Auto merge of #83813 - cbeuw:remap-std, r=michaelwoerister

Fix `--remap-path-prefix` not correctly remapping `rust-src` component paths and unify handling of path mapping with virtualized paths

This PR fixes #73167 ("Binaries end up containing path to the rust-src component despite `--remap-path-prefix`") by preventing real local filesystem paths from reaching compilation output if the path is supposed to be remapped.

`RealFileName::Named` introduced in #72767 is now renamed as `LocalPath`, because this variant wraps a (most likely) valid local filesystem path.

`RealFileName::Devirtualized` is renamed as `Remapped` to be used for remapped path from a real path via `--remap-path-prefix` argument, as well as real path inferred from a virtualized (during compiler bootstrapping) `/rustc/...` path. The `local_path` field is now an `Option<PathBuf>`, as it will be set to `None` before serialisation, so it never reaches any build output. Attempting to serialise a non-`None` `local_path` will cause an assertion faliure.

When a path is remapped, a `RealFileName::Remapped` variant is created. The original path is preserved in `local_path` field and the remapped path is saved in `virtual_name` field. Previously, the `local_path` is directly modified which goes against its purpose of "suitable for reading from the file system on the local host".

`rustc_span::SourceFile`'s fields `unmapped_path` (introduced by #44940) and `name_was_remapped` (introduced by #41508 when `--remap-path-prefix` feature originally added) are removed, as these two pieces of information can be inferred from the `name` field: if it's anything other than a `FileName::Real(_)`, or if it is a `FileName::Real(RealFileName::LocalPath(_))`, then clearly `name_was_remapped` would've been false and `unmapped_path` would've been `None`. If it is a `FileName::Real(RealFileName::Remapped{local_path, virtual_name})`, then `name_was_remapped` would've been true and `unmapped_path` would've been `Some(local_path)`.

cc `@eddyb` who implemented `/rustc/...` path devirtualisation

39 files changed, 372 insertions, 232 deletions

diff --git a/compiler/rustc_builtin_macros/src/source_util.rs b/compiler/rustc_builtin_macros/src/source_util.rs
index 4aafcb2fb6d..ccb9f15b0f5 100644
--- a/compiler/rustc_builtin_macros/src/source_util.rs
+++ b/compiler/rustc_builtin_macros/src/source_util.rs
@@ -61,7 +61,9 @@ pub fn expand_file(
 
     let topmost = cx.expansion_cause().unwrap_or(sp);
     let loc = cx.source_map().lookup_char_pos(topmost.lo());
-    base::MacEager::expr(cx.expr_str(topmost, Symbol::intern(&loc.file.name.to_string())))
+    base::MacEager::expr(
+        cx.expr_str(topmost, Symbol::intern(&loc.file.name.prefer_remapped().to_string_lossy())),
+    )
 }
 
 pub fn expand_stringify(
diff --git a/compiler/rustc_codegen_cranelift/src/common.rs b/compiler/rustc_codegen_cranelift/src/common.rs
index 92e4435565e..c12d6d0f141 100644
--- a/compiler/rustc_codegen_cranelift/src/common.rs
+++ b/compiler/rustc_codegen_cranelift/src/common.rs
@@ -334,7 +334,9 @@ impl<'tcx> FunctionCx<'_, '_, 'tcx> {
         let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
         let caller = self.tcx.sess.source_map().lookup_char_pos(topmost.lo());
         let const_loc = self.tcx.const_caller_location((
-            rustc_span::symbol::Symbol::intern(&caller.file.name.to_string()),
+            rustc_span::symbol::Symbol::intern(
+                &caller.file.name.prefer_remapped().to_string_lossy(),
+            ),
             caller.line as u32,
             caller.col_display as u32 + 1,
         ));
diff --git a/compiler/rustc_codegen_cranelift/src/debuginfo/line_info.rs b/compiler/rustc_codegen_cranelift/src/debuginfo/line_info.rs
index 8578ab33ced..9eb06770630 100644
--- a/compiler/rustc_codegen_cranelift/src/debuginfo/line_info.rs
+++ b/compiler/rustc_codegen_cranelift/src/debuginfo/line_info.rs
@@ -66,7 +66,7 @@ fn line_program_add_file(
 ) -> FileId {
     match &file.name {
         FileName::Real(path) => {
-            let (dir_path, file_name) = split_path_dir_and_file(path.stable_name());
+            let (dir_path, file_name) = split_path_dir_and_file(path.remapped_path_if_available());
             let dir_name = osstr_as_utf8_bytes(dir_path.as_os_str());
             let file_name = osstr_as_utf8_bytes(file_name);
 
@@ -87,7 +87,7 @@ fn line_program_add_file(
         filename => {
             let dir_id = line_program.default_directory();
             let dummy_file_name = LineString::new(
-                filename.to_string().into_bytes(),
+                filename.prefer_remapped().to_string().into_bytes(),
                 line_program.encoding(),
                 line_strings,
             );
diff --git a/compiler/rustc_codegen_cranelift/src/debuginfo/mod.rs b/compiler/rustc_codegen_cranelift/src/debuginfo/mod.rs
index dc8bc8d9cb7..61e54a76f29 100644
--- a/compiler/rustc_codegen_cranelift/src/debuginfo/mod.rs
+++ b/compiler/rustc_codegen_cranelift/src/debuginfo/mod.rs
@@ -64,7 +64,7 @@ impl<'tcx> DebugContext<'tcx> {
         // FIXME: how to get version when building out of tree?
         // Normally this would use option_env!("CFG_VERSION").
         let producer = format!("cg_clif (rustc {})", "unknown version");
-        let comp_dir = tcx.sess.working_dir.0.to_string_lossy().into_owned();
+        let comp_dir = tcx.sess.working_dir.to_string_lossy(false).into_owned();
         let (name, file_info) = match tcx.sess.local_crate_source_file.clone() {
             Some(path) => {
                 let name = path.to_string_lossy().into_owned();
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
index 280d9a4d370..5cf831475d4 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs
@@ -760,12 +760,12 @@ fn hex_encode(data: &[u8]) -> String {
 }
 
 pub fn file_metadata(cx: &CodegenCx<'ll, '_>, source_file: &SourceFile) -> &'ll DIFile {
-    debug!("file_metadata: file_name: {}", source_file.name);
+    debug!("file_metadata: file_name: {:?}", source_file.name);
 
     let hash = Some(&source_file.src_hash);
-    let file_name = Some(source_file.name.to_string());
+    let file_name = Some(source_file.name.prefer_remapped().to_string());
     let directory = if source_file.is_real_file() && !source_file.is_imported() {
-        Some(cx.sess().working_dir.0.to_string_lossy().to_string())
+        Some(cx.sess().working_dir.to_string_lossy(false).to_string())
     } else {
         // If the path comes from an upstream crate we assume it has been made
         // independent of the compiler's working directory one way or another.
@@ -993,7 +993,7 @@ pub fn compile_unit_metadata(
     let producer = format!("clang LLVM ({})", rustc_producer);
 
