rustfmt is slow on pest generated grammars

[matklad]

Rustfmt takes about a minute to reformat 26k file (generated pest grammar).

File in question: https://github.com/steveklabnik/semver-parser/blob/0c43dbb12d35f9b58142b3976391eb52ea71725f/src/generated.rs

To Reproduce

Run this test in the semver-parser repo:

https://github.com/steveklabnik/semver-parser/blob/0c43dbb12d35f9b58142b3976391eb52ea71725f/tests/genpest.rs#L14

Expected behavior

Formatting takes low-digits seconds.

Meta

• rustfmt version: rustfmt 1.4.20-stable (48f6c32 2020-08-09)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: https://github.com/steveklabnik/semver-parser/blob/0c43dbb12d35f9b58142b3976391eb52ea71725f/tests/genpest.rs#L68-L84

[Volker-Weissmann]

This bug seems to have a connection with #4867

[ytmimi]

Reopen since we had to revert #5139 in #5403