This has been bugging me for a while. I find complex "if any of these
are true" conditions easier to think about than complex "if all of these
are true" conditions, because you can stop as soon as one is true.

It has a single call site. This change makes the two `needs_collect`
conditions more similar to each other, and therefore easier to
understand.

And make it non-`pub`.

I have always found `is_complete` an unhelpful name. The new name (and
inverted sense) fits in better with the conditions at its call sites.

The current code is this:
```
self.capture_state.replace_ranges.push((start_pos..end_pos, Some(target)));
self.capture_state.replace_ranges.extend(inner_attr_replace_ranges);
```
What's not obvious is that every range in `inner_attr_replace_ranges`
must be a strict sub-range of `start_pos..end_pos`. Which means, in
`LazyAttrTokenStreamImpl::to_attr_token_stream`, they will be done
first, and then the `start_pos..end_pos` replacement will just overwrite
them. So they aren't needed.

Imagine you have replace ranges (2..20,X) and (5..15,Y), and these tokens:
```
a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x
```
If we replace (5..15,Y) first, then (2..20,X) we get this sequence
```
a,b,c,d,e,Y,_,_,_,_,_,_,_,_,_,p,q,r,s,t,u,v,w,x
a,b,X,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,u,v,w,x
```
which is what we want.

If we do it in the other order, we get this:
```
a,b,X,_,_,_,_,_,_,_,_,_,_,_,_,p,q,r,s,t,u,v,w,x
a,b,X,_,_,Y,_,_,_,_,_,_,_,_,_,_,_,_,_,_,u,v,w,x
```
which is wrong. So it's true that we need the `.rev()` but the comment
is wrong about why.

A fully imperative style is easier to read than a half-iterator,
half-imperative style. Also, rename `inner_attr` as `attr` because it
might be an outer attribute.

…put, too

Support ?Trait bounds in supertraits and dyn Trait under a feature gate

This patch allows `maybe` polarity bounds under a feature gate. The only language change here is that corresponding hard errors are replaced by feature gates. Example:
```rust
#![feature(allow_maybe_polarity)]
...
trait Trait1 : ?Trait { ... } // ok
fn foo(_: Box<(dyn Trait2   ?Trait)>) {} // ok
fn bar<T: ?Sized   ?Trait>(_: &T) {} // ok
```
Maybe bounds still don't do anything (except for `Sized` trait), however this patch will allow us to [experiment with default auto traits](rust-lang#120706 (comment)).

This is a part of the [MCP: Low level components for async drop](rust-lang/compiler-team#727)

…wiser

allow overwriting the output of `rustc --version`

Our wonderful bisection folk [have to work around](rust-lang#123276 (comment)) crates that do incomplete nightly/feature detection, as otherwise the bisection just points to where the feature detection breaks, and not to the actual breakage they are looking for.

This is also annoying behaviour to nightly users who did not opt-in to those nightly features. Most nightly users want to be in control of the nightly breakage they get, by

* choosing when to update rustc
* choosing when to update dependencies
* choosing which nightly features they are willing to take the breakage for

The reason this breakage occurs is that the build script of some crates run `rustc --version`, and if the version looks like nightly or dev, it will enable nightly features. These nightly features may break in random ways whenever we change something in nightly, so every release of such a crate will only work with a small range of nightly releases. This causes bisection to fail whenever it tries an unsupported nightly, even though that crate is not related to the bisection at all, but is just an unrelated dependency.

This PR (and the policy I want to establish with this FCP) is only for situations like the `version_check`'s `supports_feature` function. It is explicitly not for `autocfg` or similar feature-detection-by-building-rust-code, irrespective of my opinions on it and the similarity of nightly breakage that can occur with such schemes. These cause much less breakage, but should the breakage become an issue, they should get covered by this policy, too.

This PR allows changing the version and release strings reported by `rustc --version` via the `RUSTC_OVERRIDE_VERSION_STRING` env var. The bisection issue is then fixed by rust-lang/cargo-bisect-rustc#335.

I mainly want to establish a compiler team policy:

> We do not consider feature detection on nightly (on stable via compiler version numbering is fine) a valid use case that we need to support, and if it causes problems, we are at liberty to do what we deem best - either actively working to prevent it or to actively ignore it. We may try to work with responsive and cooperative authors, but are not obligated to.

Should they subvert the workarounds that nightly users or cargo-bisect-rustc can use, we should be able to land rustc PRs that target the specific crates that cause issues for us and outright replace their build script's logic to disable nightly detection.

I am not including links to crates, PRs or issues here, as I don't actually care about the specific use cases and don't want to make it trivial to go there and leave comments. This discussion is going to be interesting enough on its own, without branching out.

…s, r=petrochenkov

Refactor complex conditions in `collect_tokens_trailing_token`

More readability improvements for this complicated function.

r? `@petrochenkov`

…r=petrochenkov

Remove unnecessary range replacements

This PR removes an unnecessary range replacement in `collect_tokens_trailing_token`, and does a couple of other small cleanups.

r? `@petrochenkov`

…etrochenkov

Remove redundant option that was just encoding that a slice was empty

There is already a sanity check ensuring we don't put empty attribute lists into the HIR:

https://github.com/rust-lang/rust/blob/6ef11b81c2c02c3c4b7556d1991a98572fe9af87/compiler/rustc_ast_lowering/src/lib.rs#L661-L667

…ix, r=tgross35

CI: do not respect custom try jobs for unrolled perf builds

Before this PR, if a pull request merged in a rollup had some `try-job` annotations, the unrolled perf builds were running the custom try jobs instead of the default job, which was wrong.

Found out [here](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/try-perf.20jobs.20respect.20try-job.20annotations).