     let name_in_debuginfo = name_in_debuginfo.to_string_lossy();
-    let work_dir = tcx.sess.working_dir.0.to_string_lossy();
+    let work_dir = tcx.sess.working_dir.to_string_lossy(false);
     let flags = "\0";
     let out_dir = &tcx.output_filenames(LOCAL_CRATE).out_directory;
     let split_name = if tcx.sess.target_can_use_split_dwarf() {
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index fd3f89a2aee..72e9163b88e 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -1144,7 +1144,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
             let caller = tcx.sess.source_map().lookup_char_pos(topmost.lo());
             let const_loc = tcx.const_caller_location((
-                Symbol::intern(&caller.file.name.to_string()),
+                Symbol::intern(&caller.file.name.prefer_remapped().to_string_lossy()),
                 caller.line as u32,
                 caller.col_display as u32 + 1,
             ));
diff --git a/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs b/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
index 6f365c07f6d..577baec21f0 100644
--- a/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
+++ b/compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs
@@ -126,7 +126,8 @@ impl AnnotateSnippetEmitterWriter {
             }
             // owned: line source, line index, annotations
             type Owned = (String, usize, Vec<crate::snippet::Annotation>);
-            let origin = primary_lo.file.name.to_string();
+            let filename = primary_lo.file.name.prefer_local();
+            let origin = filename.to_string_lossy();
             let annotated_files: Vec<Owned> = annotated_files
                 .into_iter()
                 .flat_map(|annotated_file| {
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 61870eede1d..c4c15136cf8 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -1324,7 +1324,7 @@ impl EmitterWriter {
                         buffer_msg_line_offset,
                         &format!(
                             "{}:{}:{}",
-                            loc.file.name,
+                            loc.file.name.prefer_local(),
                             sm.doctest_offset_line(&loc.file.name, loc.line),
                             loc.col.0 + 1,
                         ),
@@ -1338,7 +1338,7 @@ impl EmitterWriter {
                         0,
                         &format!(
                             "{}:{}:{}: ",
-                            loc.file.name,
+                            loc.file.name.prefer_local(),
                             sm.doctest_offset_line(&loc.file.name, loc.line),
                             loc.col.0 + 1,
                         ),
@@ -1362,12 +1362,12 @@ impl EmitterWriter {
                     };
                     format!(
                         "{}:{}{}",
-                        annotated_file.file.name,
+                        annotated_file.file.name.prefer_local(),
                         sm.doctest_offset_line(&annotated_file.file.name, first_line.line_index),
                         col
                     )
                 } else {
-                    annotated_file.file.name.to_string()
+                    format!("{}", annotated_file.file.name.prefer_local())
                 };
                 buffer.append(buffer_msg_line_offset + 1, &loc, Style::LineAndColumn);
                 for _ in 0..max_line_num_len {
diff --git a/compiler/rustc_errors/src/json.rs b/compiler/rustc_errors/src/json.rs
index 40277006462..5d175a3ade9 100644
--- a/compiler/rustc_errors/src/json.rs
+++ b/compiler/rustc_errors/src/json.rs
@@ -468,7 +468,7 @@ impl DiagnosticSpan {
         });
 
         DiagnosticSpan {
-            file_name: start.file.name.to_string(),
+            file_name: start.file.name.prefer_local().to_string(),
             byte_start: start.file.original_relative_byte_pos(span.lo()).0,
             byte_end: start.file.original_relative_byte_pos(span.hi()).0,
             line_start: start.line,
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index 0e966d0bf79..e1c218c6408 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -1084,13 +1084,18 @@ impl<'a> ExtCtxt<'a> {
         // after macro expansion (that is, they are unhygienic).
         if !path.is_absolute() {
             let callsite = span.source_callsite();
-            let mut result = match self.source_map().span_to_unmapped_path(callsite) {
-                FileName::Real(name) => name.into_local_path(),
+            let mut result = match self.source_map().span_to_filename(callsite) {
+                FileName::Real(name) => name
+                    .into_local_path()
+                    .expect("attempting to resolve a file path in an external file"),
                 FileName::DocTest(path, _) => path,
                 other => {
                     return Err(self.struct_span_err(
                         span,
-                        &format!("cannot resolve relative path in non-file source `{}`", other),
+                        &format!(
+                            "cannot resolve relative path in non-file source `{}`",
+                            other.prefer_local()
+                        ),
                     ));
                 }
             };
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 3347c93948c..f5c6bb3db65 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -361,9 +361,11 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
     // FIXME: Avoid visiting the crate as a `Mod` item,
     // make crate a first class expansion target instead.
     pub fn expand_crate(&mut self, mut krate: ast::Crate) -> ast::Crate {
-        let file_path = match self.cx.source_map().span_to_unmapped_path(krate.span) {
-            FileName::Real(name) => name.into_local_path(),
-            other => PathBuf::from(other.to_string()),
+        let file_path = match self.cx.source_map().span_to_filename(krate.span) {
+            FileName::Real(name) => name
+                .into_local_path()
+                .expect("attempting to resolve a file path in an external file"),
+            other => PathBuf::from(other.prefer_local().to_string()),
         };
         let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
         self.cx.root_path = dir_path.clone();
diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index f6645448fcc..7bf6502c976 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -638,10 +638,11 @@ impl server::SourceFile for Rustc<'_> {
         match file.name {
             FileName::Real(ref name) => name
                 .local_path()
+                .expect("attempting to get a file path in an imported file in `proc_macro::SourceFile::path`")
                 .to_str()
                 .expect("non-UTF8 file path in `proc_macro::SourceFile::path`")
                 .to_string(),
-            _ => file.name.to_string(),
+            _ => file.name.prefer_local().to_string(),
         }
     }
     fn is_real(&mut self, file: &Self::SourceFile) -> bool {
@@ -785,7 +786,7 @@ fn ident_name_compatibility_hack(
         if let ExpnKind::Macro { name: macro_name, .. } = orig_span.ctxt().outer_expn_data().kind {
             let source_map = rustc.sess.source_map();
             let filename = source_map.span_to_filename(orig_span);
-            if let FileName::Real(RealFileName::Named(path)) = filename {
+            if let FileName::Real(RealFileName::LocalPath(path)) = filename {
                 let matches_prefix = |prefix, filename| {
                     // Check for a path that ends with 'prefix*/src/<filename>'
                     let mut iter = path.components().rev();
@@ -848,7 +849,7 @@ fn ident_name_compatibility_hack(
                 if macro_name == sym::tuple_from_req && matches_prefix("actix-web", "extract.rs") {
                     let snippet = source_map.span_to_snippet(orig_span);
                     if snippet.as_deref() == Ok("$T") {
-                        if let FileName::Real(RealFileName::Named(macro_path)) =
+                        if let FileName::Real(RealFileName::LocalPath(macro_path)) =
                             source_map.span_to_filename(rustc.def_site)
                         {
                             if macro_path.to_string_lossy().contains("pin-project-internal-0.") {
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 1cafb2fe1a2..680f6af63f2 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -1604,13 +1604,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             match (&terr, expected == found) {
                 (TypeError::Sorts(values), extra) => {
                     let sort_string = |ty: Ty<'tcx>| match (extra, ty.kind()) {
-                        (true, ty::Opaque(def_id, _)) => format!(
-                            " (opaque type at {})",
-                            self.tcx
+                        (true, ty::Opaque(def_id, _)) => {
+                            let pos = self
+                                .tcx
                                 .sess
                                 .source_map()
-                                .mk_substr_filename(self.tcx.def_span(*def_id)),
-                        ),
+                                .lookup_char_pos(self.tcx.def_span(*def_id).lo());
+                            format!(
+                                " (opaque type at <{}:{}:{}>)",
+                                pos.file.name.prefer_local(),
+                                pos.line,
+                                pos.col.to_usize() + 1,
+                            )
+                        }
                         (true, _) => format!(" ({})", ty.sort_string(self.tcx)),
                         (false, _) => "".to_string(),
                     };
diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 96dbdce1126..9005325f0b4 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -35,7 +35,6 @@ use rustc_session::output::{filename_for_input, filename_for_metadata};
 use rustc_session::search_paths::PathKind;
 use rustc_session::Session;
 use rustc_span::symbol::{Ident, Symbol};
-use rustc_span::{FileName, RealFileName};
 use rustc_trait_selection::traits;
 use rustc_typeck as typeck;
 use tracing::{info, warn};
@@ -531,10 +530,10 @@ fn output_conflicts_with_dir(output_paths: &[PathBuf]) -> Option<PathBuf> {
     check_output(output_paths, check)
 }
 
