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.

9 files changed, 82 insertions, 41 deletions

diff --git a/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs b/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
index f0699a56f98..949f52ef6b5 100644
--- a/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
+++ b/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
@@ -9,8 +9,8 @@ use crate::emitter::FileWithAnnotatedLines;
 use crate::snippet::Line;
 use crate::translation::{to_fluent_args, Translate};
 use crate::{
-    CodeSuggestion, Diagnostic, DiagnosticMessage, Emitter, FluentBundle, LazyFallbackBundle,
-    Level, MultiSpan, Style, SubDiagnostic,
+    CodeSuggestion, Diagnostic, DiagnosticMessage, Emitter, ErrCode, FluentBundle,
+    LazyFallbackBundle, Level, MultiSpan, Style, SubDiagnostic,
 };
 use annotate_snippets::{Annotation, AnnotationType, Renderer, Slice, Snippet, SourceAnnotation};
 use rustc_data_structures::sync::Lrc;
@@ -127,7 +127,7 @@ impl AnnotateSnippetEmitter {
         level: &Level,
         messages: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<String>,
+        code: &Option<ErrCode>,
         msp: &MultiSpan,
         _children: &[SubDiagnostic],
         _suggestions: &[CodeSuggestion],
@@ -178,6 +178,7 @@ impl AnnotateSnippetEmitter {
                         .collect::<Vec<Owned>>()
                 })
                 .collect();
+            let code = code.map(|code| code.to_string());
             let snippet = Snippet {
                 title: Some(Annotation {
                     label: Some(&message),
diff --git a/compiler/rustc_errors/src/codes.rs b/compiler/rustc_errors/src/codes.rs
new file mode 100644
index 00000000000..947cf27ca79
--- /dev/null
+++ b/compiler/rustc_errors/src/codes.rs
@@ -0,0 +1,39 @@
+//! This module defines the following.
+//! - The `ErrCode` type.
+//! - A constant for every error code, with a name like `E0123`.
+//! - A static table `DIAGNOSTICS` pairing every error code constant with its
+//!   long description text.
+
+use std::fmt;
+
+rustc_index::newtype_index! {
+    #[max = 9999] // Because all error codes have four digits.
+    #[orderable]
+    #[encodable]
+    #[debug_format = "ErrCode({})"]
+    pub struct ErrCode {}
+}
+
+impl fmt::Display for ErrCode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "E{:04}", self.as_u32())
+    }
+}
+
+macro_rules! define_error_code_constants_and_diagnostics_table {
+    ($($name:ident: $num:literal,)*) => (
+        $(
+            pub const $name: $crate::ErrCode = $crate::ErrCode::from_u32($num);
+        )*
+        pub static DIAGNOSTICS: &[($crate::ErrCode, &str)] = &[
+            $( (
+                $name,
+                include_str!(
+                    concat!("../../rustc_error_codes/src/error_codes/", stringify!($name), ".md")
+                )
+            ), )*
+        ];
+    )
+}
+
+rustc_error_codes::error_codes!(define_error_code_constants_and_diagnostics_table);
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 4934bc2450c..8a32e29ddc9 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -1,7 +1,8 @@
 use crate::snippet::Style;
 use crate::{
-    CodeSuggestion, DelayedBugKind, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee, Level,
-    MultiSpan, SubdiagnosticMessage, Substitution, SubstitutionPart, SuggestionStyle,
+    CodeSuggestion, DelayedBugKind, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee,
+    ErrCode, Level, MultiSpan, SubdiagnosticMessage, Substitution, SubstitutionPart,
+    SuggestionStyle,
 };
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_error_messages::fluent_value_from_str_list_sep_by_and;
@@ -104,7 +105,7 @@ pub struct Diagnostic {
     pub(crate) level: Level,
 
     pub messages: Vec<(DiagnosticMessage, Style)>,
-    pub code: Option<String>,
+    pub code: Option<ErrCode>,
     pub span: MultiSpan,
     pub children: Vec<SubDiagnostic>,
     pub suggestions: Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
@@ -893,8 +894,8 @@ impl Diagnostic {
         self
     }
 
-    pub fn code(&mut self, s: String) -> &mut Self {
-        self.code = Some(s);
+    pub fn code(&mut self, code: ErrCode) -> &mut Self {
+        self.code = Some(code);
         self
     }
 
