Stop using `String` for error codes.

Error codes are integers, but `String` is used everywhere to represent
them. Gross!

This commit introduces `ErrCode`, an integral newtype for error codes,
replacing `String`. It also introduces a constant for every error code,
e.g. `E0123`, and removes the `error_code!` macro. The constants are
imported wherever used with `use rustc_errors::codes::*`.

With the old code, we have three different ways to specify an error code
at a use point:
```
error_code!(E0123)  // macro call

struct_span_code_err!(dcx, span, E0123, "msg");  // bare ident arg to macro call

\#[diag(name, code = "E0123")]  // string
struct Diag;
```

With the new code, they all use the `E0123` constant.
```
E0123  // constant

struct_span_code_err!(dcx, span, E0123, "msg");  // constant

\#[diag(name, code = E0123)]  // constant
struct Diag;
```

The commit also changes the structure of the error code definitions:
- `rustc_error_codes` now just defines a higher-order macro listing the
  used error codes and nothing else.
- Because that's now the only thing in the `rustc_error_codes` crate, I
  moved it into the `lib.rs` file and removed the `error_codes.rs` file.
- `rustc_errors` uses that macro to define everything, e.g. the error
  code constants and the `DIAGNOSTIC_TABLES`. This is in its new
  `codes.rs` file.

8 files changed, 71 insertions, 74 deletions

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index b855ec8f920..7eb7c8c2bca 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -19,7 +19,7 @@ use rustc_ast::{self as ast, AssocItem, AssocItemKind, MetaItemKind, StmtKind};
 use rustc_ast::{Block, ForeignItem, ForeignItemKind, Impl, Item, ItemKind, NodeId};
 use rustc_attr as attr;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_code_err, Applicability};
+use rustc_errors::{codes::*, struct_span_code_err, Applicability};
 use rustc_expand::expand::AstFragment;
 use rustc_hir::def::{self, *};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 66dfee6c062..f3a6eb65a72 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -6,8 +6,8 @@ use rustc_ast::{MetaItemKind, NestedMetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    pluralize, report_ambiguity_error, struct_span_code_err, Applicability, DiagCtxt, Diagnostic,
-    DiagnosticBuilder, ErrorGuaranteed, MultiSpan, SuggestionStyle,
+    codes::*, pluralize, report_ambiguity_error, struct_span_code_err, Applicability, DiagCtxt,
+    Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, SuggestionStyle,
 };
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
index 5d9269d8f54..50ff09feb4d 100644
--- a/compiler/rustc_resolve/src/errors.rs
+++ b/compiler/rustc_resolve/src/errors.rs
@@ -1,3 +1,4 @@
+use rustc_errors::codes::*;
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{
     symbol::{Ident, Symbol},
@@ -7,16 +8,16 @@ use rustc_span::{
 use crate::{late::PatternSource, Res};
 
 #[derive(Diagnostic)]
-#[diag(resolve_parent_module_reset_for_binding, code = "E0637")]
+#[diag(resolve_parent_module_reset_for_binding, code = E0637)]
 pub(crate) struct ParentModuleResetForBinding;
 
 #[derive(Diagnostic)]
-#[diag(resolve_ampersand_used_without_explicit_lifetime_name, code = "E0637")]
+#[diag(resolve_ampersand_used_without_explicit_lifetime_name, code = E0637)]
 #[note]
 pub(crate) struct AmpersandUsedWithoutExplicitLifetimeName(#[primary_span] pub(crate) Span);
 
 #[derive(Diagnostic)]
-#[diag(resolve_underscore_lifetime_name_cannot_be_used_here, code = "E0637")]
+#[diag(resolve_underscore_lifetime_name_cannot_be_used_here, code = E0637)]
 #[note]
 pub(crate) struct UnderscoreLifetimeNameCannotBeUsedHere(#[primary_span] pub(crate) Span);
 
@@ -33,7 +34,7 @@ pub(crate) struct CrateRootNamesMustBeNamedExplicitly(#[primary_span] pub(crate)
 pub(crate) struct ResolutionError(#[primary_span] pub(crate) Span);
 
