Auto merge of #66453 - Centril:rollup-w1ohzxs, r=Centril

Rollup of 5 pull requests

Successful merges:

 - #66350 (protect creation of destructors by a mutex)
 - #66407 (Add more tests for fixed ICEs)
 - #66415 (Add --force-run-in-process unstable option to libtest)
 - #66427 (Move the JSON error emitter to librustc_errors)
 - #66441 (libpanic_unwind for Miri: make sure we have the SEH lang items when needed)

Failed merges:

r? @ghost

5 files changed, 1264 insertions, 0 deletions

diff --git a/src/libsyntax_pos/Cargo.toml b/src/libsyntax_pos/Cargo.toml
index 378f7a955a3..2cac76085d2 100644
--- a/src/libsyntax_pos/Cargo.toml
+++ b/src/libsyntax_pos/Cargo.toml
@@ -18,3 +18,4 @@ arena = { path = "../libarena" }
 scoped-tls = "1.0"
 unicode-width = "0.1.4"
 cfg-if = "0.1.2"
+log = "0.4"
diff --git a/src/libsyntax_pos/fatal_error.rs b/src/libsyntax_pos/fatal_error.rs
new file mode 100644
index 00000000000..cf7c677d59d
--- /dev/null
+++ b/src/libsyntax_pos/fatal_error.rs
@@ -0,0 +1,30 @@
+/// 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, Debug)]
+#[must_use]
+pub struct FatalError;
+
+pub struct FatalErrorMarker;
+
+// Don't implement Send on FatalError. This makes it impossible to panic!(FatalError).
+// We don't want to invoke the panic handler and print a backtrace for fatal errors.
+impl !Send for FatalError {}
+
+impl FatalError {
+    pub fn raise(self) -> ! {
+        std::panic::resume_unwind(Box::new(FatalErrorMarker))
+    }
+}
+
+impl std::fmt::Display for FatalError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "parser fatal error")
+    }
+}
+
+impl std::error::Error for FatalError {
+    fn description(&self) -> &str {
+        "The parser has encountered a fatal error"
+    }
+}
diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index a762d8af49a..b88d6dbc3f3 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -16,6 +16,8 @@
 
 use rustc_serialize::{Encodable, Decodable, Encoder, Decoder};
 
+pub mod source_map;
+
 pub mod edition;
 use edition::Edition;
 pub mod hygiene;
@@ -29,6 +31,7 @@ pub mod symbol;
 pub use symbol::{Symbol, sym};
 
 mod analyze_source_file;
+pub mod fatal_error;
 
 use rustc_data_structures::stable_hasher::StableHasher;
 use rustc_data_structures::sync::{Lrc, Lock};
diff --git a/src/libsyntax_pos/source_map.rs b/src/libsyntax_pos/source_map.rs
new file mode 100644
index 00000000000..77d9807225e
--- /dev/null
+++ b/src/libsyntax_pos/source_map.rs
@@ -0,0 +1,1009 @@
+//! The `SourceMap` tracks all the source code used within a single crate, mapping
+//! from integer byte positions to the original source code location. Each bit
+//! of source parsed during crate parsing (typically files, in-memory strings,
+//! or various bits of macro expansion) cover a continuous range of bytes in the
+//! `SourceMap` and are represented by `SourceFile`s. Byte positions are stored in
+//! `Span` and used pervasively in the compiler. They are absolute positions
+//! within the `SourceMap`, which upon request can be converted to line and column
+//! information, source code snippets, etc.
+
+pub use crate::*;
+pub use crate::hygiene::{ExpnKind, ExpnData};
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::stable_hasher::StableHasher;
+use rustc_data_structures::sync::{Lrc, Lock, LockGuard, MappedLockGuard};
+use std::cmp;
+use std::hash::Hash;
+use std::path::{Path, PathBuf};
+
+use std::env;
+use std::fs;
+use std::io;
+use log::debug;
+
+#[cfg(test)]
+mod tests;
+
+/// Returns the span itself if it doesn't come from a macro expansion,
+/// otherwise return the call site span up to the `enclosing_sp` by
+/// following the `expn_data` chain.
+pub fn original_sp(sp: Span, enclosing_sp: Span) -> Span {
+    let expn_data1 = sp.ctxt().outer_expn_data();
+    let expn_data2 = enclosing_sp.ctxt().outer_expn_data();
+    if expn_data1.is_root() ||
+       !expn_data2.is_root() && expn_data1.call_site == expn_data2.call_site {
+        sp
+    } else {
+        original_sp(expn_data1.call_site, enclosing_sp)
+    }
+}
+
+#[derive(Clone, RustcEncodable, RustcDecodable, Debug, Copy)]
+pub struct Spanned<T> {
+    pub node: T,
+    pub span: Span,
+}
+
+pub fn respan<T>(sp: Span, t: T) -> Spanned<T> {
+    Spanned {node: t, span: sp}
+}
+
+pub fn dummy_spanned<T>(t: T) -> Spanned<T> {
+    respan(DUMMY_SP, t)
+}
+
+// _____________________________________________________________________________
+// SourceFile, MultiByteChar, FileName, FileLines
+//
+
+/// An abstraction over the fs operations used by the Parser.
+pub trait FileLoader {
+    /// Query the existence of a file.
+    fn file_exists(&self, path: &Path) -> bool;
+
+    /// Returns an absolute path to a file, if possible.
+    fn abs_path(&self, path: &Path) -> Option<PathBuf>;
+
+    /// Read the contents of an UTF-8 file into memory.
+    fn read_file(&self, path: &Path) -> io::Result<String>;
+}
+
+/// A FileLoader that uses std::fs to load real files.
+pub struct RealFileLoader;
+
+impl FileLoader for RealFileLoader {
+    fn file_exists(&self, path: &Path) -> bool {
+        fs::metadata(path).is_ok()
+    }
+
+    fn abs_path(&self, path: &Path) -> Option<PathBuf> {
+        if path.is_absolute() {
+            Some(path.to_path_buf())
+        } else {
+            env::current_dir()
+                .ok()
+                .map(|cwd| cwd.join(path))
+        }
+    }
+
+    fn read_file(&self, path: &Path) -> io::Result<String> {
+        fs::read_to_string(path)
+    }
+}
+
+// This is a `SourceFile` identifier that is used to correlate `SourceFile`s between
+// subsequent compilation sessions (which is something we need to do during
+// incremental compilation).
