…dant error

rust-lang#97343

…lacrum

Remove impossible panic note from `Vec::append`

Neither the number of elements in a vector can overflow a `usize`, nor
can the amount of elements in two vectors.

…tly, r=Mark-Simulacrum

Print stderr consistently

Solves rust-lang#96712

I tried to follow what I perceived as the general consensus for error messages in boostrap i.e messages that were ..
* resulting from an Err(...) =>
* literally called as "Error: ...."
* by the end of the block scope forced to run a panic! or process::exit with a guaranteed non-zero error code.

rename `TyKind` to `RegionKind` in comment in rustc_middle

…rrors

Add regression test for rust-lang#81899

Closes rust-lang#81899
r? `@compiler-errors`

…r=compiler-errors

Modify `derive(Debug)` to use `Self` in struct literal to avoid redundant error

Reduce verbosity in rust-lang#97343.

…=notriddle

Add more eslint rules

The last one is the most useful of this batch. :)

Here are the links for the eslint rules:

 * [arrow-parens](https://eslint.org/docs/rules/arrow-parens)
 * [no-unused-vars](https://eslint.org/docs/rules/no-unused-vars)
 * [eqeqeq](https://eslint.org/docs/rules/eqeqeq)

r? `@notriddle`