-fn escape_dep_filename(filename: &FileName) -> String {
+fn escape_dep_filename(filename: &String) -> String {
     // Apparently clang and gcc *only* escape spaces:
     // http://llvm.org/klaus/clang/commit/9d50634cfc268ecc9a7250226dd5ca0e945240d4
-    filename.to_string().replace(" ", "\\ ")
+    filename.replace(" ", "\\ ")
 }
 
 // Makefile comments only need escaping newlines and `\`.
@@ -574,7 +573,7 @@ fn write_out_deps(
             .iter()
             .filter(|fmap| fmap.is_real_file())
             .filter(|fmap| !fmap.is_imported())
-            .map(|fmap| escape_dep_filename(&fmap.unmapped_path.as_ref().unwrap_or(&fmap.name)))
+            .map(|fmap| escape_dep_filename(&fmap.name.prefer_local().to_string()))
             .collect();
 
         if let Some(ref backend) = sess.opts.debugging_opts.codegen_backend {
@@ -586,16 +585,13 @@ fn write_out_deps(
                 for cnum in resolver.cstore().crates_untracked() {
                     let source = resolver.cstore().crate_source_untracked(cnum);
                     if let Some((path, _)) = source.dylib {
-                        let file_name = FileName::Real(RealFileName::Named(path));
-                        files.push(escape_dep_filename(&file_name));
+                        files.push(escape_dep_filename(&path.display().to_string()));
                     }
                     if let Some((path, _)) = source.rlib {
-                        let file_name = FileName::Real(RealFileName::Named(path));
-                        files.push(escape_dep_filename(&file_name));
+                        files.push(escape_dep_filename(&path.display().to_string()));
                     }
                     if let Some((path, _)) = source.rmeta {
-                        let file_name = FileName::Real(RealFileName::Named(path));
-                        files.push(escape_dep_filename(&file_name));
+                        files.push(escape_dep_filename(&path.display().to_string()));
                     }
                 }
             });
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index fd13cb3d59a..17dc6fd1d52 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -724,6 +724,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(profile_emit, Some(PathBuf::from("abc")));
     tracked!(relax_elf_relocations, Some(true));
     tracked!(relro_level, Some(RelroLevel::Full));
+    tracked!(simulate_remapped_rust_src_base, Some(PathBuf::from("/rustc/abc")));
     tracked!(report_delayed_bugs, true);
     tracked!(sanitizer, SanitizerSet::ADDRESS);
     tracked!(sanitizer_memory_track_origins, 2);
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 4a01731b512..e2e523fad66 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1651,9 +1651,11 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             if let Some(virtual_dir) = virtual_rust_source_base_dir {
                 if let Some(real_dir) = &sess.opts.real_rust_source_base_dir {
                     if let rustc_span::FileName::Real(old_name) = name {
-                        if let rustc_span::RealFileName::Named(one_path) = old_name {
-                            if let Ok(rest) = one_path.strip_prefix(virtual_dir) {
-                                let virtual_name = one_path.clone();
+                        if let rustc_span::RealFileName::Remapped { local_path: _, virtual_name } =
+                            old_name
+                        {
+                            if let Ok(rest) = virtual_name.strip_prefix(virtual_dir) {
+                                let virtual_name = virtual_name.clone();
 
                                 // The std library crates are in
                                 // `$sysroot/lib/rustlib/src/rust/library`, whereas other crates
@@ -1689,8 +1691,8 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                                     virtual_name.display(),
                                     new_path.display(),
                                 );
-                                let new_name = rustc_span::RealFileName::Devirtualized {
-                                    local_path: new_path,
+                                let new_name = rustc_span::RealFileName::Remapped {
+                                    local_path: Some(new_path),
                                     virtual_name,
                                 };
                                 *old_name = new_name;
@@ -1710,7 +1712,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                     // containing the information we need.
                     let rustc_span::SourceFile {
                         mut name,
-                        name_was_remapped,
                         src_hash,
                         start_pos,
                         end_pos,
@@ -1722,11 +1723,34 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                         ..
                     } = source_file_to_import;
 
