1 //! The Rust Linkage Model and Symbol Names
2 //! =======================================
4 //! The semantic model of Rust linkage is, broadly, that "there's no global
5 //! namespace" between crates. Our aim is to preserve the illusion of this
6 //! model despite the fact that it's not *quite* possible to implement on
7 //! modern linkers. We initially didn't use system linkers at all, but have
8 //! been convinced of their utility.
10 //! There are a few issues to handle:
12 //! - Linkers operate on a flat namespace, so we have to flatten names.
13 //! We do this using the C++ namespace-mangling technique. Foo::bar
16 //! - Symbols for distinct items with the same *name* need to get different
17 //! linkage-names. Examples of this are monomorphizations of functions or
18 //! items within anonymous scopes that end up having the same path.
20 //! - Symbols in different crates but with same names "within" the crate need
21 //! to get different linkage-names.
23 //! - Symbol names should be deterministic: Two consecutive runs of the
24 //! compiler over the same code base should produce the same symbol names for
27 //! - Symbol names should not depend on any global properties of the code base,
28 //! so that small modifications to the code base do not result in all symbols
29 //! changing. In previous versions of the compiler, symbol names incorporated
30 //! the SVH (Stable Version Hash) of the crate. This scheme turned out to be
31 //! infeasible when used in conjunction with incremental compilation because
32 //! small code changes would invalidate all symbols generated previously.
34 //! - Even symbols from different versions of the same crate should be able to
35 //! live next to each other without conflict.
37 //! In order to fulfill the above requirements the following scheme is used by
40 //! The main tool for avoiding naming conflicts is the incorporation of a 64-bit
41 //! hash value into every exported symbol name. Anything that makes a difference
42 //! to the symbol being named, but does not show up in the regular path needs to
43 //! be fed into this hash:
45 //! - Different monomorphizations of the same item have the same path but differ
46 //! in their concrete type parameters, so these parameters are part of the
47 //! data being digested for the symbol hash.
49 //! - Rust allows items to be defined in anonymous scopes, such as in
50 //! `fn foo() { { fn bar() {} } { fn bar() {} } }`. Both `bar` functions have
51 //! the path `foo::bar`, since the anonymous scopes do not contribute to the
52 //! path of an item. The compiler already handles this case via so-called
53 //! disambiguating `DefPaths` which use indices to distinguish items with the
54 //! same name. The DefPaths of the functions above are thus `foo[0]::bar[0]`
55 //! and `foo[0]::bar[1]`. In order to incorporate this disambiguation
56 //! information into the symbol name too, these indices are fed into the
57 //! symbol hash, so that the above two symbols would end up with different
60 //! The two measures described above suffice to avoid intra-crate conflicts. In
61 //! order to also avoid inter-crate conflicts two more measures are taken:
63 //! - The name of the crate containing the symbol is prepended to the symbol
64 //! name, i.e., symbols are "crate qualified". For example, a function `foo` in
65 //! module `bar` in crate `baz` would get a symbol name like
66 //! `baz::bar::foo::{hash}` instead of just `bar::foo::{hash}`. This avoids
67 //! simple conflicts between functions from different crates.
69 //! - In order to be able to also use symbols from two versions of the same
70 //! crate (which naturally also have the same name), a stronger measure is
71 //! required: The compiler accepts an arbitrary "disambiguator" value via the
72 //! `-C metadata` command-line argument. This disambiguator is then fed into
73 //! the symbol hash of every exported item. Consequently, the symbols in two
74 //! identical crates but with different disambiguators are not in conflict
75 //! with each other. This facility is mainly intended to be used by build
78 //! A note on symbol name stability
79 //! -------------------------------
80 //! Previous versions of the compiler resorted to feeding NodeIds into the
81 //! symbol hash in order to disambiguate between items with the same path. The
82 //! current version of the name generation algorithm takes great care not to do
83 //! that, since NodeIds are notoriously unstable: A small change to the
84 //! code base will offset all NodeIds after the change and thus, much as using
85 //! the SVH in the hash, invalidate an unbounded number of symbol names. This
86 //! makes re-using previously compiled code for incremental compilation
87 //! virtually impossible. Thus, symbol hash generation exclusively relies on
88 //! DefPaths which are much more robust in the face of changes to the code base.
