compiler/rustc_symbol_mangling/src/lib.rs

//! The Rust Linkage Model and Symbol Names
//! =======================================
//!
//! The semantic model of Rust linkage is, broadly, that "there's no global
//! namespace" between crates. Our aim is to preserve the illusion of this
//! model despite the fact that it's not *quite* possible to implement on
//! modern linkers. We initially didn't use system linkers at all, but have
//! been convinced of their utility.
//!
//! There are a few issues to handle:
//!
//!  - Linkers operate on a flat namespace, so we have to flatten names.
//!    We do this using the C++ namespace-mangling technique. Foo::bar
//!    symbols and such.
//!
//!  - Symbols for distinct items with the same *name* need to get different
//!    linkage-names. Examples of this are monomorphizations of functions or
//!    items within anonymous scopes that end up having the same path.
//!
//!  - Symbols in different crates but with same names "within" the crate need
//!    to get different linkage-names.
//!
//!  - Symbol names should be deterministic: Two consecutive runs of the
//!    compiler over the same code base should produce the same symbol names for
//!    the same items.
//!
//!  - Symbol names should not depend on any global properties of the code base,
//!    so that small modifications to the code base do not result in all symbols
//!    changing. In previous versions of the compiler, symbol names incorporated
//!    the SVH (Stable Version Hash) of the crate. This scheme turned out to be
//!    infeasible when used in conjunction with incremental compilation because
//!    small code changes would invalidate all symbols generated previously.
//!
//!  - Even symbols from different versions of the same crate should be able to
//!    live next to each other without conflict.
//!
//! In order to fulfill the above requirements the following scheme is used by
//! the compiler:
//!
//! The main tool for avoiding naming conflicts is the incorporation of a 64-bit
//! hash value into every exported symbol name. Anything that makes a difference
//! to the symbol being named, but does not show up in the regular path needs to
//! be fed into this hash:
//!
//! - Different monomorphizations of the same item have the same path but differ
//!   in their concrete type parameters, so these parameters are part of the
//!   data being digested for the symbol hash.
//!
//! - Rust allows items to be defined in anonymous scopes, such as in
//!   `fn foo() { { fn bar() {} } { fn bar() {} } }`. Both `bar` functions have
//!   the path `foo::bar`, since the anonymous scopes do not contribute to the
//!   path of an item. The compiler already handles this case via so-called
//!   disambiguating `DefPaths` which use indices to distinguish items with the
//!   same name. The DefPaths of the functions above are thus `foo[0]::bar[0]`
//!   and `foo[0]::bar[1]`. In order to incorporate this disambiguation
//!   information into the symbol name too, these indices are fed into the
//!   symbol hash, so that the above two symbols would end up with different
//!   hash values.
//!
//! The two measures described above suffice to avoid intra-crate conflicts. In
//! order to also avoid inter-crate conflicts two more measures are taken:
//!
//! - The name of the crate containing the symbol is prepended to the symbol
//!   name, i.e., symbols are "crate qualified". For example, a function `foo` in
//!   module `bar` in crate `baz` would get a symbol name like
//!   `baz::bar::foo::{hash}` instead of just `bar::foo::{hash}`. This avoids
//!   simple conflicts between functions from different crates.
//!
//! - In order to be able to also use symbols from two versions of the same
//!   crate (which naturally also have the same name), a stronger measure is
//!   required: The compiler accepts an arbitrary "disambiguator" value via the
//!   `-C metadata` command-line argument. This disambiguator is then fed into
//!   the symbol hash of every exported item. Consequently, the symbols in two
//!   identical crates but with different disambiguators are not in conflict
//!   with each other. This facility is mainly intended to be used by build
//!   tools like Cargo.
//!
//! A note on symbol name stability
//! -------------------------------
//! Previous versions of the compiler resorted to feeding NodeIds into the
//! symbol hash in order to disambiguate between items with the same path. The
//! current version of the name generation algorithm takes great care not to do
//! that, since NodeIds are notoriously unstable: A small change to the
//! code base will offset all NodeIds after the change and thus, much as using
//! the SVH in the hash, invalidate an unbounded number of symbol names. This
//! makes re-using previously compiled code for incremental compilation
//! virtually impossible. Thus, symbol hash generation exclusively relies on
//! DefPaths which are much more robust in the face of changes to the code base.

use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
use rustc_middle::mono::{InstantiationMode, MonoItem};
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, Instance, InstanceKind, TyCtxt};
use rustc_session::config::SymbolManglingVersion;
use tracing::debug;

mod export;
mod hashed;
mod legacy;
mod v0;

pub mod test;

pub use v0::{mangle_cgu, mangle_internal_symbol};

