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    //! Support for symbolication using the `gimli` crate on crates.io
    //!
    //! This implementation is largely a work in progress and is off by default for
    //! all platforms, but it's hoped to be developed over time! Long-term this is
    //! intended to wholesale replace the `libbacktrace.rs` implementation.

    use self::gimli::read::EndianSlice;
    use self::gimli::LittleEndian as Endian;
    use self::mmap::Mmap;
    use self::stash::Stash;
    use super::BytesOrWideString;
    use super::ResolveWhat;
    use super::SymbolName;
    use addr2line::gimli;
    use core::convert::TryInto;
    use core::mem;
    use core::u32;
    use libc::c_void;
    use mystd::ffi::OsString;
    use mystd::fs::File;
    use mystd::path::Path;
    use mystd::prelude::v1::*;

    #[cfg(backtrace_in_libstd)]
    mod mystd {
        pub use crate::*;
    }
    #[cfg(not(backtrace_in_libstd))]
    extern crate std as mystd;

    cfg_if::cfg_if! {
        if #[cfg(windows)] {
            #[path = "gimli/mmap_windows.rs"]
            mod mmap;
        } else if #[cfg(any(
            target_os = "android",
            target_os = "freebsd",
            target_os = "fuchsia",
            target_os = "ios",
            target_os = "linux",
            target_os = "macos",
            target_os = "openbsd",
            target_os = "solaris",
        ))] {
            #[path = "gimli/mmap_unix.rs"]
            mod mmap;
        } else {
            #[path = "gimli/mmap_fake.rs"]
            mod mmap;
        }
    }

    mod stash;

    const MAPPINGS_CACHE_SIZE: usize = 4;

    struct Context<'a> {
        dwarf: addr2line::Context<EndianSlice<'a, Endian>>,
        object: Object<'a>,
    }

    struct Mapping {
        // 'static lifetime is a lie to hack around lack of support for self-referential structs.
        cx: Context<'static>,
        _map: Mmap,
        _stash: Stash,
    }

    fn cx<'data>(stash: &'data Stash, object: Object<'data>) -> Option<Context<'data>> {
        fn load_section<'data, S>(stash: &'data Stash, obj: &Object<'data>) -> S
        where
            S: gimli::Section<gimli::EndianSlice<'data, Endian>>,
        {
            let data = obj.section(stash, S::section_name()).unwrap_or(&[]);
            S::from(EndianSlice::new(data, Endian))
        }

        let dwarf = addr2line::Context::from_sections(
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            load_section(stash, &object),
            gimli::EndianSlice::new(&[], Endian),
        )
        .ok()?;
        Some(Context { dwarf, object })
    }

    macro_rules! mk {
        (Mapping { $map:expr, $inner:expr, $stash:expr }) => {{
            fn assert_lifetimes<'a>(_: &'a Mmap, _: &Context<'a>, _: &'a Stash) {}
            assert_lifetimes(&$map, &$inner, &$stash);
            Mapping {
                // Convert to 'static lifetimes since the symbols should
                // only borrow `map` and `stash` and we're preserving them below.
                cx: unsafe { core::mem::transmute::<Context<'_>, Context<'static>>($inner) },
                _map: $map,
                _stash: $stash,
            }
        }};
    }

    fn mmap(path: &Path) -> Option<Mmap> {
        let file = File::open(path).ok()?;
        let len = file.metadata().ok()?.len().try_into().ok()?;
        unsafe { Mmap::map(&file, len) }
    }

    cfg_if::cfg_if! {
        if #[cfg(windows)] {
            use core::mem::MaybeUninit;
            use super::super::windows::*;
            use mystd::os::windows::prelude::*;
            use alloc::vec;

            mod coff;
            use self::coff::Object;