+                    // If this file is under $sysroot/lib/rustlib/src/ but has not been remapped
+                    // during rust bootstrapping by `remap-debuginfo = true`, and the user
+                    // wish to simulate that behaviour by -Z simulate-remapped-rust-src-base,
+                    // then we change `name` to a similar state as if the rust was bootstrapped
+                    // with `remap-debuginfo = true`.
+                    // This is useful for testing so that tests about the effects of
+                    // `try_to_translate_virtual_to_real` don't have to worry about how the
+                    // compiler is bootstrapped.
+                    if let Some(virtual_dir) =
+                        &sess.opts.debugging_opts.simulate_remapped_rust_src_base
+                    {
+                        if let Some(real_dir) = &sess.opts.real_rust_source_base_dir {
+                            if let rustc_span::FileName::Real(ref mut old_name) = name {
+                                if let rustc_span::RealFileName::LocalPath(local) = old_name {
+                                    if let Ok(rest) = local.strip_prefix(real_dir) {
+                                        *old_name = rustc_span::RealFileName::Remapped {
+                                            local_path: None,
+                                            virtual_name: virtual_dir.join(rest),
+                                        };
+                                    }
+                                }
+                            }
+                        }
+                    }
+
                     // If this file's path has been remapped to `/rustc/$hash`,
                     // we might be able to reverse that (also see comments above,
                     // on `try_to_translate_virtual_to_real`).
-                    // FIXME(eddyb) we could check `name_was_remapped` here,
-                    // but in practice it seems to be always `false`.
                     try_to_translate_virtual_to_real(&mut name);
 
                     let source_length = (end_pos - start_pos).to_usize();
@@ -1751,7 +1775,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
 
                     let local_version = sess.source_map().new_imported_source_file(
                         name,
-                        name_was_remapped,
                         src_hash,
                         name_hash,
                         source_length,
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index c61417f9998..19c713665c7 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -28,9 +28,12 @@ use rustc_middle::ty::codec::TyEncoder;
 use rustc_middle::ty::{self, SymbolName, Ty, TyCtxt};
 use rustc_serialize::{opaque, Encodable, Encoder};
 use rustc_session::config::CrateType;
-use rustc_span::hygiene::{ExpnDataEncodeMode, HygieneEncodeContext, MacroKind};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{self, ExternalSource, FileName, SourceFile, Span, SyntaxContext};
+use rustc_span::{
+    hygiene::{ExpnDataEncodeMode, HygieneEncodeContext, MacroKind},
+    RealFileName,
+};
 use rustc_target::abi::VariantIdx;
 use std::hash::Hash;
 use std::num::NonZeroUsize;
@@ -466,7 +469,6 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let source_map = self.tcx.sess.source_map();
         let all_source_files = source_map.files();
 
-        let (working_dir, _cwd_remapped) = self.tcx.sess.working_dir.clone();
         // By replacing the `Option` with `None`, we ensure that we can't
         // accidentally serialize any more `Span`s after the source map encoding
         // is done.
@@ -485,18 +487,41 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             })
             .map(|(_, source_file)| {
                 let mut adapted = match source_file.name {
-                    // This path of this SourceFile has been modified by
-                    // path-remapping, so we use it verbatim (and avoid
-                    // cloning the whole map in the process).
-                    _ if source_file.name_was_remapped => source_file.clone(),
-
-                    // Otherwise expand all paths to absolute paths because
-                    // any relative paths are potentially relative to a
-                    // wrong directory.
-                    FileName::Real(ref name) => {
-                        let name = name.stable_name();
+                    FileName::Real(ref realname) => {
                         let mut adapted = (**source_file).clone();
-                        adapted.name = Path::new(&working_dir).join(name).into();
+                        adapted.name = FileName::Real(match realname {
+                            RealFileName::LocalPath(path_to_file) => {
+                                // Prepend path of working directory onto potentially
+                                // relative paths, because they could become relative
+                                // to a wrong directory.
+                                let working_dir = &self.tcx.sess.working_dir;
+                                match working_dir {
+                                    RealFileName::LocalPath(absolute) => {
+                                        // If working_dir has not been remapped, then we emit a
+                                        // LocalPath variant as it's likely to be a valid path
+                                        RealFileName::LocalPath(
+                                            Path::new(absolute).join(path_to_file),
+                                        )
+                                    }
+                                    RealFileName::Remapped { local_path: _, virtual_name } => {
+                                        // If working_dir has been remapped, then we emit
+                                        // Remapped variant as the expanded path won't be valid
+                                        RealFileName::Remapped {
+                                            local_path: None,
+                                            virtual_name: Path::new(virtual_name)
+                                                .join(path_to_file),
+                                        }
+                                    }
+                                }
+                            }
+                            RealFileName::Remapped { local_path: _, virtual_name } => {
+                                RealFileName::Remapped {
+                                    // We do not want any local path to be exported into metadata
+                                    local_path: None,
+                                    virtual_name: virtual_name.clone(),
+                                }
+                            }
+                        });
                         adapted.name_hash = {
                             let mut hasher: StableHasher = StableHasher::new();
                             adapted.name.hash(&mut hasher);
diff --git a/compiler/rustc_middle/src/hir/map/collector.rs b/compiler/rustc_middle/src/hir/map/collector.rs
index 719bbf04c95..342cc9a7397 100644
--- a/compiler/rustc_middle/src/hir/map/collector.rs
+++ b/compiler/rustc_middle/src/hir/map/collector.rs
@@ -197,7 +197,7 @@ impl<'a, 'hir> NodeCollector<'a, 'hir> {
                     span,
                     "inconsistent DepNode at `{:?}` for `{}`: \
                      current_dep_node_owner={} ({:?}), hir_id.owner={} ({:?})",
-                    self.source_map.span_to_string(span),
+                    self.source_map.span_to_diagnostic_string(span),
                     node_str,
                     self.definitions
                         .def_path(self.current_dep_node_owner)
diff --git a/compiler/rustc_middle/src/ich/impls_syntax.rs b/compiler/rustc_middle/src/ich/impls_syntax.rs
index b93b25d6b5c..2d8f661ef59 100644
--- a/compiler/rustc_middle/src/ich/impls_syntax.rs
+++ b/compiler/rustc_middle/src/ich/impls_syntax.rs
@@ -61,8 +61,6 @@ impl<'a> HashStable<StableHashingContext<'a>> for SourceFile {
         let SourceFile {
             name: _, // We hash the smaller name_hash instead of this
             name_hash,
-            name_was_remapped,
-            unmapped_path: _,
             cnum,
             // Do not hash the source as it is not encoded
             src: _,
@@ -77,7 +75,6 @@ impl<'a> HashStable<StableHashingContext<'a>> for SourceFile {
         } = *self;
 
         (name_hash as u64).hash_stable(hcx, hasher);
-        name_was_remapped.hash_stable(hcx, hasher);
 
         src_hash.hash_stable(hcx, hasher);
 
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index de0c4f2e30e..1cc7f235d7d 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -2374,7 +2374,10 @@ impl<'tcx> Debug for Rvalue<'tcx> {
                                 )
                             } else {
                                 let span = tcx.hir().span(hir_id);
-                                format!("[closure@{}]", tcx.sess.source_map().span_to_string(span))
+                                format!(
+                                    "[closure@{}]",
+                                    tcx.sess.source_map().span_to_diagnostic_string(span)
+                                )
                             };
                             let mut struct_fmt = fmt.debug_struct(&name);
 
