From 36dfac75601b246dc22a6a27ee793dd9ef0b8c0e Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Fri, 2 Feb 2024 19:31:18 -0800
Subject: [PATCH 1/2] Expose locals from `let` expressions to the debugger

---
 src/libexpr/eval.cc    | 13 +++++++++++++
 src/libexpr/nixexpr.cc |  9 +++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 91fd3ddf8..4241dca6a 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1322,6 +1322,19 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
     for (auto & i : attrs->attrs)
         env2.values[displ++] = i.second.e->maybeThunk(state, i.second.inherited ? env : env2);
 
+    auto dts = state.debugRepl
+        ? makeDebugTraceStacker(
+            state,
+            *this,
+            env2,
+            getPos()
+                ? std::make_shared<Pos>(state.positions[getPos()])
+                : nullptr,
+            "while evaluating a '%1%' expression",
+            "let"
+        )
+        : nullptr;
+
     body->eval(state, env2, v);
 }
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 6fe4ba81b..492e131d0 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -409,9 +409,6 @@ void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debugRepl)
-        es.exprEnvs.insert(std::make_pair(this, env));
-
     auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
 
     Displacement displ = 0;
@@ -423,6 +420,9 @@ void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     for (auto & i : attrs->attrs)
         i.second.e->bindVars(es, i.second.inherited ? env : newEnv);
 
+    if (es.debugRepl)
+        es.exprEnvs.insert(std::make_pair(this, newEnv));
+
     body->bindVars(es, newEnv);
 }
 
@@ -447,9 +447,6 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
             break;
         }
 
-    if (es.debugRepl)
-        es.exprEnvs.insert(std::make_pair(this, env));
-
     attrs->bindVars(es, env);
     auto newEnv = std::make_shared<StaticEnv>(this, env.get());
     body->bindVars(es, newEnv);

From b63a8d7c46e7a59c3e133c94af24dfcf517fe50b Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Mon, 5 Feb 2024 13:15:29 -0800
Subject: [PATCH 2/2] Add release note

---
 .../rl-next/debugger-locals-for-let-expressions.md       | 9 +++++++++
 1 file changed, 9 insertions(+)
 create mode 100644 doc/manual/rl-next/debugger-locals-for-let-expressions.md

diff --git a/doc/manual/rl-next/debugger-locals-for-let-expressions.md b/doc/manual/rl-next/debugger-locals-for-let-expressions.md
new file mode 100644
index 000000000..736208724
--- /dev/null
+++ b/doc/manual/rl-next/debugger-locals-for-let-expressions.md
@@ -0,0 +1,9 @@
+---
+synopsis: "`--debugger` can now access bindings from `let` expressions"
+prs: 9918
+issues: 8827.
+---
+
+Breakpoints and errors in the bindings of a `let` expression can now access
+those bindings in the debugger. Previously, only the body of `let` expressions
+could access those bindings.