RFC: Rust Symbol Mangling (v0)

[michaelwoerister]

Rendered
Tracking issue
Reference Implementation
Pre-RFC

Summary

This RFC proposes a new mangling scheme that describes what the symbol names generated by the Rust compiler. This new scheme has a number of advantages over the existing one which has grown over time without a clear direction. The new scheme is consistent, does not depend on compiler internals, and the information it stores in symbol names can be decoded again which provides an improved experience for users of external tools that work with Rust symbol names. The new scheme is based on the name mangling scheme from the [Itanium C++ ABI][itanium-mangling].

Motivation

Due to its ad-hoc nature, the compiler's current name mangling scheme has a
number of drawbacks:

• It depends on compiler internals and its results cannot be replicated by another compiler implementation or external tool.
• Information about generic parameters and other things is lost in the mangling process. One cannot extract the type arguments of a monomorphized function from its symbol name.
• The current scheme is inconsistent: most paths use Itanium style encoding, but some don't.
• The symbol names it generates can contain . characters which is not generally supported on all platforms. [1][2][3]

The proposed scheme solves these problems:

• It is defined in terms of the language, not in terms of compiler data-structures that can change at any given point in time.
• It encodes information about generic parameters in a reversible way.
• It has a consistent definition that does not rely on pretty-printing certain language constructs.
• It generates symbols that only consist of the characters A-Z, a-z, 0-9, and _.

This should make it easier for third party tools to work with Rust binaries.

[eddyb]

For the record, I'll be starting the compiler implementation/integration work ASAP, to get this RFC in rustc nightly, and later on, in other tools (such as GDB, LLDB, etc.).

Doing this at the same time as the RFC will give us the ability to collect data at scale, and figure out edge cases and performance tradeoffs we might miss otherwise.

[eddyb]

In the interest of keeping this RFC sufficiently detached from current implementation details, can we use some more general placeholder notation, such as <impl>, instead of {{impl}}?

[michaelwoerister]

It's just an example of how not to do it. The {{xyz}} notation is meant to remind of what some templating engines use, not what the compiler did at some point. But I can change it to <impl> if you prefer that.

[eddyb]

Similarly here, we should avoid braces. What does C++ do for its lambdas?

[michaelwoerister]

GCC uses something with braces and indices too:

int square(int num) {
    auto foo = [num]() -> int { return num * num; };
    return foo();
}

The closure is demangled as square(int)::{lambda()#1}::operator()() const
(see https://godbolt.org/z/TaXWCe)

[eddyb]

Do debuggers work well with it? If so, how? Can we do some tests to see what works and what doesn't?

[michaelwoerister]

I assume that debuggers treat lambdas as regular operator() methods. What kind of tests did you have in mind?

[eddyb]

I'm referring to the problems @m4b mentions in #2603 (comment), regarding debuggers not being able to let you refer to symbol names that contain { (or perhaps only {{?).

If we change {{closure}} in the compiler with some other notation, we can see how well GDB and LLDB interact with the symbol names.

Although it's possible debuggers only handle such symbol names when they come from a mangling, which would mean debuggers should just pick a demangling that works for them, right?

[eddyb]

Wouldn't that include all the statics and functions? Seems a bit excessive.

[michaelwoerister]

It does, but I don't think there's a way around it. Otherwise you get conflicts for examples like:

fn foo() {
    fn bar() {}
}

mod foo {
    fn bar() {}
}

Note though that this is only for "lossless" demanglings. For most user-facing demanglings, like in debuggers or backtraces, the suffix can just be omitted. I suggest that demanglers support lossless or verbose option that is usually set to false.

[eddyb]

Clone::clone can't take type parameters.

[michaelwoerister]

Yeah, I'll come up with a better example.

[eternaleye]

Borrow<T> could work well

EDIT: Wait, no, missed you wanted the type param on the method.

[eddyb]

Shouldn't these mention type parameters?

[arielb1]

Sure enough. You want to be able to distinguish between these 2 cases (this code compiles today):

struct Foo<U,V>(U,V);

impl<U: Fn()> Foo<U, u32> {
    fn foo() {}
}

impl<U: Fn()> Foo<u32, U> {
    fn foo() {}
}

[michaelwoerister]

OK, we'll have to take care of that then.

[michaelwoerister]

I had a little chat with @nikomatsakis about this yesterday and the outcome was that:

• we always should encode type parameters in paths like this one and
• we should also always encode parameter bounds in some form because there is no way to find out if they are needed for disambiguation without looking at other impls -- which we want to avoid. The bounds could be encoded in a numeric disambiguator though.

The consequences this has on symbol syntax should be small. We just have to find the best spot for adding parameter bounds.