+#[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
+pub struct StableSourceFileId(u128);
+
+impl StableSourceFileId {
+    pub fn new(source_file: &SourceFile) -> StableSourceFileId {
+        StableSourceFileId::new_from_pieces(&source_file.name,
+                                            source_file.name_was_remapped,
+                                            source_file.unmapped_path.as_ref())
+    }
+
+    pub fn new_from_pieces(name: &FileName,
+                           name_was_remapped: bool,
+                           unmapped_path: Option<&FileName>) -> StableSourceFileId {
+        let mut hasher = StableHasher::new();
+
+        name.hash(&mut hasher);
+        name_was_remapped.hash(&mut hasher);
+        unmapped_path.hash(&mut hasher);
+
+        StableSourceFileId(hasher.finish())
+    }
+}
+
+// _____________________________________________________________________________
+// SourceMap
+//
+
+#[derive(Default)]
+pub(super) struct SourceMapFiles {
+    source_files: Vec<Lrc<SourceFile>>,
+    stable_id_to_source_file: FxHashMap<StableSourceFileId, Lrc<SourceFile>>
+}
+
+pub struct SourceMap {
+    files: Lock<SourceMapFiles>,
+    file_loader: Box<dyn FileLoader + Sync + Send>,
+    // This is used to apply the file path remapping as specified via
+    // `--remap-path-prefix` to all `SourceFile`s allocated within this `SourceMap`.
+    path_mapping: FilePathMapping,
+}
+
+impl SourceMap {
+    pub fn new(path_mapping: FilePathMapping) -> SourceMap {
+        SourceMap {
+            files: Default::default(),
+            file_loader: Box::new(RealFileLoader),
+            path_mapping,
+        }
+    }
+
+    pub fn with_file_loader(file_loader: Box<dyn FileLoader + Sync + Send>,
+                            path_mapping: FilePathMapping)
+                            -> SourceMap {
+        SourceMap {
+            files: Default::default(),
+            file_loader,
+            path_mapping,
+        }
+    }
+
+    pub fn path_mapping(&self) -> &FilePathMapping {
+        &self.path_mapping
+    }
+
+    pub fn file_exists(&self, path: &Path) -> bool {
+        self.file_loader.file_exists(path)
+    }
+
+    pub fn load_file(&self, path: &Path) -> io::Result<Lrc<SourceFile>> {
+        let src = self.file_loader.read_file(path)?;
+        let filename = path.to_owned().into();
+        Ok(self.new_source_file(filename, src))
+    }
+
+    /// Loads source file as a binary blob.
+    ///
+    /// Unlike `load_file`, guarantees that no normalization like BOM-removal
+    /// takes place.
+    pub fn load_binary_file(&self, path: &Path) -> io::Result<Vec<u8>> {
+        // Ideally, this should use `self.file_loader`, but it can't
+        // deal with binary files yet.
+        let bytes = fs::read(path)?;
+
+        // We need to add file to the `SourceMap`, so that it is present
+        // in dep-info. There's also an edge case that file might be both
+        // loaded as a binary via `include_bytes!` and as proper `SourceFile`
+        // via `mod`, so we try to use real file contents and not just an
+        // empty string.
+        let text = std::str::from_utf8(&bytes).unwrap_or("")
+            .to_string();
+        self.new_source_file(path.to_owned().into(), text);
+        Ok(bytes)
+    }
+
+    pub fn files(&self) -> MappedLockGuard<'_, Vec<Lrc<SourceFile>>> {
+        LockGuard::map(self.files.borrow(), |files| &mut files.source_files)
+    }
+
+    pub fn source_file_by_stable_id(&self, stable_id: StableSourceFileId) ->
+    Option<Lrc<SourceFile>> {
+        self.files.borrow().stable_id_to_source_file.get(&stable_id).map(|sf| sf.clone())
+    }
+
+    fn next_start_pos(&self) -> usize {
+        match self.files.borrow().source_files.last() {
+            None => 0,
+            // Add one so there is some space between files. This lets us distinguish
+            // positions in the `SourceMap`, even in the presence of zero-length files.
+            Some(last) => last.end_pos.to_usize() + 1,
+        }
+    }
+
+    /// Creates a new `SourceFile`.
+    /// If a file already exists in the `SourceMap` with the same ID, that file is returned
+    /// unmodified.
+    pub fn new_source_file(&self, filename: FileName, src: String) -> Lrc<SourceFile> {
+        self.try_new_source_file(filename, src)
+            .unwrap_or_else(|OffsetOverflowError| {
+                eprintln!("fatal error: rustc does not support files larger than 4GB");
+                crate::fatal_error::FatalError.raise()
+            })
+    }
+
+    fn try_new_source_file(
+        &self,
+        filename: FileName,
+        src: String
+    ) -> Result<Lrc<SourceFile>, OffsetOverflowError> {
+        let start_pos = self.next_start_pos();
+
+        // The path is used to determine the directory for loading submodules and
+        // include files, so it must be before remapping.
+        // Note that filename may not be a valid path, eg it may be `<anon>` etc,
+        // but this is okay because the directory determined by `path.pop()` will
+        // be empty, so the working directory will be used.
+        let unmapped_path = filename.clone();
+
+        let (filename, was_remapped) = match filename {
+            FileName::Real(filename) => {
+                let (filename, was_remapped) = self.path_mapping.map_prefix(filename);
+                (FileName::Real(filename), was_remapped)
+            },
+            other => (other, false),
+        };
+
+        let file_id = StableSourceFileId::new_from_pieces(&filename,
+                                                       was_remapped,
+                                                       Some(&unmapped_path));
+
+        let lrc_sf = match self.source_file_by_stable_id(file_id) {
+            Some(lrc_sf) => lrc_sf,
+            None => {
+                let source_file = Lrc::new(SourceFile::new(
+                    filename,
+                    was_remapped,
+                    unmapped_path,
+                    src,
+                    Pos::from_usize(start_pos),
+                )?);
+
+                let mut files = self.files.borrow_mut();
+
+                files.source_files.push(source_file.clone());
+                files.stable_id_to_source_file.insert(file_id, source_file.clone());
+
+                source_file
+            }
+        };
+        Ok(lrc_sf)
+    }
+
+    /// Allocates a new `SourceFile` representing a source file from an external
+    /// crate. The source code of such an "imported `SourceFile`" is not available,
+    /// but we still know enough to generate accurate debuginfo location
+    /// information for things inlined from other crates.
