`{{root}}` is supposed to be an internal-only name but it shows up in
the output.

(I'm working towards a more general fix -- a universal "joiner" function
that can be used all over the place -- but I'm not there yet, so let's
fix this one in-place for now.)

This avoids some symbol interning and `to_string` conversions.

- `Ident::from_str_and_span` -> `Ident::new` when the string is
  pre-interned.
- `Ident::from_str` -> `Ident::with_dummy_span` when the string is
  pre-interned.
- `_d` and `_e` are unused.

"{{root}}" is an internal-only name, and cannot appear in Rust code
being formatted.

…rochenkov

Some symbol and PathRoot cleanups

I'm looking into unifying how we join and print paths. Here are some preliminary cleanups.

r? `@petrochenkov`

Rollup of 11 pull requests

Successful merges:

 - #142478 (install docs for each target in different directory)
 - #142629 (Add config builder for bootstrap tests)
 - #142715 (correct template for `#[align]` attribute)
 - #142720 (De-dup common code from `ExternalCrate` methods)
 - #142736 (add issue template for rustdoc)
 - #142743 (rustc-dev-guide subtree update)
 - #142744 (Add a mailmap entry for y21)
 - #142758 (Make sure to rebuild rustdoc if `src/rustdoc-json-types` is changed)
 - #142764 (Convert `ilog(10)` to `ilog10()`)
 - #142767 (Some symbol and PathRoot cleanups)
 - #142769 (remove equivalent new method on context)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 10 pull requests

Successful merges:

 - #142629 (Add config builder for bootstrap tests)
 - #142715 (correct template for `#[align]` attribute)
 - #142720 (De-dup common code from `ExternalCrate` methods)
 - #142736 (add issue template for rustdoc)
 - #142743 (rustc-dev-guide subtree update)
 - #142744 (Add a mailmap entry for y21)
 - #142758 (Make sure to rebuild rustdoc if `src/rustdoc-json-types` is changed)
 - #142764 (Convert `ilog(10)` to `ilog10()`)
 - #142767 (Some symbol and PathRoot cleanups)
 - #142769 (remove equivalent new method on context)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of #142767 - nnethercote:symbol-cleanups, r=petrochenkov

Some symbol and PathRoot cleanups

I'm looking into unifying how we join and print paths. Here are some preliminary cleanups.

r? ``@petrochenkov``

…rochenkov

Some symbol and PathRoot cleanups

I'm looking into unifying how we join and print paths. Here are some preliminary cleanups.

r? ``@petrochenkov``