From 44efb05edfcba3baf7a3abcf10085c7647a909ad Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Wed, 11 Apr 2018 15:16:54 +0100
Subject: [PATCH] Add cs_fold1 for better derives

---
 src/libsyntax_ext/deriving/generic/mod.rs | 109 ++++++++++++++++++----
 1 file changed, 92 insertions(+), 17 deletions(-)

diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index 4126ce79f3555..11bd72691c568 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -1676,12 +1676,55 @@ impl<'a> TraitDef<'a> {
 
 // helpful premade recipes
 
+pub fn cs_fold_fields<'a, F>(use_foldl: bool,
+                             mut f: F,
+                             base: P<Expr>,
+                             cx: &mut ExtCtxt,
+                             all_fields: &[FieldInfo<'a>])
+                             -> P<Expr>
+    where F: FnMut(&mut ExtCtxt, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>
+{
+    if use_foldl {
+        all_fields.iter().fold(base, |old, field| {
+            f(cx, field.span, old, field.self_.clone(), &field.other)
+        })
+    } else {
+        all_fields.iter().rev().fold(base, |old, field| {
+            f(cx, field.span, old, field.self_.clone(), &field.other)
+        })
+    }
+}
+
+pub fn cs_fold_enumnonmatch(mut enum_nonmatch_f: EnumNonMatchCollapsedFunc,
+                            cx: &mut ExtCtxt,
+                            trait_span: Span,
+                            substructure: &Substructure)
+                            -> P<Expr>
+{
+    match *substructure.fields {
+        EnumNonMatchingCollapsed(ref all_args, _, tuple) => {
+            enum_nonmatch_f(cx,
+                            trait_span,
+                            (&all_args[..], tuple),
+                            substructure.nonself_args)
+        }
+        _ => cx.span_bug(trait_span, "cs_fold_enumnonmatch expected an EnumNonMatchingCollapsed")
+    }
+}
+
+pub fn cs_fold_static(cx: &mut ExtCtxt,
+                      trait_span: Span)
+                      -> P<Expr>
+{
+    cx.span_bug(trait_span, "static function in `derive`")
+}
+
 /// Fold the fields. `use_foldl` controls whether this is done
 /// left-to-right (`true`) or right-to-left (`false`).
 pub fn cs_fold<F>(use_foldl: bool,
-                  mut f: F,
+                  f: F,
                   base: P<Expr>,
-                  mut enum_nonmatch_f: EnumNonMatchCollapsedFunc,
+                  enum_nonmatch_f: EnumNonMatchCollapsedFunc,
                   cx: &mut ExtCtxt,
                   trait_span: Span,
                   substructure: &Substructure)
@@ -1691,26 +1734,58 @@ pub fn cs_fold<F>(use_foldl: bool,
     match *substructure.fields {
         EnumMatching(.., ref all_fields) |
         Struct(_, ref all_fields) => {
-            if use_foldl {
-                all_fields.iter().fold(base, |old, field| {
-                    f(cx, field.span, old, field.self_.clone(), &field.other)
-                })
-            } else {
-                all_fields.iter().rev().fold(base, |old, field| {
-                    f(cx, field.span, old, field.self_.clone(), &field.other)
-                })
-            }
+            cs_fold_fields(use_foldl, f, base, cx, all_fields)
         }
-        EnumNonMatchingCollapsed(ref all_args, _, tuple) => {
-            enum_nonmatch_f(cx,
-                            trait_span,
-                            (&all_args[..], tuple),
-                            substructure.nonself_args)
+        EnumNonMatchingCollapsed(..) => {
+            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
+        }
+        StaticEnum(..) | StaticStruct(..) => {
+            cs_fold_static(cx, trait_span)
         }
-        StaticEnum(..) | StaticStruct(..) => cx.span_bug(trait_span, "static function in `derive`"),
     }
 }
 
+/// Special version of `cs_fold` that uses the result of a function call on the first field
+/// as the base case when is at least 1 field, and the usual base case when there are zero fields.
+pub fn cs_fold1<F, B>(use_foldl: bool,
+                      f: F,
+                      mut b: B,
+                      enum_nonmatch_f: EnumNonMatchCollapsedFunc,
+                      cx: &mut ExtCtxt,
+                      trait_span: Span,
+                      substructure: &Substructure)
+                      -> P<Expr>
+    where F: FnMut(&mut ExtCtxt, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
+          B: FnMut(&mut ExtCtxt, Option<(Span, P<Expr>, &[P<Expr>])>) -> P<Expr>
+{
+    match *substructure.fields {
+        EnumMatching(.., ref all_fields) |
+        Struct(_, ref all_fields) => {
+            let (base, all_fields) = match (all_fields.is_empty(), use_foldl) {
+                (false, true) => {
+                    let field = &all_fields[0];
+                    let args = (field.span, field.self_.clone(), &field.other[..]);
+                    (b(cx, Some(args)), &all_fields[1..])
+                }
+                (false, false) => {
+                    let idx = all_fields.len() - 1;
+                    let field = &all_fields[idx];
+                    let args = (field.span, field.self_.clone(), &field.other[..]);
+                    (b(cx, Some(args)), &all_fields[..idx])
+                }
+                (true, _) => (b(cx, None), &all_fields[..])
+            };
+
+            cs_fold_fields(use_foldl, f, base, cx, all_fields)
+        }
+        EnumNonMatchingCollapsed(..) => {
+            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
+        }
+        StaticEnum(..) | StaticStruct(..) => {
+            cs_fold_static(cx, trait_span)
+        }
+    }
+}
 
 /// Call the method that is being derived on all the fields, and then
 /// process the collected results. i.e.