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 174ec481ae9..cfb9cfd86f9 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -667,7 +667,12 @@ pub trait PrettyPrinter<'tcx>:
                     if let Some(did) = did.as_local() {
                         let hir_id = self.tcx().hir().local_def_id_to_hir_id(did);
                         let span = self.tcx().hir().span(hir_id);
-                        p!(write("@{}", self.tcx().sess.source_map().span_to_string(span)));
+                        p!(write(
+                            "@{}",
+                            // This may end up in stderr diagnostics but it may also be emitted
+                            // into MIR. Hence we use the remapped path if available
+                            self.tcx().sess.source_map().span_to_embeddable_string(span)
+                        ));
                     } else {
                         p!(write("@"), print_def_path(did, substs));
                     }
@@ -702,7 +707,12 @@ pub trait PrettyPrinter<'tcx>:
                             p!("@", print_def_path(did.to_def_id(), substs));
                         } else {
                             let span = self.tcx().hir().span(hir_id);
-                            p!(write("@{}", self.tcx().sess.source_map().span_to_string(span)));
+                            p!(write(
+                                "@{}",
+                                // This may end up in stderr diagnostics but it may also be emitted
+                                // into MIR. Hence we use the remapped path if available
+                                self.tcx().sess.source_map().span_to_embeddable_string(span)
+                            ));
                         }
                     } else {
                         p!(write("@"), print_def_path(did, substs));
@@ -1407,7 +1417,13 @@ impl<F: fmt::Write> Printer<'tcx> for FmtPrinter<'_, 'tcx, F> {
                 if !self.empty_path {
                     write!(self, "::")?;
                 }
-                write!(self, "<impl at {}>", self.tcx.sess.source_map().span_to_string(span))?;
+                write!(
+                    self,
+                    "<impl at {}>",
+                    // This may end up in stderr diagnostics but it may also be emitted
+                    // into MIR. Hence we use the remapped path if available
+                    self.tcx.sess.source_map().span_to_embeddable_string(span)
+                )?;
                 self.empty_path = false;
 
                 return Ok(self);
diff --git a/compiler/rustc_mir/src/borrow_check/region_infer/dump_mir.rs b/compiler/rustc_mir/src/borrow_check/region_infer/dump_mir.rs
index 86d9db294bf..5892ef37eba 100644
--- a/compiler/rustc_mir/src/borrow_check/region_infer/dump_mir.rs
+++ b/compiler/rustc_mir/src/borrow_check/region_infer/dump_mir.rs
@@ -76,7 +76,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         for constraint in &constraints {
             let OutlivesConstraint { sup, sub, locations, category } = constraint;
             let (name, arg) = match locations {
-                Locations::All(span) => ("All", tcx.sess.source_map().span_to_string(*span)),
+                Locations::All(span) => {
+                    ("All", tcx.sess.source_map().span_to_embeddable_string(*span))
+                }
                 Locations::Single(loc) => ("Single", format!("{:?}", loc)),
             };
             with_msg(&format!("{:?}: {:?} due to {:?} at {}({})", sup, sub, category, name, arg))?;
diff --git a/compiler/rustc_mir/src/interpret/eval_context.rs b/compiler/rustc_mir/src/interpret/eval_context.rs
index 2d83d6cfbdc..e9dd7a3fe68 100644
--- a/compiler/rustc_mir/src/interpret/eval_context.rs
+++ b/compiler/rustc_mir/src/interpret/eval_context.rs
@@ -263,7 +263,13 @@ impl<'tcx> fmt::Display for FrameInfo<'tcx> {
             }
             if !self.span.is_dummy() {
                 let lo = tcx.sess.source_map().lookup_char_pos(self.span.lo());
-                write!(f, " at {}:{}:{}", lo.file.name, lo.line, lo.col.to_usize() + 1)?;
+                write!(
+                    f,
+                    " at {}:{}:{}",
+                    lo.file.name.prefer_local(),
+                    lo.line,
+                    lo.col.to_usize() + 1
+                )?;
             }
             Ok(())
         })
diff --git a/compiler/rustc_mir/src/interpret/intrinsics/caller_location.rs b/compiler/rustc_mir/src/interpret/intrinsics/caller_location.rs
index 4dfdc08b875..2b996cf62a3 100644
--- a/compiler/rustc_mir/src/interpret/intrinsics/caller_location.rs
+++ b/compiler/rustc_mir/src/interpret/intrinsics/caller_location.rs
@@ -106,7 +106,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
         let caller = self.tcx.sess.source_map().lookup_char_pos(topmost.lo());
         (
-            Symbol::intern(&caller.file.name.to_string()),
+            Symbol::intern(&caller.file.name.prefer_remapped().to_string_lossy()),
             u32::try_from(caller.line).unwrap(),
             u32::try_from(caller.col_display).unwrap().checked_add(1).unwrap(),
         )
diff --git a/compiler/rustc_mir/src/transform/coverage/mod.rs b/compiler/rustc_mir/src/transform/coverage/mod.rs
index aca11de4516..918d6ee4b0c 100644
--- a/compiler/rustc_mir/src/transform/coverage/mod.rs
+++ b/compiler/rustc_mir/src/transform/coverage/mod.rs
@@ -169,8 +169,8 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
         debug!(
             "instrumenting {:?}, fn sig span: {}, body span: {}",
             def_id,
-            source_map.span_to_string(fn_sig_span),
-            source_map.span_to_string(body_span)
+            source_map.span_to_diagnostic_string(fn_sig_span),
+            source_map.span_to_diagnostic_string(body_span)
         );
 
         let mut graphviz_data = debug::GraphvizData::new();
@@ -311,7 +311,7 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
         let tcx = self.tcx;
         let source_map = tcx.sess.source_map();
         let body_span = self.body_span;
-        let file_name = Symbol::intern(&self.source_file.name.to_string());
+        let file_name = Symbol::intern(&self.source_file.name.prefer_remapped().to_string_lossy());
 
         let mut bcb_counters = IndexVec::from_elem_n(None, self.basic_coverage_blocks.num_nodes());
         for covspan in coverage_spans {
@@ -332,8 +332,8 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
                 "Calling make_code_region(file_name={}, source_file={:?}, span={}, body_span={})",
                 file_name,
                 self.source_file,
-                source_map.span_to_string(span),
-                source_map.span_to_string(body_span)
+                source_map.span_to_diagnostic_string(span),
+                source_map.span_to_diagnostic_string(body_span)
             );
 
             inject_statement(
diff --git a/compiler/rustc_mir/src/util/pretty.rs b/compiler/rustc_mir/src/util/pretty.rs
index 3b88aec16b2..955e85c944a 100644
--- a/compiler/rustc_mir/src/util/pretty.rs
+++ b/compiler/rustc_mir/src/util/pretty.rs
@@ -445,7 +445,10 @@ impl Visitor<'tcx> for ExtraComments<'tcx> {
             ty::Tuple(tys) if tys.is_empty() => {}
             _ => {
                 self.push("mir::Constant");
-                self.push(&format!("+ span: {}", self.tcx.sess.source_map().span_to_string(*span)));
+                self.push(&format!(
+                    "+ span: {}",
+                    self.tcx.sess.source_map().span_to_embeddable_string(*span)
+                ));
                 if let Some(user_ty) = user_ty {
                     self.push(&format!("+ user_ty: {:?}", user_ty));
                 }
@@ -516,7 +519,7 @@ impl Visitor<'tcx> for ExtraComments<'tcx> {
 }
 
 fn comment(tcx: TyCtxt<'_>, SourceInfo { span, scope }: SourceInfo) -> String {
-    format!("scope {} at {}", scope.index(), tcx.sess.source_map().span_to_string(span))
+    format!("scope {} at {}", scope.index(), tcx.sess.source_map().span_to_embeddable_string(span))
 }
 