[eddyb]

we should also always encode parameter bounds

I still think that's not ideal, and I'd prefer having a disambiguated path to the impl and/or to the type parameters (either of which would be hidden in the non-verbose mode).

[michaelwoerister]

Can you give your reasons why having the path to the impl is better than encoding the bounds? I assume because it's less complicated?

[eddyb]

Similarly here, with the extraneous <U>.

[rpjohnst]

Bringing this comment up again: https://internals.rust-lang.org/t/pre-rfc-a-new-symbol-mangling-scheme/8501/4?u=rpjohnst. Would it make sense to move the trait's self type into its argument list? It reorders things from how they are displayed in error messages, but simplifies the grammar a bit.

[eddyb]

I agree, we already treat <X as Trait<Y, Z>> as sugar for Trait applied with [X, Y, Z], there's no real reason to have it separate here.

[michaelwoerister]

I didn't forget about the suggestion but unfortunately, while implementing it, it turned out that it makes the demangler a lot more complicated -- at least if we want to stick to the <X as Trait> demangling. If we mangle trait methods as foo::bar::Trait<SelfType, X, Y, Z>::method, the demangler cannot know that it is dealing with a trait method when it starts demangling the path at foo. It could only discover that when it gets to Trait and would then have to rewind and store the already generated output (foo::bar::Trait) on the heap, demangle the self-type, then copy back the trait path and continue demangling the trait's type arguments. It can only know that Trait is a trait if we put a special marker on the identifier, so traits would again be special cased. As a consequence, I thought, if we have to special case traits one way are the other, we can as well do it in a way that allows for efficient demangling and doesn't need the extra kind of logic.

The situation would be different if we actually wanted to demangle trait methods to foo::bar::Trait<SelfType, X, Y, Z>::method. But I don't think we want to do that, right?

[eddyb]

Oh, wait, an on-the-fly demangler needs to have everything in demangled order, right.
Is this only needed for <X as Trait<Y, Z>>, or are there other "out of order" constructs?

[michaelwoerister]

As far as I can tell <X as Trait<Y, Z>> is the only case.

[arielb1]

Theoretically, you could also have stuff like this:

struct Foo<T>(T);

impl<T> Foo<T> where T: FnOnce() -> u32 {
    fn foo() {
        static ABC: u32 = 0;
    }
}

impl<T> Foo<T> where T: FnOnce() -> f32 {
    fn foo() {
        static ABC: u32 = 0;
    }
}

It is not supported by today's coherence, but it might be supported someday in the future.

I suppose that for now it is enough to also let this case use the <Foo<T>>'N format for either or both impls.

[michaelwoerister]

Yes, the RFC proposes to use a numeric disambiguator for keeping the two impls apart -- until specialization is finalized, at which point the disambiguator would be replaced with something more human-readable, which probably amounts to an encoding of the where clauses.

[arielb1]

That code does not depend on specialization, as there is no overlap.

[michaelwoerister]

@m4b Let's discuss closures a bit. I want to get them fixed. The RFC proposes to demangle them as some::function::{closure}[3] where [3] means that it is there fourth closure within some::function. Am I correct in assuming that you don't find this readable enough and would prefer something like some::function::{closure at line 77}?

[michaelwoerister]

@bstrie Yes, I'm aware of these problems. I personally prefer to the indexing approach (I find C++'s {lambda()#1} notation quite appealing, actually). I'm still interested in hearing alternative suggestions.

[zackw]

I’m an end-user interested in cross-language interop, and I have some experience with implementation of the Itanium C++ ABI. I would like to provide a few notes on this RFC.

1. As I said in the pre-RFC discussion, I think the Rust mangling should be intentionally incompatible with all known C++ ABIs, because the linker should not resolve a call to extern void foo(int, int) from C++ as targeting a Rust function with the signature pub fn foo(i32, i32) -> () unless that function was specifically declared as extern "C++".

   The current proposal achieves this by using _R as the prefix for mangled names, and that’s plenty good enough, but I think it should be written down as a concrete reason not to go for C++ ABI compatibility.

2. I think functions’ mangled names should always encode the full type signature of the function, even though Rust does not have function overloading. This would be a safety feature. It would trap cross-crate type mismatches between caller and callee at link time. (I have the impression this is supposed to be one of the purposes of crate disambiguators, but I do not trust them to do the job completely. Also, putting the type signatures into the mangled names would enable the linker to give better error messages.)

3. The Unicode handling is underspecified. RFC 2457 and its stabilization issue indicate that issues like normalization, the subset of characters allowed in identifiers, etc. are being handled at the language level, but a demangler needs to know what to do with arbitrary nonsense produced by tools other than a correctly-implemented Rust compiler: e.g. a buggy Rust compiler, the extern "Rust" support in some other compiler (also arguably buggy, but still), and people writing assembly language by hand.