/// This function computes the symbol name for the given `instance` and the
/// given instantiating crate. That is, if you know that instance X is
/// instantiated in crate Y, this is the symbol name this instance would have.
pub fn symbol_name_for_instance_in_crate<'tcx>(
    tcx: TyCtxt<'tcx>,
    instance: Instance<'tcx>,
    instantiating_crate: CrateNum,
) -> String {
    compute_symbol_name(tcx, instance, || instantiating_crate)
}

pub fn provide(providers: &mut Providers) {
    *providers = Providers { symbol_name: symbol_name_provider, ..*providers };
}

// The `symbol_name` query provides the symbol name for calling a given
// instance from the local crate. In particular, it will also look up the
// correct symbol name of instances from upstream crates.
fn symbol_name_provider<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> ty::SymbolName<'tcx> {
    let symbol_name = compute_symbol_name(tcx, instance, || {
        // This closure determines the instantiating crate for instances that
        // need an instantiating-crate-suffix for their symbol name, in order
        // to differentiate between local copies.
        if is_generic(instance) {
            // For generics we might find re-usable upstream instances. If there
            // is one, we rely on the symbol being instantiated locally.
            instance.upstream_monomorphization(tcx).unwrap_or(LOCAL_CRATE)
        } else {
            // For non-generic things that need to avoid naming conflicts, we
            // always instantiate a copy in the local crate.
            LOCAL_CRATE
        }
    });

    ty::SymbolName::new(tcx, &symbol_name)
}

pub fn typeid_for_trait_ref<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_ref: ty::ExistentialTraitRef<'tcx>,
) -> String {
    v0::mangle_typeid_for_trait_ref(tcx, trait_ref)
}

pub fn symbol_name_from_attrs<'tcx>(
    tcx: TyCtxt<'tcx>,
    instance_kind: InstanceKind<'tcx>,
) -> Option<String> {
    let def_id = instance_kind.def_id();

    if let Some(def_id) = def_id.as_local() {
        if tcx.proc_macro_decls_static(()) == Some(def_id) {
            let stable_crate_id = tcx.stable_crate_id(LOCAL_CRATE);
            return Some(rustc_session::generate_proc_macro_decls_symbol(stable_crate_id));
        }
    }

    // FIXME(eddyb) Precompute a custom symbol name based on attributes.
    let attrs = if tcx.def_kind(def_id).has_codegen_attrs() {
        &tcx.codegen_instance_attrs(instance_kind)
    } else {
        CodegenFnAttrs::EMPTY
    };

    if attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) {
        // Items marked as #[rustc_std_internal_symbol] need to have a fixed
        // symbol name because it is used to import items from another crate
        // without a direct dependency. As such it is not possible to look up
        // the mangled name for the `Instance` from the crate metadata of the
        // defining crate.
        // Weak lang items automatically get #[rustc_std_internal_symbol]
        // applied by the code computing the CodegenFnAttrs.
        // We are mangling all #[rustc_std_internal_symbol] items as a
        // combination of the rustc version and the unmangled linkage name.
        // This is to ensure that if we link against a staticlib compiled by a
        // different rustc version, we don't get symbol conflicts or even UB
        // due to a different implementation/ABI. Rust staticlibs currently
        // export all symbols, including those that are hidden in cdylibs.
        // We are using the v0 symbol mangling scheme here as we need to be
        // consistent across all crates and in some contexts the legacy symbol
        // mangling scheme can't be used. For example both the GCC backend and
        // Rust-for-Linux don't support some of the characters used by the
        // legacy symbol mangling scheme.
        let name = if let Some(name) = attrs.symbol_name { name } else { tcx.item_name(def_id) };

        return Some(v0::mangle_internal_symbol(tcx, name.as_str()));
    }

    let wasm_import_module_exception_force_mangling = {
        // * On the wasm32 targets there is a bug (or feature) in LLD [1] where the
        //   same-named symbol when imported from different wasm modules will get
        //   hooked up incorrectly. As a result foreign symbols, on the wasm target,
        //   with a wasm import module, get mangled. Additionally our codegen will
        //   deduplicate symbols based purely on the symbol name, but for wasm this
        //   isn't quite right because the same-named symbol on wasm can come from
        //   different modules. For these reasons if `#[link(wasm_import_module)]`
        //   is present we mangle everything on wasm because the demangled form will
        //   show up in the `wasm-import-name` custom attribute in LLVM IR.
        //
        // [1]: https://bugs.llvm.org/show_bug.cgi?id=44316
        //
        // So, on wasm if a foreign item loses its `#[no_mangle]`, it might *still*
        // be mangled if we're forced to. Note: I don't like this.
        // These kinds of exceptions should be added during the `codegen_attrs` query.
        // However, we don't have the wasm import module map there yet.
        tcx.is_foreign_item(def_id)
            && tcx.sess.target.is_like_wasm
            && tcx.wasm_import_module_map(def_id.krate).contains_key(&def_id)
    };

    if !wasm_import_module_exception_force_mangling {
        if let Some(name) = attrs.symbol_name {
            // Use provided name
            return Some(name.to_string());
        }

        if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
            // Don't mangle
            return Some(tcx.item_name(def_id).to_string());
        }
    }

