pervasive changes for non-flattening for loops

(It's possible we can remove some of the .flats here or there without
damage, but I erred on the side of caution on some of them.)

src/core/Any.pm

@@ -182,6 +182,11 @@ my class Any { # declared in BOOTSTRAP
     proto method for (|) { * }
     multi method for(Whatever) is rw { self }
     multi method for($block, :$label) is rw {
+        MapIter.new(self, $block, Bool::False, :$label).list;
+    }
+    proto method flatmap (|) { * }
+    multi method flatmap(Whatever) is rw { self }
+    multi method flatmap($block, :$label) is rw {
         MapIter.new(self, $block, Bool::True, :$label).list
     }
     method nodemap($block) is rw { nodemap($block, self) }

src/core/Capture.pm

@@ -22,7 +22,7 @@ my class Capture { # declared in BOOTSTRAP
         if $!list {
             $WHICH ~= '|';
             $WHICH ~= ( '(' ~ $_.WHICH ~ ')' )
-              for $!list;
+              for flat $!list;
         }
         if $!hash {
             $WHICH ~= '|';

src/core/HashIter.pm

@@ -117,7 +117,7 @@ my class HashIter is Iterator {
                         nqp::push($rpa, Pair.new(
                             :value($k),
                             :key($_)))
-                                for nqp::hllize(nqp::iterval($pairish)).list;
+                                for flat nqp::hllize(nqp::iterval($pairish)).list;
                     }
                 }
                 else {
@@ -127,7 +127,7 @@ my class HashIter is Iterator {
                         nqp::push($rpa, Pair.new(
                             :value($k),
                             :key($_)))
-                                for nqp::hllize(nqp::iterval($pairish)).list;
+                                for flat nqp::hllize(nqp::iterval($pairish)).list;
                     }
                 }
             }

src/core/Mu.pm

@@ -307,7 +307,7 @@ my class Mu { # declared in BOOTSTRAP
     multi method perl(Mu:U:) { self.^name }
     multi method perl(Mu:D:) {
         my @attrs;
-        for self.^attributes().grep: { .has_accessor } -> $attr {
+        for self.^attributes().flat.grep: { .has_accessor } -> $attr {
             my $name := substr($attr.Str,2);
             @attrs.push: $name
                         ~ ' => '
@@ -322,7 +322,7 @@ my class Mu { # declared in BOOTSTRAP
         return DUMP(self, :$indent-step) unless %ctx;
 
         my Mu $attrs := nqp::list();
-        for self.^attributes -> $attr {
+        for self.^attributes.flat -> $attr {
             my str $name       = $attr.name;
             my str $acc_name   = nqp::substr($name, 2, nqp::chars($name) - 2);
             my str $build_name = $attr.has_accessor ?? $acc_name !! $name;
@@ -402,7 +402,7 @@ my class Mu { # declared in BOOTSTRAP
     method clone(*%twiddles) {
         my $cloned := nqp::clone(nqp::decont(self));
         if %twiddles.elems {
-            for self.^attributes() -> $attr {
+            for self.^attributes.flat -> $attr {
                 my $name := $attr.name;
                 my $package := $attr.package;
                 unless nqp::objprimspec($attr.type) {
@@ -417,7 +417,7 @@ my class Mu { # declared in BOOTSTRAP
             }
         }
         else {
-            for self.^attributes() -> $attr {
+            for self.^attributes.flat -> $attr {
                 unless nqp::objprimspec($attr.type) {
                     my $name     := $attr.name;
                     my $package  := $attr.package;
@@ -433,7 +433,7 @@ my class Mu { # declared in BOOTSTRAP
 
