special-casing 'static when winnowing trivial candidates

[aliemjay]

The following code compiles (adapted from tests/ui/trivial-bounds/trivial-bounds-object.rs):

// check-pass
trait A { fn test(&self); }

fn foo(x: &dyn A)
where
    dyn A + 'static: A, // Using this bound would lead to a lifetime error.
{
    x.test();
}

while this doesn't:

trait A { fn test(&self); }

fn foo<'s>(x: &dyn A)
where
    dyn A + 's: A, // Using this bound would lead to a lifetime error.
{
    x.test();
    //~^ ERROR explicit lifetime required in the type of `x`
}

There are two candidates to satisfy the bound dyn A + '?0: A:
for the first case:

1. ObjectCandidate(_); no region constraints
2. ParamCandidate(dyn A + 'static: A); requires '?0 = 'static

for the seecond one:

1. ObjectCandidate(_); no region constraints
2. ParamCandidate(dyn A + 's: A); requires '?0 = 's

The different behavior arises because the param candidate in the first case is considered "global", unlike second case. Global param candidates are dropped in favor of any other candidate. This behavior was introduced by #51042 here:

rust/compiler/rustc_trait_selection/src/traits/select/mod.rs

Line 1816 in 604f185

fn candidate_should_be_dropped_in_favor_of(

[matthewjasper]

To be clear, the behaviour predates #51042. Prior to that PR, global bounds were simply filtered out of the ParamEnv.