Allow any pretty printed line to have at least 60 chars

[dtolnay]

Follow-up to #93155. The rustc AST pretty printer has a tendency to get stuck in "vertical smear mode" when formatting highly nested code, where it puts a linebreak at every possible linebreak opportunity once the indentation goes beyond the pretty printer's target line width:

...
                                                              ((&([("test"
                                                                       as
                                                                       &str)]
                                                                     as
                                                                     [&str; 1])
                                                                   as
                                                                   &[&str; 1]),
                                                               (&([]
                                                                     as
                                                                     [ArgumentV1; 0])
                                                                   as
                                                                   &[ArgumentV1; 0]))
...

...
                                                                          [(1
                                                                               as
                                                                               i32),
                                                                           (2
                                                                               as
                                                                               i32),
                                                                           (3
                                                                               as
                                                                               i32)]
                                                                             as
                                                                             [i32; 3]
...

This is less common after #93155 because that PR greatly reduced the total amount of indentation, but the "vertical smear mode" failure mode is still just as present when you have deeply nested modules, functions, or trait impls, such as in the case of macro-expanded code from -Zunpretty=expanded.

Vertical smear mode is never the best way to format highly indented code though. It does not prevent the target line width from being exceeded, and it produces output that is less readable than just a longer line.

This PR makes the pretty printing algorithm allow a minimum of 60 chars on every line independent of indentation. So as code gets more indented, the right margin eventually recedes to make room for formatting without vertical smear.

├─────────────────────────────────────┤
├─────────────────────────────────────┤
├─────────────────────────────────────┤
  ├───────────────────────────────────┤
    ├─────────────────────────────────┤
      ├───────────────────────────────┤
        ├─────────────────────────────┤
          ├───────────────────────────┤
            ├───────────────────────────┤
              ├───────────────────────────┤
            ├───────────────────────────┤
          ├───────────────────────────┤
        ├─────────────────────────────┤
      ├───────────────────────────────┤
    ├─────────────────────────────────┤
  ├───────────────────────────────────┤
├─────────────────────────────────────┤

[rust-highfive]

r? @estebank

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[nagisa]

@bors r+ Nice!
r? @nagisa

[bors]

📌 Commit 7739fca has been approved by nagisa