            // For loading native libraries on Windows, see some discussion on
            // rust-lang/rust#71060 for the various strategies here.
            fn native_libraries() -> Vec<Library> {
                let mut ret = Vec::new();
                unsafe { add_loaded_images(&mut ret); }
                return ret;
            }

            unsafe fn add_loaded_images(ret: &mut Vec<Library>) {
                let snap = CreateToolhelp32Snapshot(TH32CS_SNAPMODULE, 0);
                if snap == INVALID_HANDLE_VALUE {
                    return;
                }

                let mut me = MaybeUninit::<MODULEENTRY32W>::zeroed().assume_init();
                me.dwSize = mem::size_of_val(&me) as DWORD;
                if Module32FirstW(snap, &mut me) == TRUE {
                    loop {
                        if let Some(lib) = load_library(&me) {
                            ret.push(lib);
                        }

                        if Module32NextW(snap, &mut me) != TRUE {
                            break;
                        }
                    }

                }

                CloseHandle(snap);
            }

            unsafe fn load_library(me: &MODULEENTRY32W) -> Option<Library> {
                let pos = me
                    .szExePath
                    .iter()
                    .position(|i| *i == 0)
                    .unwrap_or(me.szExePath.len());
                let name = OsString::from_wide(&me.szExePath[..pos]);

                // MinGW libraries currently don't support ASLR
                // (rust-lang/rust#16514), but DLLs can still be relocated around in
                // the address space. It appears that addresses in debug info are
                // all as-if this library was loaded at its "image base", which is a
                // field in its COFF file headers. Since this is what debuginfo
                // seems to list we parse the symbol table and store addresses as if
                // the library was loaded at "image base" as well.
                //
                // The library may not be loaded at "image base", however.
                // (presumably something else may be loaded there?) This is where
                // the `bias` field comes into play, and we need to figure out the
                // value of `bias` here. Unfortunately though it's not clear how to
                // acquire this from a loaded module. What we do have, however, is
                // the actual load address (`modBaseAddr`).
                //
                // As a bit of a cop-out for now we mmap the file, read the file
                // header information, then drop the mmap. This is wasteful because
                // we'll probably reopen the mmap later, but this should work well
                // enough for now.
                //
                // Once we have the `image_base` (desired load location) and the
                // `base_addr` (actual load location) we can fill in the `bias`
                // (difference between the actual and desired) and then the stated
                // address of each segment is the `image_base` since that's what the
                // file says.
                //
                // For now it appears that unlike ELF/MachO we can make do with one
                // segment per library, using `modBaseSize` as the whole size.
                let mmap = mmap(name.as_ref())?;
                let image_base = coff::get_image_base(&mmap)?;
                let base_addr = me.modBaseAddr as usize;
                Some(Library {
                    name,
                    bias: base_addr.wrapping_sub(image_base),
                    segments: vec![LibrarySegment {
                        stated_virtual_memory_address: image_base,
                        len: me.modBaseSize as usize,
                    }],
                })
            }
        } else if #[cfg(target_os = "macos")] {
            // macOS uses the Mach-O file format and uses DYLD-specific APIs to
            // load a list of native libraries that are part of the appplication.

            use mystd::os::unix::prelude::*;
            use mystd::ffi::{OsStr, CStr};

            mod macho;
            use self::macho::Object;

            #[allow(deprecated)]
            fn native_libraries() -> Vec<Library> {
                let mut ret = Vec::new();
                let images = unsafe { libc::_dyld_image_count() };
                for i in 0..images {
                    ret.extend(native_library(i));
                }
                return ret;
            }

            #[allow(deprecated)]
            fn native_library(i: u32) -> Option<Library> {
                use object::macho;
                use object::read::macho::{MachHeader, Segment};
                use object::{Bytes, NativeEndian};

                // Fetch the name of this library which corresponds to the path of
                // where to load it as well.
                let name = unsafe {
                    let name = libc::_dyld_get_image_name(i);
                    if name.is_null() {
                        return None;
                    }
                    CStr::from_ptr(name)
                };

