rust/src/librustc_trans/back/symbol_names.rs

// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! 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` commandline 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 monomorphize::Instance;

use rustc::middle::weak_lang_items;
use rustc::hir::def_id::DefId;
use rustc::hir::map as hir_map;
use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
use rustc::ty::fold::TypeVisitor;
use rustc::ty::item_path::{self, ItemPathBuffer, RootMode};
use rustc::ty::subst::Substs;
use rustc::hir::map::definitions::DefPathData;
use rustc::util::common::record_time;

use syntax::attr;

use std::fmt::Write;

fn get_symbol_hash<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,

                             // the DefId of the item this name is for
                             def_id: Option<DefId>,

                             // type of the item, without any generic
                             // parameters substituted; this is
                             // included in the hash as a kind of
                             // safeguard.
                             item_type: Ty<'tcx>,

                             // values for generic type parameters,
                             // if any.
                             substs: Option<&'tcx Substs<'tcx>>)
                             -> String {
    debug!("get_symbol_hash(def_id={:?}, parameters={:?})", def_id, substs);

    let mut hasher = ty::util::TypeIdHasher::<u64>::new(tcx);

    record_time(&tcx.sess.perf_stats.symbol_hash_time, || {
        // the main symbol name is not necessarily unique; hash in the
        // compiler's internal def-path, guaranteeing each symbol has a
        // truly unique path
        hasher.hash(def_id.map(|def_id| tcx.def_path_hash(def_id)));

        // Include the main item-type. Note that, in this case, the
        // assertions about `needs_subst` may not hold, but this item-type
        // ought to be the same for every reference anyway.
        assert!(!item_type.has_erasable_regions());
        hasher.visit_ty(item_type);

        // also include any type parameters (for generic items)
        if let Some(substs) = substs {
            assert!(!substs.has_erasable_regions());
            assert!(!substs.needs_subst());
            substs.visit_with(&mut hasher);

            // 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.
            if substs.types().next().is_some() {
                hasher.hash(tcx.crate_name.as_str());
                hasher.hash(tcx.sess.local_crate_disambiguator().as_str());
            }
        }
    });

    // 64 bits should be enough to avoid collisions.
    format!("h{:016x}", hasher.finish())
}

pub fn symbol_name<'a, 'tcx>(instance: Instance<'tcx>,
                             tcx: TyCtxt<'a, 'tcx, 'tcx>) -> String {
    let def_id = instance.def_id();
    let substs = instance.substs;

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

    let node_id = tcx.hir.as_local_node_id(def_id);

    if let Some(id) = node_id {
        if tcx.sess.plugin_registrar_fn.get() == Some(id) {
            let idx = def_id.index;
            let disambiguator = tcx.sess.local_crate_disambiguator();
            return tcx.sess.generate_plugin_registrar_symbol(disambiguator, idx);
        }
        if tcx.sess.derive_registrar_fn.get() == Some(id) {
            let idx = def_id.index;
            let disambiguator = tcx.sess.local_crate_disambiguator();
            return tcx.sess.generate_derive_registrar_symbol(disambiguator, idx);
        }
    }

    // FIXME(eddyb) Precompute a custom symbol name based on attributes.
    let attrs = tcx.get_attrs(def_id);
    let is_foreign = if let Some(id) = node_id {
        match tcx.hir.get(id) {
            hir_map::NodeForeignItem(_) => true,
            _ => false
        }
    } else {
        tcx.sess.cstore.is_foreign_item(def_id)
    };

    if let Some(name) = weak_lang_items::link_name(&attrs) {
        return name.to_string();
    }

    if is_foreign {
        if let Some(name) = attr::first_attr_value_str_by_name(&attrs, "link_name") {
            return name.to_string();
        }
        // Don't mangle foreign items.
        return tcx.item_name(def_id).as_str().to_string();
    }

    if let Some(name) = attr::find_export_name_attr(tcx.sess.diagnostic(), &attrs) {
        // Use provided name
        return name.to_string();
    }

    if attr::contains_name(&attrs, "no_mangle") {
        // Don't mangle
        return tcx.item_name(def_id).as_str().to_string();
    }

