Doc usage FAQ
Short answer: set the RUST_LOG environment variable to the name of your source file, sans extension.
rustc hello.rs
export RUST_LOG=hello
./helloLong answer: RUST_LOG takes a 'logging spec' that consists of a comma-separated list of paths, where a path consists of the crate name and sequence of module names, each separated by double-colons. For standalone .rs files the crate is implicitly named after the source file, so in the above example we were setting RUST_LOG to the name of the hello crate. Multiple paths can be combined to control the exact logging you want to see. For example, when debugging linking in the compiler you might set RUST_LOG=rustc::metadata::creader,rustc::util::filesearch,rustc::back::rpath
Also try setting RUST_LOG to ::help. For a full description see the language reference.
What does it mean when a program exits with leaked memory in rust main loop (2 objects)' failed, rt/memory_region.cpp:99 2 objects?
This message indicates a memory leak, and is mostly likely to happen on rarely exercised failure paths. Note that failure unwinding is not yet implemented on windows so this is expected. If you see this on Linux or Mac it's a compiler bug; please report it.
This happens when the snapshot compiler failed to download correctly so doesn't have the correct hash. To fix it just delete the mentioned tar.bz2 file from the dl folder in your build directory and run make again.
Short answer: your gdb is too old to understand our hip new stacks. Upgrade to a newer version (7.3.1 is known to work).
Long answer: Rust uses 'spaghetti stacks' (a linked list of stacks) to allow tasks to start very small but recurse arbitrarily deep when necessary. As a result, new frames don't always decrease the stack pointer like gdb expects but instead may jump around the heap to different stack segments. Newer versions of gdb recognize that the special function called __morestack may change the stack pointer to a different stack.