rust/src/libsyntax/diagnostic.rs

// Copyright 2012 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.

pub use self::Level::*;
pub use self::RenderSpan::*;
pub use self::ColorConfig::*;
use self::Destination::*;

use codemap::{COMMAND_LINE_SP, COMMAND_LINE_EXPN, Pos, Span};
use codemap;
use diagnostics;

use std::cell::{RefCell, Cell};
use std::cmp;
use std::fmt;
use std::io::prelude::*;
use std::io;
use term::WriterWrapper;
use term;
use libc;

/// maximum number of lines we will print for each error; arbitrary.
const MAX_LINES: usize = 6;

#[derive(Clone)]
pub enum RenderSpan {
    /// A FullSpan renders with both with an initial line for the
    /// message, prefixed by file:linenum, followed by a summary of
    /// the source code covered by the span.
    FullSpan(Span),

    /// Similar to a FullSpan, but the cited position is the end of
    /// the span, instead of the start. Used, at least, for telling
    /// compiletest/runtest to look at the last line of the span
    /// (since `end_highlight_lines` displays an arrow to the end
    /// of the span).
    EndSpan(Span),

    /// A suggestion renders with both with an initial line for the
    /// message, prefixed by file:linenum, followed by a summary
    /// of hypothetical source code, where the `String` is spliced
    /// into the lines in place of the code covered by the span.
    Suggestion(Span, String),

    /// A FileLine renders with just a line for the message prefixed
    /// by file:linenum.
    FileLine(Span),
}

impl RenderSpan {
    fn span(&self) -> Span {
        match *self {
            FullSpan(s) |
            Suggestion(s, _) |
            EndSpan(s) |
            FileLine(s) =>
                s
        }
    }
}

#[derive(Clone, Copy)]
pub enum ColorConfig {
    Auto,
    Always,
    Never
}

pub trait Emitter {
    fn emit(&mut self, cmsp: Option<(&codemap::CodeMap, Span)>,
            msg: &str, code: Option<&str>, lvl: Level);
    fn custom_emit(&mut self, cm: &codemap::CodeMap,
                   sp: RenderSpan, msg: &str, lvl: Level);
}

/// Used as a return value to signify a fatal error occurred. (It is also
/// used as the argument to panic at the moment, but that will eventually
/// not be true.)
#[derive(Copy, Clone)]
#[must_use]
pub struct FatalError;

/// Signifies that the compiler died with an explicit call to `.bug`
/// or `.span_bug` rather than a failed assertion, etc.
#[derive(Copy, Clone)]
pub struct ExplicitBug;

/// A span-handler is like a handler but also
/// accepts span information for source-location
/// reporting.
pub struct SpanHandler {
    pub handler: Handler,
    pub cm: codemap::CodeMap,
}

impl SpanHandler {
    pub fn span_fatal(&self, sp: Span, msg: &str) -> FatalError {
        self.handler.emit(Some((&self.cm, sp)), msg, Fatal);
        return FatalError;
    }
    pub fn span_fatal_with_code(&self, sp: Span, msg: &str, code: &str) -> FatalError {
        self.handler.emit_with_code(Some((&self.cm, sp)), msg, code, Fatal);
        return FatalError;
    }
    pub fn span_err(&self, sp: Span, msg: &str) {
        self.handler.emit(Some((&self.cm, sp)), msg, Error);
        self.handler.bump_err_count();
    }
    pub fn span_err_with_code(&self, sp: Span, msg: &str, code: &str) {
        self.handler.emit_with_code(Some((&self.cm, sp)), msg, code, Error);
        self.handler.bump_err_count();
    }
    pub fn span_warn(&self, sp: Span, msg: &str) {
        self.handler.emit(Some((&self.cm, sp)), msg, Warning);
    }
    pub fn span_warn_with_code(&self, sp: Span, msg: &str, code: &str) {
        self.handler.emit_with_code(Some((&self.cm, sp)), msg, code, Warning);
    }
    pub fn span_note(&self, sp: Span, msg: &str) {
        self.handler.emit(Some((&self.cm, sp)), msg, Note);
    }
    pub fn span_end_note(&self, sp: Span, msg: &str) {
        self.handler.custom_emit(&self.cm, EndSpan(sp), msg, Note);
    }
    pub fn span_help(&self, sp: Span, msg: &str) {
        self.handler.emit(Some((&self.cm, sp)), msg, Help);
    }
    /// Prints out a message with a suggested edit of the code.
    ///
    /// See `diagnostic::RenderSpan::Suggestion` for more information.
    pub fn span_suggestion(&self, sp: Span, msg: &str, suggestion: String) {
        self.handler.custom_emit(&self.cm, Suggestion(sp, suggestion), msg, Help);
    }
    pub fn fileline_note(&self, sp: Span, msg: &str) {
        self.handler.custom_emit(&self.cm, FileLine(sp), msg, Note);
    }
    pub fn fileline_help(&self, sp: Span, msg: &str) {
        self.handler.custom_emit(&self.cm, FileLine(sp), msg, Help);
    }
    pub fn span_bug(&self, sp: Span, msg: &str) -> ! {
        self.handler.emit(Some((&self.cm, sp)), msg, Bug);
        panic!(ExplicitBug);
    }
    pub fn span_unimpl(&self, sp: Span, msg: &str) -> ! {
        self.span_bug(sp, &format!("unimplemented {}", msg));
    }
    pub fn handler<'a>(&'a self) -> &'a Handler {
        &self.handler
    }
}

