FCW [T;0]: Default

[SciMind2460]

In #145457, @WaffleLapkin has shown that it may be OK for rust to break backwards compatibility for #61415, i.e. ([T; N]: Default where T: Default.
This lint makes it so that [T; 0]: Default where T: !Default is warned against.

Please review, I am new to the rustc process and would love to correct errors!

[rustbot]

r? @petrochenkov

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[rustbot]

⚠️ Warning ⚠️

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • 55edd51

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/rust-lang/rust.git master
  $ git push --force-with-lease
  

[SciMind2460]

Apparently there are merge commits. Closing so a fresh PR without merge commits can be made.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt check
error: expected `{`, found keyword `if`
##[error]  --> /checkout/compiler/rustc_lint/src/impl_default_on_zero_size_nondefault_arrays.rs:87:33
   |
87 | ...                   if let Some(0) = len_const.try_eval_usize(cx.tcx, cx.param_env) {
   |                       ^^ expected `{`
   |
note: the `if` expression is missing a block after this condition
  --> /checkout/compiler/rustc_lint/src/impl_default_on_zero_size_nondefault_arrays.rs:86:32
   |
86 | ...                   if let ty::tyKind::Array(t_ty, len_const) = ty.kind()
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: you might have meant to write this as part of a block
   |
87 ~                                 { if let Some(0) = len_const.try_eval_usize(cx.tcx, cx.param_env) {
88 |                                     if !cx.tcx.infer_ctxt().type_implements_trait(default_id, [t_ty], cx.param_env)  {
...
95 |                                     }
96 ~                                } }
   |