…ed vec.

I based this solution on my reading of:

https://rethinkdb.com/blog/make-debugging-easier-with-custom-pretty-printers#what-is-still-to-be-done

That post claims that there is no clean way to check for garbage pointers, and
so this PR adopts the same solution of tentatively attempting to convert a
dererence to a string, which throws a clean exception on garbage that we can
catch and recover from.

I only made the change to vec and not the other pretty printers because I wanted
to focus my effort on the simplest thing that would resolve issue rust-lang#64343. In
particular, I *considered* generalizing this fix to work on the other datatypes
in the pretty-printing support library, but I don't want to invest effort in
that until after we resolve our overall debugging support strategy; see also
issues rust-lang#60826 and rust-lang#65564.

While the redesign is in progress (rust-lang#62886), clarify the purpose of replace_zero and replace_one.

add fn type_name_of_val

This function is often useful during testing and mirrors `align_of_val` and `size_of_val`.

# Example

Showing the default type of integers.

```rust
let x = 7;
println!("per default, integers have the type: {}", std::any::type_name_of_val(&x));
```

To my knowledge this can currently not be done without defining a function similar to `type_name_of_val`.

…-check, r=Mark-Simulacrum

Update tidy check for error codes testing

Now that all the error codes have been grouped into one file, we moved the long error explanations into markdown files, which means that we need to read those markdown file to also count their code blocks as well.

So 2 "big" things happened here:
* No more need to keep the current error code when reading the `error_codes.rs` file since it's been put into the equivalent markdown file.
* Need to instead read the markdown file (but it's simpler since I can just look for code blocks directly).

…r, r=alexcrichton

made gdb pretty-printing more robust when printing uninitialized vec

made gdb pretty-printing more robust when printing uninitialized vec

I based this solution on my reading of:

https://rethinkdb.com/blog/make-debugging-easier-with-custom-pretty-printers#what-is-still-to-be-done

That post claims that there is no clean way to check for garbage pointers, and
so this PR adopts the same solution of tentatively attempting to convert a
dererence to a string, which throws a clean exception on garbage that we can
catch and recover from.

I only made the change to vec and not the other pretty printers because I wanted
to focus my effort on the simplest thing that would resolve issue rust-lang#64343. In
particular, I *considered* generalizing this fix to work on the other datatypes
in the pretty-printing support library, but I don't want to invest effort in
that until after we resolve our overall debugging support strategy; see also
issues rust-lang#60826 and rust-lang#65564.

Fix rust-lang#64343

Clarify Step Documentation

While the redesign is in progress (rust-lang#62886), clarify the purpose of replace_zero and replace_one.

First, "returning itself" is technically impossible due to the function signature of &mut self -> Self. A clone or copy operation must be used. So this is now explicitly stated in the documentation.

Second, the added docs give some guidance about the actual contract around implementation of replace_zero and replace one. Specifically, the only usage is to create a range with no more steps, by setting start to replace_one and end to replace_zero. So the only property that is actually used is `replace_one > replace_zero`. See rust-lang#42168 (comment)

The new documentation does not say that is the *only* contract, and so it should not be considered an api change. It just highlights the most important detail for implementors.

The redesign doesn't seem to be landing any time soon, so this is a stopgap measure to reduce confusion in the meantime.