+    pub fn new_imported_source_file(
+        &self,
+        filename: FileName,
+        name_was_remapped: bool,
+        crate_of_origin: u32,
+        src_hash: u128,
+        name_hash: u128,
+        source_len: usize,
+        mut file_local_lines: Vec<BytePos>,
+        mut file_local_multibyte_chars: Vec<MultiByteChar>,
+        mut file_local_non_narrow_chars: Vec<NonNarrowChar>,
+        mut file_local_normalized_pos: Vec<NormalizedPos>,
+    ) -> Lrc<SourceFile> {
+        let start_pos = self.next_start_pos();
+
+        let end_pos = Pos::from_usize(start_pos + source_len);
+        let start_pos = Pos::from_usize(start_pos);
+
+        for pos in &mut file_local_lines {
+            *pos = *pos + start_pos;
+        }
+
+        for mbc in &mut file_local_multibyte_chars {
+            mbc.pos = mbc.pos + start_pos;
+        }
+
+        for swc in &mut file_local_non_narrow_chars {
+            *swc = *swc + start_pos;
+        }
+
+        for nc in &mut file_local_normalized_pos {
+            nc.pos = nc.pos + start_pos;
+        }
+
+        let source_file = Lrc::new(SourceFile {
+            name: filename,
+            name_was_remapped,
+            unmapped_path: None,
+            crate_of_origin,
+            src: None,
+            src_hash,
+            external_src: Lock::new(ExternalSource::AbsentOk),
+            start_pos,
+            end_pos,
+            lines: file_local_lines,
+            multibyte_chars: file_local_multibyte_chars,
+            non_narrow_chars: file_local_non_narrow_chars,
+            normalized_pos: file_local_normalized_pos,
+            name_hash,
+        });
+
+        let mut files = self.files.borrow_mut();
+
+        files.source_files.push(source_file.clone());
+        files.stable_id_to_source_file.insert(StableSourceFileId::new(&source_file),
+                                              source_file.clone());
+
+        source_file
+    }
+
+    pub fn mk_substr_filename(&self, sp: Span) -> String {
+        let pos = self.lookup_char_pos(sp.lo());
+        format!("<{}:{}:{}>",
+                 pos.file.name,
+                 pos.line,
+                 pos.col.to_usize() + 1)
+    }
+
+    // If there is a doctest offset, applies it to the line.
+    pub fn doctest_offset_line(&self, file: &FileName, orig: usize) -> usize {
+        return match file {
+            FileName::DocTest(_, offset) => {
+                return if *offset >= 0 {
+                    orig + *offset as usize
+                } else {
+                    orig - (-(*offset)) as usize
+                }
+            },
+            _ => orig
+        }
+    }
+
+    /// Looks up source information about a `BytePos`.
+    pub fn lookup_char_pos(&self, pos: BytePos) -> Loc {
+        let chpos = self.bytepos_to_file_charpos(pos);
+        match self.lookup_line(pos) {
+            Ok(SourceFileAndLine { sf: f, line: a }) => {
+                let line = a + 1; // Line numbers start at 1
+                let linebpos = f.lines[a];
+                let linechpos = self.bytepos_to_file_charpos(linebpos);
+                let col = chpos - linechpos;
+
+                let col_display = {
+                    let start_width_idx = f
+                        .non_narrow_chars
+                        .binary_search_by_key(&linebpos, |x| x.pos())
+                        .unwrap_or_else(|x| x);
+                    let end_width_idx = f
+                        .non_narrow_chars
+                        .binary_search_by_key(&pos, |x| x.pos())
+                        .unwrap_or_else(|x| x);
+                    let special_chars = end_width_idx - start_width_idx;
+                    let non_narrow: usize = f
+                        .non_narrow_chars[start_width_idx..end_width_idx]
+                        .into_iter()
+                        .map(|x| x.width())
+                        .sum();
+                    col.0 - special_chars + non_narrow
+                };
+                debug!("byte pos {:?} is on the line at byte pos {:?}",
+                       pos, linebpos);
+                debug!("char pos {:?} is on the line at char pos {:?}",
+                       chpos, linechpos);
+                debug!("byte is on line: {}", line);
+                assert!(chpos >= linechpos);
+                Loc {
+                    file: f,
+                    line,
+                    col,
+                    col_display,
+                }
+            }
+            Err(f) => {
+                let col_display = {
+                    let end_width_idx = f
+                        .non_narrow_chars
+                        .binary_search_by_key(&pos, |x| x.pos())
+                        .unwrap_or_else(|x| x);
+                    let non_narrow: usize = f
+                        .non_narrow_chars[0..end_width_idx]
+                        .into_iter()
+                        .map(|x| x.width())
+                        .sum();
+                    chpos.0 - end_width_idx + non_narrow
+                };
+                Loc {
+                    file: f,
+                    line: 0,
+                    col: chpos,
+                    col_display,
+                }
+            }
+        }
+    }
+
+    // If the corresponding `SourceFile` is empty, does not return a line number.
+    pub fn lookup_line(&self, pos: BytePos) -> Result<SourceFileAndLine, Lrc<SourceFile>> {
+        let idx = self.lookup_source_file_idx(pos);
+
+        let f = (*self.files.borrow().source_files)[idx].clone();
+
+        match f.lookup_line(pos) {
+            Some(line) => Ok(SourceFileAndLine { sf: f, line }),
+            None => Err(f)
+        }
+    }
+
+    /// Returns `Some(span)`, a union of the LHS and RHS span. The LHS must precede the RHS. If
+    /// there are gaps between LHS and RHS, the resulting union will cross these gaps.
+    /// For this to work,
+    ///
+    ///    * the syntax contexts of both spans much match,
+    ///    * the LHS span needs to end on the same line the RHS span begins,
+    ///    * the LHS span must start at or before the RHS span.
+    pub fn merge_spans(&self, sp_lhs: Span, sp_rhs: Span) -> Option<Span> {
+        // Ensure we're at the same expansion ID.
+        if sp_lhs.ctxt() != sp_rhs.ctxt() {
+            return None;
+        }
+
+        let lhs_end = match self.lookup_line(sp_lhs.hi()) {
+            Ok(x) => x,
+            Err(_) => return None
+        };
+        let rhs_begin = match self.lookup_line(sp_rhs.lo()) {
+            Ok(x) => x,
+            Err(_) => return None
+        };
+
+        // If we must cross lines to merge, don't merge.
