Read in rmeta crates

[rust.git] / src / librustc_metadata / locator.rs

// Copyright 2012-2015 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.

//! Finds crate binaries and loads their metadata
//!
//! Might I be the first to welcome you to a world of platform differences,
//! version requirements, dependency graphs, conflicting desires, and fun! This
//! is the major guts (along with metadata::creader) of the compiler for loading
//! crates and resolving dependencies. Let's take a tour!
//!
//! # The problem
//!
//! Each invocation of the compiler is immediately concerned with one primary
//! problem, to connect a set of crates to resolved crates on the filesystem.
//! Concretely speaking, the compiler follows roughly these steps to get here:
//!
//! 1. Discover a set of `extern crate` statements.
//! 2. Transform these directives into crate names. If the directive does not
//!    have an explicit name, then the identifier is the name.
//! 3. For each of these crate names, find a corresponding crate on the
//!    filesystem.
//!
//! Sounds easy, right? Let's walk into some of the nuances.
//!
//! ## Transitive Dependencies
//!
//! Let's say we've got three crates: A, B, and C. A depends on B, and B depends
//! on C. When we're compiling A, we primarily need to find and locate B, but we
//! also end up needing to find and locate C as well.
//!
//! The reason for this is that any of B's types could be composed of C's types,
//! any function in B could return a type from C, etc. To be able to guarantee
//! that we can always typecheck/translate any function, we have to have
//! complete knowledge of the whole ecosystem, not just our immediate
//! dependencies.
//!
//! So now as part of the "find a corresponding crate on the filesystem" step
//! above, this involves also finding all crates for *all upstream
//! dependencies*. This includes all dependencies transitively.
//!
//! ## Rlibs and Dylibs
//!
//! The compiler has two forms of intermediate dependencies. These are dubbed
//! rlibs and dylibs for the static and dynamic variants, respectively. An rlib
//! is a rustc-defined file format (currently just an ar archive) while a dylib
//! is a platform-defined dynamic library. Each library has a metadata somewhere
//! inside of it.
//!
//! A third kind of dependency is an rmeta file. These are rlibs, which contain
//! metadata, but no code. To a first approximation, these are treated in the
//! same way as rlibs. Where there is both an rlib and an rmeta file, the rlib
//! gets priority (even if the rmeta file is newer). An rmeta file is only
//! useful for checking a downstream crate, attempting to link one will cause an
//! error.
//!
//! When translating a crate name to a crate on the filesystem, we all of a
//! sudden need to take into account both rlibs and dylibs! Linkage later on may
//! use either one of these files, as each has their pros/cons. The job of crate
//! loading is to discover what's possible by finding all candidates.
//!
//! Most parts of this loading systems keep the dylib/rlib as just separate
//! variables.
//!
//! ## Where to look?
//!
//! We can't exactly scan your whole hard drive when looking for dependencies,
//! so we need to places to look. Currently the compiler will implicitly add the
//! target lib search path ($prefix/lib/rustlib/$target/lib) to any compilation,
//! and otherwise all -L flags are added to the search paths.
//!
//! ## What criterion to select on?
//!
//! This a pretty tricky area of loading crates. Given a file, how do we know
//! whether it's the right crate? Currently, the rules look along these lines:
//!
//! 1. Does the filename match an rlib/dylib pattern? That is to say, does the
//!    filename have the right prefix/suffix?
//! 2. Does the filename have the right prefix for the crate name being queried?
//!    This is filtering for files like `libfoo*.rlib` and such.
//! 3. Is the file an actual rust library? This is done by loading the metadata
//!    from the library and making sure it's actually there.
//! 4. Does the name in the metadata agree with the name of the library?
//! 5. Does the target in the metadata agree with the current target?
//! 6. Does the SVH match? (more on this later)
//!
//! If the file answers `yes` to all these questions, then the file is
//! considered as being *candidate* for being accepted. It is illegal to have
//! more than two candidates as the compiler has no method by which to resolve
//! this conflict. Additionally, rlib/dylib candidates are considered
//! separately.
//!
//! After all this has happened, we have 1 or two files as candidates. These
//! represent the rlib/dylib file found for a library, and they're returned as