                // Load the image header of this library and delegate to `object` to
                // parse all the load commands so we can figure out all the segments
                // involved here.
                let (mut load_commands, endian) = unsafe {
                    let header = libc::_dyld_get_image_header(i);
                    if header.is_null() {
                        return None;
                    }
                    match (*header).magic {
                        macho::MH_MAGIC => {
                            let endian = NativeEndian;
                            let header = &*(header as *const macho::MachHeader32<NativeEndian>);
                            let data = core::slice::from_raw_parts(
                                header as *const _ as *const u8,
                                mem::size_of_val(header) + header.sizeofcmds.get(endian) as usize
                            );
                            (header.load_commands(endian, Bytes(data)).ok()?, endian)
                        }
                        macho::MH_MAGIC_64 => {
                            let endian = NativeEndian;
                            let header = &*(header as *const macho::MachHeader64<NativeEndian>);
                            let data = core::slice::from_raw_parts(
                                header as *const _ as *const u8,
                                mem::size_of_val(header) + header.sizeofcmds.get(endian) as usize
                            );
                            (header.load_commands(endian, Bytes(data)).ok()?, endian)
                        }
                        _ => return None,
                    }
                };

                // Iterate over the segments and register known regions for segments
                // that we find. Additionally record information bout text segments
                // for processing later, see comments below.
                let mut segments = Vec::new();
                let mut first_text = 0;
                let mut text_fileoff_zero = false;
                while let Some(cmd) = load_commands.next().ok()? {
                    if let Some((seg, _)) = cmd.segment_32().ok()? {
                        if seg.name() == b"__TEXT" {
                            first_text = segments.len();
                            if seg.fileoff(endian) == 0 && seg.filesize(endian) > 0 {
                                text_fileoff_zero = true;
                            }
                        }
                        segments.push(LibrarySegment {
                            len: seg.vmsize(endian).try_into().ok()?,
                            stated_virtual_memory_address: seg.vmaddr(endian).try_into().ok()?,
                        });
                    }
                    if let Some((seg, _)) = cmd.segment_64().ok()? {
                        if seg.name() == b"__TEXT" {
                            first_text = segments.len();
                            if seg.fileoff(endian) == 0 && seg.filesize(endian) > 0 {
                                text_fileoff_zero = true;
                            }
                        }
                        segments.push(LibrarySegment {
                            len: seg.vmsize(endian).try_into().ok()?,
                            stated_virtual_memory_address: seg.vmaddr(endian).try_into().ok()?,
                        });
                    }
                }

                // Determine the "slide" for this library which ends up being the
                // bias we use to figure out where in memory objects are loaded.
                // This is a bit of a weird computation though and is the result of
                // trying a few things in the wild and seeing what sticks.
                //
                // The general idea is that the `bias` plus a segment's
                // `stated_virtual_memory_address` is going to be where in the
                // actual address space the segment resides. The other thing we rely
                // on though is that a real address minus the `bias` is the index to
                // look up in the symbol table and debuginfo.
                //
                // It turns out, though, that for system loaded libraries these
                // calculations are incorrect. For native executables, however, it
                // appears correct. Lifting some logic from LLDB's source it has
                // some special-casing for the first `__TEXT` section loaded from
                // file offset 0 with a nonzero size. For whatever reason when this
                // is present it appears to mean that the symbol table is relative
                // to just the vmaddr slide for the library. If it's *not* present
                // then the symbol table is relative to the the vmaddr slide plus
                // the segment's stated address.
                //
                // To handle this situation if we *don't* find a text section at
                // file offset zero then we increase the bias by the first text
                // sections's stated address and decrease all stated addresses by
                // that amount as well. That way the symbol table is always appears
                // relative to the library's bias amount. This appears to have the
                // right results for symbolizing via the symbol table.
                //
                // Honestly I'm not entirely sure whether this is right or if
                // there's something else that should indicate how to do this. For
                // now though this seems to work well enough (?) and we should
                // always be able to tweak this over time if necessary.
                //
                // For some more information see #318
                let mut slide = unsafe { libc::_dyld_get_image_vmaddr_slide(i) as usize };
                if !text_fileoff_zero {
                    let adjust = segments[first_text].stated_virtual_memory_address;
                    for segment in segments.iter_mut() {
                        segment.stated_virtual_memory_address -= adjust;
                    }
                    slide += adjust;
                }