/// A handler deals with errors; certain errors
/// (fatal, bug, unimpl) may cause immediate exit,
/// others log errors for later reporting.
pub struct Handler {
    err_count: Cell<usize>,
    emit: RefCell<Box<Emitter + Send>>,
    pub can_emit_warnings: bool
}

impl Handler {
    pub fn fatal(&self, msg: &str) -> ! {
        self.emit.borrow_mut().emit(None, msg, None, Fatal);
        panic!(FatalError);
    }
    pub fn err(&self, msg: &str) {
        self.emit.borrow_mut().emit(None, msg, None, Error);
        self.bump_err_count();
    }
    pub fn bump_err_count(&self) {
        self.err_count.set(self.err_count.get() + 1);
    }
    pub fn err_count(&self) -> usize {
        self.err_count.get()
    }
    pub fn has_errors(&self) -> bool {
        self.err_count.get() > 0
    }
    pub fn abort_if_errors(&self) {
        let s;
        match self.err_count.get() {
          0 => return,
          1 => s = "aborting due to previous error".to_string(),
          _   => {
            s = format!("aborting due to {} previous errors",
                        self.err_count.get());
          }
        }
        self.fatal(&s[..]);
    }
    pub fn warn(&self, msg: &str) {
        self.emit.borrow_mut().emit(None, msg, None, Warning);
    }
    pub fn note(&self, msg: &str) {
        self.emit.borrow_mut().emit(None, msg, None, Note);
    }
    pub fn help(&self, msg: &str) {
        self.emit.borrow_mut().emit(None, msg, None, Help);
    }
    pub fn bug(&self, msg: &str) -> ! {
        self.emit.borrow_mut().emit(None, msg, None, Bug);
        panic!(ExplicitBug);
    }
    pub fn unimpl(&self, msg: &str) -> ! {
        self.bug(&format!("unimplemented {}", msg));
    }
    pub fn emit(&self,
                cmsp: Option<(&codemap::CodeMap, Span)>,
                msg: &str,
                lvl: Level) {
        if lvl == Warning && !self.can_emit_warnings { return }
        self.emit.borrow_mut().emit(cmsp, msg, None, lvl);
    }
    pub fn emit_with_code(&self,
                          cmsp: Option<(&codemap::CodeMap, Span)>,
                          msg: &str,
                          code: &str,
                          lvl: Level) {
        if lvl == Warning && !self.can_emit_warnings { return }
        self.emit.borrow_mut().emit(cmsp, msg, Some(code), lvl);
    }
    pub fn custom_emit(&self, cm: &codemap::CodeMap,
                       sp: RenderSpan, msg: &str, lvl: Level) {
        if lvl == Warning && !self.can_emit_warnings { return }
        self.emit.borrow_mut().custom_emit(cm, sp, msg, lvl);
    }
}

pub fn mk_span_handler(handler: Handler, cm: codemap::CodeMap) -> SpanHandler {
    SpanHandler {
        handler: handler,
        cm: cm,
    }
}

pub fn default_handler(color_config: ColorConfig,
                       registry: Option<diagnostics::registry::Registry>,
                       can_emit_warnings: bool) -> Handler {
    mk_handler(can_emit_warnings, Box::new(EmitterWriter::stderr(color_config, registry)))
}

pub fn mk_handler(can_emit_warnings: bool, e: Box<Emitter + Send>) -> Handler {
    Handler {
        err_count: Cell::new(0),
        emit: RefCell::new(e),
        can_emit_warnings: can_emit_warnings
    }
}