@@ -903,8 +904,8 @@ impl Diagnostic {
         self
     }
 
-    pub fn get_code(&self) -> Option<&str> {
-        self.code.as_deref()
+    pub fn get_code(&self) -> Option<ErrCode> {
+        self.code
     }
 
     pub fn primary_message(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
@@ -990,7 +991,7 @@ impl Diagnostic {
         &Level,
         &[(DiagnosticMessage, Style)],
         Vec<(&Cow<'static, str>, &DiagnosticArgValue<'static>)>,
-        &Option<String>,
+        &Option<ErrCode>,
         &Option<IsLint>,
         &MultiSpan,
         &Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 87e2d295c7f..8bfb1816486 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -1,7 +1,7 @@
 use crate::diagnostic::IntoDiagnosticArg;
 use crate::{DiagCtxt, Level, MultiSpan, StashKey};
 use crate::{
-    Diagnostic, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed, ExplicitBug,
+    Diagnostic, DiagnosticMessage, DiagnosticStyledString, ErrCode, ErrorGuaranteed, ExplicitBug,
     SubdiagnosticMessage,
 };
 use rustc_lint_defs::Applicability;
@@ -399,7 +399,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         name: String, has_future_breakage: bool,
     ));
     forward!((code, with_code)(
-        s: String,
+        code: ErrCode,
     ));
     forward!((arg, with_arg)(
         name: impl Into<Cow<'static, str>>, arg: impl IntoDiagnosticArg,
@@ -439,12 +439,7 @@ impl<G: EmissionGuarantee> Drop for DiagnosticBuilder<'_, G> {
 
 #[macro_export]
 macro_rules! struct_span_code_err {
-    ($dcx:expr, $span:expr, $code:ident, $($message:tt)*) => ({
-        $dcx.struct_span_err($span, format!($($message)*)).with_code($crate::error_code!($code))
+    ($dcx:expr, $span:expr, $code:expr, $($message:tt)*) => ({
+        $dcx.struct_span_err($span, format!($($message)*)).with_code($code)
     })
 }
-
-#[macro_export]
-macro_rules! error_code {
-    ($code:ident) => {{ stringify!($code).to_owned() }};
-}
diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index d58d05095cd..df3fd4af4b4 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -1,7 +1,7 @@
 use crate::diagnostic::DiagnosticLocation;
 use crate::{fluent_generated as fluent, AddToDiagnostic};
 use crate::{
-    DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic,
+    DiagCtxt, DiagnosticArgValue, DiagnosticBuilder, EmissionGuarantee, ErrCode, IntoDiagnostic,
     IntoDiagnosticArg, Level,
 };
 use rustc_ast as ast;
@@ -93,6 +93,7 @@ into_diagnostic_arg_using_display!(
     &TargetTriple,
     SplitDebuginfo,
     ExitStatus,
+    ErrCode,
 );
 
 into_diagnostic_arg_for_number!(i8, u8, i16, u16, i32, u32, i64, u64, i128, u128, isize, usize);
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 23efdaea0eb..9f76c1dd248 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -17,8 +17,8 @@ use crate::styled_buffer::StyledBuffer;
 use crate::translation::{to_fluent_args, Translate};
 use crate::{
     diagnostic::DiagnosticLocation, CodeSuggestion, DiagCtxt, Diagnostic, DiagnosticMessage,
-    FluentBundle, LazyFallbackBundle, Level, MultiSpan, SubDiagnostic, SubstitutionHighlight,
-    SuggestionStyle, TerminalUrl,
+    ErrCode, FluentBundle, LazyFallbackBundle, Level, MultiSpan, SubDiagnostic,
+    SubstitutionHighlight, SuggestionStyle, TerminalUrl,
 };
 use rustc_lint_defs::pluralize;
 
@@ -1309,7 +1309,7 @@ impl HumanEmitter {
         msp: &MultiSpan,
         msgs: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<String>,
+        code: &Option<ErrCode>,
         level: &Level,
         max_line_num_len: usize,
         is_secondary: bool,
@@ -1340,9 +1340,9 @@ impl HumanEmitter {
                 buffer.append(0, "[", Style::Level(*level));
                 let code = if let TerminalUrl::Yes = self.terminal_url {
                     let path = "https://doc.rust-lang.org/error_codes";
-                    Cow::Owned(format!("\x1b]8;;{path}/{code}.html\x07{code}\x1b]8;;\x07"))
+                    format!("\x1b]8;;{path}/{code}.html\x07{code}\x1b]8;;\x07")
                 } else {
-                    Cow::Borrowed(code)
+                    code.to_string()
                 };
                 buffer.append(0, &code, Style::Level(*level));
                 buffer.append(0, "]", Style::Level(*level));
@@ -2076,7 +2076,7 @@ impl HumanEmitter {
         level: &Level,
         messages: &[(DiagnosticMessage, Style)],
         args: &FluentArgs<'_>,
-        code: &Option<String>,
+        code: &Option<ErrCode>,
         span: &MultiSpan,
         children: &[SubDiagnostic],
         suggestions: &[CodeSuggestion],
diff --git a/compiler/rustc_errors/src/json.rs b/compiler/rustc_errors/src/json.rs
index 51b064f4c61..6f922998279 100644
--- a/compiler/rustc_errors/src/json.rs
+++ b/compiler/rustc_errors/src/json.rs
@@ -400,10 +400,10 @@ impl Diagnostic {
 
         let translated_message = je.translate_messages(&diag.messages, &args);
 