                Some(Library {
                    name: OsStr::from_bytes(name.to_bytes()).to_owned(),
                    segments,
                    bias: slide,
                })
            }
        } else if #[cfg(any(
            target_os = "linux",
            target_os = "fuchsia",
        ))] {
            // Other Unix (e.g. Linux) platforms use ELF as an object file format
            // and typically implement an API called `dl_iterate_phdr` to load
            // native libraries.

            use mystd::os::unix::prelude::*;
            use mystd::ffi::{OsStr, CStr};

            mod elf;
            use self::elf::Object;

            fn native_libraries() -> Vec<Library> {
                let mut ret = Vec::new();
                unsafe {
                    libc::dl_iterate_phdr(Some(callback), &mut ret as *mut _ as *mut _);
                }
                return ret;
            }

            unsafe extern "C" fn callback(
                info: *mut libc::dl_phdr_info,
                _size: libc::size_t,
                vec: *mut libc::c_void,
            ) -> libc::c_int {
                let libs = &mut *(vec as *mut Vec<Library>);
                let name = if (*info).dlpi_name.is_null() || *(*info).dlpi_name == 0{
                    if libs.is_empty() {
                        mystd::env::current_exe().map(|e| e.into()).unwrap_or_default()
                    } else {
                        OsString::new()
                    }
                } else {
                    let bytes = CStr::from_ptr((*info).dlpi_name).to_bytes();
                    OsStr::from_bytes(bytes).to_owned()
                };
                let headers = core::slice::from_raw_parts((*info).dlpi_phdr, (*info).dlpi_phnum as usize);
                libs.push(Library {
                    name,
                    segments: headers
                        .iter()
                        .map(|header| LibrarySegment {
                            len: (*header).p_memsz as usize,
                            stated_virtual_memory_address: (*header).p_vaddr as usize,
                        })
                        .collect(),
                    bias: (*info).dlpi_addr as usize,
                });
                0
            }
        } else if #[cfg(target_env = "libnx")] {
            // DevkitA64 doesn't natively support debug info, but the build system will place debug
            // info at the path `romfs:/debug_info.elf`.
            mod elf;
            use self::elf::Object;

            fn native_libraries() -> Vec<Library> {
                extern "C" {
                    static __start__: u8;
                }

                let bias = unsafe { &__start__ } as *const u8 as usize;

                let mut ret = Vec::new();
                let mut segments = Vec::new();
                segments.push(LibrarySegment {
                    stated_virtual_memory_address: 0,
                    len: usize::max_value() - bias,
                });

                let path = "romfs:/debug_info.elf";
                ret.push(Library {
                    name: path.into(),
                    segments,
                    bias,
                });

                ret
            }
        } else {
            // Everything else should use ELF, but doesn't know how to load native
            // libraries.

            use mystd::os::unix::prelude::*;
            mod elf;
            use self::elf::Object;

            fn native_libraries() -> Vec<Library> {
                Vec::new()
            }
        }
    }

    #[derive(Default)]
    struct Cache {
        /// All known shared libraries that have been loaded.
        libraries: Vec<Library>,

