Prevent main from having a where clause.

[matthewjasper]

Closes #50714

Should this have a crater run?

cc #48557, #48214

r? @nikomatsakis

[varkor]

You should add this check for start functions too:

rust/src/librustc_typeck/lib.rs

Lines 251 to 258 in 27acb9b

	if !ps.params.is_empty() => {
	struct_span_err!(tcx.sess, ps.span, E0132,
	"start function is not allowed to have type parameters")
	.span_label(ps.span,
	"start function cannot have type parameters")
	.emit();
	return;
	}

[nikomatsakis]

So, this cannot affect libraries, so the potential impact is pretty limited. I suspect we can get by without a crater run.

[nikomatsakis]

r=me once we check start functions as well like @varkor suggested

@bors delegate=matthewjasper

[bors]

✌️ @matthewjasper can now approve this pull request

[varkor]

Your tests look correct, but this isn't a correct example of a start function.

[matthewjasper]

@bors r+

[bors]

📌 Commit 218a81b has been approved by matthewjasper

[kennytm]

@bors p=4

[bors]

⌛ Testing commit 218a81b with merge a308575...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matthewjasper
Pushing a308575 to master...