//! being found.
//!
//! ### What about versions?
//!
//! A lot of effort has been put forth to remove versioning from the compiler.
//! There have been forays in the past to have versioning baked in, but it was
//! largely always deemed insufficient to the point that it was recognized that
//! it's probably something the compiler shouldn't do anyway due to its
//! complicated nature and the state of the half-baked solutions.
//!
//! With a departure from versioning, the primary criterion for loading crates
//! is just the name of a crate. If we stopped here, it would imply that you
//! could never link two crates of the same name from different sources
//! together, which is clearly a bad state to be in.
//!
//! To resolve this problem, we come to the next section!
//!
//! # Expert Mode
//!
//! A number of flags have been added to the compiler to solve the "version
//! problem" in the previous section, as well as generally enabling more
//! powerful usage of the crate loading system of the compiler. The goal of
//! these flags and options are to enable third-party tools to drive the
//! compiler with prior knowledge about how the world should look.
//!
//! ## The `--extern` flag
//!
//! The compiler accepts a flag of this form a number of times:
//!
//! ```text
//! --extern crate-name=path/to/the/crate.rlib
//! ```
//!
//! This flag is basically the following letter to the compiler:
//!
//! > Dear rustc,
//! >
//! > When you are attempting to load the immediate dependency `crate-name`, I
//! > would like you to assume that the library is located at
//! > `path/to/the/crate.rlib`, and look nowhere else. Also, please do not
//! > assume that the path I specified has the name `crate-name`.
//!
//! This flag basically overrides most matching logic except for validating that
//! the file is indeed a rust library. The same `crate-name` can be specified
//! twice to specify the rlib/dylib pair.
//!
//! ## Enabling "multiple versions"
//!
//! This basically boils down to the ability to specify arbitrary packages to
//! the compiler. For example, if crate A wanted to use Bv1 and Bv2, then it
//! would look something like:
//!
//! ```ignore
//! extern crate b1;
//! extern crate b2;
//!
//! fn main() {}
//! ```
//!
//! and the compiler would be invoked as:
//!
//! ```text
//! rustc a.rs --extern b1=path/to/libb1.rlib --extern b2=path/to/libb2.rlib
//! ```
//!
//! In this scenario there are two crates named `b` and the compiler must be
//! manually driven to be informed where each crate is.
//!
//! ## Frobbing symbols
//!
//! One of the immediate problems with linking the same library together twice
//! in the same problem is dealing with duplicate symbols. The primary way to
//! deal with this in rustc is to add hashes to the end of each symbol.
//!
//! In order to force hashes to change between versions of a library, if
//! desired, the compiler exposes an option `-C metadata=foo`, which is used to
//! initially seed each symbol hash. The string `foo` is prepended to each
//! string-to-hash to ensure that symbols change over time.
//!
//! ## Loading transitive dependencies
//!
//! Dealing with same-named-but-distinct crates is not just a local problem, but
//! one that also needs to be dealt with for transitive dependencies. Note that
//! in the letter above `--extern` flags only apply to the *local* set of
//! dependencies, not the upstream transitive dependencies. Consider this
//! dependency graph:
//!
//! ```text
//! A.1   A.2
//! |     |
//! |     |
//! B     C
//!  \   /
//!   \ /
//!    D
//! ```
//!
//! In this scenario, when we compile `D`, we need to be able to distinctly
//! resolve `A.1` and `A.2`, but an `--extern` flag cannot apply to these
//! transitive dependencies.
//!
//! Note that the key idea here is that `B` and `C` are both *already compiled*.
//! That is, they have already resolved their dependencies. Due to unrelated
//! technical reasons, when a library is compiled, it is only compatible with
//! the *exact same* version of the upstream libraries it was compiled against.
//! We use the "Strict Version Hash" to identify the exact copy of an upstream
//! library.
//!
//! With this knowledge, we know that `B` and `C` will depend on `A` with
//! different SVH values, so we crawl the normal `-L` paths looking for
//! `liba*.rlib` and filter based on the contained SVH.
//!
//! In the end, this ends up not needing `--extern` to specify upstream
//! transitive dependencies.
//!
//! # Wrapping up
//!
//! That's the general overview of loading crates in the compiler, but it's by
//! no means all of the necessary details. Take a look at the rest of
//! metadata::locator or metadata::creader for all the juicy details!

