Regression in the Debug formatter

[pietroalbini]

The Debug formatter output was recently changed in #30967 to always include a decimal point, and apparently some crates' test suites depends on the exact output of the formatter. I don't know if it's worth to revert the change, but I reported it just in case.

• caldyn regressed from stable to beta (crater log)
• earlgrey regressed from stable to beta (crater log 1 and crater log 2)
• euclid regressed from stable to beta (crater log 1 and crater log 2), but it's fixed on master
• expecttest regressed from stable to beta (crater log), but it's fixed on master
• mkv regressed from stable to beta (crater log)
• postgis regressed from stable to beta (crater log)
• pretty_assertions regressed from stable to beta (crater log), but it's fixed on master
• scanlex regressed from stable to beta (crater log)
• spectral regressed from stable to beta (crater log)
• vectors regressed from stable to beta (crater log)

cc @dtolnay @Luthaf @rodolf0

[pietroalbini]

euclid and expecttest are also affected (but euclid now it's fixed on master).

cc @zummenix

[sfackler]

We make zero guarantees as to what Debug implementations look like.

[Luthaf]

I'm fine with this, I'll change my tests =)

[zummenix]

I'll change my tests too. @pietroalbini thanks for cc me!

[pietroalbini]

mkv is also affected. cc @vi

[pietroalbini]

postgis is also affected. cc @andelf @pka
pretty_assertions is also affected. cc @colin-kiegel
scanlex is also affected. cc @stevedonovan

[colin-kiegel]

Thanks for the heads up!

If you want to restore "consistent" behaviour across old and future rust versions, you can just replace {:?} with either {:.0?} or {:.1?}. This way you can still write testcases that depend on the exact debug output for floats:

print!("{:?}",   0.0); // stable: "0",   nightly "0.0"
print!("{:.0?}", 0.0); // stable: "0",   nightly "0"
print!("{:.1?}", 0.0); // stable: "0.0", nightly "0.0"

[vi]

Shall there be a [clippy] lint against using Debug output for assert_eq!?

[pietroalbini]

This also affects spectral (crater log). cc @cfrancia
This also affects vectors (crater log). cc @deepthought