+        if lhs_end.line != rhs_begin.line {
+            return None;
+        }
+
+        // Ensure these follow the expected order and that we don't overlap.
+        if (sp_lhs.lo() <= sp_rhs.lo()) && (sp_lhs.hi() <= sp_rhs.lo()) {
+            Some(sp_lhs.to(sp_rhs))
+        } else {
+            None
+        }
+    }
+
+    pub fn span_to_string(&self, sp: Span) -> String {
+        if self.files.borrow().source_files.is_empty() && sp.is_dummy() {
+            return "no-location".to_string();
+        }
+
+        let lo = self.lookup_char_pos(sp.lo());
+        let hi = self.lookup_char_pos(sp.hi());
+        format!("{}:{}:{}: {}:{}",
+            lo.file.name,
+            lo.line,
+            lo.col.to_usize() + 1,
+            hi.line,
+            hi.col.to_usize() + 1,
+        )
+    }
+
+    pub fn span_to_filename(&self, sp: Span) -> FileName {
+        self.lookup_char_pos(sp.lo()).file.name.clone()
+    }
+
+    pub fn span_to_unmapped_path(&self, sp: Span) -> FileName {
+        self.lookup_char_pos(sp.lo()).file.unmapped_path.clone()
+            .expect("`SourceMap::span_to_unmapped_path` called for imported `SourceFile`?")
+    }
+
+    pub fn is_multiline(&self, sp: Span) -> bool {
+        let lo = self.lookup_char_pos(sp.lo());
+        let hi = self.lookup_char_pos(sp.hi());
+        lo.line != hi.line
+    }
+
+    pub fn span_to_lines(&self, sp: Span) -> FileLinesResult {
+        debug!("span_to_lines(sp={:?})", sp);
+
+        let lo = self.lookup_char_pos(sp.lo());
+        debug!("span_to_lines: lo={:?}", lo);
+        let hi = self.lookup_char_pos(sp.hi());
+        debug!("span_to_lines: hi={:?}", hi);
+
+        if lo.file.start_pos != hi.file.start_pos {
+            return Err(SpanLinesError::DistinctSources(DistinctSources {
+                begin: (lo.file.name.clone(), lo.file.start_pos),
+                end: (hi.file.name.clone(), hi.file.start_pos),
+            }));
+        }
+        assert!(hi.line >= lo.line);
+
+        let mut lines = Vec::with_capacity(hi.line - lo.line + 1);
+
+        // The span starts partway through the first line,
+        // but after that it starts from offset 0.
+        let mut start_col = lo.col;
+
+        // For every line but the last, it extends from `start_col`
+        // and to the end of the line. Be careful because the line
+        // numbers in Loc are 1-based, so we subtract 1 to get 0-based
+        // lines.
+        for line_index in lo.line-1 .. hi.line-1 {
+            let line_len = lo.file.get_line(line_index)
+                                  .map(|s| s.chars().count())
+                                  .unwrap_or(0);
+            lines.push(LineInfo { line_index,
+                                  start_col,
+                                  end_col: CharPos::from_usize(line_len) });
+            start_col = CharPos::from_usize(0);
+        }
+
+        // For the last line, it extends from `start_col` to `hi.col`:
+        lines.push(LineInfo { line_index: hi.line - 1,
+                              start_col,
+                              end_col: hi.col });
+
+        Ok(FileLines {file: lo.file, lines})
+    }
+
+    /// Extracts the source surrounding the given `Span` using the `extract_source` function. The
+    /// extract function takes three arguments: a string slice containing the source, an index in
+    /// the slice for the beginning of the span and an index in the slice for the end of the span.
+    fn span_to_source<F>(&self, sp: Span, extract_source: F) -> Result<String, SpanSnippetError>
+        where F: Fn(&str, usize, usize) -> Result<String, SpanSnippetError>
+    {
+        let local_begin = self.lookup_byte_offset(sp.lo());
+        let local_end = self.lookup_byte_offset(sp.hi());
+
+        if local_begin.sf.start_pos != local_end.sf.start_pos {
+            return Err(SpanSnippetError::DistinctSources(DistinctSources {
+                begin: (local_begin.sf.name.clone(),
+                        local_begin.sf.start_pos),
+                end: (local_end.sf.name.clone(),
+                      local_end.sf.start_pos)
+            }));
+        } else {
+            self.ensure_source_file_source_present(local_begin.sf.clone());
+
+            let start_index = local_begin.pos.to_usize();
+            let end_index = local_end.pos.to_usize();
+            let source_len = (local_begin.sf.end_pos -
+                              local_begin.sf.start_pos).to_usize();
+
+            if start_index > end_index || end_index > source_len {
+                return Err(SpanSnippetError::MalformedForSourcemap(
+                    MalformedSourceMapPositions {
+                        name: local_begin.sf.name.clone(),
+                        source_len,
+                        begin_pos: local_begin.pos,
+                        end_pos: local_end.pos,
+                    }));
+            }
+
+            if let Some(ref src) = local_begin.sf.src {
+                return extract_source(src, start_index, end_index);
+            } else if let Some(src) = local_begin.sf.external_src.borrow().get_source() {
+                return extract_source(src, start_index, end_index);
+            } else {
+                return Err(SpanSnippetError::SourceNotAvailable {
+                    filename: local_begin.sf.name.clone()
+                });
+            }
+        }
+    }
+
+    /// Returns the source snippet as `String` corresponding to the given `Span`.
+    pub fn span_to_snippet(&self, sp: Span) -> Result<String, SpanSnippetError> {
+        self.span_to_source(sp, |src, start_index, end_index| src.get(start_index..end_index)
+            .map(|s| s.to_string())
+            .ok_or_else(|| SpanSnippetError::IllFormedSpan(sp)))
+    }
+
+    pub fn span_to_margin(&self, sp: Span) -> Option<usize> {
+        match self.span_to_prev_source(sp) {
+            Err(_) => None,
+            Ok(source) => source.split('\n').last().map(|last_line| {
+                last_line.len() - last_line.trim_start().len()
+            })
+        }
+    }
+
+    /// Returns the source snippet as `String` before the given `Span`.