#[derive(Copy, PartialEq, Clone, Debug)]
pub enum Level {
    Bug,
    Fatal,
    Error,
    Warning,
    Note,
    Help,
}

impl fmt::Display for Level {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use std::fmt::Display;

        match *self {
            Bug => "error: internal compiler error".fmt(f),
            Fatal | Error => "error".fmt(f),
            Warning => "warning".fmt(f),
            Note => "note".fmt(f),
            Help => "help".fmt(f),
        }
    }
}

impl Level {
    fn color(self) -> term::color::Color {
        match self {
            Bug | Fatal | Error => term::color::BRIGHT_RED,
            Warning => term::color::BRIGHT_YELLOW,
            Note => term::color::BRIGHT_GREEN,
            Help => term::color::BRIGHT_CYAN,
        }
    }
}

fn print_maybe_styled(w: &mut EmitterWriter,
                      msg: &str,
                      color: term::attr::Attr) -> io::Result<()> {
    match w.dst {
        Terminal(ref mut t) => {
            try!(t.attr(color));
            // If `msg` ends in a newline, we need to reset the color before
            // the newline. We're making the assumption that we end up writing
            // to a `LineBufferedWriter`, which means that emitting the reset
            // after the newline ends up buffering the reset until we print
            // another line or exit. Buffering the reset is a problem if we're
            // sharing the terminal with any other programs (e.g. other rustc
            // instances via `make -jN`).
            //
            // Note that if `msg` contains any internal newlines, this will
            // result in the `LineBufferedWriter` flushing twice instead of
            // once, which still leaves the opportunity for interleaved output
            // to be miscolored. We assume this is rare enough that we don't
            // have to worry about it.
            if msg.ends_with("\n") {
                try!(t.write_all(msg[..msg.len()-1].as_bytes()));
                try!(t.reset());
                try!(t.write_all(b"\n"));
            } else {
                try!(t.write_all(msg.as_bytes()));
                try!(t.reset());
            }
            Ok(())
        }
        Raw(ref mut w) => w.write_all(msg.as_bytes()),
    }
}

fn print_diagnostic(dst: &mut EmitterWriter, topic: &str, lvl: Level,
                    msg: &str, code: Option<&str>) -> io::Result<()> {
    if !topic.is_empty() {
        try!(write!(&mut dst.dst, "{} ", topic));
    }

    try!(print_maybe_styled(dst,
                            &format!("{}: ", lvl.to_string()),
                            term::attr::ForegroundColor(lvl.color())));
    try!(print_maybe_styled(dst,
                            &format!("{}", msg),
                            term::attr::Bold));

    match code {
        Some(code) => {
            let style = term::attr::ForegroundColor(term::color::BRIGHT_MAGENTA);
            try!(print_maybe_styled(dst, &format!(" [{}]", code.clone()), style));
        }
        None => ()
    }
    try!(write!(&mut dst.dst, "\n"));
    Ok(())
}

pub struct EmitterWriter {
    dst: Destination,
    registry: Option<diagnostics::registry::Registry>
}

enum Destination {
    Terminal(Box<term::Terminal<WriterWrapper> + Send>),
    Raw(Box<Write + Send>),
}

impl EmitterWriter {
    pub fn stderr(color_config: ColorConfig,
                  registry: Option<diagnostics::registry::Registry>) -> EmitterWriter {
        let stderr = io::stderr();

        let use_color = match color_config {
            Always => true,
            Never  => false,
            Auto   => stderr_isatty(),
        };

        if use_color {
            let dst = match term::stderr() {
                Some(t) => Terminal(t),
                None    => Raw(Box::new(stderr)),
            };
            EmitterWriter { dst: dst, registry: registry }
        } else {
            EmitterWriter { dst: Raw(Box::new(stderr)), registry: registry }
        }
    }

    pub fn new(dst: Box<Write + Send>,
               registry: Option<diagnostics::registry::Registry>) -> EmitterWriter {
        EmitterWriter { dst: Raw(dst), registry: registry }
    }
}