        /// Mappings cache where we retain parsed dwarf information.
        ///
        /// This list has a fixed capacity for its entire liftime which never
        /// increases. The `usize` element of each pair is an index into `libraries`
        /// above where `usize::max_value()` represents the current executable. The
        /// `Mapping` is corresponding parsed dwarf information.
        ///
        /// Note that this is basically an LRU cache and we'll be shifting things
        /// around in here as we symbolize addresses.
        mappings: Vec<(usize, Mapping)>,
    }

    struct Library {
        name: OsString,
        /// Segments of this library loaded into memory, and where they're loaded.
        segments: Vec<LibrarySegment>,
        /// The "bias" of this library, typically where it's loaded into memory.
        /// This value is added to each segment's stated address to get the actual
        /// virtual memory address that the segment is loaded into. Additionally
        /// this bias is subtracted from real virtual memory addresses to index into
        /// debuginfo and the symbol table.
        bias: usize,
    }

    struct LibrarySegment {
        /// The stated address of this segment in the object file. This is not
        /// actually where the segment is loaded, but rather this address plus the
        /// containing library's `bias` is where to find it.
        stated_virtual_memory_address: usize,
        /// The size of ths segment in memory.
        len: usize,
    }

    // unsafe because this is required to be externally synchronized
    pub unsafe fn clear_symbol_cache() {
        Cache::with_global(|cache| cache.mappings.clear());
    }

    impl Cache {
        fn new() -> Cache {
            Cache {
                mappings: Vec::with_capacity(MAPPINGS_CACHE_SIZE),
                libraries: native_libraries(),
            }
        }

        // unsafe because this is required to be externally synchronized
        unsafe fn with_global(f: impl FnOnce(&mut Self)) {
            // A very small, very simple LRU cache for debug info mappings.
            //
            // The hit rate should be very high, since the typical stack doesn't cross
            // between many shared libraries.
            //
            // The `addr2line::Context` structures are pretty expensive to create. Its
            // cost is expected to be amortized by subsequent `locate` queries, which
            // leverage the structures built when constructing `addr2line::Context`s to
            // get nice speedups. If we didn't have this cache, that amortization would
            // never happen, and symbolicating backtraces would be ssssllllooooowwww.
            static mut MAPPINGS_CACHE: Option<Cache> = None;

            f(MAPPINGS_CACHE.get_or_insert_with(|| Cache::new()))
        }

        fn avma_to_svma(&self, addr: *const u8) -> Option<(usize, *const u8)> {
            self.libraries
                .iter()
                .enumerate()
                .filter_map(|(i, lib)| {
                    // First up, test if this `lib` has any segment containing the
                    // `addr` (handling relocation). If this check passes then we
                    // can continue below and actually translate the address.
                    //
                    // Note that we're using `wrapping_add` here to avoid overflow
                    // checks. It's been seen in the wild that the SVMA + bias
                    // computation overflows. It seems a bit odd that would happen
                    // but there's not a huge amount we can do about it other than
                    // probably just ignore those segments since they're likely
                    // pointing off into space. This originally came up in
                    // rust-lang/backtrace-rs#329.
                    if !lib.segments.iter().any(|s| {
                        let svma = s.stated_virtual_memory_address;
                        let start = svma.wrapping_add(lib.bias);
                        let end = start.wrapping_add(s.len);
                        let address = addr as usize;
                        start <= address && address < end
                    }) {
                        return None;
                    }

                    // Now that we know `lib` contains `addr`, we can offset with
                    // the bias to find the stated virutal memory address.
                    let svma = (addr as usize).wrapping_sub(lib.bias);
                    Some((i, svma as *const u8))
                })
                .next()
        }

        fn mapping_for_lib<'a>(&'a mut self, lib: usize) -> Option<&'a Context<'a>> {
            let idx = self.mappings.iter().position(|(idx, _)| *idx == lib);

            // Invariant: after this conditional completes without early returning
            // from an error, the cache entry for this path is at index 0.