+    pub fn span_to_prev_source(&self, sp: Span) -> Result<String, SpanSnippetError> {
+        self.span_to_source(sp, |src, start_index, _| src.get(..start_index)
+            .map(|s| s.to_string())
+            .ok_or_else(|| SpanSnippetError::IllFormedSpan(sp)))
+    }
+
+    /// Extends the given `Span` to just after the previous occurrence of `c`. Return the same span
+    /// if no character could be found or if an error occurred while retrieving the code snippet.
+    pub fn span_extend_to_prev_char(&self, sp: Span, c: char) -> Span {
+        if let Ok(prev_source) = self.span_to_prev_source(sp) {
+            let prev_source = prev_source.rsplit(c).nth(0).unwrap_or("").trim_start();
+            if !prev_source.is_empty() && !prev_source.contains('\n') {
+                return sp.with_lo(BytePos(sp.lo().0 - prev_source.len() as u32));
+            }
+        }
+
+        sp
+    }
+
+    /// Extends the given `Span` to just after the previous occurrence of `pat` when surrounded by
+    /// whitespace. Returns the same span if no character could be found or if an error occurred
+    /// while retrieving the code snippet.
+    pub fn span_extend_to_prev_str(&self, sp: Span, pat: &str, accept_newlines: bool) -> Span {
+        // assure that the pattern is delimited, to avoid the following
+        //     fn my_fn()
+        //           ^^^^ returned span without the check
+        //     ---------- correct span
+        for ws in &[" ", "\t", "\n"] {
+            let pat = pat.to_owned() + ws;
+            if let Ok(prev_source) = self.span_to_prev_source(sp) {
+                let prev_source = prev_source.rsplit(&pat).nth(0).unwrap_or("").trim_start();
+                if !prev_source.is_empty() && (!prev_source.contains('\n') || accept_newlines) {
+                    return sp.with_lo(BytePos(sp.lo().0 - prev_source.len() as u32));
+                }
+            }
+        }
+
+        sp
+    }
+
+    /// Given a `Span`, tries to get a shorter span ending before the first occurrence of `char`
+    /// `c`.
+    pub fn span_until_char(&self, sp: Span, c: char) -> Span {
+        match self.span_to_snippet(sp) {
+            Ok(snippet) => {
+                let snippet = snippet.split(c).nth(0).unwrap_or("").trim_end();
+                if !snippet.is_empty() && !snippet.contains('\n') {
+                    sp.with_hi(BytePos(sp.lo().0 + snippet.len() as u32))
+                } else {
+                    sp
+                }
+            }
+            _ => sp,
+        }
+    }
+
+    /// Given a `Span`, tries to get a shorter span ending just after the first occurrence of `char`
+    /// `c`.
+    pub fn span_through_char(&self, sp: Span, c: char) -> Span {
+        if let Ok(snippet) = self.span_to_snippet(sp) {
+            if let Some(offset) = snippet.find(c) {
+                return sp.with_hi(BytePos(sp.lo().0 + (offset + c.len_utf8()) as u32));
+            }
+        }
+        sp
+    }
+
+    /// Given a `Span`, gets a new `Span` covering the first token and all its trailing whitespace
+    /// or the original `Span`.
+    ///
+    /// If `sp` points to `"let mut x"`, then a span pointing at `"let "` will be returned.
+    pub fn span_until_non_whitespace(&self, sp: Span) -> Span {
+        let mut whitespace_found = false;
+
+        self.span_take_while(sp, |c| {
+            if !whitespace_found && c.is_whitespace() {
+                whitespace_found = true;
+            }
+
+            if whitespace_found && !c.is_whitespace() {
+                false
+            } else {
+                true
+            }
+        })
+    }
+
+    /// Given a `Span`, gets a new `Span` covering the first token without its trailing whitespace
+    /// or the original `Span` in case of error.
+    ///
+    /// If `sp` points to `"let mut x"`, then a span pointing at `"let"` will be returned.
+    pub fn span_until_whitespace(&self, sp: Span) -> Span {
+        self.span_take_while(sp, |c| !c.is_whitespace())
+    }
+
+    /// Given a `Span`, gets a shorter one until `predicate` yields `false`.
+    pub fn span_take_while<P>(&self, sp: Span, predicate: P) -> Span
+        where P: for <'r> FnMut(&'r char) -> bool
+    {
+        if let Ok(snippet) = self.span_to_snippet(sp) {
+            let offset = snippet.chars()
+                .take_while(predicate)
+                .map(|c| c.len_utf8())
+                .sum::<usize>();
+
+            sp.with_hi(BytePos(sp.lo().0 + (offset as u32)))
+        } else {
+            sp
+        }
+    }
+
+    pub fn def_span(&self, sp: Span) -> Span {
+        self.span_until_char(sp, '{')
+    }
+
+    /// Returns a new span representing just the start point of this span.
+    pub fn start_point(&self, sp: Span) -> Span {
+        let pos = sp.lo().0;
+        let width = self.find_width_of_character_at_span(sp, false);
+        let corrected_start_position = pos.checked_add(width).unwrap_or(pos);
+        let end_point = BytePos(cmp::max(corrected_start_position, sp.lo().0));
+        sp.with_hi(end_point)
+    }
+
+    /// Returns a new span representing just the end point of this span.
+    pub fn end_point(&self, sp: Span) -> Span {
+        let pos = sp.hi().0;
+
+        let width = self.find_width_of_character_at_span(sp, false);
+        let corrected_end_position = pos.checked_sub(width).unwrap_or(pos);
+
+        let end_point = BytePos(cmp::max(corrected_end_position, sp.lo().0));
+        sp.with_lo(end_point)
+    }
+
+    /// Returns a new span representing the next character after the end-point of this span.
+    pub fn next_point(&self, sp: Span) -> Span {
+        let start_of_next_point = sp.hi().0;
+
+        let width = self.find_width_of_character_at_span(sp, true);
+        // If the width is 1, then the next span should point to the same `lo` and `hi`. However,
+        // in the case of a multibyte character, where the width != 1, the next span should
+        // span multiple bytes to include the whole character.
+        let end_of_next_point = start_of_next_point.checked_add(
+            width - 1).unwrap_or(start_of_next_point);
+
+        let end_of_next_point = BytePos(cmp::max(sp.lo().0 + 1, end_of_next_point));
+        Span::new(BytePos(start_of_next_point), end_of_next_point, sp.ctxt())
+    }
+
+    /// Finds the width of a character, either before or after the provided span.
