For the purposes of this guide, fuzzing is any testing methodology that involves compiling a wide variety of programs in an attempt to uncover bugs in rustc. Fuzzing is often used to find internal compiler errors (ICEs). Fuzzing can be beneficial, because it can find bugs before users run into them and provide small, self-contained programs that make the bug easier to track down. However, some common mistakes can reduce the helpfulness of fuzzing and end up making contributors' lives harder. To maximize your positive impact on the Rust project, please read this guide before reporting fuzzer-generated bugs!


In a nutshell

Please do:

  • Ensure the bug is still present on the latest nightly rustc
  • Include a reasonably minimal, standalone example along with any bug report
  • Include all of the information requested in the bug report template
  • Search for existing reports with the same message and query stack
  • Format the test case with rustfmt, if it maintains the bug
  • Indicate that the bug was found by fuzzing

Please don't:

  • Don't report lots of bugs that use internal features, including but not limited to custom_mir, lang_items, no_core, and rustc_attrs.
  • Don't seed your fuzzer with inputs that are known to crash rustc (details below).


If you're not sure whether or not an ICE is a duplicate of one that's already been reported, please go ahead and report it and link to issues you think might be related. In general, ICEs on the same line but with different query stacks are usually distinct bugs. For example, #109020 and #109129 had similar error messages:

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <[closure@src/main.rs:36:25: 36:28] as std::ops::FnOnce<(Emplacable<()>,)>>::Output, maybe try to call `try_normalize_erasing_regions` instead
error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize <() as Project>::Assoc, maybe try to call `try_normalize_erasing_regions` instead

but different query stacks:

query stack during panic:
#0 [fn_abi_of_instance] computing call ABI of `<[closure@src/main.rs:36:25: 36:28] as core::ops::function::FnOnce<(Emplacable<()>,)>>::call_once - shim(vtable)`
end of query stack
query stack during panic:
#0 [check_mod_attrs] checking attributes in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack

Building a corpus

When building a corpus, be sure to avoid collecting tests that are already known to crash rustc. A fuzzer that is seeded with such tests is more likely to generate bugs with the same root cause, wasting everyone's time. The simplest way to avoid this is to loop over each file in the corpus, see if it causes an ICE, and remove it if so.

To build a corpus, you may want to use:

  • The rustc/rust-analyzer/clippy test suites (or even source code) --- though avoid tests that are already known to cause failures, which often begin with comments like // failure-status: 101 or // known-bug: #NNN.
  • The already-fixed ICEs in Glacier --- though avoid the unfixed ones in ices/!

Extra credit

Here are a few things you can do to help the Rust project after filing an ICE.

  • Bisect the bug to figure out when it was introduced
  • Fix "distractions": problems with the test case that don't contribute to triggering the ICE, such as syntax errors or borrow-checking errors
  • Minimize the test case (see below)
  • Add the minimal test case to Glacier


It is helpful to carefully minimize the fuzzer-generated input. When minimizing, be careful to preserve the original error, and avoid introducing distracting problems such as syntax, type-checking, or borrow-checking errors.

There are some tools that can help with minimization. If you're not sure how to avoid introducing syntax, type-, and borrow-checking errors while using these tools, post both the complete and minimized test cases. Generally, syntax-aware tools give the best results in the least amount of time. treereduce-rust and picireny are syntax-aware. halfempty is not, but is generally a high-quality tool.

Effective fuzzing

When fuzzing rustc, you may want to avoid generating machine code, since this is mostly done by LLVM. Try --emit=mir instead.

A variety of compiler flags can uncover different issues. -Zmir-opt-level=4 will turn on MIR optimization passes that are not run by default, potentially uncovering interesting bugs. -Zvalidate-mir can help uncover such bugs.

If you're fuzzing a compiler you built, you may want to build it with -C target-cpu=native or even PGO/BOLT to squeeze out a few more executions per second. Of course, it's best to try multiple build configurations and see what actually results in superior throughput.

You may want to build rustc from source with debug assertions to find additional bugs, though this is a trade-off: it can slow down fuzzing by requiring extra work for every execution. To enable debug assertions, add this to config.toml when compiling rustc:

debug-assertions = true

ICEs that require debug assertions to reproduce should be tagged requires-debug-assertions.

Existing projects

  • fuzz-rustc demonstrates how to fuzz rustc with libfuzzer
  • icemaker runs rustc and other tools on a large number of source files with a variety of flags to catch ICEs
  • tree-splicer generates new source files by combining existing ones while maintaining correct syntax