-        let code = if let Some(code) = &diag.code {
+        let code = if let Some(code) = diag.code {
             Some(DiagnosticCode {
                 code: code.to_string(),
-                explanation: je.registry.as_ref().unwrap().try_find_description(&code).ok(),
+                explanation: je.registry.as_ref().unwrap().try_find_description(code).ok(),
             })
         } else if let Some(IsLint { name, .. }) = &diag.is_lint {
             Some(DiagnosticCode { code: name.to_string(), explanation: None })
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index a64ad2beb6c..6d9208341a5 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -14,6 +14,7 @@
 #![feature(error_reporter)]
 #![feature(extract_if)]
 #![feature(let_chains)]
+#![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(rustc_attrs)]
@@ -30,6 +31,7 @@ extern crate tracing;
 
 extern crate self as rustc_errors;
 
+pub use codes::*;
 pub use diagnostic::{
     AddToDiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgValue,
     DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
@@ -78,6 +80,7 @@ use std::path::{Path, PathBuf};
 use Level::*;
 
 pub mod annotate_snippet_emitter_writer;
+pub mod codes;
 mod diagnostic;
 mod diagnostic_builder;
 mod diagnostic_impls;
@@ -446,10 +449,10 @@ struct DiagCtxtInner {
     /// This set contains the code of all emitted diagnostics to avoid
     /// emitting the same diagnostic with extended help (`--teach`) twice, which
     /// would be unnecessary repetition.
-    taught_diagnostics: FxHashSet<String>,
+    taught_diagnostics: FxHashSet<ErrCode>,
 
     /// Used to suggest rustc --explain `<error code>`
-    emitted_diagnostic_codes: FxIndexSet<String>,
+    emitted_diagnostic_codes: FxIndexSet<ErrCode>,
 
     /// This set contains a hash of every diagnostic that has been emitted by
     /// this `DiagCtxt`. These hashes is used to avoid emitting the same error
@@ -1004,9 +1007,9 @@ impl DiagCtxt {
             let mut error_codes = inner
                 .emitted_diagnostic_codes
                 .iter()
-                .filter_map(|code| {
+                .filter_map(|&code| {
                     if registry.try_find_description(code).is_ok().clone() {
-                        Some(code.clone())
+                        Some(code.to_string())
                     } else {
                         None
                     }
@@ -1052,8 +1055,8 @@ impl DiagCtxt {
     ///
     /// Used to suppress emitting the same error multiple times with extended explanation when
     /// calling `-Zteach`.
-    pub fn must_teach(&self, code: &str) -> bool {
-        self.inner.borrow_mut().taught_diagnostics.insert(code.to_string())
+    pub fn must_teach(&self, code: ErrCode) -> bool {
+        self.inner.borrow_mut().taught_diagnostics.insert(code)
     }
 
     pub fn force_print_diagnostic(&self, db: Diagnostic) {
@@ -1313,8 +1316,8 @@ impl DiagCtxtInner {
 
         let mut guaranteed = None;
         (*TRACK_DIAGNOSTIC)(diagnostic, &mut |mut diagnostic| {
-            if let Some(ref code) = diagnostic.code {
-                self.emitted_diagnostic_codes.insert(code.clone());
+            if let Some(code) = diagnostic.code {
+                self.emitted_diagnostic_codes.insert(code);
             }
 
             let already_emitted = {
diff --git a/compiler/rustc_errors/src/registry.rs b/compiler/rustc_errors/src/registry.rs
index f26d8e7ebdc..8834d04d971 100644
--- a/compiler/rustc_errors/src/registry.rs
+++ b/compiler/rustc_errors/src/registry.rs
@@ -1,3 +1,4 @@
+use crate::ErrCode;
 use rustc_data_structures::fx::FxHashMap;
 
 #[derive(Debug)]
@@ -5,17 +6,17 @@ pub struct InvalidErrorCode;
 
 #[derive(Clone)]
 pub struct Registry {
-    long_descriptions: FxHashMap<&'static str, &'static str>,
+    long_descriptions: FxHashMap<ErrCode, &'static str>,
 }
 
 impl Registry {
-    pub fn new(long_descriptions: &[(&'static str, &'static str)]) -> Registry {
+    pub fn new(long_descriptions: &[(ErrCode, &'static str)]) -> Registry {
         Registry { long_descriptions: long_descriptions.iter().copied().collect() }
     }
 
     /// Returns `InvalidErrorCode` if the code requested does not exist in the
     /// registry.
-    pub fn try_find_description(&self, code: &str) -> Result<&'static str, InvalidErrorCode> {
-        self.long_descriptions.get(code).copied().ok_or(InvalidErrorCode)
+    pub fn try_find_description(&self, code: ErrCode) -> Result<&'static str, InvalidErrorCode> {
+        self.long_descriptions.get(&code).copied().ok_or(InvalidErrorCode)
     }
 }