Regression parsing closing angle brackets near function ptr return type

[dtolnay]

Alternate title: "Rust is literally C++"　https://wg21.link/N1757

As of nightly-2020-10-29, rustc fails to parse the following code. The previous nightly and all past stable releases since 1.0.0 are not affected.

pub fn f() -> Option<fn() -> Option<bool>> {
    None
}

error: unmatched angle bracket
 --> src/main.rs:1:42
  |
1 | pub fn f() -> Option<fn() -> Option<bool>> {
  |                                          ^^ help: remove extra angle bracket

error: expected one of `,` or `>`, found `{`
 --> src/main.rs:1:44
  |
1 | pub fn f() -> Option<fn() -> Option<bool>> {
  |                                            ^ expected one of `,` or `>`

The relevant commit range is 07e968b...31ee872.

[dtolnay]

Mentioning @estebank @varkor because #78379 "Tweak invalid `fn` header and body parsing" sounds extremely relevant.

[camelid]

Assigning P-critical and removing I-prioritize as discussed in the prioritization working group.

[camelid]

searched nightlies: from nightly-2020-10-27 to nightly-2020-10-29
regressed nightly: nightly-2020-10-29
searched commits: from 07e968b to 31ee872
regressed commit: db241bb

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-apple-darwin
Reproduce with:

cargo bisect-rustc --preserve --regress=error --start=2020-10-27 

---

Yeah, seems likely that #78379 is the culprit.

[estebank]

Yep, this was me :-/

I'll have a PR up in the next hour.