 #[derive(Diagnostic)]
-#[diag(resolve_generic_params_from_outer_item, code = "E0401")]
+#[diag(resolve_generic_params_from_outer_item, code = E0401)]
 pub(crate) struct GenericParamsFromOuterItem {
     #[primary_span]
     #[label]
@@ -67,7 +68,7 @@ pub(crate) struct GenericParamsFromOuterItemSugg {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_name_is_already_used_as_generic_parameter, code = "E0403")]
+#[diag(resolve_name_is_already_used_as_generic_parameter, code = E0403)]
 pub(crate) struct NameAlreadyUsedInParameterList {
     #[primary_span]
     #[label]
@@ -78,7 +79,7 @@ pub(crate) struct NameAlreadyUsedInParameterList {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_method_not_member_of_trait, code = "E0407")]
+#[diag(resolve_method_not_member_of_trait, code = E0407)]
 pub(crate) struct MethodNotMemberOfTrait {
     #[primary_span]
     #[label]
@@ -102,7 +103,7 @@ pub(crate) struct AssociatedFnWithSimilarNameExists {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_type_not_member_of_trait, code = "E0437")]
+#[diag(resolve_type_not_member_of_trait, code = E0437)]
 pub(crate) struct TypeNotMemberOfTrait {
     #[primary_span]
     #[label]
@@ -126,7 +127,7 @@ pub(crate) struct AssociatedTypeWithSimilarNameExists {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_const_not_member_of_trait, code = "E0438")]
+#[diag(resolve_const_not_member_of_trait, code = E0438)]
 pub(crate) struct ConstNotMemberOfTrait {
     #[primary_span]
     #[label]
@@ -150,7 +151,7 @@ pub(crate) struct AssociatedConstWithSimilarNameExists {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_variable_bound_with_different_mode, code = "E0409")]
+#[diag(resolve_variable_bound_with_different_mode, code = E0409)]
 pub(crate) struct VariableBoundWithDifferentMode {
     #[primary_span]
     #[label]
@@ -161,7 +162,7 @@ pub(crate) struct VariableBoundWithDifferentMode {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_ident_bound_more_than_once_in_parameter_list, code = "E0415")]
+#[diag(resolve_ident_bound_more_than_once_in_parameter_list, code = E0415)]
 pub(crate) struct IdentifierBoundMoreThanOnceInParameterList {
     #[primary_span]
     #[label]
@@ -170,7 +171,7 @@ pub(crate) struct IdentifierBoundMoreThanOnceInParameterList {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_ident_bound_more_than_once_in_same_pattern, code = "E0416")]
+#[diag(resolve_ident_bound_more_than_once_in_same_pattern, code = E0416)]
 pub(crate) struct IdentifierBoundMoreThanOnceInSamePattern {
     #[primary_span]
     #[label]
@@ -179,7 +180,7 @@ pub(crate) struct IdentifierBoundMoreThanOnceInSamePattern {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_undeclared_label, code = "E0426")]
+#[diag(resolve_undeclared_label, code = E0426)]
 pub(crate) struct UndeclaredLabel {
     #[primary_span]
     #[label]
@@ -217,7 +218,7 @@ pub(crate) struct UnreachableLabelWithSimilarNameExists {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_self_import_can_only_appear_once_in_the_list, code = "E0430")]
+#[diag(resolve_self_import_can_only_appear_once_in_the_list, code = E0430)]
 pub(crate) struct SelfImportCanOnlyAppearOnceInTheList {
     #[primary_span]
     #[label]
@@ -225,7 +226,7 @@ pub(crate) struct SelfImportCanOnlyAppearOnceInTheList {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_self_import_only_in_import_list_with_non_empty_prefix, code = "E0431")]
+#[diag(resolve_self_import_only_in_import_list_with_non_empty_prefix, code = E0431)]
 pub(crate) struct SelfImportOnlyInImportListWithNonEmptyPrefix {
     #[primary_span]
     #[label]
@@ -233,7 +234,7 @@ pub(crate) struct SelfImportOnlyInImportListWithNonEmptyPrefix {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_cannot_capture_dynamic_environment_in_fn_item, code = "E0434")]
+#[diag(resolve_cannot_capture_dynamic_environment_in_fn_item, code = E0434)]
 #[help]
 pub(crate) struct CannotCaptureDynamicEnvironmentInFnItem {
     #[primary_span]
@@ -241,7 +242,7 @@ pub(crate) struct CannotCaptureDynamicEnvironmentInFnItem {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_attempt_to_use_non_constant_value_in_constant, code = "E0435")]
+#[diag(resolve_attempt_to_use_non_constant_value_in_constant, code = E0435)]
 pub(crate) struct AttemptToUseNonConstantValueInConstant<'a> {
     #[primary_span]
     pub(crate) span: Span,
@@ -283,7 +284,7 @@ pub(crate) struct AttemptToUseNonConstantValueInConstantWithoutSuggestion<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_self_imports_only_allowed_within, code = "E0429")]
+#[diag(resolve_self_imports_only_allowed_within, code = E0429)]
 pub(crate) struct SelfImportsOnlyAllowedWithin {
     #[primary_span]
     pub(crate) span: Span,
@@ -317,7 +318,7 @@ pub(crate) struct SelfImportsOnlyAllowedWithinMultipartSuggestion {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_binding_shadows_something_unacceptable, code = "E0530")]
+#[diag(resolve_binding_shadows_something_unacceptable, code = E0530)]
 pub(crate) struct BindingShadowsSomethingUnacceptable<'a> {
     #[primary_span]
     #[label]
@@ -346,7 +347,7 @@ pub(crate) struct BindingShadowsSomethingUnacceptableSuggestion {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_forward_declared_generic_param, code = "E0128")]
+#[diag(resolve_forward_declared_generic_param, code = E0128)]
 pub(crate) struct ForwardDeclaredGenericParam {
     #[primary_span]
     #[label]
@@ -354,7 +355,7 @@ pub(crate) struct ForwardDeclaredGenericParam {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_param_in_ty_of_const_param, code = "E0770")]
+#[diag(resolve_param_in_ty_of_const_param, code = E0770)]
 pub(crate) struct ParamInTyOfConstParam {
     #[primary_span]
     #[label]
@@ -376,7 +377,7 @@ pub(crate) enum ParamKindInTyOfConstParam {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_self_in_generic_param_default, code = "E0735")]
+#[diag(resolve_self_in_generic_param_default, code = E0735)]
 pub(crate) struct SelfInGenericParamDefault {
     #[primary_span]
     #[label]
@@ -412,7 +413,7 @@ pub(crate) enum ParamKindInNonTrivialAnonConst {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_unreachable_label, code = "E0767")]
+#[diag(resolve_unreachable_label, code = E0767)]
 #[note]
 pub(crate) struct UnreachableLabel {
     #[primary_span]
@@ -495,7 +496,7 @@ pub(crate) struct BindingInNeverPattern {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_trait_impl_duplicate, code = "E0201")]
+#[diag(resolve_trait_impl_duplicate, code = E0201)]
 pub(crate) struct TraitImplDuplicate {
     #[primary_span]
     #[label]
@@ -518,11 +519,11 @@ pub(crate) struct Relative2018 {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_ancestor_only, code = "E0742")]
+#[diag(resolve_ancestor_only, code = E0742)]
 pub(crate) struct AncestorOnly(#[primary_span] pub(crate) Span);
 
