syntax: Trait object types permit extra parentheses on the first bound

[petrochenkov]

// Accepted while it should not.
type A = Box<(Trait1) + Trait2 + 'lifetime>;

// Correct syntax.
type A = Box<Trait1 + Trait2 + 'lifetime>;

The syntax for object types is the same as for other bounds - Bound + Bound + Bound + .... and parentheses are not a part of bound syntax.

This is a stable-stable regression, introduced in Rust 1.6 by #29870.
cc #39169 #39179

Current status

• Refactor parsing of trait object types #40043 - (Bound) + Bound + .... is still parsed , but with a warning. This is a "hardcoded" warning and not a lint, lints cannot be reported from libsyntax.
• PR ? turns this syntax into a hard error

[brson]

Needs a P-tag. Thanks for the fixes @petrochenkov

[nikomatsakis]

I'm removing Nominated -- it's not clear that we want to actually enforce this rule. See the discussion on #40043