Auto merge of rust-lang#55067 - ljedrz:generic_iterator_related_improvements, r=petrochenkov

A few iterator-related improvements

- typeck: don't collect into a vector when unnecessary
- create only one vector when winnowing candidates
- change a cloning map to `into_iter`

Author: bors

Diff for: src/librustc/lib.rs

@@ -71,6 +71,7 @@
 #![feature(in_band_lifetimes)]
 #![feature(macro_at_most_once_rep)]
 #![feature(crate_visibility_modifier)]
+#![feature(transpose_result)]
 
 #![recursion_limit="512"]
 

Diff for: src/librustc/traits/select.rs

@@ -1368,8 +1368,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
 
         // Winnow, but record the exact outcome of evaluation, which
         // is needed for specialization. Propagate overflow if it occurs.
-        let candidates: Result<Vec<Option<EvaluatedCandidate<'_>>>, _> = candidates
-            .into_iter()
+        let mut candidates = candidates.into_iter()
             .map(|c| match self.evaluate_candidate(stack, &c) {
                 Ok(eval) if eval.may_apply() => Ok(Some(EvaluatedCandidate {
                     candidate: c,
@@ -1378,10 +1377,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 Ok(_) => Ok(None),
                 Err(OverflowError) => Err(Overflow),
             })
-            .collect();
-
-        let mut candidates: Vec<EvaluatedCandidate<'_>> =
-            candidates?.into_iter().filter_map(|c| c).collect();
+           .flat_map(Result::transpose)
+           .collect::<Result<Vec<_>, _>>()?;
 
         debug!(
             "winnowed to {} candidates for {:?}: {:?}",
@@ -1390,7 +1387,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             candidates
         );
 
-        // If there are STILL multiple candidate, we can further
+        // If there are STILL multiple candidates, we can further
         // reduce the list by dropping duplicates -- including
         // resolving specializations.
         if candidates.len() > 1 {

Diff for: src/librustc/ty/util.rs

@@ -681,7 +681,7 @@ impl<'a, 'tcx> ty::TyS<'tcx> {
                 match (r1, r2) {
                     (Representability::SelfRecursive(v1),
                      Representability::SelfRecursive(v2)) => {
-                        Representability::SelfRecursive(v1.iter().map(|s| *s).chain(v2).collect())
+                        Representability::SelfRecursive(v1.into_iter().chain(v2).collect())
                     }
                     (r1, r2) => cmp::max(r1, r2)
                 }

Diff for: src/librustc_typeck/astconv.rs

@@ -1592,8 +1592,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         debug!("ty_of_fn");
 
         let tcx = self.tcx();
-        let input_tys: Vec<Ty> =
-            decl.inputs.iter().map(|a| self.ty_of_arg(a, None)).collect();
+        let input_tys =
+            decl.inputs.iter().map(|a| self.ty_of_arg(a, None));
 
         let output_ty = match decl.output {
             hir::Return(ref output) => self.ast_ty_to_ty(output),
@@ -1603,7 +1603,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx>+'o {
         debug!("ty_of_fn: output_ty={:?}", output_ty);
 
         let bare_fn_ty = ty::Binder::bind(tcx.mk_fn_sig(
-            input_tys.into_iter(),
+            input_tys,
             output_ty,
             decl.variadic,
             unsafety,

Diff for: src/librustc_typeck/check/_match.rs

@@ -626,7 +626,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
 
         // Typecheck the patterns first, so that we get types for all the
         // bindings.
-        let all_arm_pats_diverge: Vec<_> = arms.iter().map(|arm| {
+        let all_arm_pats_diverge = arms.iter().map(|arm| {
             let mut all_pats_diverge = Diverges::WarnedAlways;
             for p in &arm.pats {
                 self.diverges.set(Diverges::Maybe);
@@ -642,7 +642,7 @@ https://doc.rust-lang.org/reference/types.html#trait-objects");
                 Diverges::Maybe => Diverges::Maybe,
                 Diverges::Always | Diverges::WarnedAlways => Diverges::WarnedAlways,
             }
-        }).collect();
+        });
 
         // Now typecheck the blocks.
         //

Diff for: src/librustc_typeck/check/mod.rs

@@ -1769,7 +1769,7 @@ fn check_transparent<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, sp: Span, def_id: De
     }
 
     // For each field, figure out if it's known to be a ZST and align(1)
-    let field_infos: Vec<_> = adt.non_enum_variant().fields.iter().map(|field| {
+    let field_infos = adt.non_enum_variant().fields.iter().map(|field| {
         let ty = field.ty(tcx, Substs::identity_for_item(tcx, field.did));
         let param_env = tcx.param_env(field.did);
         let layout = tcx.layout_of(param_env.and(ty));
@@ -1778,19 +1778,19 @@ fn check_transparent<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, sp: Span, def_id: De
         let zst = layout.map(|layout| layout.is_zst()).unwrap_or(false);
         let align1 = layout.map(|layout| layout.align.abi() == 1).unwrap_or(false);
         (span, zst, align1)
-    }).collect();
+    });
 
-    let non_zst_fields = field_infos.iter().filter(|(_span, zst, _align1)| !*zst);
+    let non_zst_fields = field_infos.clone().filter(|(_span, zst, _align1)| !*zst);
     let non_zst_count = non_zst_fields.clone().count();
     if non_zst_count != 1 {
-        let field_spans: Vec<_> = non_zst_fields.map(|(span, _zst, _align1)| *span).collect();
+        let field_spans: Vec<_> = non_zst_fields.map(|(span, _zst, _align1)| span).collect();
         struct_span_err!(tcx.sess, sp, E0690,
                          "transparent struct needs exactly one non-zero-sized field, but has {}",
                          non_zst_count)
         .span_note(field_spans, "non-zero-sized field")
         .emit();
     }
-    for &(span, zst, align1) in &field_infos {
+    for (span, zst, align1) in field_infos {
         if zst && !align1 {
             span_err!(tcx.sess, span, E0691,
                       "zero-sized field in transparent struct has alignment larger than 1");