 /// Prints local variables in a scope tree.
@@ -617,7 +620,7 @@ fn write_scope_tree(
                 "{0:1$} // at {2}",
                 indented_header,
                 ALIGN,
-                tcx.sess.source_map().span_to_string(span),
+                tcx.sess.source_map().span_to_embeddable_string(span),
             )?;
         } else {
             writeln!(w, "{}", indented_header)?;
@@ -1004,7 +1007,7 @@ fn write_user_type_annotations(
             "| {:?}: {:?} at {}",
             index.index(),
             annotation.user_ty,
-            tcx.sess.source_map().span_to_string(annotation.span)
+            tcx.sess.source_map().span_to_embeddable_string(annotation.span)
         )?;
     }
     if !body.user_type_annotations.is_empty() {
diff --git a/compiler/rustc_mir/src/util/spanview.rs b/compiler/rustc_mir/src/util/spanview.rs
index 9abfa4a8dc6..2103f2f0934 100644
--- a/compiler/rustc_mir/src/util/spanview.rs
+++ b/compiler/rustc_mir/src/util/spanview.rs
@@ -628,7 +628,7 @@ fn tooltip<'tcx>(
 ) -> String {
     let source_map = tcx.sess.source_map();
     let mut text = Vec::new();
-    text.push(format!("{}: {}:", spanview_id, &source_map.span_to_string(span)));
+    text.push(format!("{}: {}:", spanview_id, &source_map.span_to_embeddable_string(span)));
     for statement in statements {
         let source_range = source_range_no_file(tcx, &statement.source_info.span);
         text.push(format!(
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index 905077a48e2..077b19fa959 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -188,8 +188,10 @@ pub fn maybe_file_to_stream(
     override_span: Option<Span>,
 ) -> Result<(TokenStream, Vec<lexer::UnmatchedBrace>), Vec<Diagnostic>> {
     let src = source_file.src.as_ref().unwrap_or_else(|| {
-        sess.span_diagnostic
-            .bug(&format!("cannot lex `source_file` without source: {}", source_file.name));
+        sess.span_diagnostic.bug(&format!(
+            "cannot lex `source_file` without source: {}",
+            source_file.name.prefer_local()
+        ));
     });
 
     let (token_trees, unmatched_braces) =
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index fa930471c21..4ceefa17bcf 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -132,9 +132,9 @@ enum LiveNodeKind {
 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
     let sm = tcx.sess.source_map();
     match lnk {
-        UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_string(s)),
-        ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)),
-        VarDefNode(s) => format!("Var def node [{}]", sm.span_to_string(s)),
+        UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_diagnostic_string(s)),
+        ExprNode(s) => format!("Expr node [{}]", sm.span_to_diagnostic_string(s)),
+        VarDefNode(s) => format!("Var def node [{}]", sm.span_to_diagnostic_string(s)),
         ClosureNode => "Closure node".to_owned(),
         ExitNode => "Exit node".to_owned(),
     }
diff --git a/compiler/rustc_passes/src/region.rs b/compiler/rustc_passes/src/region.rs
index 14a373c5942..c133f1a0417 100644
--- a/compiler/rustc_passes/src/region.rs
+++ b/compiler/rustc_passes/src/region.rs
@@ -717,7 +717,7 @@ impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
         debug!(
             "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
             owner_id,
-            self.tcx.sess.source_map().span_to_string(body.value.span),
+            self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
             body_id,
             self.cx.parent
         );
diff --git a/compiler/rustc_resolve/src/late/lifetimes.rs b/compiler/rustc_resolve/src/late/lifetimes.rs
index bf69defb4e6..efa5d7e7b11 100644
--- a/compiler/rustc_resolve/src/late/lifetimes.rs
+++ b/compiler/rustc_resolve/src/late/lifetimes.rs
@@ -3451,7 +3451,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
     fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
         debug!(
             node = ?self.tcx.hir().node_to_string(lifetime_ref.hir_id),
-            span = ?self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
+            span = ?self.tcx.sess.source_map().span_to_diagnostic_string(lifetime_ref.span)
         );
         self.map.defs.insert(lifetime_ref.hir_id, def);
 
diff --git a/compiler/rustc_save_analysis/src/dump_visitor.rs b/compiler/rustc_save_analysis/src/dump_visitor.rs
index 12c77e0c8a6..54b6a121585 100644
--- a/compiler/rustc_save_analysis/src/dump_visitor.rs
+++ b/compiler/rustc_save_analysis/src/dump_visitor.rs
@@ -190,7 +190,7 @@ impl<'tcx> DumpVisitor<'tcx> {
         };
 
         let data = CompilationOptions {
-            directory: self.tcx.sess.working_dir.0.clone(),
+            directory: self.tcx.sess.working_dir.remapped_path_if_available().into(),
             program,
             arguments,
             output: self.save_ctxt.compilation_output(crate_name),
@@ -1112,7 +1112,7 @@ impl<'tcx> DumpVisitor<'tcx> {
                 name: String::new(),
                 qualname,
                 span,
-                value: filename.to_string(),
+                value: filename.prefer_remapped().to_string(),
                 children,
                 parent: None,
                 decl_id: None,
diff --git a/compiler/rustc_save_analysis/src/lib.rs b/compiler/rustc_save_analysis/src/lib.rs
index 7256e68b87a..bcd125ae01d 100644
--- a/compiler/rustc_save_analysis/src/lib.rs
+++ b/compiler/rustc_save_analysis/src/lib.rs
@@ -80,7 +80,7 @@ impl<'tcx> SaveContext<'tcx> {
         let end = sm.lookup_char_pos(span.hi());
 
