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.

1 files changed, 7 insertions, 7 deletions

diff --git a/compiler/rustc_lint/src/errors.rs b/compiler/rustc_lint/src/errors.rs
index 841d282a099..3bd0c1b8031 100644
--- a/compiler/rustc_lint/src/errors.rs
+++ b/compiler/rustc_lint/src/errors.rs
@@ -1,11 +1,11 @@
 use crate::fluent_generated as fluent;
-use rustc_errors::{AddToDiagnostic, Diagnostic, SubdiagnosticMessage};
+use rustc_errors::{codes::*, AddToDiagnostic, Diagnostic, SubdiagnosticMessage};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::lint::Level;
 use rustc_span::{Span, Symbol};
 
 #[derive(Diagnostic)]
-#[diag(lint_overruled_attribute, code = "E0453")]
+#[diag(lint_overruled_attribute, code = E0453)]
 pub struct OverruledAttribute<'a> {
     #[primary_span]
     pub span: Span,
@@ -48,7 +48,7 @@ impl AddToDiagnostic for OverruledAttributeSub {
 }
 
 #[derive(Diagnostic)]
-#[diag(lint_malformed_attribute, code = "E0452")]
+#[diag(lint_malformed_attribute, code = E0452)]
 pub struct MalformedAttribute {
     #[primary_span]
     pub span: Span,
@@ -67,7 +67,7 @@ pub enum MalformedAttributeSub {
 }
 
 #[derive(Diagnostic)]
-#[diag(lint_unknown_tool_in_scoped_lint, code = "E0710")]
+#[diag(lint_unknown_tool_in_scoped_lint, code = E0710)]
 pub struct UnknownToolInScopedLint {
     #[primary_span]
     pub span: Option<Span>,
@@ -78,7 +78,7 @@ pub struct UnknownToolInScopedLint {
 }
 
 #[derive(Diagnostic)]
-#[diag(lint_builtin_ellipsis_inclusive_range_patterns, code = "E0783")]
+#[diag(lint_builtin_ellipsis_inclusive_range_patterns, code = E0783)]
 pub struct BuiltinEllipsisInclusiveRangePatterns {
     #[primary_span]
     pub span: Span,
@@ -95,13 +95,13 @@ pub struct RequestedLevel<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(lint_unsupported_group, code = "E0602")]
+#[diag(lint_unsupported_group, code = E0602)]
 pub struct UnsupportedGroup {
     pub lint_group: String,
 }
 
 #[derive(Diagnostic)]
-#[diag(lint_check_name_unknown_tool, code = "E0602")]
+#[diag(lint_check_name_unknown_tool, code = E0602)]
 pub struct CheckNameUnknownTool<'a> {
     pub tool_name: Symbol,
     #[subdiagnostic]