Debugging the Compiler

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Types of Compiler Issues
Setting Up for Debugging
Debugging Internal Compiler Errors (ICEs)
Reproducing an ICE
Using GDB with the Compiler
Debugging with LLDB
Debug Logging
Using RUSTC_LOG
Compiler Flags for Debugging
Debugging Specific Components
Borrow Checker
Type Checker
Code Generation
Debugging Tests
Running Individual Tests
Updating Test Expectations
Debugging Miscompilations
Isolating the Problem
Tools for Miscompilations
Advanced Debugging Techniques
Using LLVM Tools
Query System Debugging
Memory Debugging
Common Debugging Scenarios
Debugging Checklist
Resources
Next Steps

This guide covers debugging techniques for the Rust compiler itself, from internal compiler errors (ICEs) to miscompilations and performance issues.

Types of Compiler Issues

ICE (Internal Compiler Error)

The compiler panicked - this is always a bug

Miscompilation

Incorrect code generation - produces wrong results

Compile-time Error

False positive or incorrect error message

Performance Regression

Compilation takes too long or uses too much memory

Setting Up for Debugging

Build with debug symbols

Configure config.toml for debugging:

[rust]
# Enable debug assertions
debug = true
debug-assertions = true
debug-assertions-std = true

# Debug info for the compiler
debuginfo-level = 1
debuginfo-level-rustc = 1
debuginfo-level-std = 1

# Backtrace on ICE
backtrace-on-ice = true

# Enable debug logging
debug-logging = true

Build the debug compiler

# Build stage 1 with debug info
./x.py build --stage 1

# Or use the stage 0 compiler for faster iteration
./x.py build --stage 0 library/std

Set up debugging tools

GDB
LLDB
RR

# Install GDB
sudo apt-get install gdb

# Set in config.toml
[build]
gdb = "gdb"

# Install LLDB
brew install llvm  # macOS

# Set in config.toml
[build]
lldb = "lldb"

# Install rr (record and replay debugger)
sudo apt-get install rr

Debugging Internal Compiler Errors (ICEs)

Reproducing an ICE

Create a minimal test case

Create a file that triggers the ICE:

// ice.rs
fn main() {
    // Minimal code that triggers the ICE
}

Compile with backtrace

# Set environment variable
export RUST_BACKTRACE=1

# Compile the test case
rustc ice.rs

# Or with your stage 1 compiler
./build/x86_64-unknown-linux-gnu/stage1/bin/rustc ice.rs

Get detailed backtrace

# Full backtrace with RUST_BACKTRACE=full
RUST_BACKTRACE=full rustc ice.rs

# With additional logging
RUSTC_LOG=debug rustc ice.rs 2>&1 | tee debug.log

Using GDB with the Compiler

Basic GDB
Breakpoints
Advanced

# Start rustc under GDB
gdb --args ./build/stage1/bin/rustc ice.rs

# In GDB:
(gdb) run
# Wait for crash
(gdb) backtrace
(gdb) info locals
(gdb) frame 3
(gdb) print variable_name

gdb --args rustc ice.rs

# Set breakpoint on function
(gdb) break rustc_middle::ty::context::TyCtxt::create_and_enter
(gdb) run

# Set breakpoint on panic
(gdb) break rust_panic
(gdb) run

# Conditional breakpoint
(gdb) break file.rs:123 if variable == 5

# Use rr for record and replay
rr record ./build/stage1/bin/rustc ice.rs
rr replay

# In replay mode, you can reverse-execute
(rr) reverse-continue
(rr) reverse-step

Debugging with LLDB

# Start rustc under LLDB
lldb ./build/stage1/bin/rustc -- ice.rs

# In LLDB:
(lldb) run
# Wait for crash
(lldb) bt
(lldb) frame variable
(lldb) frame select 3

Debug Logging

Using RUSTC_LOG

The compiler has extensive debug logging:

