Fix "tried to pop from empty stack" with BlockExpr and const by im-0 · Pull Request #7916 · RustPython/RustPython

im-0 · 2026-05-18T19:03:42Z

Constant expressions without side effects are optimized away unless in an interactive mode. This causes panic when last statement in a block is a constant and Mode::BlockExpr is used as this constant value must be returned to caller.

---- vm::python_run::tests::test_block_expr_return_const stdout ----
[crates/vm/src/frame.rs:9653:9] self = ExecutingFrame {
    code: code: <code object <module> at ??? file "<embedded>", line 1>,
    stack_len: 0,
}

thread 'vm::python_run::tests::test_block_expr_return_const' (2640752) panicked at crates/vm/src/frame.rs:9410:18:
tried to pop from empty stack
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:689:5
   1: core::panicking::panic_fmt
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/panicking.rs:80:14
   2: rustpython_vm::frame::ExecutingFrame::fatal
             at ./src/frame.rs:9654:9
   3: rustpython_vm::frame::ExecutingFrame::pop_stackref_opt
             at ./src/frame.rs:9410:18
   4: rustpython_vm::frame::ExecutingFrame::pop_stackref
             at ./src/frame.rs:9420:18
   5: rustpython_vm::frame::ExecutingFrame::pop_value
             at ./src/frame.rs:9435:14
   6: rustpython_vm::frame::ExecutingFrame::execute_instruction
             at ./src/frame.rs:3391:34
   7: rustpython_vm::frame::ExecutingFrame::run
             at ./src/frame.rs:1623:31
   8: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::run::{{closure}}
             at ./src/frame.rs:1098:44
   9: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::with_exec
             at ./src/frame.rs:1093:9
  10: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::run
             at ./src/frame.rs:1098:14
  11: rustpython_vm::vm::VirtualMachine::run_frame::{{closure}}
             at ./src/vm/mod.rs:1201:44
  12: rustpython_vm::vm::VirtualMachine::with_frame_impl::{{closure}}::{{closure}}
             at ./src/vm/mod.rs:1610:60
  13: rustpython_vm::vm::VirtualMachine::dispatch_traced_frame
             at ./src/vm/mod.rs:1689:22
  14: rustpython_vm::vm::VirtualMachine::with_frame_impl::{{closure}}
             at ./src/vm/mod.rs:1610:22
  15: rustpython_vm::vm::VirtualMachine::with_recursion
             at ./src/vm/mod.rs:1547:9
  16: rustpython_vm::vm::VirtualMachine::with_frame_impl
             at ./src/vm/mod.rs:1572:14
  17: rustpython_vm::vm::VirtualMachine::with_frame
             at ./src/vm/mod.rs:1555:14
  18: rustpython_vm::vm::VirtualMachine::run_frame
             at ./src/vm/mod.rs:1201:20
  19: rustpython_vm::vm::VirtualMachine::run_code_obj
             at ./src/vm/mod.rs:1120:14
  20: rustpython_vm::vm::python_run::<impl rustpython_vm::vm::VirtualMachine>::run_block_expr
             at ./src/vm/python_run.rs:51:14
  21: rustpython_vm::vm::python_run::tests::test_block_expr_return_const::{{closure}}
             at ./src/vm/python_run.rs:190:47
  22: rustpython_vm::vm::interpreter::Interpreter::enter::{{closure}}
             at ./src/vm/interpreter.rs:366:39
  23: rustpython_vm::vm::thread::set_current_vm::{{closure}}
             at ./src/vm/thread.rs:107:9
  24: std::thread::local::LocalKey<T>::try_with
             at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/local.rs:513:12
  25: std::thread::local::LocalKey<T>::with
             at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/local.rs:477:20
  26: rustpython_vm::vm::thread::set_current_vm
             at ./src/vm/thread.rs:102:14
  27: rustpython_vm::vm::thread::enter_vm
             at ./src/vm/thread.rs:132:5
  28: rustpython_vm::vm::interpreter::Interpreter::enter
             at ./src/vm/interpreter.rs:366:9
  29: rustpython_vm::vm::python_run::tests::test_block_expr_return_const
             at ./src/vm/python_run.rs:188:23
  30: rustpython_vm::vm::python_run::tests::test_block_expr_return_const::{{closure}}
             at ./src/vm/python_run.rs:187:38
  31: core::ops::function::FnOnce::call_once
             at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
  32: <fn() -> core::result::Result<(), alloc::string::String> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/ops/function.rs:250:5

Simple test case included.

Summary by CodeRabbit

Refactor
- Improved compilation efficiency for constant expressions in statement blocks.

test_block_expr_return_const() panics currently.