    None
}

/// Computes the symbol name for the given instance. This function will call
/// `compute_instantiating_crate` if it needs to factor the instantiating crate
/// into the symbol name.
fn compute_symbol_name<'tcx>(
    tcx: TyCtxt<'tcx>,
    instance: Instance<'tcx>,
    compute_instantiating_crate: impl FnOnce() -> CrateNum,
) -> String {
    let def_id = instance.def_id();
    let args = instance.args;
    let def_kind = tcx.def_kind(instance.def_id());
    if let DefKind::Fn | DefKind::AssocFn = def_kind {
        debug_assert!(tcx.constness(instance.def_id()) != hir::Constness::Const { always: true });
    }

    debug!("symbol_name(def_id={:?}, args={:?})", def_id, args);

    if let Some(symbol) = symbol_name_from_attrs(tcx, instance.def) {
        return symbol;
    }

    // If we're dealing with an instance of a function that's inlined from
    // another crate but we're marking it as globally shared to our
    // compilation (aka we're not making an internal copy in each of our
    // codegen units) then this symbol may become an exported (but hidden
    // visibility) symbol. This means that multiple crates may do the same
    // and we want to be sure to avoid any symbol conflicts here.
    let is_globally_shared_function = matches!(
        def_kind,
        DefKind::Fn
            | DefKind::AssocFn
            | DefKind::Closure
            | DefKind::SyntheticCoroutineBody
            | DefKind::Ctor(..)
    ) && matches!(
        MonoItem::Fn(instance).instantiation_mode(tcx),
        InstantiationMode::GloballyShared { may_conflict: true }
    );

    // If this is an instance of a generic function, we also hash in
    // the ID of the instantiating crate. This avoids symbol conflicts
    // in case the same instances is emitted in two crates of the same
    // project.
    let avoid_cross_crate_conflicts = is_generic(instance) || is_globally_shared_function;

    let instantiating_crate = avoid_cross_crate_conflicts.then(compute_instantiating_crate);

    // Pick the crate responsible for the symbol mangling version, which has to:
    // 1. be stable for each instance, whether it's being defined or imported
    // 2. obey each crate's own `-C symbol-mangling-version`, as much as possible
    // We solve these as follows:
    // 1. because symbol names depend on both `def_id` and `instantiating_crate`,
    // both their `CrateNum`s are stable for any given instance, so we can pick
    // either and have a stable choice of symbol mangling version
    // 2. we favor `instantiating_crate` where possible (i.e. when `Some`)
    let mangling_version_crate = instantiating_crate.unwrap_or(def_id.krate);
    let mangling_version = if mangling_version_crate == LOCAL_CRATE {
        tcx.sess.opts.get_symbol_mangling_version()
    } else {
        tcx.symbol_mangling_version(mangling_version_crate)
    };

    let symbol = match tcx.is_exportable(def_id) {
        true => format!(
            "{}.{}",
            v0::mangle(tcx, instance, instantiating_crate, true),
            export::compute_hash_of_export_fn(tcx, instance)
        ),
        false => match mangling_version {
            SymbolManglingVersion::Legacy => {
                let mangled_name = legacy::mangle(tcx, instance, instantiating_crate);

                let mangled_name_too_long = {
                    // The PDB debug info format cannot store mangled symbol names for which its
                    // internal record exceeds u16::MAX bytes, a limit multiple Rust projects have been
                    // hitting due to the verbosity of legacy name mangling. Depending on the linker version
                    // in use, such symbol names can lead to linker crashes or incomprehensible linker error
                    // about a limit being hit.
                    // Mangle those symbols with v0 mangling instead, which gives us more room to breathe
                    // as v0 mangling is more compact.
                    // Empirical testing has shown the limit for the symbol name to be 65521 bytes; use
                    // 65000 bytes to leave some room for prefixes / suffixes as well as unknown scenarios
                    // with a different limit.
                    const MAX_SYMBOL_LENGTH: usize = 65000;

                    tcx.sess.target.uses_pdb_debuginfo() && mangled_name.len() > MAX_SYMBOL_LENGTH
                };

                if mangled_name_too_long {
                    v0::mangle(tcx, instance, instantiating_crate, false)
                } else {
                    mangled_name
                }
            }
            SymbolManglingVersion::V0 => v0::mangle(tcx, instance, instantiating_crate, false),
            SymbolManglingVersion::Hashed => {
                hashed::mangle(tcx, instance, instantiating_crate, || {
                    v0::mangle(tcx, instance, instantiating_crate, false)
                })
            }
        },
    };

    debug_assert!(
        rustc_demangle::try_demangle(&symbol).is_ok(),
        "compute_symbol_name: `{symbol}` cannot be demangled"
    );

    symbol
}

fn is_generic<'tcx>(instance: Instance<'tcx>) -> bool {
    instance.args.non_erasable_generics().next().is_some()
}