   I think this RFC should say that demanglers should check whether the result of decoding the punycode is a valid Rust identifier according to the rules that end up getting stabilized in the RFC 2457 process, and display the punycode string if it doesn’t qualify. For instance, _RN15mycrate_4a3b56d9godel_fgdu6escher4bachVE, which uses the NFD encoding of gödel, should be decoded as mycrate[4a3b56d]::[godel_fgd]::escher::bach.

   Also, please add a cross-reference to RFC 2457.

4. In my opinion, the possibility of Unicode sequences that are not valid Rust identifiers being shoved into a mangled name by a tool that doesn’t follow all of the appropriate rules is a strong reason not to allow the use of raw UTF-8 in mangled names.

5. ABI markers are simultaneously under- and overspecified. There’s what appears to be an exhaustive list of codes for calling conventions that are currently supported on any architecture, including several mutually exclusive groups, e.g. you’ll never need "m" and "i" on the same computer, I hope. And at the same time it doesn’t say what it means if the ABI marker is not present at all. I would suggest cutting this down drastically, e.g.

   // If the <abi> is not present, the function uses the usual Rust calling
   // convention for this architecture and OS.
   <abi> = "K" (
       "c" |     // Usual C calling convention for this arch and OS
       "j" |     // Rust intrinsic calling convention
       "i" |     // Interrupt handler for this architecture
       // Other single-lowercase-letter codes may be defined by each
       // architecture and OS; for instance, "s" could mean the Win32
       // "stdcall" convention.
   )
   

   Also, please add cross-references to the exact definitions of each of the calling conventions that are referenced by name.

6. It appears to me that the N prefix and E suffix on <abs-path> are unnecessary, and the E suffix may also be unnecessary in several of the other places where it’s used. The Itanium C++ ABI only uses N ... E notation when it’s necessary to disambiguate “nested names,” e.g. when a name needs to be encoded as part of a template parameter.

Thanks for listening.

[eddyb]

It would trap cross-crate type mismatches between caller and callee at link time.

This can't handle all possible ABI-impacting details, only shallow ones, and on top of that, rustc already has better detection of incompatible crates, solving most, if not all, linking concerns.
C++ has this issue because of header files, but Rust doesn't have any equivalent features.

So to me, it seems like this would just increase symbol name size, without many (any?) benefits.

[Wilfred]

Spelling pedantry: I think this should be predict

[rocallahan]

What justifies the additional complexity of the "does not itself add any new information" rule for node equivalence? Is this a microoptimization or does it make things easier to implement?

[jsgf]

This all seems a bit arbitrary. Given that in principle there could be an unbounded number of ABIs, it seems like we should splurge on using a real string here rather than a single character. I'm also going to guess that these will be relatively rare, so space isn't a consideration?

[eddyb]

I'd also favor encoding the ABI "string" (ideally as an identifier, replacing - with _, etc.)

This makes me wonder if Rust should've used extern(C) fn syntax instead of extern "C" fn, but it's too late now.

[michaelwoerister]

I responded in #2603 (comment).

[jsgf]

Is this only a single digit? What if more than 62 things need disambiguation? I can imagine such things arising in generated code.
I'd propose {<base-62-digit>}.

[eddyb]

Meta-nit: in a post-regex world, I find EBNF somewhat unintuitive: it took me a while to even notice that by {...} you meant "replace ? with *", initially I thought you were talking about "{" ... "}".

cc @Centril (who started using "lyg" syntax instead)

[michaelwoerister]

Whoops, that's just a mistake in the grammar. It should be {<base-62-digit>} indeed.

[michaelwoerister]

Yeah, I don't really care which notation is used :P

[jsgf]

Likewise seems safer to make this {...} (notwithstanding other comments about compression).

[jsgf]

I'd be very tempted to omit this kind of ad-hoc compression scheme in favour of using a standard algorithm like zstd (say, zstd with a well-defined domain-specific dictionary). (cc @Cyan4973 - are there any examples of using zstd for per-symbol compression?)

Historically C++ demanglers have been very fragile, and I suspect a big part of that is due to the implementation complexity of the Itanium ABI compression mechanism.

Aside from the implementation issues, because this is so coupled with the definition of the mangling scheme itself, it means that any future evolution of mangling needs to also take compression into account. Using a completely separate compression layer makes this a non-issue. The other nice thing about making compression largely isolated from the rest of the encoding is that it means it can be added in a second pass as an extension once we have some experience with uncompressed mangling - maybe it wouldn't be so bad?

The main problem with using an external library is that any Rust demangler introduces another dependency. This is particularly worth considering when integrating Rust demangler support into other tools like binutils/llvm/perf/valgrind/etc.

