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, 1 insertions, 1 deletions

diff --git a/compiler/rustc_macros/src/diagnostics/mod.rs b/compiler/rustc_macros/src/diagnostics/mod.rs
index 135b9e32fe7..33dffe6998a 100644
--- a/compiler/rustc_macros/src/diagnostics/mod.rs
+++ b/compiler/rustc_macros/src/diagnostics/mod.rs
@@ -20,7 +20,7 @@ use synstructure::Structure;
 /// # extern crate rust_middle;
 /// # use rustc_middle::ty::Ty;
 /// #[derive(Diagnostic)]
-/// #[diag(borrowck_move_out_of_borrow, code = "E0505")]
+/// #[diag(borrowck_move_out_of_borrow, code = E0505)]
 /// pub struct MoveOutOfBorrowError<'tcx> {
 ///     pub name: Ident,
 ///     pub ty: Ty<'tcx>,