From 8420d1c7449eaee281f83a9a02c2553f28ccf76d Mon Sep 17 00:00:00 2001
From: Akash h g <akashhga@gmail.com>
Date: Sat, 26 Sep 2026 15:31:26 +0530
Subject: [PATCH] repl: fix multi-line attribute set in variable binding (issue
 #16310)
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When a user typed  = { followed by Enter in nix repl, the REPL
produced a confusing error:

  error: syntax error, unexpected '=', expecting end of file
       at «string»:1:3:
            1| a = {
             |   ^

The root cause was in NixRepl::parseReplBindings: the first parse
attempt (without semicolon) caught *all* ParseErrors silently. When the
input was an incomplete attrset binding (e.g.  = {), the parse fails
with 'unexpected end of file' — an incomplete-input signal. Because
this was swallowed, the code fell through to the semicolon-retry, which
produced a different syntax error (not 'unexpected end of file'), so
the second catch returned
ullptr. Back in processLine,
ullptr
caused the line to be evaluated as a Nix expression, which correctly
rejected  = { as invalid expression syntax.

Fix: mirror the isIncompleteInput check already present in the
second catch block into the first catch block, throwing
IncompleteReplExpr so the main loop prompts for continuation.

This is analogous to the fix for multi-line strings in bindings (issue
#15801, PR #15892). The single-line case  = { b = 1; } is
unaffected — only incomplete inputs (missing closing }) trigger the
continuation prompt.

Add two characterisation tests:
- multiline-attrset-binding: variable bound to a multi-line attrset
- multiline-attrset-expr: multi-line attrset expression (regression guard)
---
 src/libcmd/repl.cc                                       | 8 +++++++-
 .../suites/repl/cases/multiline-attrset-binding.expected | 9 +++++++++
 .../suites/repl/cases/multiline-attrset-binding.in       | 5 +++++
 .../suites/repl/cases/multiline-attrset-expr.expected    | 7 +++++++
 .../suites/repl/cases/multiline-attrset-expr.in          | 4 ++++
 5 files changed, 32 insertions(+), 1 deletion(-)
 create mode 100644 tests/functional/suites/repl/cases/multiline-attrset-binding.expected
 create mode 100644 tests/functional/suites/repl/cases/multiline-attrset-binding.in
 create mode 100644 tests/functional/suites/repl/cases/multiline-attrset-expr.expected
 create mode 100644 tests/functional/suites/repl/cases/multiline-attrset-expr.in

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 9e3d99d75d13..d33ac8ac76d2 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -902,7 +902,13 @@ ExprAttrs * NixRepl::parseReplBindings(std::string s)
     // Try parsing as bindings
     try {
         return state->parseReplBindings(s, basePath, staticEnv);
-    } catch (ParseError &) {
+    } catch (ParseError & e) {
+        // If the input is incomplete (e.g. `a = {` without closing `}`),
+        // signal the main loop to prompt for continuation instead of falling
+        // through to expression evaluation (which would produce a confusing
+        // "unexpected '='" error).
+        if (isIncompleteInput(e))
+            throw IncompleteReplExpr(e.msg());
     }
 
     // Try with semicolon appended (for `inherit foo` shorthand)
diff --git a/tests/functional/suites/repl/cases/multiline-attrset-binding.expected b/tests/functional/suites/repl/cases/multiline-attrset-binding.expected
new file mode 100644
index 000000000000..52542b688894
--- /dev/null
+++ b/tests/functional/suites/repl/cases/multiline-attrset-binding.expected
@@ -0,0 +1,9 @@
+Nix <nix version>
+Type :? for help.
+
+nix-repl> a = {
+        >   b = 1;
+        > }
+
+nix-repl> a
+{ b = 1; }
diff --git a/tests/functional/suites/repl/cases/multiline-attrset-binding.in b/tests/functional/suites/repl/cases/multiline-attrset-binding.in
new file mode 100644
index 000000000000..742f7cdec687
--- /dev/null
+++ b/tests/functional/suites/repl/cases/multiline-attrset-binding.in
@@ -0,0 +1,5 @@
+# COM: Multi-line attribute set in binding should trigger continuation (issue #16310)
+a = {
+  b = 1;
+}
+a
diff --git a/tests/functional/suites/repl/cases/multiline-attrset-expr.expected b/tests/functional/suites/repl/cases/multiline-attrset-expr.expected
new file mode 100644
index 000000000000..e644eddd32a3
--- /dev/null
+++ b/tests/functional/suites/repl/cases/multiline-attrset-expr.expected
@@ -0,0 +1,7 @@
+Nix <nix version>
+Type :? for help.
+
+nix-repl> {
+        >   x = 1;
+        > }
+{ x = 1; }
diff --git a/tests/functional/suites/repl/cases/multiline-attrset-expr.in b/tests/functional/suites/repl/cases/multiline-attrset-expr.in
new file mode 100644
index 000000000000..57c538a6fb61
--- /dev/null
+++ b/tests/functional/suites/repl/cases/multiline-attrset-expr.in
@@ -0,0 +1,4 @@
+# COM: Multi-line attribute set expression (not binding) should still trigger continuation (issue #16310)
+{
+  x = 1;
+}