Basic Logging
Query Logging
Filtering

# Enable all debug logs
RUSTC_LOG=debug rustc file.rs

# Specific module
RUSTC_LOG=rustc_middle::ty=debug rustc file.rs

# Multiple modules
RUSTC_LOG=rustc_borrowck=debug,rustc_mir=trace rustc file.rs

# Log all queries
RUSTC_LOG=rustc_query_impl=debug rustc file.rs

# Log specific query
RUSTC_LOG=rustc_query_impl::queries::type_of=trace rustc file.rs

# With timestamps
RUSTC_LOG_COLOR=always RUSTC_LOG=debug rustc file.rs

# Only errors and warnings
RUSTC_LOG=warn rustc file.rs

# Everything except noise
RUSTC_LOG=debug,rustc_metadata=info rustc file.rs

# Save to file
RUSTC_LOG=debug rustc file.rs 2>&1 | tee debug.log

Compiler Flags for Debugging

# Dump HIR
rustc -Z unpretty=hir-tree file.rs

# Dump MIR
rustc -Z dump-mir=all file.rs

# Dump MIR for specific function
rustc -Z dump-mir=main file.rs

# Dump LLVM IR
rustc --emit=llvm-ir file.rs

Debugging Specific Components

Borrow Checker

NLL Debugging
Borrow Errors

# Dump NLL facts
rustc -Z dump-mir=all -Z dump-mir-dir=mir_dump file.rs

# Polonius facts
rustc -Z polonius -Z dump-mir=all file.rs

# Borrow check logging
RUSTC_LOG=rustc_borrowck=debug rustc file.rs

# Explain borrow check errors
rustc -Z teach file.rs

# Verbose borrow check
RUSTC_LOG=rustc_borrowck::diagnostics=trace rustc file.rs

Type Checker

# Type checking logs
RUSTC_LOG=rustc_hir_analysis=debug rustc file.rs

# Trait resolution
RUSTC_LOG=rustc_trait_selection=debug rustc file.rs

# Print trait solver tree
rustc -Z dump-solver-proof-tree file.rs

Code Generation

LLVM Backend
MIR Optimization
Codegen Units

# LLVM module dump
rustc --emit=llvm-ir -C llvm-args=-print-after-all file.rs

# LLVM optimization remarks
rustc -C llvm-args="-pass-remarks=inline" file.rs

# Disable optimizations
rustc -C opt-level=0 file.rs

# Dump MIR before and after optimizations
rustc -Z dump-mir=all -Z dump-mir-dir=mir file.rs

# Disable MIR optimizations
rustc -Z mir-opt-level=0 file.rs

# Specific MIR pass
RUSTC_LOG=rustc_mir_transform=debug rustc file.rs

# Single codegen unit
rustc -C codegen-units=1 file.rs

# List codegen units
rustc -Z print-mono-items=lazy file.rs

Debugging Tests

Running Individual Tests

Find the failing test

# Run UI tests
./x.py test tests/ui

# Find specific test
./x.py test tests/ui --test-args my-test

Run with verbose output

# Verbose test output
./x.py test tests/ui/my-test.rs --test-args --verbose

# Show compiler output
./x.py test tests/ui/my-test.rs --test-args --nocapture

Debug the test

# Get the rustc command from test output
# Then run it manually:
RUST_BACKTRACE=1 ./build/stage1/bin/rustc tests/ui/my-test.rs

# Or under GDB:
gdb --args ./build/stage1/bin/rustc tests/ui/my-test.rs

Updating Test Expectations

# Update stderr expectations
./x.py test tests/ui --bless

# Update specific test
./x.py test tests/ui/my-test.rs --bless

Debugging Miscompilations

Isolating the Problem

Create minimal reproducer

Reduce the code to the smallest example that shows the bug:

# Use cargo-minimize or manual reduction
cargo install cargo-minimize
cargo minimize --bin my-bin

Identify optimization level

# Test different optimization levels
rustc -C opt-level=0 file.rs  # No optimization
rustc -C opt-level=1 file.rs  # Basic
rustc -C opt-level=2 file.rs  # Default
rustc -C opt-level=3 file.rs  # Aggressive