 #[derive(Diagnostic)]
-#[diag(resolve_expected_found, code = "E0577")]
+#[diag(resolve_expected_found, code = E0577)]
 pub(crate) struct ExpectedFound {
     #[primary_span]
     #[label]
@@ -532,7 +533,7 @@ pub(crate) struct ExpectedFound {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_indeterminate, code = "E0578")]
+#[diag(resolve_indeterminate, code = E0578)]
 pub(crate) struct Indeterminate(#[primary_span] pub(crate) Span);
 
 #[derive(Diagnostic)]
@@ -714,7 +715,7 @@ pub(crate) struct CannotDetermineMacroResolution {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_cannot_be_reexported_private, code = "E0364")]
+#[diag(resolve_cannot_be_reexported_private, code = E0364)]
 pub(crate) struct CannotBeReexportedPrivate {
     #[primary_span]
     pub(crate) span: Span,
@@ -722,7 +723,7 @@ pub(crate) struct CannotBeReexportedPrivate {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_cannot_be_reexported_crate_public, code = "E0364")]
+#[diag(resolve_cannot_be_reexported_crate_public, code = E0364)]
 pub(crate) struct CannotBeReexportedCratePublic {
     #[primary_span]
     pub(crate) span: Span,
@@ -730,7 +731,7 @@ pub(crate) struct CannotBeReexportedCratePublic {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_cannot_be_reexported_private, code = "E0365")]
+#[diag(resolve_cannot_be_reexported_private, code = E0365)]
 #[note(resolve_consider_declaring_with_pub)]
 pub(crate) struct CannotBeReexportedPrivateNS {
     #[primary_span]
@@ -740,7 +741,7 @@ pub(crate) struct CannotBeReexportedPrivateNS {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_cannot_be_reexported_crate_public, code = "E0365")]
+#[diag(resolve_cannot_be_reexported_crate_public, code = E0365)]
 #[note(resolve_consider_declaring_with_pub)]
 pub(crate) struct CannotBeReexportedCratePublicNS {
     #[primary_span]
@@ -779,7 +780,7 @@ pub(crate) struct ItemsInTraitsAreNotImportable {
 }
 
