Auto merge of #44866 - mdevlamynck:impl-trait, r=eddyb
First step toward implementing impl Trait in argument position
First step implementing #44721.
Add a flag to hir and ty TypeParameterDef and raise an error when using
explicit type parameters when calling a function using impl Trait in
argument position.
I don't know if there is a procedure to add an error code so I just took an available code. Is that ok ?
r? @nikomatsakis