rustc_metadata

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Finds crate binaries and loads their metadata

Might I be the first to welcome you to a world of platform differences, version requirements, dependency graphs, conflicting desires, and fun! This is the major guts (along with metadata::creader) of the compiler for loading crates and resolving dependencies. Let’s take a tour!

§The problem

Each invocation of the compiler is immediately concerned with one primary problem, to connect a set of crates to resolved crates on the filesystem. Concretely speaking, the compiler follows roughly these steps to get here:

  1. Discover a set of extern crate statements.
  2. Transform these directives into crate names. If the directive does not have an explicit name, then the identifier is the name.
  3. For each of these crate names, find a corresponding crate on the filesystem.

Sounds easy, right? Let’s walk into some of the nuances.

§Transitive Dependencies

Let’s say we’ve got three crates: A, B, and C. A depends on B, and B depends on C. When we’re compiling A, we primarily need to find and locate B, but we also end up needing to find and locate C as well.

The reason for this is that any of B’s types could be composed of C’s types, any function in B could return a type from C, etc. To be able to guarantee that we can always type-check/translate any function, we have to have complete knowledge of the whole ecosystem, not just our immediate dependencies.

So now as part of the “find a corresponding crate on the filesystem” step above, this involves also finding all crates for all upstream dependencies. This includes all dependencies transitively.

§Rlibs and Dylibs

The compiler has two forms of intermediate dependencies. These are dubbed rlibs and dylibs for the static and dynamic variants, respectively. An rlib is a rustc-defined file format (currently just an ar archive) while a dylib is a platform-defined dynamic library. Each library has a metadata somewhere inside of it.

A third kind of dependency is an rmeta file. These are metadata files and do not contain any code, etc. To a first approximation, these are treated in the same way as rlibs. Where there is both an rlib and an rmeta file, the rlib gets priority (even if the rmeta file is newer). An rmeta file is only useful for checking a downstream crate, attempting to link one will cause an error.

When translating a crate name to a crate on the filesystem, we all of a sudden need to take into account both rlibs and dylibs! Linkage later on may use either one of these files, as each has their pros/cons. The job of crate loading is to discover what’s possible by finding all candidates.

Most parts of this loading systems keep the dylib/rlib as just separate variables.

§Where to look?

We can’t exactly scan your whole hard drive when looking for dependencies, so we need to places to look. Currently the compiler will implicitly add the target lib search path ($prefix/lib/rustlib/$target/lib) to any compilation, and otherwise all -L flags are added to the search paths.

§What criterion to select on?

This is a pretty tricky area of loading crates. Given a file, how do we know whether it’s the right crate? Currently, the rules look along these lines:

  1. Does the filename match an rlib/dylib pattern? That is to say, does the filename have the right prefix/suffix?
  2. Does the filename have the right prefix for the crate name being queried? This is filtering for files like libfoo*.rlib and such. If the crate we’re looking for was originally compiled with -C extra-filename, the extra filename will be included in this prefix to reduce reading metadata from crates that would otherwise share our prefix.
  3. Is the file an actual rust library? This is done by loading the metadata from the library and making sure it’s actually there.
  4. Does the name in the metadata agree with the name of the library?
  5. Does the target in the metadata agree with the current target?
  6. Does the SVH match? (more on this later)

If the file answers yes to all these questions, then the file is considered as being candidate for being accepted. It is illegal to have more than two candidates as the compiler has no method by which to resolve this conflict. Additionally, rlib/dylib candidates are considered separately.

After all this has happened, we have 1 or two files as candidates. These represent the rlib/dylib file found for a library, and they’re returned as being found.

§What about versions?

A lot of effort has been put forth to remove versioning from the compiler. There have been forays in the past to have versioning baked in, but it was largely always deemed insufficient to the point that it was recognized that it’s probably something the compiler shouldn’t do anyway due to its complicated nature and the state of the half-baked solutions.

With a departure from versioning, the primary criterion for loading crates is just the name of a crate. If we stopped here, it would imply that you could never link two crates of the same name from different sources together, which is clearly a bad state to be in.

To resolve this problem, we come to the next section!

§Expert Mode

A number of flags have been added to the compiler to solve the “version problem” in the previous section, as well as generally enabling more powerful usage of the crate loading system of the compiler. The goal of these flags and options are to enable third-party tools to drive the compiler with prior knowledge about how the world should look.

§The --extern flag

The compiler accepts a flag of this form a number of times:

--extern crate-name=path/to/the/crate.rlib

This flag is basically the following letter to the compiler:

Dear rustc,

When you are attempting to load the immediate dependency crate-name, I would like you to assume that the library is located at path/to/the/crate.rlib, and look nowhere else. Also, please do not assume that the path I specified has the name crate-name.

This flag basically overrides most matching logic except for validating that the file is indeed a rust library. The same crate-name can be specified twice to specify the rlib/dylib pair.

§Enabling “multiple versions”

This basically boils down to the ability to specify arbitrary packages to the compiler. For example, if crate A wanted to use Bv1 and Bv2, then it would look something like:

ⓘ
extern crate b1;
extern crate b2;

fn main() {}

and the compiler would be invoked as:

rustc a.rs --extern b1=path/to/libb1.rlib --extern b2=path/to/libb2.rlib

In this scenario there are two crates named b and the compiler must be manually driven to be informed where each crate is.

§Frobbing symbols

One of the immediate problems with linking the same library together twice in the same problem is dealing with duplicate symbols. The primary way to deal with this in rustc is to add hashes to the end of each symbol.

In order to force hashes to change between versions of a library, if desired, the compiler exposes an option -C metadata=foo, which is used to initially seed each symbol hash. The string foo is prepended to each string-to-hash to ensure that symbols change over time.

§Loading transitive dependencies

Dealing with same-named-but-distinct crates is not just a local problem, but one that also needs to be dealt with for transitive dependencies. Note that in the letter above --extern flags only apply to the local set of dependencies, not the upstream transitive dependencies. Consider this dependency graph:

A.1   A.2
|     |
|     |
B     C
 \   /
  \ /
   D

In this scenario, when we compile D, we need to be able to distinctly resolve A.1 and A.2, but an --extern flag cannot apply to these transitive dependencies.

Note that the key idea here is that B and C are both already compiled. That is, they have already resolved their dependencies. Due to unrelated technical reasons, when a library is compiled, it is only compatible with the exact same version of the upstream libraries it was compiled against. We use the “Strict Version Hash” to identify the exact copy of an upstream library.

With this knowledge, we know that B and C will depend on A with different SVH values, so we crawl the normal -L paths looking for liba*.rlib and filter based on the contained SVH.

In the end, this ends up not needing --extern to specify upstream transitive dependencies.

§Wrapping up

That’s the general overview of loading crates in the compiler, but it’s by no means all of the necessary details. Take a look at the rest of metadata::locator or metadata::creader for all the juicy details!

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