 #[derive(Diagnostic)]
-#[diag(resolve_is_not_directly_importable, code = "E0253")]
+#[diag(resolve_is_not_directly_importable, code = E0253)]
 pub(crate) struct IsNotDirectlyImportable {
     #[primary_span]
     #[label]
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index ad60caed354..30fb35238c3 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -17,7 +17,7 @@ use crate::{NameBinding, NameBindingData, NameBindingKind, PathResult};
 use rustc_ast::NodeId;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::intern::Interned;
-use rustc_errors::{pluralize, struct_span_code_err, Applicability, MultiSpan};
+use rustc_errors::{codes::*, pluralize, struct_span_code_err, Applicability, MultiSpan};
 use rustc_hir::def::{self, DefKind, PartialRes};
 use rustc_middle::metadata::ModChild;
 use rustc_middle::metadata::Reexport;
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 18926463d6b..0743ff03864 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -16,7 +16,9 @@ use rustc_ast::ptr::P;
 use rustc_ast::visit::{self, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
-use rustc_errors::{Applicability, DiagnosticArgValue, IntoDiagnosticArg};
+use rustc_errors::{
+    codes::*, struct_span_code_err, Applicability, DiagnosticArgValue, ErrCode, IntoDiagnosticArg,
+};
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
@@ -533,21 +535,20 @@ impl<'a> PathSource<'a> {
         }
     }
 
-    fn error_code(self, has_unexpected_resolution: bool) -> String {
-        use rustc_errors::error_code;
+    fn error_code(self, has_unexpected_resolution: bool) -> ErrCode {
         match (self, has_unexpected_resolution) {
-            (PathSource::Trait(_), true) => error_code!(E0404),
-            (PathSource::Trait(_), false) => error_code!(E0405),
-            (PathSource::Type, true) => error_code!(E0573),
-            (PathSource::Type, false) => error_code!(E0412),
-            (PathSource::Struct, true) => error_code!(E0574),
-            (PathSource::Struct, false) => error_code!(E0422),
-            (PathSource::Expr(..), true) | (PathSource::Delegation, true) => error_code!(E0423),
-            (PathSource::Expr(..), false) | (PathSource::Delegation, false) => error_code!(E0425),
-            (PathSource::Pat | PathSource::TupleStruct(..), true) => error_code!(E0532),
-            (PathSource::Pat | PathSource::TupleStruct(..), false) => error_code!(E0531),
-            (PathSource::TraitItem(..), true) => error_code!(E0575),
-            (PathSource::TraitItem(..), false) => error_code!(E0576),
+            (PathSource::Trait(_), true) => E0404,
+            (PathSource::Trait(_), false) => E0405,
+            (PathSource::Type, true) => E0573,
+            (PathSource::Type, false) => E0412,
+            (PathSource::Struct, true) => E0574,
+            (PathSource::Struct, false) => E0422,
+            (PathSource::Expr(..), true) | (PathSource::Delegation, true) => E0423,
+            (PathSource::Expr(..), false) | (PathSource::Delegation, false) => E0425,
+            (PathSource::Pat | PathSource::TupleStruct(..), true) => E0532,
+            (PathSource::Pat | PathSource::TupleStruct(..), false) => E0531,
+            (PathSource::TraitItem(..), true) => E0575,
+            (PathSource::TraitItem(..), false) => E0576,
         }
     }
 }
@@ -1673,13 +1674,8 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     } else {
                         ("`'_` cannot be used here", "`'_` is a reserved lifetime name")
                     };
-                    let mut diag = rustc_errors::struct_span_code_err!(
-                        self.r.dcx(),
-                        lifetime.ident.span,
-                        E0637,
-                        "{}",
-                        msg,
-                    );
+                    let mut diag =
+                        struct_span_code_err!(self.r.dcx(), lifetime.ident.span, E0637, "{}", msg,);
                     diag.span_label(lifetime.ident.span, note);
                     if elided {
                         for rib in self.lifetime_ribs[i..].iter().rev() {
@@ -1863,7 +1859,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     LifetimeRibKind::AnonymousCreateParameter { report_in_path: true, .. }
                     | LifetimeRibKind::AnonymousWarn(_) => {
                         let sess = self.r.tcx.sess;
-                        let mut err = rustc_errors::struct_span_code_err!(
+                        let mut err = struct_span_code_err!(
                             sess.dcx(),
                             path_span,
                             E0726,
@@ -2608,7 +2604,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             }
 
