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//! 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.
#![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
#![feature(never_type)]
#![recursion_limit = "256"]
#![allow(rustc::potential_query_instability)]
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]
#[macro_use]
extern crate rustc_middle;
#[macro_use]
extern crate tracing;
use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
use rustc_fluent_macro::fluent_messages;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
use rustc_middle::mir::mono::{InstantiationMode, MonoItem};
use rustc_middle::query::Providers;
use rustc_middle::ty::{self, Instance, TyCtxt};
use rustc_session::config::SymbolManglingVersion;
mod legacy;
mod v0;
pub mod errors;
pub mod test;
pub mod typeid;
fluent_messages! { "../messages.ftl" }
/// 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, tcx) {
// 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::PolyExistentialTraitRef<'tcx>,
) -> String {
v0::mangle_typeid_for_trait_ref(tcx, trait_ref)
}
/// 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;
debug!("symbol_name(def_id={:?}, args={:?})", def_id, args);
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 tcx.sess.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_fn_attrs(def_id)
} else {
CodegenFnAttrs::EMPTY
};
// Foreign items by default use no mangling for their symbol name. There's a
// few exceptions to this rule though:
//
// * This can be overridden with the `#[link_name]` attribute
//
// * 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
if 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 let Some(name) = attrs.link_name {
return name.to_string();
}
return tcx.item_name(def_id).to_string();
}
if let Some(name) = attrs.export_name {
// Use provided name
return name.to_string();
}
if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
// Don't mangle
return tcx.item_name(def_id).to_string();
}
// 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!(
tcx.def_kind(instance.def_id()),
DefKind::Fn | DefKind::AssocFn | DefKind::Closure | DefKind::Generator | 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, tcx) || 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 mangling_version {
SymbolManglingVersion::Legacy => legacy::mangle(tcx, instance, instantiating_crate),
SymbolManglingVersion::V0 => v0::mangle(tcx, instance, instantiating_crate),
};
debug_assert!(
rustc_demangle::try_demangle(&symbol).is_ok(),
"compute_symbol_name: `{symbol}` cannot be demangled"
);
symbol
}
fn is_generic<'tcx>(instance: Instance<'tcx>, tcx: TyCtxt<'tcx>) -> bool {
instance.args.non_erasable_generics(tcx, instance.def_id()).next().is_some()
}