90 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
91 #![feature(never_type)]
93 #![feature(or_patterns)]
94 #![feature(in_band_lifetimes)]
95 #![recursion_limit = "256"]
98 extern crate rustc_middle
;
100 use rustc_hir
::def_id
::{CrateNum, LOCAL_CRATE}
;
102 use rustc_middle
::middle
::codegen_fn_attrs
::CodegenFnAttrFlags
;
103 use rustc_middle
::mir
::mono
::{InstantiationMode, MonoItem}
;
104 use rustc_middle
::ty
::query
::Providers
;
105 use rustc_middle
::ty
::subst
::SubstsRef
;
106 use rustc_middle
::ty
::{self, Instance, TyCtxt}
;
107 use rustc_session
::config
::SymbolManglingVersion
;
109 use rustc_span
::symbol
::Symbol
;
118 /// This function computes the symbol name for the given `instance` and the
119 /// given instantiating crate. That is, if you know that instance X is
120 /// instantiated in crate Y, this is the symbol name this instance would have.
121 pub fn symbol_name_for_instance_in_crate(
123 instance
: Instance
<'tcx
>,
124 instantiating_crate
: CrateNum
,
126 compute_symbol_name(tcx
, instance
, || instantiating_crate
)
129 pub fn provide(providers
: &mut Providers
<'_
>) {
130 *providers
= Providers { symbol_name: symbol_name_provider, ..*providers }
;
133 // The `symbol_name` query provides the symbol name for calling a given
134 // instance from the local crate. In particular, it will also look up the
135 // correct symbol name of instances from upstream crates.
136 fn symbol_name_provider(tcx
: TyCtxt
<'tcx
>, instance
: Instance
<'tcx
>) -> ty
::SymbolName
{
137 let symbol_name
= compute_symbol_name(tcx
, instance
, || {
138 // This closure determines the instantiating crate for instances that
139 // need an instantiating-crate-suffix for their symbol name, in order
140 // to differentiate between local copies.
141 if is_generic(instance
.substs
) {
142 // For generics we might find re-usable upstream instances. If there
143 // is one, we rely on the symbol being instantiated locally.
144 instance
.upstream_monomorphization(tcx
).unwrap_or(LOCAL_CRATE
)
146 // For non-generic things that need to avoid naming conflicts, we
147 // always instantiate a copy in the local crate.
152 ty
::SymbolName { name: Symbol::intern(&symbol_name) }
155 /// Computes the symbol name for the given instance. This function will call
156 /// `compute_instantiating_crate` if it needs to factor the instantiating crate
157 /// into the symbol name.
158 fn compute_symbol_name(
160 instance
: Instance
<'tcx
>,
161 compute_instantiating_crate
: impl FnOnce() -> CrateNum
,
163 let def_id
= instance
.def_id();
164 let substs
= instance
.substs
;
166 debug
!("symbol_name(def_id={:?}, substs={:?})", def_id
, substs
);
168 // FIXME(eddyb) Precompute a custom symbol name based on attributes.