    // We want to compute the "type" of this item. Unfortunately, some
    // kinds of items (e.g., closures) don't have an entry in the
    // item-type array. So walk back up the find the closest parent
    // that DOES have an entry.
    let mut ty_def_id = def_id;
    let instance_ty;
    loop {
        let key = tcx.def_key(ty_def_id);
        match key.disambiguated_data.data {
            DefPathData::TypeNs(_) |
            DefPathData::ValueNs(_) => {
                instance_ty = tcx.type_of(ty_def_id);
                break;
            }
            _ => {
                // if we're making a symbol for something, there ought
                // to be a value or type-def or something in there
                // *somewhere*
                ty_def_id.index = key.parent.unwrap_or_else(|| {
                    bug!("finding type for {:?}, encountered def-id {:?} with no \
                          parent", def_id, ty_def_id);
                });
            }
        }
    }

    // Erase regions because they may not be deterministic when hashed
    // and should not matter anyhow.
    let instance_ty = tcx.erase_regions(&instance_ty);

    let hash = get_symbol_hash(tcx, Some(def_id), instance_ty, Some(substs));

    let mut buffer = SymbolPathBuffer::new();
    item_path::with_forced_absolute_paths(|| {
        tcx.push_item_path(&mut buffer, def_id);
    });
    buffer.finish(&hash)
}

// Follow C++ namespace-mangling style, see
// http://en.wikipedia.org/wiki/Name_mangling for more info.
//
// It turns out that on macOS you can actually have arbitrary symbols in
// function names (at least when given to LLVM), but this is not possible
// when using unix's linker. Perhaps one day when we just use a linker from LLVM
// we won't need to do this name mangling. The problem with name mangling is
// that it seriously limits the available characters. For example we can't
// have things like &T in symbol names when one would theoretically
// want them for things like impls of traits on that type.
//
// To be able to work on all platforms and get *some* reasonable output, we
// use C++ name-mangling.
struct SymbolPathBuffer {
    result: String,
    temp_buf: String
}

impl SymbolPathBuffer {
    fn new() -> Self {
        let mut result = SymbolPathBuffer {
            result: String::with_capacity(64),
            temp_buf: String::with_capacity(16)
        };
        result.result.push_str("_ZN"); // _Z == Begin name-sequence, N == nested
        result
    }

    fn finish(mut self, hash: &str) -> String {
        // end name-sequence
        self.push(hash);
        self.result.push('E');
        self.result
    }
}

impl ItemPathBuffer for SymbolPathBuffer {
    fn root_mode(&self) -> &RootMode {
        const ABSOLUTE: &'static RootMode = &RootMode::Absolute;
        ABSOLUTE
    }

    fn push(&mut self, text: &str) {
        self.temp_buf.clear();
        let need_underscore = sanitize(&mut self.temp_buf, text);
        let _ = write!(self.result, "{}", self.temp_buf.len() + (need_underscore as usize));
        if need_underscore {
            self.result.push('_');
        }
        self.result.push_str(&self.temp_buf);
    }
}

pub fn exported_name_from_type_and_prefix<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                                    t: Ty<'tcx>,
                                                    prefix: &str)
                                                    -> String {
    let hash = get_symbol_hash(tcx, None, t, None);
    let mut buffer = SymbolPathBuffer::new();
    buffer.push(prefix);
    buffer.finish(&hash)
}

// Name sanitation. LLVM will happily accept identifiers with weird names, but
// gas doesn't!
// gas accepts the following characters in symbols: a-z, A-Z, 0-9, ., _, $
//
// returns true if an underscore must be added at the start
pub fn sanitize(result: &mut String, s: &str) -> bool {
    for c in s.chars() {
        match c {
            // Escape these with $ sequences
            '@' => result.push_str("$SP$"),
            '*' => result.push_str("$BP$"),
            '&' => result.push_str("$RF$"),
            '<' => result.push_str("$LT$"),
            '>' => result.push_str("$GT$"),
            '(' => result.push_str("$LP$"),
            ')' => result.push_str("$RP$"),
            ',' => result.push_str("$C$"),

            // '.' doesn't occur in types and functions, so reuse it
            // for ':' and '-'
            '-' | ':' => result.push('.'),

            // These are legal symbols
            'a' ... 'z'
            | 'A' ... 'Z'
            | '0' ... '9'
            | '_' | '.' | '$' => result.push(c),

            _ => {
                result.push('$');
                for c in c.escape_unicode().skip(1) {
                    match c {
                        '{' => {},
                        '}' => result.push('$'),
                        c => result.push(c),
                    }
                }
            }
        }
    }

    // Underscore-qualify anything that didn't start as an ident.
    !result.is_empty() &&
        result.as_bytes()[0] != '_' as u8 &&
        ! (result.as_bytes()[0] as char).is_xid_start()
}