             if param.ident.name == kw::UnderscoreLifetime {
-                rustc_errors::struct_span_code_err!(
+                struct_span_code_err!(
                     self.r.dcx(),
                     param.ident.span,
                     E0637,
@@ -2622,7 +2618,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             }
 
             if param.ident.name == kw::StaticLifetime {
-                rustc_errors::struct_span_code_err!(
+                struct_span_code_err!(
                     self.r.dcx(),
                     param.ident.span,
                     E0262,
@@ -3164,10 +3160,10 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         // The method kind does not correspond to what appeared in the trait, report.
         let path = &self.current_trait_ref.as_ref().unwrap().1.path;
         let (code, kind) = match kind {
-            AssocItemKind::Const(..) => (rustc_errors::error_code!(E0323), "const"),
-            AssocItemKind::Fn(..) => (rustc_errors::error_code!(E0324), "method"),
-            AssocItemKind::Type(..) => (rustc_errors::error_code!(E0325), "type"),
-            AssocItemKind::Delegation(..) => (rustc_errors::error_code!(E0324), "method"),
+            AssocItemKind::Const(..) => (E0323, "const"),
+            AssocItemKind::Fn(..) => (E0324, "method"),
+            AssocItemKind::Type(..) => (E0325, "type"),
+            AssocItemKind::Delegation(..) => (E0324, "method"),
             AssocItemKind::MacCall(..) => span_bug!(span, "unexpanded macro"),
         };
         let trait_path = path_names_to_string(path);
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index abb0a7a465a..a4c041f1ff2 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -16,8 +16,8 @@ use rustc_ast::{
 use rustc_ast_pretty::pprust::where_bound_predicate_to_string;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
-    MultiSpan, SuggestionStyle,
+    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    ErrorGuaranteed, MultiSpan, SuggestionStyle,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{self, CtorKind, CtorOf, DefKind};
@@ -922,8 +922,8 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         path: &[Segment],
         span: Span,
     ) {
-        if let Some(err_code) = &err.code {
-            if err_code == &rustc_errors::error_code!(E0425) {
+        if let Some(err_code) = err.code {
+            if err_code == E0425 {
                 for label_rib in &self.label_ribs {
                     for (label_ident, node_id) in &label_rib.bindings {
                         let ident = path.last().unwrap().ident;
@@ -946,7 +946,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                         }
                     }
                 }
-            } else if err_code == &rustc_errors::error_code!(E0412) {
+            } else if err_code == E0412 {
                 if let Some(correct) = Self::likely_rust_type(path) {
                     err.span_suggestion(
                         span,
@@ -970,7 +970,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         if !is_self_type(path, source.namespace()) {
             return false;
         }
-        err.code(rustc_errors::error_code!(E0411));
+        err.code(E0411);
         err.span_label(span, "`Self` is only available in impls, traits, and type definitions");
         if let Some(item_kind) = self.diagnostic_metadata.current_item {
             if !item_kind.ident.span.is_dummy() {
@@ -999,7 +999,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         }
 
         debug!("smart_resolve_path_fragment: E0424, source={:?}", source);
-        err.code(rustc_errors::error_code!(E0424));
+        err.code(E0424);
         err.span_label(
             span,
             match source {
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 925c3c0e27d..9d09d060b59 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -35,7 +35,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{FreezeReadGuard, Lrc};
-use rustc_errors::{Applicability, DiagnosticBuilder};
+use rustc_errors::{Applicability, DiagnosticBuilder, ErrCode};
 use rustc_expand::base::{DeriveResolutions, SyntaxExtension, SyntaxExtensionKind};
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
@@ -256,7 +256,7 @@ enum ResolutionError<'a> {
         kind: &'static str,
         trait_path: String,
         trait_item_span: Span,
-        code: String,
+        code: ErrCode,
     },
     /// Error E0201: multiple impl items for the same trait item.
     TraitImplDuplicate { name: Symbol, trait_item_span: Span, old_span: Span },
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index 1c085ddf57b..a6bf289a76a 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -15,7 +15,7 @@ use rustc_ast_pretty::pprust;
 use rustc_attr::StabilityLevel;
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_code_err, Applicability};
+use rustc_errors::{codes::*, struct_span_code_err, Applicability};
 use rustc_expand::base::{Annotatable, DeriveResolutions, Indeterminate, ResolverExpand};
 use rustc_expand::base::{SyntaxExtension, SyntaxExtensionKind};
 use rustc_expand::compile_declarative_macro;