Constant expressions without side effects are optimized away unless in an interactive mode[1]. This causes panic when last statement in a block is a constant and Mode::BlockExpr is used as this constant value must be returned to caller. Example panic: ---- vm::python_run::tests::test_block_expr_return_const stdout ---- [crates/vm/src/frame.rs:9653:9] self = ExecutingFrame { code: code: <code object <module> at ??? file "<embedded>", line 1>, stack_len: 0, } thread 'vm::python_run::tests::test_block_expr_return_const' (2640752) panicked at crates/vm/src/frame.rs:9410:18: tried to pop from empty stack stack backtrace: 0: __rustc::rust_begin_unwind at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/std/src/panicking.rs:689:5 1: core::panicking::panic_fmt at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/panicking.rs:80:14 2: rustpython_vm::frame::ExecutingFrame::fatal at ./src/frame.rs:9654:9 3: rustpython_vm::frame::ExecutingFrame::pop_stackref_opt at ./src/frame.rs:9410:18 4: rustpython_vm::frame::ExecutingFrame::pop_stackref at ./src/frame.rs:9420:18 5: rustpython_vm::frame::ExecutingFrame::pop_value at ./src/frame.rs:9435:14 6: rustpython_vm::frame::ExecutingFrame::execute_instruction at ./src/frame.rs:3391:34 7: rustpython_vm::frame::ExecutingFrame::run at ./src/frame.rs:1623:31 8: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::run::{{closure}} at ./src/frame.rs:1098:44 9: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::with_exec at ./src/frame.rs:1093:9 10: rustpython_vm::frame::<impl rustpython_vm::object::core::Py<rustpython_vm::frame::Frame>>::run at ./src/frame.rs:1098:14 11: rustpython_vm::vm::VirtualMachine::run_frame::{{closure}} at ./src/vm/mod.rs:1201:44 12: rustpython_vm::vm::VirtualMachine::with_frame_impl::{{closure}}::{{closure}} at ./src/vm/mod.rs:1610:60 13: rustpython_vm::vm::VirtualMachine::dispatch_traced_frame at ./src/vm/mod.rs:1689:22 14: rustpython_vm::vm::VirtualMachine::with_frame_impl::{{closure}} at ./src/vm/mod.rs:1610:22 15: rustpython_vm::vm::VirtualMachine::with_recursion at ./src/vm/mod.rs:1547:9 16: rustpython_vm::vm::VirtualMachine::with_frame_impl at ./src/vm/mod.rs:1572:14 17: rustpython_vm::vm::VirtualMachine::with_frame at ./src/vm/mod.rs:1555:14 18: rustpython_vm::vm::VirtualMachine::run_frame at ./src/vm/mod.rs:1201:20 19: rustpython_vm::vm::VirtualMachine::run_code_obj at ./src/vm/mod.rs:1120:14 20: rustpython_vm::vm::python_run::<impl rustpython_vm::vm::VirtualMachine>::run_block_expr at ./src/vm/python_run.rs:51:14 21: rustpython_vm::vm::python_run::tests::test_block_expr_return_const::{{closure}} at ./src/vm/python_run.rs:190:47 22: rustpython_vm::vm::interpreter::Interpreter::enter::{{closure}} at ./src/vm/interpreter.rs:366:39 23: rustpython_vm::vm::thread::set_current_vm::{{closure}} at ./src/vm/thread.rs:107:9 24: std::thread::local::LocalKey<T>::try_with at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/local.rs:513:12 25: std::thread::local::LocalKey<T>::with at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/thread/local.rs:477:20 26: rustpython_vm::vm::thread::set_current_vm at ./src/vm/thread.rs:102:14 27: rustpython_vm::vm::thread::enter_vm at ./src/vm/thread.rs:132:5 28: rustpython_vm::vm::interpreter::Interpreter::enter at ./src/vm/interpreter.rs:366:9 29: rustpython_vm::vm::python_run::tests::test_block_expr_return_const at ./src/vm/python_run.rs:188:23 30: rustpython_vm::vm::python_run::tests::test_block_expr_return_const::{{closure}} at ./src/vm/python_run.rs:187:38 31: core::ops::function::FnOnce::call_once at /home/im/.rustup/toolchains/stable-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5 32: <fn() -> core::result::Result<(), alloc::string::String> as core::ops::function::FnOnce<()>>::call_once at /rustc/59807616e1fa2540724bfbac14d7976d7e4a3860/library/core/src/ops/function.rs:250:5 [1] https://github.com/RustPython/RustPython/blob/883ce9d273db98b435eb3f51128925a1255ba88d/crates/codegen/src/compile.rs#L2990-L2997

coderabbitai · 2026-05-18T19:04:00Z

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crates/codegen/src/compile.rs
crates/vm/src/vm/python_run.rs

📝 Walkthrough

Walkthrough

The PR refines how the compiler handles trailing expressions in statement blocks. When compiling in non-interactive mode and the trailing expression qualifies as const, it now compiles the expression directly instead of unconditionally discarding it. Two unit tests validate this behavior for both const and variable-dependent block expressions.

Changes

Const Expression Optimization in Trailing Statements

Layer / File(s)	Summary
Compiler handling of trailing const expressions `crates/codegen/src/compile.rs`	The match arm for `ast::Stmt::Expr` destructures the expression value and conditionally compiles it when `!self.interactive` and the expression passes `Self::is_const_expression(value)`; otherwise reverts to the prior behavior of popping the `PopTop` instruction.
Block expression evaluation tests `crates/vm/src/vm/python_run.rs`	Unit tests verify `VirtualMachine::run_block_expr` correctly evaluates constant block expressions (e.g., `"1"`) and non-constant expressions that reference globals (e.g., `"2 + x"` with `x = 3`).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

RustPython/RustPython#7541: Expands const-expression detection logic that this PR depends on to gate the new trailing statement compilation.

Poem

🐰 Const expressions hop and skip,
No more wasted PopTop trips,
Tests confirm they land just right,
Blocks compile with cleaner light!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

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Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main fix: handling const expressions in BlockExpr mode to prevent stack underflow.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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ShaharNaveh

TYSM!
welcome to the project:)

youknowone

Thanks! good to have test for this

im-0 added 2 commits May 18, 2026 20:40

Add tests for Mode::BlockExpr and VirtualMachine::run_block_expr

899fac4

test_block_expr_return_const() panics currently.

ShaharNaveh approved these changes May 18, 2026

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youknowone approved these changes May 19, 2026

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youknowone merged commit 150b910 into RustPython:main May 19, 2026
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Fix "tried to pop from empty stack" with BlockExpr and const#7916

Fix "tried to pop from empty stack" with BlockExpr and const#7916
youknowone merged 2 commits into
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im-0:fix-block-expr

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im-0 commented May 18, 2026 •

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