#[cfg(unix)]
fn stderr_isatty() -> bool {
    unsafe { libc::isatty(libc::STDERR_FILENO) != 0 }
}
#[cfg(windows)]
fn stderr_isatty() -> bool {
    const STD_ERROR_HANDLE: libc::DWORD = -12i32 as libc::DWORD;
    extern "system" {
        fn GetStdHandle(which: libc::DWORD) -> libc::HANDLE;
        fn GetConsoleMode(hConsoleHandle: libc::HANDLE,
                          lpMode: libc::LPDWORD) -> libc::BOOL;
    }
    unsafe {
        let handle = GetStdHandle(STD_ERROR_HANDLE);
        let mut out = 0;
        GetConsoleMode(handle, &mut out) != 0
    }
}

impl Write for Destination {
    fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
        match *self {
            Terminal(ref mut t) => t.write(bytes),
            Raw(ref mut w) => w.write(bytes),
        }
    }
    fn flush(&mut self) -> io::Result<()> {
        match *self {
            Terminal(ref mut t) => t.flush(),
            Raw(ref mut w) => w.flush(),
        }
    }
}

impl Emitter for EmitterWriter {
    fn emit(&mut self,
            cmsp: Option<(&codemap::CodeMap, Span)>,
            msg: &str, code: Option<&str>, lvl: Level) {
        let error = match cmsp {
            Some((cm, COMMAND_LINE_SP)) => emit(self, cm,
                                                FileLine(COMMAND_LINE_SP),
                                                msg, code, lvl),
            Some((cm, sp)) => emit(self, cm, FullSpan(sp), msg, code, lvl),
            None => print_diagnostic(self, "", lvl, msg, code),
        };

        match error {
            Ok(()) => {}
            Err(e) => panic!("failed to print diagnostics: {:?}", e),
        }
    }

    fn custom_emit(&mut self, cm: &codemap::CodeMap,
                   sp: RenderSpan, msg: &str, lvl: Level) {
        match emit(self, cm, sp, msg, None, lvl) {
            Ok(()) => {}
            Err(e) => panic!("failed to print diagnostics: {:?}", e),
        }
    }
}

fn emit(dst: &mut EmitterWriter, cm: &codemap::CodeMap, rsp: RenderSpan,
        msg: &str, code: Option<&str>, lvl: Level) -> io::Result<()> {
    let sp = rsp.span();

    // We cannot check equality directly with COMMAND_LINE_SP
    // since PartialEq is manually implemented to ignore the ExpnId
    let ss = if sp.expn_id == COMMAND_LINE_EXPN {
        "<command line option>".to_string()
    } else if let EndSpan(_) = rsp {
        let span_end = Span { lo: sp.hi, hi: sp.hi, expn_id: sp.expn_id};
        cm.span_to_string(span_end)
    } else {
        cm.span_to_string(sp)
    };

    try!(print_diagnostic(dst, &ss[..], lvl, msg, code));

    match rsp {
        FullSpan(_) => {
            try!(highlight_lines(dst, cm, sp, lvl, cm.span_to_lines(sp)));
            try!(print_macro_backtrace(dst, cm, sp));
        }
        EndSpan(_) => {
            try!(end_highlight_lines(dst, cm, sp, lvl, cm.span_to_lines(sp)));
            try!(print_macro_backtrace(dst, cm, sp));
        }
        Suggestion(_, ref suggestion) => {
            try!(highlight_suggestion(dst, cm, sp, suggestion));
            try!(print_macro_backtrace(dst, cm, sp));
        }
        FileLine(..) => {
            // no source text in this case!
        }
    }

    match code {
        Some(code) =>
            match dst.registry.as_ref().and_then(|registry| registry.find_description(code)) {
                Some(_) => {
                    try!(print_diagnostic(dst, &ss[..], Help,
                                          &format!("pass `--explain {}` to see a detailed \
                                                   explanation", code), None));
                }
                None => ()
            },
        None => (),
    }
    Ok(())
}

fn highlight_suggestion(err: &mut EmitterWriter,
                        cm: &codemap::CodeMap,
                        sp: Span,
                        suggestion: &str)
                        -> io::Result<()>
{
    let lines = cm.span_to_lines(sp);
    assert!(!lines.lines.is_empty());

