group inherit by source during Expr::show

for plain inherits this is really just a stylistic choice, but for
inherit-from it actually fixes an exponential size increase problem
during expr printing (as may happen during assertion failure reporting,
on during duplicate attr detection in the parser)

(cherry picked from commit ecf8b12d60ad2929f9998666cf0966475b91e291)
Change-Id: Ie55f0cb01a37e766414c31f8d40f51c2c7d106b0

1 files changed, 29 insertions, 3 deletions

diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 525b2440b..c9d42cea5 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -77,10 +77,36 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
         std::string_view sa = symbols[a->first], sb = symbols[b->first];
         return sa < sb;
     });
+    std::vector<Symbol> inherits;
+    std::map<Expr *, std::vector<Symbol>> inheritsFrom;
     for (auto & i : sorted) {
-        if (i->second.inherited())
-            str << "inherit " << symbols[i->first] << " " << "; ";
-        else {
+        switch (i->second.kind) {
+        case AttrDef::Kind::Plain:
+            break;
+        case AttrDef::Kind::Inherited:
+            inherits.push_back(i->first);
+            break;
+        case AttrDef::Kind::InheritedFrom: {
+            auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
+            inheritsFrom[select.e].push_back(i->first);
+            break;
+        }
+        }
+    }
+    if (!inherits.empty()) {
+        str << "inherit";
+        for (auto sym : inherits) str << " " << symbols[sym];
+        str << "; ";
+    }
+    for (const auto & [from, syms] : inheritsFrom) {
+        str << "inherit (";
+        from->show(symbols, str);
+        str << ")";
+        for (auto sym : syms) str << " " << symbols[sym];
+        str << "; ";
+    }
+    for (auto & i : sorted) {
+        if (i->second.kind == AttrDef::Kind::Plain) {
             str << symbols[i->first] << " = ";
             i->second.e->show(symbols, str);
             str << "; ";