[1.38] Regression: custom attribute panicked

[petrochenkov]

#63628 (comment)

custom attribute panicked

[INFO] [stderr]    Compiling display-as v0.4.6 (/opt/crater/workdir)
[INFO] [stderr] error: custom attribute panicked
[INFO] [stderr]    --> tests/format_as.rs:104:5
[INFO] [stderr]     |
[INFO] [stderr] 104 |     #[with_template("Foo " self.0)]
[INFO] [stderr]     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stderr]     |
[INFO] [stderr]     = help: message: with_template must be applied to an impl that ends in '{}', not { }

One more victim to the pretty-printing changes (#62667), whitespace-related in this case.

Here's what the macro does:

    if last.to_string() != "{  }" {
        /* report error */
    }

Pretty unreliable check, isn't it?

[estebank]

I feel this is a reasonable breaking change.

[nikomatsakis]

Discussed in compiler triage meeting today. Closing as expected and reasonable.