169 let is_foreign
= if let Some(def_id
) = def_id
.as_local() {
170 if tcx
.plugin_registrar_fn(LOCAL_CRATE
) == Some(def_id
.to_def_id()) {
171 let disambiguator
= tcx
.sess
.local_crate_disambiguator();
172 return tcx
.sess
.generate_plugin_registrar_symbol(disambiguator
);
174 if tcx
.proc_macro_decls_static(LOCAL_CRATE
) == Some(def_id
.to_def_id()) {
175 let disambiguator
= tcx
.sess
.local_crate_disambiguator();
176 return tcx
.sess
.generate_proc_macro_decls_symbol(disambiguator
);
178 let hir_id
= tcx
.hir().as_local_hir_id(def_id
);
179 match tcx
.hir().get(hir_id
) {
180 Node
::ForeignItem(_
) => true,
184 tcx
.is_foreign_item(def_id
)
187 let attrs
= tcx
.codegen_fn_attrs(def_id
);
189 // Foreign items by default use no mangling for their symbol name. There's a
190 // few exceptions to this rule though:
192 // * This can be overridden with the `#[link_name]` attribute
194 // * On the wasm32 targets there is a bug (or feature) in LLD [1] where the
195 // same-named symbol when imported from different wasm modules will get
196 // hooked up incorrectly. As a result foreign symbols, on the wasm target,
197 // with a wasm import module, get mangled. Additionally our codegen will
198 // deduplicate symbols based purely on the symbol name, but for wasm this
199 // isn't quite right because the same-named symbol on wasm can come from
200 // different modules. For these reasons if `#[link(wasm_import_module)]`
201 // is present we mangle everything on wasm because the demangled form will
202 // show up in the `wasm-import-name` custom attribute in LLVM IR.
204 // [1]: https://bugs.llvm.org/show_bug.cgi?id=44316
206 if tcx
.sess
.target
.target
.arch
!= "wasm32"
207 || !tcx
.wasm_import_module_map(def_id
.krate
).contains_key(&def_id
)
209 if let Some(name
) = attrs
.link_name
{
210 return name
.to_string();
212 return tcx
.item_name(def_id
).to_string();
216 if let Some(name
) = attrs
.export_name
{
218 return name
.to_string();
221 if attrs
.flags
.contains(CodegenFnAttrFlags
::NO_MANGLE
) {
223 return tcx
.item_name(def_id
).to_string();
226 let avoid_cross_crate_conflicts
=
227 // If this is an instance of a generic function, we also hash in
228 // the ID of the instantiating crate. This avoids symbol conflicts
229 // in case the same instances is emitted in two crates of the same
231 is_generic(substs
) ||
233 // If we're dealing with an instance of a function that's inlined from
234 // another crate but we're marking it as globally shared to our
235 // compliation (aka we're not making an internal copy in each of our
236 // codegen units) then this symbol may become an exported (but hidden
237 // visibility) symbol. This means that multiple crates may do the same
238 // and we want to be sure to avoid any symbol conflicts here.
239 match MonoItem
::Fn(instance
).instantiation_mode(tcx
) {
240 InstantiationMode
::GloballyShared { may_conflict: true }
=> true,
244 let instantiating_crate
=
245 if avoid_cross_crate_conflicts { Some(compute_instantiating_crate()) }
else { None }
;
247 // Pick the crate responsible for the symbol mangling version, which has to:
248 // 1. be stable for each instance, whether it's being defined or imported
249 // 2. obey each crate's own `-Z symbol-mangling-version`, as much as possible
250 // We solve these as follows:
251 // 1. because symbol names depend on both `def_id` and `instantiating_crate`,
252 // both their `CrateNum`s are stable for any given instance, so we can pick
253 // either and have a stable choice of symbol mangling version
254 // 2. we favor `instantiating_crate` where possible (i.e. when `Some`)
255 let mangling_version_crate
= instantiating_crate
.unwrap_or(def_id
.krate
);
256 let mangling_version
= if mangling_version_crate
== LOCAL_CRATE
{
257 tcx
.sess
.opts
.debugging_opts
.symbol_mangling_version
259 tcx
.symbol_mangling_version(mangling_version_crate
)
262 match mangling_version
{
263 SymbolManglingVersion
::Legacy
=> legacy
::mangle(tcx
, instance
, instantiating_crate
),
264 SymbolManglingVersion
::V0
=> v0
::mangle(tcx
, instance
, instantiating_crate
),
268 fn is_generic(substs
: SubstsRef
<'_
>) -> bool
{
269 substs
.non_erasable_generics().next().is_some()