         SpanData {
-            file_name: start.file.name.to_string().into(),
+            file_name: start.file.name.prefer_remapped().to_string().into(),
             byte_start: span.lo().0,
             byte_end: span.hi().0,
             line_start: Row::new_one_indexed(start.line as u32),
@@ -290,7 +290,7 @@ impl<'tcx> SaveContext<'tcx> {
                     name: item.ident.to_string(),
                     qualname,
                     span: self.span_from_span(item.ident.span),
-                    value: filename.to_string(),
+                    value: filename.prefer_remapped().to_string(),
                     parent: None,
                     children: m
                         .item_ids
diff --git a/compiler/rustc_save_analysis/src/span_utils.rs b/compiler/rustc_save_analysis/src/span_utils.rs
index edcd4925773..1947b04f441 100644
--- a/compiler/rustc_save_analysis/src/span_utils.rs
+++ b/compiler/rustc_save_analysis/src/span_utils.rs
@@ -16,8 +16,7 @@ impl<'a> SpanUtils<'a> {
 
     pub fn make_filename_string(&self, file: &SourceFile) -> String {
         match &file.name {
-            FileName::Real(name) if !file.name_was_remapped => {
-                let path = name.local_path();
+            FileName::Real(RealFileName::LocalPath(path)) => {
                 if path.is_absolute() {
                     self.sess
                         .source_map()
@@ -27,12 +26,15 @@ impl<'a> SpanUtils<'a> {
                         .display()
                         .to_string()
                 } else {
-                    self.sess.working_dir.0.join(&path).display().to_string()
+                    self.sess
+                        .working_dir
+                        .remapped_path_if_available()
+                        .join(&path)
+                        .display()
+                        .to_string()
                 }
             }
-            // If the file name is already remapped, we assume the user
-            // configured it the way they wanted to, so use that directly
-            filename => filename.to_string(),
+            filename => filename.prefer_remapped().to_string(),
         }
     }
 
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index ca8bb3af2ba..d2145b2865b 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1201,6 +1201,9 @@ options! {
         "whether ELF relocations can be relaxed"),
     relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
         "choose which RELRO level to use"),
+    simulate_remapped_rust_src_base: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],
+        "simulate the effect of remap-debuginfo = true at bootstrapping by remapping path \
+        to rust's source base directory. only meant for testing purposes"),
     report_delayed_bugs: bool = (false, parse_bool, [TRACKED],
         "immediately print bugs registered with `delay_span_bug` (default: no)"),
     sanitizer: SanitizerSet = (SanitizerSet::empty(), parse_sanitizers, [TRACKED],
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 6d74723993b..1348b02b878 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -23,8 +23,8 @@ use rustc_errors::registry::Registry;
 use rustc_errors::{Diagnostic, DiagnosticBuilder, DiagnosticId, ErrorReported};
 use rustc_lint_defs::FutureBreakage;
 pub use rustc_span::crate_disambiguator::CrateDisambiguator;
-use rustc_span::edition::Edition;
 use rustc_span::source_map::{FileLoader, MultiSpan, RealFileLoader, SourceMap, Span};
+use rustc_span::{edition::Edition, RealFileName};
 use rustc_span::{sym, SourceFileHashAlgorithm, Symbol};
 use rustc_target::asm::InlineAsmArch;
 use rustc_target::spec::{CodeModel, PanicStrategy, RelocModel, RelroLevel};
@@ -125,9 +125,8 @@ pub struct Session {
     /// The name of the root source file of the crate, in the local file system.
     /// `None` means that there is no source file.
     pub local_crate_source_file: Option<PathBuf>,
-    /// The directory the compiler has been executed in plus a flag indicating
-    /// if the value stored here has been affected by path remapping.
-    pub working_dir: (PathBuf, bool),
+    /// The directory the compiler has been executed in
+    pub working_dir: RealFileName,
 
     /// Set of `(DiagnosticId, Option<Span>, message)` tuples tracking
     /// (sub)diagnostics that have been set once, but should not be set again,
@@ -1356,7 +1355,12 @@ pub fn build_session(
     let working_dir = env::current_dir().unwrap_or_else(|e| {
         parse_sess.span_diagnostic.fatal(&format!("Current directory is invalid: {}", e)).raise()
     });
-    let working_dir = file_path_mapping.map_prefix(working_dir);
+    let (path, remapped) = file_path_mapping.map_prefix(working_dir.clone());
+    let working_dir = if remapped {
+        RealFileName::Remapped { local_path: Some(working_dir), virtual_name: path }
+    } else {
+        RealFileName::LocalPath(path)
+    };
 
     let cgu_reuse_tracker = if sopts.debugging_opts.query_dep_graph {
         CguReuseTracker::new()
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index d56b434ca85..0301b364591 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -114,52 +114,112 @@ pub fn with_default_session_globals<R>(f: impl FnOnce() -> R) -> R {
 // deserialization.
 scoped_tls::scoped_thread_local!(pub static SESSION_GLOBALS: SessionGlobals);
 
-// FIXME: Perhaps this should not implement Rustc{Decodable, Encodable}
-//
 // FIXME: We should use this enum or something like it to get rid of the
 // use of magic `/rust/1.x/...` paths across the board.
-#[derive(Debug, Eq, PartialEq, Clone, Ord, PartialOrd, Hash)]
-#[derive(HashStable_Generic, Decodable, Encodable)]
+#[derive(Debug, Eq, PartialEq, Clone, Ord, PartialOrd)]
+#[derive(HashStable_Generic, Decodable)]
 pub enum RealFileName {
-    Named(PathBuf),
-    /// For de-virtualized paths (namely paths into libstd that have been mapped
-    /// to the appropriate spot on the local host's file system),
-    Devirtualized {
-        /// `local_path` is the (host-dependent) local path to the file.
-        local_path: PathBuf,
+    LocalPath(PathBuf),
+    /// For remapped paths (namely paths into libstd that have been mapped
+    /// to the appropriate spot on the local host's file system, and local file
+    /// system paths that have been remapped with `FilePathMapping`),
+    Remapped {
+        /// `local_path` is the (host-dependent) local path to the file. This is
+        /// None if the file was imported from another crate
+        local_path: Option<PathBuf>,
         /// `virtual_name` is the stable path rustc will store internally within
         /// build artifacts.
         virtual_name: PathBuf,
     },
 }
 
+impl Hash for RealFileName {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        // To prevent #70924 from happening again we should only hash the
+        // remapped (virtualized) path if that exists. This is because
+        // virtualized paths to sysroot crates (/rust/$hash or /rust/$version)
+        // remain stable even if the corresponding local_path changes
+        self.remapped_path_if_available().hash(state)
+    }
+}
+
+// This is functionally identical to #[derive(Encodable)], with the exception of
+// an added assert statement
+impl<S: Encoder> Encodable<S> for RealFileName {
+    fn encode(&self, encoder: &mut S) -> Result<(), S::Error> {
+        encoder.emit_enum("RealFileName", |encoder| match *self {
+            RealFileName::LocalPath(ref local_path) => {
+                encoder.emit_enum_variant("LocalPath", 0, 1, |encoder| {
+                    Ok({
+                        encoder.emit_enum_variant_arg(0, |encoder| local_path.encode(encoder))?;
+                    })
+                })
+            }
+
+            RealFileName::Remapped { ref local_path, ref virtual_name } => encoder
+                .emit_enum_variant("Remapped", 1, 2, |encoder| {
+                    // For privacy and build reproducibility, we must not embed host-dependant path in artifacts
+                    // if they have been remapped by --remap-path-prefix
+                    assert!(local_path.is_none());
+                    Ok({
+                        encoder.emit_enum_variant_arg(0, |encoder| local_path.encode(encoder))?;
+                        encoder.emit_enum_variant_arg(1, |encoder| virtual_name.encode(encoder))?;
+                    })
+                }),
+        })
+    }
+}
+
 impl RealFileName {
-    /// Returns the path suitable for reading from the file system on the local host.
-    /// Avoid embedding this in build artifacts; see `stable_name()` for that.
-    pub fn local_path(&self) -> &Path {
+    /// Returns the path suitable for reading from the file system on the local host,
+    /// if this information exists.
+    /// Avoid embedding this in build artifacts; see `remapped_path_if_available()` for that.
+    pub fn local_path(&self) -> Option<&Path> {
         match self {
-            RealFileName::Named(p)
-            | RealFileName::Devirtualized { local_path: p, virtual_name: _ } => &p,
+            RealFileName::LocalPath(p) => Some(p),
+            RealFileName::Remapped { local_path: p, virtual_name: _ } => {
+                p.as_ref().map(PathBuf::as_path)
+            }
         }
     }
 