    // To build up the result, we want to take the snippet from the first
    // line that precedes the span, prepend that with the suggestion, and
    // then append the snippet from the last line that trails the span.
    let fm = &lines.file;

    let first_line = &lines.lines[0];
    let prefix = fm.get_line(first_line.line_index)
                   .map(|l| &l[..first_line.start_col.0])
                   .unwrap_or("");

    let last_line = lines.lines.last().unwrap();
    let suffix = fm.get_line(last_line.line_index)
                   .map(|l| &l[last_line.end_col.0..])
                   .unwrap_or("");

    let complete = format!("{}{}{}", prefix, suggestion, suffix);

    // print the suggestion without any line numbers, but leave
    // space for them. This helps with lining up with previous
    // snippets from the actual error being reported.
    let fm = &*lines.file;
    let mut lines = complete.lines();
    for (line, line_index) in lines.by_ref().take(MAX_LINES).zip(first_line.line_index..) {
        let elided_line_num = format!("{}", line_index+1);
        try!(write!(&mut err.dst, "{0}:{1:2$} {3}\n",
                    fm.name, "", elided_line_num.len(), line));
    }

    // if we elided some lines, add an ellipsis
    if lines.next().is_some() {
        let elided_line_num = format!("{}", first_line.line_index + MAX_LINES + 1);
        try!(write!(&mut err.dst, "{0:1$} {0:2$} ...\n",
                    "", fm.name.len(), elided_line_num.len()));
    }

    Ok(())
}

fn highlight_lines(err: &mut EmitterWriter,
                   cm: &codemap::CodeMap,
                   sp: Span,
                   lvl: Level,
                   lines: codemap::FileLines)
                   -> io::Result<()>
{
    let fm = &*lines.file;

    let line_strings: Option<Vec<&str>> =
        lines.lines.iter()
                   .map(|info| fm.get_line(info.line_index))
                   .collect();

    let line_strings = match line_strings {
        None => { return Ok(()); }
        Some(line_strings) => line_strings
    };

    // Display only the first MAX_LINES lines.
    let all_lines = lines.lines.len();
    let display_lines = cmp::min(all_lines, MAX_LINES);
    let display_line_infos = &lines.lines[..display_lines];
    let display_line_strings = &line_strings[..display_lines];

    // Print the offending lines
    for (line_info, line) in display_line_infos.iter().zip(display_line_strings.iter()) {
        try!(write!(&mut err.dst, "{}:{} {}\n",
                    fm.name,
                    line_info.line_index + 1,
                    line));
    }

    // If we elided something, put an ellipsis.
    if display_lines < all_lines {
        let last_line_index = display_line_infos.last().unwrap().line_index;
        let s = format!("{}:{} ", fm.name, last_line_index + 1);
        try!(write!(&mut err.dst, "{0:1$}...\n", "", s.len()));
    }

    // FIXME (#3260)
    // If there's one line at fault we can easily point to the problem
    if lines.lines.len() == 1 {
        let lo = cm.lookup_char_pos(sp.lo);
        let mut digits = 0;
        let mut num = (lines.lines[0].line_index + 1) / 10;

        // how many digits must be indent past?
        while num > 0 { num /= 10; digits += 1; }

        let mut s = String::new();
        // Skip is the number of characters we need to skip because they are
        // part of the 'filename:line ' part of the previous line.
        let skip = fm.name.width(false) + digits + 3;
        for _ in 0..skip {
            s.push(' ');
        }
        if let Some(orig) = fm.get_line(lines.lines[0].line_index) {
            let mut col = skip;
            let mut lastc = ' ';
            let mut iter = orig.chars().enumerate();
            for (pos, ch) in iter.by_ref() {
                lastc = ch;
                if pos >= lo.col.to_usize() { break; }
                // Whenever a tab occurs on the previous line, we insert one on
                // the error-point-squiggly-line as well (instead of a space).
                // That way the squiggly line will usually appear in the correct
                // position.
                match ch {
                    '\t' => {
                        col += 8 - col%8;
                        s.push('\t');
                    },
                    c => for _ in 0..c.width(false).unwrap_or(0) {
                        col += 1;
                        s.push(' ');
                    },
                }
            }

            try!(write!(&mut err.dst, "{}", s));
            let mut s = String::from_str("^");
            let count = match lastc {
                // Most terminals have a tab stop every eight columns by default
                '\t' => 8 - col%8,
                _ => lastc.width(false).unwrap_or(0),
            };
            col += count;
            s.extend(::std::iter::repeat('~').take(count));

            let hi = cm.lookup_char_pos(sp.hi);
            if hi.col != lo.col {
                for (pos, ch) in iter {
                    if pos >= hi.col.to_usize() { break; }
                    let count = match ch {
                        '\t' => 8 - col%8,
                        _ => ch.width(false).unwrap_or(0),
                    };
                    col += count;
                    s.extend(::std::iter::repeat('~').take(count));
                }
            }

            if s.len() > 1 {
                // One extra squiggly is replaced by a "^"
                s.pop();
            }

            try!(print_maybe_styled(err,
                                    &format!("{}\n", s),
                                    term::attr::ForegroundColor(lvl.color())));
        }
    }
    Ok(())
}

