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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.
use rustc::hir::def_id::LOCAL_CRATE;
use rustc::hir::Node;
use rustc::hir::CodegenFnAttrFlags;
use rustc::session::config::SymbolManglingVersion;
use rustc::ty::query::Providers;
use rustc::ty::{self, TyCtxt, Instance};
use rustc::mir::mono::{MonoItem, InstantiationMode};
use syntax_pos::symbol::InternedString;
use log::debug;
mod legacy;
mod v0;
pub fn provide(providers: &mut Providers<'_>) {
*providers = Providers {
symbol_name: |tcx, instance| ty::SymbolName {
name: symbol_name(tcx, instance),
},
..*providers
};
}
fn symbol_name(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> InternedString {
let def_id = instance.def_id();
let substs = instance.substs;
debug!("symbol_name(def_id={:?}, substs={:?})", def_id, substs);
let hir_id = tcx.hir().as_local_hir_id(def_id);
if def_id.is_local() {
if tcx.plugin_registrar_fn(LOCAL_CRATE) == Some(def_id) {
let disambiguator = tcx.sess.local_crate_disambiguator();
return
InternedString::intern(&tcx.sess.generate_plugin_registrar_symbol(disambiguator));
}
if tcx.proc_macro_decls_static(LOCAL_CRATE) == Some(def_id) {
let disambiguator = tcx.sess.local_crate_disambiguator();
return
InternedString::intern(&tcx.sess.generate_proc_macro_decls_symbol(disambiguator));
}
}
// FIXME(eddyb) Precompute a custom symbol name based on attributes.
let is_foreign = if let Some(id) = hir_id {
match tcx.hir().get(id) {
Node::ForeignItem(_) => true,
_ => false,
}
} else {
tcx.is_foreign_item(def_id)
};
let attrs = tcx.codegen_fn_attrs(def_id);
if is_foreign {
if let Some(name) = attrs.link_name {
return name.as_interned_str();
}
// Don't mangle foreign items.
return tcx.item_name(def_id).as_interned_str();
}
if let Some(name) = &attrs.export_name {
// Use provided name
return name.as_interned_str();
}
if attrs.flags.contains(CodegenFnAttrFlags::NO_MANGLE) {
// Don't mangle
return tcx.item_name(def_id).as_interned_str();
}
let is_generic = substs.non_erasable_generics().next().is_some();
let avoid_cross_crate_conflicts =
// 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.
is_generic ||
// 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
// compliation (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.
match MonoItem::Fn(instance).instantiation_mode(tcx) {
InstantiationMode::GloballyShared { may_conflict: true } => true,
_ => false,
};
let instantiating_crate = if avoid_cross_crate_conflicts {
Some(if is_generic {
if !def_id.is_local() && tcx.sess.opts.share_generics() {
// If we are re-using a monomorphization from another crate,
// we have to compute the symbol hash accordingly.
let upstream_monomorphizations = tcx.upstream_monomorphizations_for(def_id);
upstream_monomorphizations
.and_then(|monos| monos.get(&substs).cloned())
.unwrap_or(LOCAL_CRATE)
} else {
LOCAL_CRATE
}
} else {
LOCAL_CRATE
})
} else {
None
};
// 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 `-Z 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.debugging_opts.symbol_mangling_version
} else {
tcx.symbol_mangling_version(mangling_version_crate)
};
let mangled = match mangling_version {
SymbolManglingVersion::Legacy => legacy::mangle(tcx, instance, instantiating_crate),
SymbolManglingVersion::V0 => v0::mangle(tcx, instance, instantiating_crate),
};
InternedString::intern(&mangled)
}
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