use cstore::MetadataBlob;
use creader::Library;
use schema::{METADATA_HEADER, rustc_version};

use rustc::hir::svh::Svh;
use rustc::session::{config, Session};
use rustc::session::filesearch::{FileSearch, FileMatches, FileDoesntMatch};
use rustc::session::search_paths::PathKind;
use rustc::util::common;
use rustc::util::nodemap::FxHashMap;

use rustc_llvm as llvm;
use rustc_llvm::{False, ObjectFile, mk_section_iter};
use rustc_llvm::archive_ro::ArchiveRO;
use errors::DiagnosticBuilder;
use syntax_pos::Span;
use rustc_back::target::Target;

use std::cmp;
use std::fmt;
use std::fs;
use std::io;
use std::path::{Path, PathBuf};
use std::ptr;
use std::slice;
use std::time::Instant;

use flate;

pub struct CrateMismatch {
    path: PathBuf,
    got: String,
}

pub struct Context<'a> {
    pub sess: &'a Session,
    pub span: Span,
    pub ident: &'a str,
    pub crate_name: &'a str,
    pub hash: Option<&'a Svh>,
    // points to either self.sess.target.target or self.sess.host, must match triple
    pub target: &'a Target,
    pub triple: &'a str,
    pub filesearch: FileSearch<'a>,
    pub root: &'a Option<CratePaths>,
    pub rejected_via_hash: Vec<CrateMismatch>,
    pub rejected_via_triple: Vec<CrateMismatch>,
    pub rejected_via_kind: Vec<CrateMismatch>,
    pub rejected_via_version: Vec<CrateMismatch>,
    pub should_match_name: bool,
    pub is_proc_macro: Option<bool>,
}

pub struct ArchiveMetadata {
    _archive: ArchiveRO,
    // points into self._archive
    data: *const [u8],
}

pub struct CratePaths {
    pub ident: String,
    pub dylib: Option<PathBuf>,
    pub rlib: Option<PathBuf>,
    pub rmeta: Option<PathBuf>,
}

pub const METADATA_FILENAME: &'static str = "rust.metadata.bin";

#[derive(Copy, Clone, PartialEq)]
enum CrateFlavor {
    Rlib,
    Rmeta,
    Dylib,
}

impl fmt::Display for CrateFlavor {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.write_str(match *self {
            CrateFlavor::Rlib => "rlib",
            CrateFlavor::Rmeta => "rmeta",
            CrateFlavor::Dylib => "dylib",
        })
    }
}

impl CratePaths {
    fn paths(&self) -> Vec<PathBuf> {
        self.dylib.iter().chain(self.rlib.iter()).chain(self.rmeta.iter()).cloned().collect()
    }
}

impl<'a> Context<'a> {
    pub fn maybe_load_library_crate(&mut self) -> Option<Library> {
        self.find_library_crate()
    }

    pub fn load_library_crate(&mut self) -> Library {
        self.find_library_crate().unwrap_or_else(|| self.report_errs())
    }

    pub fn report_errs(&mut self) -> ! {
        let add = match self.root {
            &None => String::new(),
            &Some(ref r) => format!(" which `{}` depends on", r.ident),
        };
        let mut err = if !self.rejected_via_hash.is_empty() {
            struct_span_err!(self.sess,
                             self.span,
                             E0460,
                             "found possibly newer version of crate `{}`{}",
                             self.ident,
                             add)
        } else if !self.rejected_via_triple.is_empty() {
            struct_span_err!(self.sess,
                             self.span,
                             E0461,
                             "couldn't find crate `{}` with expected target triple {}{}",
                             self.ident,
                             self.triple,
                             add)
        } else if !self.rejected_via_kind.is_empty() {
            struct_span_err!(self.sess,
                             self.span,
                             E0462,
                             "found staticlib `{}` instead of rlib or dylib{}",
                             self.ident,
                             add)
        } else if !self.rejected_via_version.is_empty() {
            struct_span_err!(self.sess,
                             self.span,
                             E0514,
                             "found crate `{}` compiled by an incompatible version of rustc{}",
                             self.ident,
                             add)
        } else {
            let mut err = struct_span_err!(self.sess,
                                           self.span,
                                           E0463,
                                           "can't find crate for `{}`{}",
                                           self.ident,
                                           add);

