Merge changes I829581a3,I0016970d,I5dac8e77,Ib7560fe5 into main

* changes:
  doc/release-notes: add for pretty printing improvements
  libexpr/print: do not show elided nested items when there are none
  libexpr/print: never show empty attrsets or derivations as «repeated»
  libexpr/print: pretty-print idempotently

5 files changed, 132 insertions, 32 deletions

diff --git a/doc/manual/rl-next/pretty-printing.md b/doc/manual/rl-next/pretty-printing.md
new file mode 100644
index 000000000..f7953f9ff
--- /dev/null
+++ b/doc/manual/rl-next/pretty-printing.md
@@ -0,0 +1,58 @@
+---
+synopsis: "Eliminate some pretty-printing surprises"
+cls: [1616, 1617, 1618]
+prs: [11100]
+credits: [alois31, roberth]
+category: Improvements
+---
+
+Some inconsistent and surprising behaviours have been eliminated from the pretty-printing used by the REPL and `nix eval`:
+* Lists and attribute sets that contain only a single item without nested structures are no longer sometimes inappropriately indented in the REPL, depending on internal state of the evaluator.
+* Empty attribute sets and derivations are no longer shown as `«repeated»`, since they are always cheap to print.
+  This matches the existing behaviour of `nix-instantiate` on empty attribute sets.
+  Empty lists were never printed as `«repeated»` already.
+* The REPL by default does not print nested attribute sets and lists, and indicates elided items with an ellipsis.
+  Previously, the ellipsis was printed even when the structure was empty, so that such items do not in fact exist.
+  Since this behaviour was confusing, it does not happen any more.
+
+Before:
+```
+nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
+[
+  [
+    3
+  ]
+  [ 3 ]
+]
+
+nix-repl> let inherit (import <nixpkgs> { }) hello; in [ hello hello ]
+[
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+  «repeated»
+]
+
+nix-repl> let x = {}; in [ x ]
+[
+  { ... }
+]
+```
+
+After:
+```
+nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
+[
+  [ 3 ]
+  [ 3 ]
+]
+
+nix-repl> let inherit (import <nixpkgs> { }) hello; in [ hello hello ]
+[
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+  «derivation /nix/store/fqs92lzychkm6p37j7fnj4d65nq9fzla-hello-2.12.1.drv»
+]
+
+nix-repl> let x = {}; in [ x ]
+[
+  { }
+]
+```
diff --git a/src/libexpr/print.cc b/src/libexpr/print.cc
index 87db004b2..b7ff50f48 100644
--- a/src/libexpr/print.cc
+++ b/src/libexpr/print.cc
@@ -264,22 +264,24 @@ private:
             return true;
         }
 
+        if (options.force) {
+            // The item is going to be forced during printing anyway, but we need its type now.
+            state.forceValue(*item, item->determinePos(noPos));
+        }
+
         // Pretty-print single-item attrsets only if they contain nested
         // structures.
         auto itemType = item->type();
-        return itemType == nList || itemType == nAttrs || itemType == nThunk;
+        return itemType == nList || itemType == nAttrs;
     }
 
     void printAttrs(Value & v, size_t depth)
     {
-        if (seen && !seen->insert(v.attrs).second) {
-            printRepeated();
-            return;
-        }
-
         if (options.force && options.derivationPaths && state.isDerivation(v)) {
             printDerivation(v);
-        } else if (depth < options.maxDepth) {
+        } else if (seen && !v.attrs->empty() && !seen->insert(v.attrs).second) {
+            printRepeated();
+        } else if (depth < options.maxDepth || v.attrs->empty()) {
             increaseIndent();
             output << "{";
 
@@ -335,10 +337,15 @@ private:
             return true;
         }
 
+        if (options.force) {
+            // The item is going to be forced during printing anyway, but we need its type now.
+            state.forceValue(*item, item->determinePos(noPos));
+        }
+
         // Pretty-print single-item lists only if they contain nested
         // structures.
         auto itemType = item->type();
-        return itemType == nList || itemType == nAttrs || itemType == nThunk;
+        return itemType == nList || itemType == nAttrs;
     }
 
     void printList(Value & v, size_t depth)
@@ -348,7 +355,7 @@ private:
             return;
         }
 
-        if (depth < options.maxDepth) {
+        if (depth < options.maxDepth || v.listSize() == 0) {
             increaseIndent();
             output << "[";
             auto listItems = v.listItems();
diff --git a/tests/functional/repl_characterization/data/idempotent.test b/tests/functional/repl_characterization/data/idempotent.test
new file mode 100644
index 000000000..4ab087d45
--- /dev/null
+++ b/tests/functional/repl_characterization/data/idempotent.test
@@ -0,0 +1,13 @@
+A previously unforced thunk in an attribute set does not lead to indentation when it won't evaluate to a nested structure:
+  nix-repl> :p let x = 1 + 2; in [ { inherit x; } { inherit x; } ]
+  [
+    { x = 3; }
+    { x = 3; }
+  ]
+
+Same for a list:
+  nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
+  [
+    [ 3 ]
+    [ 3 ]
+  ]
diff --git a/tests/functional/repl_characterization/repl_characterization.cc b/tests/functional/repl_characterization/repl_characterization.cc
index c91d6c1e3..f2cf0ca3d 100644
--- a/tests/functional/repl_characterization/repl_characterization.cc
+++ b/tests/functional/repl_characterization/repl_characterization.cc
@@ -185,5 +185,6 @@ REPL_TEST(repl_overlays_error);
 REPL_TEST(repl_printing);
 REPL_TEST(stack_vars);
 REPL_TEST(errors);
+REPL_TEST(idempotent);
 