Check MIR vs LLVM

# Dump MIR
rustc -Z dump-mir=all file.rs

# Dump LLVM IR
rustc --emit=llvm-ir file.rs

# Compare optimized vs unoptimized
rustc -C opt-level=0 --emit=llvm-ir -o unopt.ll file.rs
rustc -C opt-level=2 --emit=llvm-ir -o opt.ll file.rs
diff unopt.ll opt.ll

Bisect to find the culprit

# Use cargo-bisect-rustc to find regression
cargo install cargo-bisect-rustc
cargo bisect-rustc --start 2024-01-01 --end 2024-06-01

Tools for Miscompilations

Miri

Detect undefined behavior:

cargo +nightly miri test
cargo +nightly miri run

Sanitizers

Detect memory errors:

RUSTFLAGS="-Z sanitizer=address" cargo build
RUSTFLAGS="-Z sanitizer=memory" cargo build

Valgrind

Memory debugging:

valgrind --leak-check=full ./target/debug/binary

cargo-bisect-rustc

Find regressions:

cargo bisect-rustc

Advanced Debugging Techniques

Using LLVM Tools

llvm-dis
llvm-opt
llvm-objdump

# Disassemble bitcode
llvm-dis output.bc -o output.ll

# Run specific LLVM passes
opt -O2 input.ll -o output.ll
opt -print-after-all input.ll 2>&1 | less

# Disassemble binary
llvm-objdump -d binary
llvm-objdump -S binary  # With source

Query System Debugging

# Track query execution
RUSTC_LOG=rustc_query_system::query::plumbing=debug rustc file.rs

# Query dependencies
rustc -Z dump-dep-graph file.rs

Memory Debugging

Massif (Heap Profiler)
DHAT (Heap Analyzer)

# Profile heap usage
valgrind --tool=massif ./build/stage1/bin/rustc file.rs

# Visualize with massif-visualizer
massif-visualizer massif.out.12345

# Analyze heap allocations
valgrind --tool=dhat ./build/stage1/bin/rustc file.rs

Common Debugging Scenarios

Debugging a panic in the compiler

# Set backtrace
export RUST_BACKTRACE=1

# Enable panic logging
export RUSTC_LOG=debug

# Run under debugger
gdb --args ./build/stage1/bin/rustc file.rs
(gdb) break rust_panic
(gdb) run

Debugging why compilation is slow

# Time each pass
rustc -Z time-passes file.rs

# Self-profile
rustc -Z self-profile file.rs

# Analyze with measureme
cargo install measureme
crox file-pid.mm_profdata

Debugging a type error

# Enable type logging
RUSTC_LOG=rustc_hir_analysis::check=debug rustc file.rs

# Dump type information
rustc -Z print-type-sizes file.rs

Debugging a borrow check error

# Enable borrow check logging
RUSTC_LOG=rustc_borrowck=debug rustc file.rs

# Dump NLL facts
rustc -Z dump-mir=all file.rs

Debugging Checklist

Resources

Debugging Chapter

Rustc dev guide debugging section

GDB Tutorial

GDB documentation

LLDB Tutorial

LLDB tutorial

Zulip #t-compiler

Ask for help on Zulip

Next Steps

Profiling

Learn about profiling the compiler

Compiler Development

Return to compiler development guide

Library Development Guide

Profiling and Performance

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Contributing Guide

Development

​Types of Compiler Issues

ICE (Internal Compiler Error)

Miscompilation

Compile-time Error

Performance Regression

​Setting Up for Debugging

​Debugging Internal Compiler Errors (ICEs)

​Reproducing an ICE

​Using GDB with the Compiler

​Debugging with LLDB

​Debug Logging

​Using RUSTC_LOG

​Compiler Flags for Debugging

​Debugging Specific Components

​Borrow Checker

​Type Checker

​Code Generation

​Debugging Tests

​Running Individual Tests

​Updating Test Expectations

​Debugging Miscompilations

​Isolating the Problem

​Tools for Miscompilations