            if (self.ident == "std" || self.ident == "core")
                && self.triple != config::host_triple() {
                err.note(&format!("the `{}` target may not be installed", self.triple));
            }
            err.span_label(self.span, &format!("can't find crate"));
            err
        };

        if !self.rejected_via_triple.is_empty() {
            let mismatches = self.rejected_via_triple.iter();
            for (i, &CrateMismatch { ref path, ref got }) in mismatches.enumerate() {
                err.note(&format!("crate `{}`, path #{}, triple {}: {}",
                                  self.ident,
                                  i + 1,
                                  got,
                                  path.display()));
            }
        }
        if !self.rejected_via_hash.is_empty() {
            err.note("perhaps that crate needs to be recompiled?");
            let mismatches = self.rejected_via_hash.iter();
            for (i, &CrateMismatch { ref path, .. }) in mismatches.enumerate() {
                err.note(&format!("crate `{}` path #{}: {}", self.ident, i + 1, path.display()));
            }
            match self.root {
                &None => {}
                &Some(ref r) => {
                    for (i, path) in r.paths().iter().enumerate() {
                        err.note(&format!("crate `{}` path #{}: {}",
                                          r.ident,
                                          i + 1,
                                          path.display()));
                    }
                }
            }
        }
        if !self.rejected_via_kind.is_empty() {
            err.help("please recompile that crate using --crate-type lib");
            let mismatches = self.rejected_via_kind.iter();
            for (i, &CrateMismatch { ref path, .. }) in mismatches.enumerate() {
                err.note(&format!("crate `{}` path #{}: {}", self.ident, i + 1, path.display()));
            }
        }
        if !self.rejected_via_version.is_empty() {
            err.help(&format!("please recompile that crate using this compiler ({})",
                              rustc_version()));
            let mismatches = self.rejected_via_version.iter();
            for (i, &CrateMismatch { ref path, ref got }) in mismatches.enumerate() {
                err.note(&format!("crate `{}` path #{}: {} compiled by {:?}",
                                  self.ident,
                                  i + 1,
                                  path.display(),
                                  got));
            }
        }

        err.emit();
        self.sess.abort_if_errors();
        unreachable!();
    }

    fn find_library_crate(&mut self) -> Option<Library> {
        // If an SVH is specified, then this is a transitive dependency that
        // must be loaded via -L plus some filtering.
        if self.hash.is_none() {
            self.should_match_name = false;
            if let Some(s) = self.sess.opts.externs.get(self.crate_name) {
                return self.find_commandline_library(s.iter());
            }
            self.should_match_name = true;
        }

        let dypair = self.dylibname();
        let staticpair = self.staticlibname();

        // want: crate_name.dir_part() + prefix + crate_name.file_part + "-"
        let dylib_prefix = format!("{}{}", dypair.0, self.crate_name);
        let rlib_prefix = format!("lib{}", self.crate_name);
        let staticlib_prefix = format!("{}{}", staticpair.0, self.crate_name);

        let mut candidates = FxHashMap();
        let mut staticlibs = vec![];

        // First, find all possible candidate rlibs and dylibs purely based on
        // the name of the files themselves. We're trying to match against an
        // exact crate name and a possibly an exact hash.
        //
        // During this step, we can filter all found libraries based on the
        // name and id found in the crate id (we ignore the path portion for
        // filename matching), as well as the exact hash (if specified). If we
        // end up having many candidates, we must look at the metadata to
        // perform exact matches against hashes/crate ids. Note that opening up
        // the metadata is where we do an exact match against the full contents
        // of the crate id (path/name/id).
        //
        // The goal of this step is to look at as little metadata as possible.
        self.filesearch.search(|path, kind| {
            let file = match path.file_name().and_then(|s| s.to_str()) {
                None => return FileDoesntMatch,
                Some(file) => file,
            };
            let (hash, found_kind) = if file.starts_with(&rlib_prefix[..]) && file.ends_with(".rlib") {
                (&file[(rlib_prefix.len())..(file.len() - ".rlib".len())], CrateFlavor::Rlib)
            } else if file.starts_with(&rlib_prefix[..]) && file.ends_with(".rmeta") {
                (&file[(rlib_prefix.len())..(file.len() - ".rmeta".len())], CrateFlavor::Rmeta)
            } else if file.starts_with(&dylib_prefix) &&
                                         file.ends_with(&dypair.1) {
                (&file[(dylib_prefix.len())..(file.len() - dypair.1.len())], CrateFlavor::Dylib)
            } else {
                if file.starts_with(&staticlib_prefix[..]) && file.ends_with(&staticpair.1) {
                    staticlibs.push(CrateMismatch {
                        path: path.to_path_buf(),
                        got: "static".to_string(),
                    });
                }
                return FileDoesntMatch;
            };
            info!("lib candidate: {}", path.display());