[eddyb]

I believe we can come up with a simple compression scheme (i.e. refer back to the byte position where the first occurrence of something was encoded).

This would allow a demangler implementation to have 0 external dependencies, and the specification would also not implicitly depend on another standard.

It might also behave better than zstd given the limitation of [a-zA-Z0-9_] (which makes bit streams less appealing), and it has the advantage that any path component name is guaranteed to show up in clear.

However, I would not be opposed to at least having a compiler mangling option which disregards the [a-zA-Z0-9_] limitation and which does the best compression it can, for use in situations where that might be advantageous (although at that point, you might be better off with symbol names just, say, hashes, and keep everything else in split debuginfo, and/or an ad-hoc mapping from hashes to symbol names).

Oh and should definitely gather data on all compression schemes we can think of (that are not too painful to implement), before we accept the RFC!

[michaelwoerister]

I've listed the zstd option as point 5 in Rational and Alternatives. I would be interested in seeing how zstd performs. But it does come with some real downsides:

• Every demangler would have to support zstd. That's another dependency that not everyone might want to pull in.
• The specification of the mangling scheme would depend on the specification of zstd. I see that there's an IETF RFC for it. That's good. But it's still rather heavyweight.
• Mangled symbol would not retain any human-readability at all.

I think one of the next steps would be to collect a body of symbol names for testing different compression schemes.

[michaelwoerister]

@jsgf I do acknowledge, btw, that an AST-independent compression scheme is clearly beneficial when it comes to evolving the grammar.

[jsgf]

@michaelwoerister Yes, I think extra dependencies and non-readability reasonable counter-arguments to using zstd.

(waving hands wildly) I'm assuming that we wouldn't bother compressing small symbols, so they would remain directly readable, and large symbols with any compression scheme would be such a soup that even if the compression scheme leaves some parts "in the clear" they're still not directly readable in any practical sense.

[nagisa]

zstd and similar bit-compression algorithms are not very suitable for compressing short strings at all, and encoding the compressed bit-stream into [a-zA-Z0-9] (required by toolchains) would lose most of the compression again.

[jsgf]

This is moot if compression (of any kind) is applied as well.

[michaelwoerister]

You mean because compressed names are unreadable in any case?

[jsgf]

Right.

[jsgf]

Given that impls can appear anywhere within the crate, would the path be to the impl itself, or to the type being impled?

Do we need distinguish between different impls, or just impls with different constraints?

Given these questions, I think the proposal below to ignore impls themselves makes sense.

[eddyb]

The type being impled doesn't have to be a path, it can be e.g. [u8], so I think the safest thing to do would be to have both a path to the impl and the full type (and optionally trait) the impl is for.

[michaelwoerister]

The PR proposes to not include the path of the impl at all. @eddyb, you would rather demangle symbols to something like my_crate::foo::impl<u32, char>::some_method?

[eddyb]

I don't understand what Self and the Trait are in that example.
What I'm thinking is mangling the equivalent information of e.g. my_crate::foo::impl'17<my_crate::foo::S as my_crate::Trait>::some_method, demangling back to that only in verbose mode, but only showing <my_crate::foo::S as my_crate::Trait>::some_method in the "user-friendly" mode.

[michaelwoerister]

OK, yes that's what I thought you meant.

[jsgf]

@zackw +1 on most of your points, but I think for 2. to matter it would mean that Rust compilation would have to change a lot. In practice with Rust code, one never sees linker errors for Rust symbols.

[Centril]

Huzzah! This RFC has been merged!

Tracking issue: rust-lang/rust#60705

[michaelwoerister]

@vitiral, thanks a lot! :)
Lot's of credit also goes to @eddyb who put a ton of work into this and really improved upon my initial proposal.

[Serentty]

So, related to the ABI discussion above, I have a question. Using this name mangling scheme, should it be possible to link Rust code to other Rust code while going through the existing C ABI, while preserving modules and other niceties?

[eddyb]

@Serentty Nope, and the mangling doesn't matter at all there - if you have a working module system, that means you need extra information (as found in .rmeta / .rlib) in the first place, so any mangling will work (as it will have everything it needs today).

If you want to link plain object files and still retain high-level features... well, that's both not planned but also technically implausible (without coming up with some special cut-down version of those features).

[Serentty]

So the RLIB files serve as Rust headers, essentially? That sounds reasonable.

[eddyb]

Updating the RFC's "v2" to "v0" as per rust-lang/rust#57967 (comment) (should've done it back then).

[sivizius]

Change very first ›Rendered‹-Link to https://github.com/rust-lang/rfcs/blob/master/text/2603-rust-symbol-name-mangling-v0.md