Use fewer origins when creating type variables.

`InferCtxt::next_{ty,const}_var*` all take an origin, but the
`param_def_id` is almost always `None`. This commit changes them to just
take a `Span` and build the origin within the method, and adds new
methods for the rare cases where `param_def_id` might not be `None`.
This avoids a lot of tedious origin building.

Specifically:
- next_ty_var{,_id_in_universe,_in_universe}: now take `Span` instead of
  `TypeVariableOrigin`
- next_ty_var_with_origin: added

- next_const_var{,_in_universe}: takes Span instead of ConstVariableOrigin
- next_const_var_with_origin: added

- next_region_var, next_region_var_in_universe: these are unchanged,
  still take RegionVariableOrigin

The API inconsistency (ty/const vs region) seems worth it for the
large conciseness improvements.

Author: nnethercote

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -11,7 +11,6 @@ use std::assert_matches::assert_matches;
 
 use itertools::Itertools;
 use rustc_hir as hir;
-use rustc_infer::infer::type_variable::TypeVariableOrigin;
 use rustc_infer::infer::{BoundRegionConversionTime, RegionVariableOrigin};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty};
@@ -74,9 +73,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 }),
             );
 
-            let next_ty_var = || {
-                self.infcx.next_ty_var(TypeVariableOrigin { span: body.span, param_def_id: None })
-            };
+            let next_ty_var = || self.infcx.next_ty_var(body.span);
             let output_ty = Ty::new_coroutine(
                 self.tcx(),
                 self.tcx().coroutine_for_closure(mir_def_id),

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -16,7 +16,6 @@ use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
 use rustc_infer::infer::region_constraints::RegionConstraintData;
-use rustc_infer::infer::type_variable::TypeVariableOrigin;
 use rustc_infer::infer::{
     BoundRegion, BoundRegionConversionTime, InferCtxt, NllRegionVariableOrigin,
 };
@@ -2356,10 +2355,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     // Types with regions are comparable if they have a common super-type.
                     ty::RawPtr(_, _) | ty::FnPtr(_) => {
                         let ty_right = right.ty(body, tcx);
-                        let common_ty = self.infcx.next_ty_var(TypeVariableOrigin {
-                            param_def_id: None,
-                            span: body.source_info(location).span,
-                        });
+                        let common_ty = self.infcx.next_ty_var(body.source_info(location).span);
                         self.sub_types(
                             ty_left,
                             common_ty,