/// Here are the differences between this and the normal `highlight_lines`:
/// `end_highlight_lines` will always put arrow on the last byte of the
/// span (instead of the first byte). Also, when the span is too long (more
/// than 6 lines), `end_highlight_lines` will print the first line, then
/// dot dot dot, then last line, whereas `highlight_lines` prints the first
/// six lines.
fn end_highlight_lines(w: &mut EmitterWriter,
                          cm: &codemap::CodeMap,
                          sp: Span,
                          lvl: Level,
                          lines: codemap::FileLines)
                          -> io::Result<()> {
    let fm = &*lines.file;

    let lines = &lines.lines[..];
    if lines.len() > MAX_LINES {
        if let Some(line) = fm.get_line(lines[0].line_index) {
            try!(write!(&mut w.dst, "{}:{} {}\n", fm.name,
                        lines[0].line_index + 1, line));
        }
        try!(write!(&mut w.dst, "...\n"));
        let last_line_index = lines[lines.len() - 1].line_index;
        if let Some(last_line) = fm.get_line(last_line_index) {
            try!(write!(&mut w.dst, "{}:{} {}\n", fm.name,
                        last_line_index + 1, last_line));
        }
    } else {
        for line_info in lines {
            if let Some(line) = fm.get_line(line_info.line_index) {
                try!(write!(&mut w.dst, "{}:{} {}\n", fm.name,
                            line_info.line_index + 1, line));
            }
        }
    }
    let last_line_start = format!("{}:{} ", fm.name, lines[lines.len()-1].line_index + 1);
    let hi = cm.lookup_char_pos(sp.hi);
    let skip = last_line_start.width(false);
    let mut s = String::new();
    for _ in 0..skip {
        s.push(' ');
    }
    if let Some(orig) = fm.get_line(lines[0].line_index) {
        let iter = orig.chars().enumerate();
        for (pos, ch) in iter {
            // Span seems to use half-opened interval, so subtract 1
            if pos >= hi.col.to_usize() - 1 { break; }
            // Whenever a tab occurs on the previous line, we insert one on
            // the error-point-squiggly-line as well (instead of a space).
            // That way the squiggly line will usually appear in the correct
            // position.
            match ch {
                '\t' => s.push('\t'),
                c => for _ in 0..c.width(false).unwrap_or(0) {
                    s.push(' ');
                },
            }
        }
    }
    s.push('^');
    s.push('\n');
    print_maybe_styled(w,
                       &s[..],
                       term::attr::ForegroundColor(lvl.color()))
}

fn print_macro_backtrace(w: &mut EmitterWriter,
                         cm: &codemap::CodeMap,
                         sp: Span)
                         -> io::Result<()> {
    let cs = try!(cm.with_expn_info(sp.expn_id, |expn_info| -> io::Result<_> {
        match expn_info {
            Some(ei) => {
                let ss = ei.callee.span.map_or(String::new(),
                                               |span| cm.span_to_string(span));
                let (pre, post) = match ei.callee.format {
                    codemap::MacroAttribute => ("#[", "]"),
                    codemap::MacroBang => ("", "!")
                };
                try!(print_diagnostic(w, &ss, Note,
                                      &format!("in expansion of {}{}{}", pre,
                                              ei.callee.name,
                                              post), None));
                let ss = cm.span_to_string(ei.call_site);
                try!(print_diagnostic(w, &ss, Note, "expansion site", None));
                Ok(Some(ei.call_site))
            }
            None => Ok(None)
    }
    }));
    cs.map_or(Ok(()), |call_site| print_macro_backtrace(w, cm, call_site))
}

pub fn expect<T, M>(diag: &SpanHandler, opt: Option<T>, msg: M) -> T where
    M: FnOnce() -> String,
{
    match opt {
        Some(t) => t,
        None => diag.handler().bug(&msg()),
    }
}