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.

compiler/rustc_session/src/errors.rs

@@ -3,7 +3,7 @@ use std::num::NonZeroU32;
 use rustc_ast::token;
 use rustc_ast::util::literal::LitError;
 use rustc_errors::{
-    error_code, DiagCtxt, DiagnosticBuilder, DiagnosticMessage, IntoDiagnostic, Level, MultiSpan,
+    codes::*, DiagCtxt, DiagnosticBuilder, DiagnosticMessage, IntoDiagnostic, Level, MultiSpan,
 };
 use rustc_macros::Diagnostic;
 use rustc_span::{Span, Symbol};
@@ -19,9 +19,7 @@ pub struct FeatureGateError {
 impl<'a> IntoDiagnostic<'a> for FeatureGateError {
     #[track_caller]
     fn into_diagnostic(self, dcx: &'a DiagCtxt, level: Level) -> DiagnosticBuilder<'a> {
-        DiagnosticBuilder::new(dcx, level, self.explain)
-            .with_span(self.span)
-            .with_code(error_code!(E0658))
+        DiagnosticBuilder::new(dcx, level, self.explain).with_span(self.span).with_code(E0658)
     }
 }
 

compiler/rustc_session/src/session.rs

@@ -22,7 +22,7 @@ use rustc_errors::emitter::{DynEmitter, HumanEmitter, HumanReadableErrorType};
 use rustc_errors::json::JsonEmitter;
 use rustc_errors::registry::Registry;
 use rustc_errors::{
-    error_code, fallback_fluent_bundle, DiagCtxt, DiagnosticBuilder, DiagnosticMessage,
+    codes::*, fallback_fluent_bundle, DiagCtxt, DiagnosticBuilder, DiagnosticMessage, ErrCode,
     ErrorGuaranteed, FatalAbort, FluentBundle, IntoDiagnostic, LazyFallbackBundle, TerminalUrl,
 };
 use rustc_macros::HashStable_Generic;
@@ -316,7 +316,7 @@ impl Session {
     ) -> DiagnosticBuilder<'a> {
         let mut err = self.dcx().create_err(err);
         if err.code.is_none() {
-            err.code(error_code!(E0658));
+            err.code(E0658);
         }
         add_feature_diagnostics(&mut err, self, feature);
         err
@@ -893,7 +893,7 @@ impl Session {
         CodegenUnits::Default(16)
     }
 
-    pub fn teach(&self, code: &str) -> bool {
+    pub fn teach(&self, code: ErrCode) -> bool {
         self.opts.unstable_opts.teach && self.dcx().must_teach(code)
     }