            let hash_str = hash.to_string();
            let slot = candidates.entry(hash_str)
                .or_insert_with(|| (FxHashMap(), FxHashMap(), FxHashMap()));
            let (ref mut rlibs, ref mut rmetas, ref mut dylibs) = *slot;
            fs::canonicalize(path)
                .map(|p| {
                    match found_kind {
                        CrateFlavor::Rlib => { rlibs.insert(p, kind); }
                        CrateFlavor::Rmeta => { rmetas.insert(p, kind); }
                        CrateFlavor::Dylib => { dylibs.insert(p, kind); }
                    }
                    FileMatches
                })
                .unwrap_or(FileDoesntMatch)
        });
        self.rejected_via_kind.extend(staticlibs);

        // We have now collected all known libraries into a set of candidates
        // keyed of the filename hash listed. For each filename, we also have a
        // list of rlibs/dylibs that apply. Here, we map each of these lists
        // (per hash), to a Library candidate for returning.
        //
        // A Library candidate is created if the metadata for the set of
        // libraries corresponds to the crate id and hash criteria that this
        // search is being performed for.
        let mut libraries = FxHashMap();
        for (_hash, (rlibs, rmetas, dylibs)) in candidates {
            let mut slot = None;
            let rlib = self.extract_one(rlibs, CrateFlavor::Rlib, &mut slot);
            let rmeta = self.extract_one(rmetas, CrateFlavor::Rmeta, &mut slot);
            let dylib = self.extract_one(dylibs, CrateFlavor::Dylib, &mut slot);
            if let Some((h, m)) = slot {
                libraries.insert(h,
                                 Library {
                                     dylib: dylib,
                                     rlib: rlib,
                                     rmeta: rmeta,
                                     metadata: m,
                                 });
            }
        }

        // Having now translated all relevant found hashes into libraries, see
        // what we've got and figure out if we found multiple candidates for
        // libraries or not.
        match libraries.len() {
            0 => None,
            1 => Some(libraries.into_iter().next().unwrap().1),
            _ => {
                let mut err = struct_span_err!(self.sess,
                                               self.span,
                                               E0464,
                                               "multiple matching crates for `{}`",
                                               self.crate_name);
                err.note("candidates:");
                for (_, lib) in libraries {
                    if let Some((ref p, _)) = lib.dylib {
                        err.note(&format!("path: {}", p.display()));
                    }
                    if let Some((ref p, _)) = lib.rlib {
                        err.note(&format!("path: {}", p.display()));
                    }
                    note_crate_name(&mut err, &lib.metadata.get_root().name);
                }
                err.emit();
                None
            }
        }
    }

    // Attempts to extract *one* library from the set `m`. If the set has no
    // elements, `None` is returned. If the set has more than one element, then
    // the errors and notes are emitted about the set of libraries.
    //
    // With only one library in the set, this function will extract it, and then
    // read the metadata from it if `*slot` is `None`. If the metadata couldn't
    // be read, it is assumed that the file isn't a valid rust library (no
    // errors are emitted).
    fn extract_one(&mut self,
                   m: FxHashMap<PathBuf, PathKind>,
                   flavor: CrateFlavor,
                   slot: &mut Option<(Svh, MetadataBlob)>)
                   -> Option<(PathBuf, PathKind)> {
        let mut ret: Option<(PathBuf, PathKind)> = None;
        let mut error = 0;

        if slot.is_some() {
            // FIXME(#10786): for an optimization, we only read one of the
            //                libraries' metadata sections. In theory we should
            //                read both, but reading dylib metadata is quite
            //                slow.
            if m.is_empty() {
                return None;
            } else if m.len() == 1 {
                return Some(m.into_iter().next().unwrap());
            }
        }