     method Capture() {
         my %attrs;
-        for self.^attributes -> $attr {
+        for self.^attributes.flat -> $attr {
             if $attr.has-accessor {
                 my $name = substr($attr.name,2);
                 unless %attrs.EXISTS-KEY($name) {
@@ -538,7 +538,7 @@ my class Mu { # declared in BOOTSTRAP
                 push @classes, @search_list;
                 my @new_search_list;
                 for @search_list -> $current {
-                    for $current.^parents(:local) -> $next {
+                    for flat $current.^parents(:local) -> $next {
                         unless @new_search_list.grep({ $^c.WHAT =:= $next.WHAT }) {
                             push @new_search_list, $next;
                         }
@@ -550,7 +550,7 @@ my class Mu { # declared in BOOTSTRAP
             sub build_ascendent(Mu $class) {
                 unless @classes.grep({ $^c.WHAT =:= $class.WHAT }) {
                     push @classes, $class;
-                    for $class.^parents(:local) {
+                    for flat $class.^parents(:local) {
                         build_ascendent($^parent);
                     }
                 }
@@ -559,7 +559,7 @@ my class Mu { # declared in BOOTSTRAP
         } elsif $descendant {
             sub build_descendent(Mu $class) {
                 unless @classes.grep({ $^c.WHAT =:= $class.WHAT }) {
-                    for $class.^parents(:local) {
+                    for flat $class.^parents(:local) {
                         build_descendent($^parent);
                     }
                     push @classes, $class;
@@ -578,7 +578,7 @@ my class Mu { # declared in BOOTSTRAP
             if (!defined($include) || $include.ACCEPTS($class)) &&
               (!defined($omit) || !$omit.ACCEPTS($class)) {
                 try {
-                    for $class.^methods(:local) -> $method {
+                    for flat $class.^methods(:local) -> $method {
                         my $check_name = $method.?name;
                         if $check_name.defined && $check_name eq $name {
                             @methods.push($method);
@@ -664,7 +664,7 @@ sub DUMP(|args (*@args, :$indent-step = 4, :%ctx?)) {
 
             my @pieces;
             {
-                for $topic {
+                for $topic.pairs {
                     @pieces.push: $_.key ~ ' => ' ~ DUMP($_.value, :$indent-step, :%ctx);
                 }
                 CATCH { default { @pieces.push: '...' } }

src/core/Parcel.pm

@@ -75,7 +75,14 @@ my class Parcel does Positional { # declared in BOOTSTRAP
     method list()  { nqp::p6list(nqp::clone($!storage), List, Mu) }
     method lol()   { nqp::p6list(nqp::clone($!storage), LoL, Mu) }
     method eager() { nqp::p6list(nqp::clone($!storage), List, Mu).eager }
-    method for(|c) { nqp::p6list(nqp::clone($!storage), List, Bool::True).map(|c) }
+    method for(|c) {
+        if nqp::elems($!storage) == 1 and !nqp::isnull(nqp::atpos($!storage,0)) and !nqp::iscont(nqp::atpos($!storage,0)) {
+            try { nqp::atpos($!storage,0).map(|c) } // nqp::p6list(nqp::clone($!storage), List, Mu).map(|c);
+        }
+        else {
+            nqp::p6list(nqp::clone($!storage), List, Mu).map(|c);
+        }
+    }
     method map(|c) { nqp::p6list(nqp::clone($!storage), List, Mu).map(|c) }
     method join(|c) { nqp::p6list(nqp::clone($!storage), List, Mu).join(|c) }
 

src/core/Pod.pm

@@ -22,7 +22,7 @@ my class Pod::Block {
             }
         }
         @chunks = $leading, $pod.^name, (%confs.perl if %confs), "\n";
-        for $pod.contents.list -> $c {
+        for $pod.contents.flat -> $c {
             if nqp::istype($c,Pod::Block) {
                 @chunks.push: pod-gist($c, $level + 2);
             }

src/core/Regex.pm

@@ -15,7 +15,7 @@ my class Regex { # declared in BOOTSTRAP
         my $dollar_slash := nqp::getlexrelcaller(
             nqp::ctxcallerskipthunks(nqp::ctx()),
             '$/');
-        for @a {
+        for flat @a {
             $dollar_slash = SELF.(Cursor.'!cursor_init'($_, :c(0))).MATCH_SAVE;
             return $dollar_slash if $dollar_slash;
         }