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.

3 files changed, 8 insertions, 6 deletions

diff --git a/compiler/rustc_borrowck/src/borrowck_errors.rs b/compiler/rustc_borrowck/src/borrowck_errors.rs
index 351976cdaea..8a275a83630 100644
--- a/compiler/rustc_borrowck/src/borrowck_errors.rs
+++ b/compiler/rustc_borrowck/src/borrowck_errors.rs
@@ -1,4 +1,4 @@
-use rustc_errors::{struct_span_code_err, DiagCtxt, DiagnosticBuilder};
+use rustc_errors::{codes::*, struct_span_code_err, DiagCtxt, DiagnosticBuilder};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::Span;
 
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index da67862a48d..2e83072b8d1 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -3,7 +3,9 @@
 use either::Either;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan};
+use rustc_errors::{
+    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan,
+};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{walk_block, walk_expr, Visitor};
diff --git a/compiler/rustc_borrowck/src/session_diagnostics.rs b/compiler/rustc_borrowck/src/session_diagnostics.rs
index e321b92603d..1685624f247 100644
--- a/compiler/rustc_borrowck/src/session_diagnostics.rs
+++ b/compiler/rustc_borrowck/src/session_diagnostics.rs
@@ -1,4 +1,4 @@
-use rustc_errors::MultiSpan;
+use rustc_errors::{codes::*, MultiSpan};
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{GenericArg, Ty};
 use rustc_span::Span;
@@ -6,7 +6,7 @@ use rustc_span::Span;
 use crate::diagnostics::RegionName;
 
 #[derive(Diagnostic)]
-#[diag(borrowck_move_unsized, code = "E0161")]
+#[diag(borrowck_move_unsized, code = E0161)]
 pub(crate) struct MoveUnsized<'tcx> {
     pub ty: Ty<'tcx>,
     #[primary_span]
@@ -281,7 +281,7 @@ pub(crate) enum CaptureVarCause {
 }
 
 #[derive(Diagnostic)]
-#[diag(borrowck_cannot_move_when_borrowed, code = "E0505")]
+#[diag(borrowck_cannot_move_when_borrowed, code = E0505)]
 pub(crate) struct MoveBorrow<'a> {
     pub place: &'a str,
     pub borrow_place: &'a str,
@@ -294,7 +294,7 @@ pub(crate) struct MoveBorrow<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(borrowck_opaque_type_non_generic_param, code = "E0792")]
+#[diag(borrowck_opaque_type_non_generic_param, code = E0792)]
 pub(crate) struct NonGenericOpaqueTypeParam<'a, 'tcx> {
     pub ty: GenericArg<'tcx>,
     pub kind: &'a str,