        let mut err: Option<DiagnosticBuilder> = None;
        for (lib, kind) in m {
            info!("{} reading metadata from: {}", flavor, lib.display());
            let (hash, metadata) = match get_metadata_section(self.target, flavor, &lib) {
                Ok(blob) => {
                    if let Some(h) = self.crate_matches(&blob, &lib) {
                        (h, blob)
                    } else {
                        info!("metadata mismatch");
                        continue;
                    }
                }
                Err(err) => {
                    info!("no metadata found: {}", err);
                    continue;
                }
            };
            // If we see multiple hashes, emit an error about duplicate candidates.
            if slot.as_ref().map_or(false, |s| s.0 != hash) {
                let mut e = struct_span_err!(self.sess,
                                             self.span,
                                             E0465,
                                             "multiple {} candidates for `{}` found",
                                             flavor,
                                             self.crate_name);
                e.span_note(self.span,
                            &format!(r"candidate #1: {}",
                                     ret.as_ref()
                                         .unwrap()
                                         .0
                                         .display()));
                if let Some(ref mut e) = err {
                    e.emit();
                }
                err = Some(e);
                error = 1;
                *slot = None;
            }
            if error > 0 {
                error += 1;
                err.as_mut().unwrap().span_note(self.span,
                                                &format!(r"candidate #{}: {}",
                                                         error,
                                                         lib.display()));
                continue;
            }
            *slot = Some((hash, metadata));
            ret = Some((lib, kind));
        }

        if error > 0 {
            err.unwrap().emit();
            None
        } else {
            ret
        }
    }

    fn crate_matches(&mut self, metadata: &MetadataBlob, libpath: &Path) -> Option<Svh> {
        let root = metadata.get_root();
        if let Some(is_proc_macro) = self.is_proc_macro {
            if root.macro_derive_registrar.is_some() != is_proc_macro {
                return None;
            }
        }

        let rustc_version = rustc_version();
        if root.rustc_version != rustc_version {
            info!("Rejecting via version: expected {} got {}",
                  rustc_version,
                  root.rustc_version);
            self.rejected_via_version.push(CrateMismatch {
                path: libpath.to_path_buf(),
                got: root.rustc_version,
            });
            return None;
        }

        if self.should_match_name {
            if self.crate_name != root.name {
                info!("Rejecting via crate name");
                return None;
            }
        }

        if root.triple != self.triple {
            info!("Rejecting via crate triple: expected {} got {}",
                  self.triple,
                  root.triple);
            self.rejected_via_triple.push(CrateMismatch {
                path: libpath.to_path_buf(),
                got: root.triple,
            });
            return None;
        }

        if let Some(myhash) = self.hash {
            if *myhash != root.hash {
                info!("Rejecting via hash: expected {} got {}", *myhash, root.hash);
                self.rejected_via_hash.push(CrateMismatch {
                    path: libpath.to_path_buf(),
                    got: myhash.to_string(),
                });
                return None;
            }
        }

        Some(root.hash)
    }


    // Returns the corresponding (prefix, suffix) that files need to have for
    // dynamic libraries
    fn dylibname(&self) -> (String, String) {
        let t = &self.target;
        (t.options.dll_prefix.clone(), t.options.dll_suffix.clone())
    }

    // Returns the corresponding (prefix, suffix) that files need to have for
    // static libraries
    fn staticlibname(&self) -> (String, String) {
        let t = &self.target;
        (t.options.staticlib_prefix.clone(), t.options.staticlib_suffix.clone())
    }

