Function rustc_hir_analysis::astconv::generics::create_substs_for_generic_args

pub fn create_substs_for_generic_args<'tcx, 'a>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    parent_substs: &[GenericArg<'tcx>],
    has_self: bool,
    self_ty: Option<Ty<'tcx>>,
    arg_count: &GenericArgCountResult,
    ctx: &mut impl CreateSubstsForGenericArgsCtxt<'a, 'tcx>
) -> SubstsRef<'tcx>

Creates the relevant generic argument substitutions corresponding to a set of generic parameters. 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 we are creating the substitutions for, and a partial set of substitutions parent_substs. In general, the substitutions for an item begin with substitutions 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_substs first to see if the parent’s substitutions are listed in there. If so, we can append those and move on. Otherwise, it invokes the three callback functions:

• 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, instantiate a GenericArg.
• inferred_kind: if no parameter was provided, and inference is enabled, then creates a suitable inference variable.