Function lower_generic_args

pub fn lower_generic_args<'tcx: 'a, 'a>(
    cx: &dyn HirTyLowerer<'tcx>,
    def_id: DefId,
    parent_args: &[GenericArg<'tcx>],
    has_self: bool,
    self_ty: Option<Ty<'tcx>>,
    arg_count: &GenericArgCountResult,
    ctx: &mut impl GenericArgsLowerer<'a, 'tcx>,
) -> GenericArgsRef<'tcx>

Lower generic arguments from the HIR to the rustc_middle::ty representation.

This is a rather complex function. Let us try to explain the role of each of its parameters:

To start, we are given the def_id of the thing whose generic parameters we are creating, and a partial set of arguments parent_args. In general, the generic arguments for an item begin with arguments for all the “parents” of that item – e.g., for a method it might include the parameters from the impl.

Therefore, the method begins by walking down these parents, starting with the outermost parent and proceed inwards until it reaches def_id. For each parent P, it will check parent_args first to see if the parent’s arguments are listed in there. If so, we can append those and move on. Otherwise, it uses the provided GenericArgsLowerer ctx which has the following methods:

• args_for_def_id: given the DefId P, supplies back the generic arguments that were given to that parent from within the path; so e.g., if you have <T as Foo>::Bar, the DefId might refer to the trait Foo, and the arguments might be [T]. The boolean value indicates whether to infer values for arguments whose values were not explicitly provided.
• provided_kind: given the generic parameter and the value from args_for_def_id, creating a GenericArg.
• inferred_kind: if no parameter was provided, and inference is enabled, then creates a suitable inference variable.