            if let Some(idx) = idx {
                // When the mapping is already in the cache, move it to the front.
                if idx != 0 {
                    let entry = self.mappings.remove(idx);
                    self.mappings.insert(0, entry);
                }
            } else {
                // When the mapping is not in the cache, create a new mapping,
                // insert it into the front of the cache, and evict the oldest cache
                // entry if necessary.
                let name = &self.libraries[lib].name;
                let mapping = Mapping::new(name.as_ref())?;

                if self.mappings.len() == MAPPINGS_CACHE_SIZE {
                    self.mappings.pop();
                }

                self.mappings.insert(0, (lib, mapping));
            }

            let cx: &'a Context<'static> = &self.mappings[0].1.cx;
            // don't leak the `'static` lifetime, make sure it's scoped to just
            // ourselves
            Some(unsafe { mem::transmute::<&'a Context<'static>, &'a Context<'a>>(cx) })
        }
    }

    pub unsafe fn resolve(what: ResolveWhat<'_>, cb: &mut dyn FnMut(&super::Symbol)) {
        let addr = what.address_or_ip();
        let mut call = |sym: Symbol<'_>| {
            // Extend the lifetime of `sym` to `'static` since we are unfortunately
            // required to here, but it's ony ever going out as a reference so no
            // reference to it should be persisted beyond this frame anyway.
            let sym = mem::transmute::<Symbol<'_>, Symbol<'static>>(sym);
            (cb)(&super::Symbol { inner: sym });
        };

        Cache::with_global(|cache| {
            let (lib, addr) = match cache.avma_to_svma(addr as *const u8) {
                Some(pair) => pair,
                None => return,
            };

            // Finally, get a cached mapping or create a new mapping for this file, and
            // evaluate the DWARF info to find the file/line/name for this address.
            let cx = match cache.mapping_for_lib(lib) {
                Some(cx) => cx,
                None => return,
            };
            let mut any_frames = false;
            if let Ok(mut frames) = cx.dwarf.find_frames(addr as u64) {
                while let Ok(Some(frame)) = frames.next() {
                    any_frames = true;
                    call(Symbol::Frame {
                        addr: addr as *mut c_void,
                        location: frame.location,
                        name: frame.function.map(|f| f.name.slice()),
                    });
                }
            }

            if !any_frames {
                if let Some(name) = cx.object.search_symtab(addr as u64) {
                    call(Symbol::Symtab {
                        addr: addr as *mut c_void,
                        name,
                    });
                }
            }
        });
    }

    pub enum Symbol<'a> {
        /// We were able to locate frame information for this symbol, and
        /// `addr2line`'s frame internally has all the nitty gritty details.
        Frame {
            addr: *mut c_void,
            location: Option<addr2line::Location<'a>>,
            name: Option<&'a [u8]>,
        },
        /// Couldn't find debug information, but we found it in the symbol table of
        /// the elf executable.
        Symtab { addr: *mut c_void, name: &'a [u8] },
    }

    impl Symbol<'_> {
        pub fn name(&self) -> Option<SymbolName<'_>> {
            match self {
                Symbol::Frame { name, .. } => {
                    let name = name.as_ref()?;
                    Some(SymbolName::new(name))
                }
                Symbol::Symtab { name, .. } => Some(SymbolName::new(name)),
            }
        }

        pub fn addr(&self) -> Option<*mut c_void> {
            match self {
                Symbol::Frame { addr, .. } => Some(*addr),
                Symbol::Symtab { .. } => None,
            }
        }

        pub fn filename_raw(&self) -> Option<BytesOrWideString<'_>> {
            match self {
                Symbol::Frame { location, .. } => {
                    let file = location.as_ref()?.file?;
                    Some(BytesOrWideString::Bytes(file.as_bytes()))
                }
                Symbol::Symtab { .. } => None,
            }
        }

        pub fn filename(&self) -> Option<&Path> {
            match self {
                Symbol::Frame { location, .. } => {
                    let file = location.as_ref()?.file?;
                    Some(Path::new(file))
                }
                Symbol::Symtab { .. } => None,
            }
        }

        pub fn lineno(&self) -> Option<u32> {
            match self {
                Symbol::Frame { location, .. } => location.as_ref()?.line,
                Symbol::Symtab { .. } => None,
            }
        }
    }