Author: jdonszelmann

compiler/rustc_error_codes/src/error_codes/E0806.md

@@ -0,0 +1,68 @@
+An externally implementable item is not compatible with its declaration.
+
+Erroneous code example:
+
+```rust,edition2021,compile_fail,E0806
+#![feature(eii)]
+
+#[eii(foo)]
+fn x();
+
+#[foo]
+fn y(a: u64) -> u64 {
+//~^ ERROR E0806
+    a
+}
+
+
+fn main() {}
+```
+
+The error here is caused by the fact that `y` implements the
+externally implementable item `foo`.
+It can only do so if the signature of the implementation `y` matches
+that of the declaration of `foo`.
+So, to fix this, `y`'s signature must be changed to match that of `x`:
+
+```rust,edition2021
+#![feature(eii)]
+
+#[eii(foo)]
+fn x();
+
+#[foo]
+fn y() {}
+
+
+fn main() {}
+```
+
+One common way this can be triggered is by using the wrong
+signature for `#[panic_handler]`.
+The signature is provided by `core`.
+
+```rust,edition2021,ignore
+#![no_std]
+
+#[panic_handler]
+fn on_panic() -> ! {
+//~^ ERROR E0806
+
+    loop {}
+}
+
+fn main() {}
+```
+
+Should be:
+
+```rust,edition2021,ignore
+#![no_std]
+
+#[panic_handler]
+fn on_panic(info: &core::panic::PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+fn main() {}
+```

compiler/rustc_error_codes/src/lib.rs

@@ -543,6 +543,7 @@ macro_rules! error_codes {
 0803,
 0804,
 0805,
+0806,
         );
     )
 }

compiler/rustc_hir_analysis/src/check/compare_eii.rs

@@ -7,7 +7,7 @@ use std::borrow::Cow;
 use std::iter;
 
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{Applicability, E0805, struct_span_code_err};
+use rustc_errors::{Applicability, E0806, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, FnSig, HirId, ItemKind};
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
@@ -291,7 +291,7 @@ fn report_number_of_arguments_mismatch<'tcx>(
     let mut err = struct_span_code_err!(
         tcx.dcx(),
         impl_span,
-        E0805,
+        E0806,
         "`{external_impl_name}` has {} but #[{eii_name}] requires it to have {}",
         potentially_plural_count(external_impl_number_args, "parameter"),
         declaration_number_args
@@ -333,7 +333,7 @@ fn report_eii_mismatch<'tcx>(
     let mut diag = struct_span_code_err!(
         tcx.dcx(),
         impl_err_span,
-        E0805,
+        E0806,
         "function `{}` has a type that is incompatible with the declaration of `#[{eii_name}]`",
         external_impl_name
     );

tests/ui/error-codes/E0806.rs

@@ -0,0 +1,12 @@
+#![feature(eii)]
+
+#[eii(foo)]
+fn x();
+
+#[foo]
+fn y(a: u64) -> u64 {
+    //~^ ERROR E0806
+    a
+}
+
+fn main() {}

tests/ui/error-codes/E0806.stderr

@@ -0,0 +1,14 @@
+error[E0806]: `y` has 1 parameter but #[foo] requires it to have 0
+  --> $DIR/E0806.rs:7:9
+   |
+LL | fn x();
+   | ------- requires 0 parameters
+LL |
+LL | #[foo]
+   | ------ required because of this attribute
+LL | fn y(a: u64) -> u64 {
+   |         ^^^ expected 0 parameters, found 1
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0806`.