+    fn find_width_of_character_at_span(&self, sp: Span, forwards: bool) -> u32 {
+        let sp = sp.data();
+        if sp.lo == sp.hi {
+            debug!("find_width_of_character_at_span: early return empty span");
+            return 1;
+        }
+
+        let local_begin = self.lookup_byte_offset(sp.lo);
+        let local_end = self.lookup_byte_offset(sp.hi);
+        debug!("find_width_of_character_at_span: local_begin=`{:?}`, local_end=`{:?}`",
+               local_begin, local_end);
+
+        if local_begin.sf.start_pos != local_end.sf.start_pos {
+            debug!("find_width_of_character_at_span: begin and end are in different files");
+            return 1;
+        }
+
+        let start_index = local_begin.pos.to_usize();
+        let end_index = local_end.pos.to_usize();
+        debug!("find_width_of_character_at_span: start_index=`{:?}`, end_index=`{:?}`",
+               start_index, end_index);
+
+        // Disregard indexes that are at the start or end of their spans, they can't fit bigger
+        // characters.
+        if (!forwards && end_index == usize::min_value()) ||
+            (forwards && start_index == usize::max_value()) {
+            debug!("find_width_of_character_at_span: start or end of span, cannot be multibyte");
+            return 1;
+        }
+
+        let source_len = (local_begin.sf.end_pos - local_begin.sf.start_pos).to_usize();
+        debug!("find_width_of_character_at_span: source_len=`{:?}`", source_len);
+        // Ensure indexes are also not malformed.
+        if start_index > end_index || end_index > source_len {
+            debug!("find_width_of_character_at_span: source indexes are malformed");
+            return 1;
+        }
+
+        let src = local_begin.sf.external_src.borrow();
+
+        // We need to extend the snippet to the end of the src rather than to end_index so when
+        // searching forwards for boundaries we've got somewhere to search.
+        let snippet = if let Some(ref src) = local_begin.sf.src {
+            let len = src.len();
+            (&src[start_index..len])
+        } else if let Some(src) = src.get_source() {
+            let len = src.len();
+            (&src[start_index..len])
+        } else {
+            return 1;
+        };
+        debug!("find_width_of_character_at_span: snippet=`{:?}`", snippet);
+
+        let mut target = if forwards { end_index + 1 } else { end_index - 1 };
+        debug!("find_width_of_character_at_span: initial target=`{:?}`", target);
+
+        while !snippet.is_char_boundary(target - start_index) && target < source_len {
+            target = if forwards {
+                target + 1
+            } else {
+                match target.checked_sub(1) {
+                    Some(target) => target,
+                    None => {
+                        break;
+                    }
+                }
+            };
+            debug!("find_width_of_character_at_span: target=`{:?}`", target);
+        }
+        debug!("find_width_of_character_at_span: final target=`{:?}`", target);
+
+        if forwards {
+            (target - end_index) as u32
+        } else {
+            (end_index - target) as u32
+        }
+    }
+
+    pub fn get_source_file(&self, filename: &FileName) -> Option<Lrc<SourceFile>> {
+        for sf in self.files.borrow().source_files.iter() {
+            if *filename == sf.name {
+                return Some(sf.clone());
+            }
+        }
+        None
+    }
+
+    /// For a global `BytePos`, computes the local offset within the containing `SourceFile`.
+    pub fn lookup_byte_offset(&self, bpos: BytePos) -> SourceFileAndBytePos {
+        let idx = self.lookup_source_file_idx(bpos);
+        let sf = (*self.files.borrow().source_files)[idx].clone();
+        let offset = bpos - sf.start_pos;
+        SourceFileAndBytePos {sf, pos: offset}
+    }
+
+    /// Converts an absolute `BytePos` to a `CharPos` relative to the `SourceFile`.
+    pub fn bytepos_to_file_charpos(&self, bpos: BytePos) -> CharPos {
+        let idx = self.lookup_source_file_idx(bpos);
+        let map = &(*self.files.borrow().source_files)[idx];
+
+        // The number of extra bytes due to multibyte chars in the `SourceFile`.
+        let mut total_extra_bytes = 0;
+
+        for mbc in map.multibyte_chars.iter() {
+            debug!("{}-byte char at {:?}", mbc.bytes, mbc.pos);
+            if mbc.pos < bpos {
+                // Every character is at least one byte, so we only
+                // count the actual extra bytes.
+                total_extra_bytes += mbc.bytes as u32 - 1;
+                // We should never see a byte position in the middle of a
+                // character.
+                assert!(bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32);
+            } else {
+                break;
+            }
+        }
+
+        assert!(map.start_pos.to_u32() + total_extra_bytes <= bpos.to_u32());
+        CharPos(bpos.to_usize() - map.start_pos.to_usize() - total_extra_bytes as usize)
+    }
+
+    // Returns the index of the `SourceFile` (in `self.files`) that contains `pos`.
+    pub fn lookup_source_file_idx(&self, pos: BytePos) -> usize {
+        self.files.borrow().source_files.binary_search_by_key(&pos, |key| key.start_pos)
+            .unwrap_or_else(|p| p - 1)
+    }
+
+    pub fn count_lines(&self) -> usize {
+        self.files().iter().fold(0, |a, f| a + f.count_lines())
+    }
+
+
+    pub fn generate_fn_name_span(&self, span: Span) -> Option<Span> {
+        let prev_span = self.span_extend_to_prev_str(span, "fn", true);
+        self.span_to_snippet(prev_span).map(|snippet| {
+            let len = snippet.find(|c: char| !c.is_alphanumeric() && c != '_')
+                .expect("no label after fn");
+            prev_span.with_hi(BytePos(prev_span.lo().0 + len as u32))
+        }).ok()
+    }
+
+    /// Takes the span of a type parameter in a function signature and try to generate a span for
+    /// the function name (with generics) and a new snippet for this span with the pointed type
+    /// parameter as a new local type parameter.
+    ///
+    /// For instance:
+    /// ```rust,ignore (pseudo-Rust)
+    /// // Given span
+    /// fn my_function(param: T)
+    /// //                    ^ Original span
+    ///
+    /// // Result
+    /// fn my_function(param: T)
+    /// // ^^^^^^^^^^^ Generated span with snippet `my_function<T>`
+    /// ```
+    ///
+    /// Attention: The method used is very fragile since it essentially duplicates the work of the
+    /// parser. If you need to use this function or something similar, please consider updating the
+    /// `SourceMap` functions and this function to something more robust.