    fn find_commandline_library<'b, LOCS>(&mut self, locs: LOCS) -> Option<Library>
        where LOCS: Iterator<Item = &'b String>
    {
        // First, filter out all libraries that look suspicious. We only accept
        // files which actually exist that have the correct naming scheme for
        // rlibs/dylibs.
        let sess = self.sess;
        let dylibname = self.dylibname();
        let mut rlibs = FxHashMap();
        let mut rmetas = FxHashMap();
        let mut dylibs = FxHashMap();
        {
            let locs = locs.map(|l| PathBuf::from(l)).filter(|loc| {
                if !loc.exists() {
                    sess.err(&format!("extern location for {} does not exist: {}",
                                      self.crate_name,
                                      loc.display()));
                    return false;
                }
                let file = match loc.file_name().and_then(|s| s.to_str()) {
                    Some(file) => file,
                    None => {
                        sess.err(&format!("extern location for {} is not a file: {}",
                                          self.crate_name,
                                          loc.display()));
                        return false;
                    }
                };
                if file.starts_with("lib") && file.ends_with(".rlib") {
                    return true;
                } else {
                    let (ref prefix, ref suffix) = dylibname;
                    if file.starts_with(&prefix[..]) && file.ends_with(&suffix[..]) {
                        return true;
                    }
                }
                sess.struct_err(&format!("extern location for {} is of an unknown type: {}",
                                         self.crate_name,
                                         loc.display()))
                    .help(&format!("file name should be lib*.rlib or {}*.{}",
                                   dylibname.0,
                                   dylibname.1))
                    .emit();
                false
            });

            // Now that we have an iterator of good candidates, make sure
            // there's at most one rlib and at most one dylib.
            for loc in locs {
                if loc.file_name().unwrap().to_str().unwrap().ends_with(".rlib") {
                    rlibs.insert(fs::canonicalize(&loc).unwrap(), PathKind::ExternFlag);
                } else if loc.file_name().unwrap().to_str().unwrap().ends_with(".rmeta") {
                    rmetas.insert(fs::canonicalize(&loc).unwrap(), PathKind::ExternFlag);
                } else {
                    dylibs.insert(fs::canonicalize(&loc).unwrap(), PathKind::ExternFlag);
                }
            }
        };

        // Extract the rlib/dylib pair.
        let mut slot = None;
        let rlib = self.extract_one(rlibs, CrateFlavor::Rlib, &mut slot);
        let rmeta = self.extract_one(rmetas, CrateFlavor::Rmeta, &mut slot);
        let dylib = self.extract_one(dylibs, CrateFlavor::Dylib, &mut slot);

        if rlib.is_none() && rmeta.is_none() && dylib.is_none() {
            return None;
        }
        match slot {
            Some((_, metadata)) => {
                Some(Library {
                    dylib: dylib,
                    rlib: rlib,
                    rmeta: rmeta,
                    metadata: metadata,
                })
            }
            None => None,
        }
    }
}

pub fn note_crate_name(err: &mut DiagnosticBuilder, name: &str) {
    err.note(&format!("crate name: {}", name));
}

impl ArchiveMetadata {
    fn new(ar: ArchiveRO) -> Option<ArchiveMetadata> {
        let data = {
            let section = ar.iter()
                .filter_map(|s| s.ok())
                .find(|sect| sect.name() == Some(METADATA_FILENAME));
            match section {
                Some(s) => s.data() as *const [u8],
                None => {
                    debug!("didn't find '{}' in the archive", METADATA_FILENAME);
                    return None;
                }
            }
        };

        Some(ArchiveMetadata {
            _archive: ar,
            data: data,
        })
    }

    pub fn as_slice<'a>(&'a self) -> &'a [u8] {
        unsafe { &*self.data }
    }
}

fn verify_decompressed_encoding_version(blob: &MetadataBlob,
                                        filename: &Path)
                                        -> Result<(), String> {
    if !blob.is_compatible() {
        Err((format!("incompatible metadata version found: '{}'",
                     filename.display())))
    } else {
        Ok(())
    }
}

// Just a small wrapper to time how long reading metadata takes.
fn get_metadata_section(target: &Target,
                        flavor: CrateFlavor,
                        filename: &Path)
                        -> Result<MetadataBlob, String> {
    let start = Instant::now();
    let ret = get_metadata_section_imp(target, flavor, filename);
    info!("reading {:?} => {:?}",
          filename.file_name().unwrap(),
          start.elapsed());
    return ret;
}

