Auto merge of rust-lang#108112 - nnethercote:clarify-iterator-interne…

…rs, r=oli-obk,compiler-errors

Clarify iterator interners

I found the iterator interners very confusing. This PR clarifies things.

r? `@compiler-errors`

clippy_lints/src/methods/needless_collect.rs

@@ -173,7 +173,7 @@ fn is_contains_sig(cx: &LateContext<'_>, call_id: HirId, iter_expr: &Expr<'_>) -
         && let Some(iter_item) = cx.tcx
             .associated_items(iter_trait)
             .find_by_name_and_kind(cx.tcx, Ident::with_dummy_span(Symbol::intern("Item")), AssocKind::Type, iter_trait)
-        && let substs = cx.tcx.mk_substs([GenericArg::from(typeck.expr_ty_adjusted(iter_expr))].into_iter())
+        && let substs = cx.tcx.intern_substs(&[GenericArg::from(typeck.expr_ty_adjusted(iter_expr))])
         && let proj_ty = cx.tcx.mk_projection(iter_item.def_id, substs)
         && let Ok(item_ty) = cx.tcx.try_normalize_erasing_regions(cx.param_env, proj_ty)
     {

clippy_lints/src/redundant_slicing.rs

@@ -11,8 +11,6 @@ use rustc_middle::ty::adjustment::{Adjust, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::subst::GenericArg;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
-use std::iter;
-
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for redundant slicing expressions which use the full range, and
@@ -136,7 +134,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantSlicing {
                 } else if let Some(target_id) = cx.tcx.lang_items().deref_target() {
                     if let Ok(deref_ty) = cx.tcx.try_normalize_erasing_regions(
                         cx.param_env,
-                        cx.tcx.mk_projection(target_id, cx.tcx.mk_substs(iter::once(GenericArg::from(indexed_ty)))),
+                        cx.tcx.mk_projection(target_id, cx.tcx.intern_substs(&[GenericArg::from(indexed_ty)])),
                     ) {
                         if deref_ty == expr_ty {
                             let snip = snippet_with_context(cx, indexed.span, ctxt, "..", &mut app).0;