+    pub fn generate_local_type_param_snippet(&self, span: Span) -> Option<(Span, String)> {
+        // Try to extend the span to the previous "fn" keyword to retrieve the function
+        // signature.
+        let sugg_span = self.span_extend_to_prev_str(span, "fn", false);
+        if sugg_span != span {
+            if let Ok(snippet) = self.span_to_snippet(sugg_span) {
+                // Consume the function name.
+                let mut offset = snippet.find(|c: char| !c.is_alphanumeric() && c != '_')
+                    .expect("no label after fn");
+
+                // Consume the generics part of the function signature.
+                let mut bracket_counter = 0;
+                let mut last_char = None;
+                for c in snippet[offset..].chars() {
+                    match c {
+                        '<' => bracket_counter += 1,
+                        '>' => bracket_counter -= 1,
+                        '(' => if bracket_counter == 0 { break; }
+                        _ => {}
+                    }
+                    offset += c.len_utf8();
+                    last_char = Some(c);
+                }
+
+                // Adjust the suggestion span to encompass the function name with its generics.
+                let sugg_span = sugg_span.with_hi(BytePos(sugg_span.lo().0 + offset as u32));
+
+                // Prepare the new suggested snippet to append the type parameter that triggered
+                // the error in the generics of the function signature.
+                let mut new_snippet = if last_char == Some('>') {
+                    format!("{}, ", &snippet[..(offset - '>'.len_utf8())])
+                } else {
+                    format!("{}<", &snippet[..offset])
+                };
+                new_snippet.push_str(
+                    &self.span_to_snippet(span).unwrap_or_else(|_| "T".to_string()));
+                new_snippet.push('>');
+
+                return Some((sugg_span, new_snippet));
+            }
+        }
+
+        None
+    }
+    pub fn ensure_source_file_source_present(&self, source_file: Lrc<SourceFile>) -> bool {
+        source_file.add_external_src(
+            || match source_file.name {
+                FileName::Real(ref name) => self.file_loader.read_file(name).ok(),
+                _ => None,
+            }
+        )
+    }
+    pub fn call_span_if_macro(&self, sp: Span) -> Span {
+        if self.span_to_filename(sp.clone()).is_macros() {
+            let v = sp.macro_backtrace();
+            if let Some(use_site) = v.last() {
+                return use_site.call_site;
+            }
+        }
+        sp
+    }
+}
+
+#[derive(Clone)]
+pub struct FilePathMapping {
+    mapping: Vec<(PathBuf, PathBuf)>,
+}
+
+impl FilePathMapping {
+    pub fn empty() -> FilePathMapping {
+        FilePathMapping {
+            mapping: vec![]
+        }
+    }
+
+    pub fn new(mapping: Vec<(PathBuf, PathBuf)>) -> FilePathMapping {
+        FilePathMapping {
+            mapping,
+        }
+    }
+
+    /// Applies any path prefix substitution as defined by the mapping.
+    /// The return value is the remapped path and a boolean indicating whether
+    /// the path was affected by the mapping.
+    pub fn map_prefix(&self, path: PathBuf) -> (PathBuf, bool) {
+        // NOTE: We are iterating over the mapping entries from last to first
+        //       because entries specified later on the command line should
+        //       take precedence.
+        for &(ref from, ref to) in self.mapping.iter().rev() {
+            if let Ok(rest) = path.strip_prefix(from) {
+                return (to.join(rest), true);
+            }
+        }
+
+        (path, false)
+    }
+}
diff --git a/src/libsyntax_pos/source_map/tests.rs b/src/libsyntax_pos/source_map/tests.rs
new file mode 100644
index 00000000000..15254336bbf
--- /dev/null
+++ b/src/libsyntax_pos/source_map/tests.rs
@@ -0,0 +1,221 @@
+use super::*;
+
+use rustc_data_structures::sync::Lrc;
+
+fn init_source_map() -> SourceMap {
+    let sm = SourceMap::new(FilePathMapping::empty());
+    sm.new_source_file(
+        PathBuf::from("blork.rs").into(),
+        "first line.\nsecond line".to_string(),
+    );
+    sm.new_source_file(
+        PathBuf::from("empty.rs").into(),
+        String::new(),
+    );
+    sm.new_source_file(
+        PathBuf::from("blork2.rs").into(),
+        "first line.\nsecond line".to_string(),
+    );
+    sm
+}
+
+/// Tests `lookup_byte_offset`.
+#[test]
+fn t3() {
+    let sm = init_source_map();
+
+    let srcfbp1 = sm.lookup_byte_offset(BytePos(23));
+    assert_eq!(srcfbp1.sf.name, PathBuf::from("blork.rs").into());
+    assert_eq!(srcfbp1.pos, BytePos(23));
+
+    let srcfbp1 = sm.lookup_byte_offset(BytePos(24));
+    assert_eq!(srcfbp1.sf.name, PathBuf::from("empty.rs").into());
+    assert_eq!(srcfbp1.pos, BytePos(0));
+
+    let srcfbp2 = sm.lookup_byte_offset(BytePos(25));
+    assert_eq!(srcfbp2.sf.name, PathBuf::from("blork2.rs").into());
+    assert_eq!(srcfbp2.pos, BytePos(0));
+}
+
+/// Tests `bytepos_to_file_charpos`.
+#[test]
+fn t4() {
+    let sm = init_source_map();
+
+    let cp1 = sm.bytepos_to_file_charpos(BytePos(22));
+    assert_eq!(cp1, CharPos(22));
+
+    let cp2 = sm.bytepos_to_file_charpos(BytePos(25));
+    assert_eq!(cp2, CharPos(0));
+}
+
+/// Tests zero-length `SourceFile`s.
+#[test]
+fn t5() {
+    let sm = init_source_map();
+
+    let loc1 = sm.lookup_char_pos(BytePos(22));
+    assert_eq!(loc1.file.name, PathBuf::from("blork.rs").into());
+    assert_eq!(loc1.line, 2);
+    assert_eq!(loc1.col, CharPos(10));
+
+    let loc2 = sm.lookup_char_pos(BytePos(25));
+    assert_eq!(loc2.file.name, PathBuf::from("blork2.rs").into());
+    assert_eq!(loc2.line, 1);
+    assert_eq!(loc2.col, CharPos(0));
+}
+
+fn init_source_map_mbc() -> SourceMap {
+    let sm = SourceMap::new(FilePathMapping::empty());
+    // "€" is a three-byte UTF8 char.