fn get_metadata_section_imp(target: &Target,
                            flavor: CrateFlavor,
                            filename: &Path)
                            -> Result<MetadataBlob, String> {
    if !filename.exists() {
        return Err(format!("no such file: '{}'", filename.display()));
    }
    if flavor == CrateFlavor::Rlib || flavor == CrateFlavor::Rmeta {
        // Use ArchiveRO for speed here, it's backed by LLVM and uses mmap
        // internally to read the file. We also avoid even using a memcpy by
        // just keeping the archive along while the metadata is in use.
        let archive = match ArchiveRO::open(filename) {
            Some(ar) => ar,
            None => {
                debug!("llvm didn't like `{}`", filename.display());
                return Err(format!("failed to read rlib metadata: '{}'", filename.display()));
            }
        };
        return match ArchiveMetadata::new(archive).map(|ar| MetadataBlob::Archive(ar)) {
            None => Err(format!("failed to read rlib metadata: '{}'", filename.display())),
            Some(blob) => {
                verify_decompressed_encoding_version(&blob, filename)?;
                Ok(blob)
            }
        };
    }
    unsafe {
        let buf = common::path2cstr(filename);
        let mb = llvm::LLVMRustCreateMemoryBufferWithContentsOfFile(buf.as_ptr());
        if mb as isize == 0 {
            return Err(format!("error reading library: '{}'", filename.display()));
        }
        let of = match ObjectFile::new(mb) {
            Some(of) => of,
            _ => {
                return Err((format!("provided path not an object file: '{}'", filename.display())))
            }
        };
        let si = mk_section_iter(of.llof);
        while llvm::LLVMIsSectionIteratorAtEnd(of.llof, si.llsi) == False {
            let mut name_buf = ptr::null();
            let name_len = llvm::LLVMRustGetSectionName(si.llsi, &mut name_buf);
            let name = slice::from_raw_parts(name_buf as *const u8, name_len as usize).to_vec();
            let name = String::from_utf8(name).unwrap();
            debug!("get_metadata_section: name {}", name);
            if read_meta_section_name(target) == name {
                let cbuf = llvm::LLVMGetSectionContents(si.llsi);
                let csz = llvm::LLVMGetSectionSize(si.llsi) as usize;
                let cvbuf: *const u8 = cbuf as *const u8;
                let vlen = METADATA_HEADER.len();
                debug!("checking {} bytes of metadata-version stamp", vlen);
                let minsz = cmp::min(vlen, csz);
                let buf0 = slice::from_raw_parts(cvbuf, minsz);
                let version_ok = buf0 == METADATA_HEADER;
                if !version_ok {
                    return Err((format!("incompatible metadata version found: '{}'",
                                        filename.display())));
                }

                let cvbuf1 = cvbuf.offset(vlen as isize);
                debug!("inflating {} bytes of compressed metadata", csz - vlen);
                let bytes = slice::from_raw_parts(cvbuf1, csz - vlen);
                match flate::inflate_bytes(bytes) {
                    Ok(inflated) => {
                        let blob = MetadataBlob::Inflated(inflated);
                        verify_decompressed_encoding_version(&blob, filename)?;
                        return Ok(blob);
                    }
                    Err(_) => {}
                }
            }
            llvm::LLVMMoveToNextSection(si.llsi);
        }
        Err(format!("metadata not found: '{}'", filename.display()))
    }
}

pub fn meta_section_name(target: &Target) -> &'static str {
    // Historical note:
    //
    // When using link.exe it was seen that the section name `.note.rustc`
    // was getting shortened to `.note.ru`, and according to the PE and COFF
    // specification:
    //
    // > Executable images do not use a string table and do not support
    // > section names longer than 8 characters
    //
    // https://msdn.microsoft.com/en-us/library/windows/hardware/gg463119.aspx
    //
    // As a result, we choose a slightly shorter name! As to why
    // `.note.rustc` works on MinGW, that's another good question...

    if target.options.is_like_osx {
        "__DATA,.rustc"
    } else {
        ".rustc"
    }
}

pub fn read_meta_section_name(_target: &Target) -> &'static str {
    ".rustc"
}

// A diagnostic function for dumping crate metadata to an output stream
pub fn list_file_metadata(target: &Target, path: &Path, out: &mut io::Write) -> io::Result<()> {
    let filename = path.file_name().unwrap().to_str().unwrap();
    let flavor = if filename.ends_with(".rlib") {
        CrateFlavor::Rlib
    } else if filename.ends_with(".rmeta") {
        CrateFlavor::Rmeta
    } else {
        CrateFlavor::Dylib
    };
    match get_metadata_section(target, flavor, path) {
        Ok(metadata) => metadata.list_crate_metadata(out),
        Err(msg) => write!(out, "{}\n", msg),
    }
}