 }; // namespace nix
diff --git a/tests/unit/libexpr/value/print.cc b/tests/unit/libexpr/value/print.cc
index cdbc8f8f9..ddf8b8b99 100644
--- a/tests/unit/libexpr/value/print.cc
+++ b/tests/unit/libexpr/value/print.cc
@@ -193,6 +193,9 @@ TEST_F(ValuePrintingTests, vBlackhole)
 
 TEST_F(ValuePrintingTests, depthAttrs)
 {
+    Value vZero;
+    vZero.mkInt(0);
+
     Value vOne;
     vOne.mkInt(1);
 
@@ -203,10 +206,16 @@ TEST_F(ValuePrintingTests, depthAttrs)
     Value vAttrsEmpty;
     vAttrsEmpty.mkAttrs(builderEmpty.finish());
 
+    BindingsBuilder builderNested(state, state.allocBindings(1));
+    builderNested.insert(state.symbols.create("zero"), &vZero);
+    Value vAttrsNested;
+    vAttrsNested.mkAttrs(builderNested.finish());
+
     BindingsBuilder builder(state, state.allocBindings(10));
     builder.insert(state.symbols.create("one"), &vOne);
     builder.insert(state.symbols.create("two"), &vTwo);
-    builder.insert(state.symbols.create("nested"), &vAttrsEmpty);
+    builder.insert(state.symbols.create("empty"), &vAttrsEmpty);
+    builder.insert(state.symbols.create("nested"), &vAttrsNested);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
@@ -220,9 +229,9 @@ TEST_F(ValuePrintingTests, depthAttrs)
     vNested.mkAttrs(builder2.finish());
 
     test(vNested, "{ nested = { ... }; one = 1; two = 2; }", PrintOptions { .maxDepth = 1 });
-    test(vNested, "{ nested = { nested = { ... }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 2 });
-    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 3 });
-    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 4 });
+    test(vNested, "{ nested = { empty = { }; nested = { ... }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 2 });
+    test(vNested, "{ nested = { empty = { }; nested = { zero = 0; }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 3 });
+    test(vNested, "{ nested = { empty = { }; nested = { zero = 0; }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 4 });
 }
 
 TEST_F(ValuePrintingTests, depthList)
@@ -641,20 +650,24 @@ TEST_F(ValuePrintingTests, ansiColorsBlackhole)
 
 TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    Value vZero;
+    vZero.mkInt(0);
+
+    BindingsBuilder innerBuilder(state, state.allocBindings(1));
+    innerBuilder.insert(state.symbols.create("x"), &vZero);
 
-    Value vEmpty;
-    vEmpty.mkAttrs(emptyBuilder.finish());
+    Value vInner;
+    vInner.mkAttrs(innerBuilder.finish());
 
     BindingsBuilder builder(state, state.allocBindings(10));
-    builder.insert(state.symbols.create("a"), &vEmpty);
-    builder.insert(state.symbols.create("b"), &vEmpty);
+    builder.insert(state.symbols.create("a"), &vInner);
+    builder.insert(state.symbols.create("b"), &vInner);
 
     Value vAttrs;
     vAttrs.mkAttrs(builder.finish());
 
     test(vAttrs,
-         "{ a = { }; b = " ANSI_MAGENTA "«repeated»" ANSI_NORMAL "; }",
+         "{ a = { x = " ANSI_CYAN "0" ANSI_NORMAL "; }; b = " ANSI_MAGENTA "«repeated»" ANSI_NORMAL "; }",
          PrintOptions {
              .ansiColors = true
          });
@@ -662,19 +675,23 @@ TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
 
 TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    Value vZero;
+    vZero.mkInt(0);
+
+    BindingsBuilder innerBuilder(state, state.allocBindings(1));
+    innerBuilder.insert(state.symbols.create("x"), &vZero);
 
-    Value vEmpty;
-    vEmpty.mkAttrs(emptyBuilder.finish());
+    Value vInner;
+    vInner.mkAttrs(innerBuilder.finish());
 
     Value vList;
     state.mkList(vList, 3);
-    vList.bigList.elems[0] = &vEmpty;
-    vList.bigList.elems[1] = &vEmpty;
+    vList.bigList.elems[0] = &vInner;
+    vList.bigList.elems[1] = &vInner;
     vList.bigList.size = 2;
 
     test(vList,
-         "[ { } " ANSI_MAGENTA "«repeated»" ANSI_NORMAL " ]",
+         "[ { x = " ANSI_CYAN "0" ANSI_NORMAL "; } " ANSI_MAGENTA "«repeated»" ANSI_NORMAL " ]",
          PrintOptions {
              .ansiColors = true
          });
@@ -682,20 +699,24 @@ TEST_F(ValuePrintingTests, ansiColorsListRepeated)
 
 TEST_F(ValuePrintingTests, listRepeated)
 {
-    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+    Value vZero;
+    vZero.mkInt(0);
+
+    BindingsBuilder innerBuilder(state, state.allocBindings(1));
+    innerBuilder.insert(state.symbols.create("x"), &vZero);
 
-    Value vEmpty;
-    vEmpty.mkAttrs(emptyBuilder.finish());
+    Value vInner;
+    vInner.mkAttrs(innerBuilder.finish());
 
     Value vList;
     state.mkList(vList, 3);
-    vList.bigList.elems[0] = &vEmpty;
-    vList.bigList.elems[1] = &vEmpty;
+    vList.bigList.elems[0] = &vInner;
+    vList.bigList.elems[1] = &vInner;
     vList.bigList.size = 2;
 
-    test(vList, "[ { } «repeated» ]", PrintOptions { });
+    test(vList, "[ { x = 0; } «repeated» ]", PrintOptions { });
     test(vList,
-         "[ { } { } ]",
+         "[ { x = 0; } { x = 0; } ]",
          PrintOptions {
              .trackRepeated = false
          });