+    sm.new_source_file(PathBuf::from("blork.rs").into(),
+                    "fir€st €€€€ line.\nsecond line".to_string());
+    sm.new_source_file(PathBuf::from("blork2.rs").into(),
+                    "first line€€.\n€ second line".to_string());
+    sm
+}
+
+/// Tests `bytepos_to_file_charpos` in the presence of multi-byte chars.
+#[test]
+fn t6() {
+    let sm = init_source_map_mbc();
+
+    let cp1 = sm.bytepos_to_file_charpos(BytePos(3));
+    assert_eq!(cp1, CharPos(3));
+
+    let cp2 = sm.bytepos_to_file_charpos(BytePos(6));
+    assert_eq!(cp2, CharPos(4));
+
+    let cp3 = sm.bytepos_to_file_charpos(BytePos(56));
+    assert_eq!(cp3, CharPos(12));
+
+    let cp4 = sm.bytepos_to_file_charpos(BytePos(61));
+    assert_eq!(cp4, CharPos(15));
+}
+
+/// Test `span_to_lines` for a span ending at the end of a `SourceFile`.
+#[test]
+fn t7() {
+    let sm = init_source_map();
+    let span = Span::with_root_ctxt(BytePos(12), BytePos(23));
+    let file_lines = sm.span_to_lines(span).unwrap();
+
+    assert_eq!(file_lines.file.name, PathBuf::from("blork.rs").into());
+    assert_eq!(file_lines.lines.len(), 1);
+    assert_eq!(file_lines.lines[0].line_index, 1);
+}
+
+/// Given a string like " ~~~~~~~~~~~~ ", produces a span
+/// converting that range. The idea is that the string has the same
+/// length as the input, and we uncover the byte positions. Note
+/// that this can span lines and so on.
+fn span_from_selection(input: &str, selection: &str) -> Span {
+    assert_eq!(input.len(), selection.len());
+    let left_index = selection.find('~').unwrap() as u32;
+    let right_index = selection.rfind('~').map(|x|x as u32).unwrap_or(left_index);
+    Span::with_root_ctxt(BytePos(left_index), BytePos(right_index + 1))
+}
+
+/// Tests `span_to_snippet` and `span_to_lines` for a span converting 3
+/// lines in the middle of a file.
+#[test]
+fn span_to_snippet_and_lines_spanning_multiple_lines() {
+    let sm = SourceMap::new(FilePathMapping::empty());
+    let inputtext = "aaaaa\nbbbbBB\nCCC\nDDDDDddddd\neee\n";
+    let selection = "     \n    ~~\n~~~\n~~~~~     \n   \n";
+    sm.new_source_file(Path::new("blork.rs").to_owned().into(), inputtext.to_string());
+    let span = span_from_selection(inputtext, selection);
+
+    // Check that we are extracting the text we thought we were extracting.
+    assert_eq!(&sm.span_to_snippet(span).unwrap(), "BB\nCCC\nDDDDD");
+
+    // Check that span_to_lines gives us the complete result with the lines/cols we expected.
+    let lines = sm.span_to_lines(span).unwrap();
+    let expected = vec![
+        LineInfo { line_index: 1, start_col: CharPos(4), end_col: CharPos(6) },
+        LineInfo { line_index: 2, start_col: CharPos(0), end_col: CharPos(3) },
+        LineInfo { line_index: 3, start_col: CharPos(0), end_col: CharPos(5) }
+        ];
+    assert_eq!(lines.lines, expected);
+}
+
+/// Test span_to_snippet for a span ending at the end of a `SourceFile`.
+#[test]
+fn t8() {
+    let sm = init_source_map();
+    let span = Span::with_root_ctxt(BytePos(12), BytePos(23));
+    let snippet = sm.span_to_snippet(span);
+
+    assert_eq!(snippet, Ok("second line".to_string()));
+}
+
+/// Test `span_to_str` for a span ending at the end of a `SourceFile`.
+#[test]
+fn t9() {
+    let sm = init_source_map();
+    let span = Span::with_root_ctxt(BytePos(12), BytePos(23));
+    let sstr =  sm.span_to_string(span);
+
+    assert_eq!(sstr, "blork.rs:2:1: 2:12");
+}
+
+/// Tests failing to merge two spans on different lines.
+#[test]
+fn span_merging_fail() {
+    let sm = SourceMap::new(FilePathMapping::empty());
+    let inputtext  = "bbbb BB\ncc CCC\n";
+    let selection1 = "     ~~\n      \n";
+    let selection2 = "       \n   ~~~\n";
+    sm.new_source_file(Path::new("blork.rs").to_owned().into(), inputtext.to_owned());
+    let span1 = span_from_selection(inputtext, selection1);
+    let span2 = span_from_selection(inputtext, selection2);
+
+    assert!(sm.merge_spans(span1, span2).is_none());
+}
+
+/// Returns the span corresponding to the `n`th occurrence of `substring` in `source_text`.
+trait SourceMapExtension {
+    fn span_substr(
+        &self,
+        file: &Lrc<SourceFile>,
+        source_text: &str,
+        substring: &str,
+        n: usize,
+    ) -> Span;
+}
+
+impl SourceMapExtension for SourceMap {
+    fn span_substr(
+        &self,
+        file: &Lrc<SourceFile>,
+        source_text: &str,
+        substring: &str,
+        n: usize,
+    ) -> Span {
+        println!(
+            "span_substr(file={:?}/{:?}, substring={:?}, n={})",
+            file.name, file.start_pos, substring, n
+        );
+        let mut i = 0;
+        let mut hi = 0;
+        loop {
+            let offset = source_text[hi..].find(substring).unwrap_or_else(|| {
+                panic!(
+                    "source_text `{}` does not have {} occurrences of `{}`, only {}",
+                    source_text, n, substring, i
+                );
+            });
+            let lo = hi + offset;
+            hi = lo + substring.len();
+            if i == n {
+                let span = Span::with_root_ctxt(
+                    BytePos(lo as u32 + file.start_pos.0),
+                    BytePos(hi as u32 + file.start_pos.0),
+                );
+                assert_eq!(&self.span_to_snippet(span).unwrap()[..], substring);
+                return span;
+            }
+            i += 1;
+        }
+    }
+}