-    /// Returns the path suitable for reading from the file system on the local host.
-    /// Avoid embedding this in build artifacts; see `stable_name()` for that.
-    pub fn into_local_path(self) -> PathBuf {
+    /// Returns the path suitable for reading from the file system on the local host,
+    /// if this information exists.
+    /// Avoid embedding this in build artifacts; see `remapped_path_if_available()` for that.
+    pub fn into_local_path(self) -> Option<PathBuf> {
         match self {
-            RealFileName::Named(p)
-            | RealFileName::Devirtualized { local_path: p, virtual_name: _ } => p,
+            RealFileName::LocalPath(p) => Some(p),
+            RealFileName::Remapped { local_path: p, virtual_name: _ } => p,
         }
     }
 
-    /// Returns the path suitable for embedding into build artifacts. Note that
-    /// a virtualized path will not correspond to a valid file system path; see
-    /// `local_path()` for something that is more likely to return paths into the
-    /// local host file system.
-    pub fn stable_name(&self) -> &Path {
+    /// Returns the path suitable for embedding into build artifacts. This would still
+    /// be a local path if it has not been remapped. A remapped path will not correspond
+    /// to a valid file system path: see `local_path_if_available()` for something that
+    /// is more likely to return paths into the local host file system.
+    pub fn remapped_path_if_available(&self) -> &Path {
         match self {
-            RealFileName::Named(p)
-            | RealFileName::Devirtualized { local_path: _, virtual_name: p } => &p,
+            RealFileName::LocalPath(p)
+            | RealFileName::Remapped { local_path: _, virtual_name: p } => &p,
+        }
+    }
+
+    /// Returns the path suitable for reading from the file system on the local host,
+    /// if this information exists. Otherwise returns the remapped name.
+    /// Avoid embedding this in build artifacts; see `remapped_path_if_available()` for that.
+    pub fn local_path_if_available(&self) -> &Path {
+        match self {
+            RealFileName::LocalPath(path)
+            | RealFileName::Remapped { local_path: None, virtual_name: path }
+            | RealFileName::Remapped { local_path: Some(path), virtual_name: _ } => path,
+        }
+    }
+
+    pub fn to_string_lossy(&self, prefer_local: bool) -> Cow<'_, str> {
+        if prefer_local {
+            self.local_path_if_available().to_string_lossy()
+        } else {
+            self.remapped_path_if_available().to_string_lossy()
         }
     }
 }
@@ -188,16 +248,24 @@ pub enum FileName {
     InlineAsm(u64),
 }
 
-impl std::fmt::Display for FileName {
+impl From<PathBuf> for FileName {
+    fn from(p: PathBuf) -> Self {
+        assert!(!p.to_string_lossy().ends_with('>'));
+        FileName::Real(RealFileName::LocalPath(p))
+    }
+}
+
+pub struct FileNameDisplay<'a> {
+    inner: &'a FileName,
+    prefer_local: bool,
+}
+
+impl fmt::Display for FileNameDisplay<'_> {
     fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         use FileName::*;
-        match *self {
-            Real(RealFileName::Named(ref path)) => write!(fmt, "{}", path.display()),
-            // FIXME: might be nice to display both components of Devirtualized.
-            // But for now (to backport fix for issue #70924), best to not
-            // perturb diagnostics so its obvious test suite still works.
-            Real(RealFileName::Devirtualized { ref local_path, virtual_name: _ }) => {
-                write!(fmt, "{}", local_path.display())
+        match *self.inner {
+            Real(ref name) => {
+                write!(fmt, "{}", name.to_string_lossy(self.prefer_local))
             }
             QuoteExpansion(_) => write!(fmt, "<quote expansion>"),
             MacroExpansion(_) => write!(fmt, "<macro expansion>"),
@@ -212,10 +280,12 @@ impl std::fmt::Display for FileName {
     }
 }
 
-impl From<PathBuf> for FileName {
-    fn from(p: PathBuf) -> Self {
-        assert!(!p.to_string_lossy().ends_with('>'));
-        FileName::Real(RealFileName::Named(p))
+impl FileNameDisplay<'_> {
+    pub fn to_string_lossy(&self) -> Cow<'_, str> {
+        match self.inner {
+            FileName::Real(ref inner) => inner.to_string_lossy(self.prefer_local),
+            _ => Cow::from(format!("{}", self)),
+        }
     }
 }
 
@@ -236,6 +306,16 @@ impl FileName {
         }
     }
 
+    pub fn prefer_remapped(&self) -> FileNameDisplay<'_> {
+        FileNameDisplay { inner: self, prefer_local: false }
+    }
+
+    // This may include transient local filesystem information.
+    // Must not be embedded in build outputs.
+    pub fn prefer_local(&self) -> FileNameDisplay<'_> {
+        FileNameDisplay { inner: self, prefer_local: true }
+    }
+
     pub fn macro_expansion_source_code(src: &str) -> FileName {
         let mut hasher = StableHasher::new();
         src.hash(&mut hasher);
@@ -796,7 +876,7 @@ pub fn debug_with_source_map(
     f: &mut fmt::Formatter<'_>,
     source_map: &SourceMap,
 ) -> fmt::Result {
-    write!(f, "{} ({:?})", source_map.span_to_string(span), span.ctxt())
+    write!(f, "{} ({:?})", source_map.span_to_diagnostic_string(span), span.ctxt())
 }
 
 pub fn default_span_debug(span: Span, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -1128,11 +1208,6 @@ pub struct SourceFile {
     /// originate from files has names between angle brackets by convention
     /// (e.g., `<anon>`).
     pub name: FileName,
-    /// `true` if the `name` field above has been modified by `--remap-path-prefix`.
-    pub name_was_remapped: bool,
-    /// The unmapped path of the file that the source came from.
-    /// Set to `None` if the `SourceFile` was imported from an external crate.
-    pub unmapped_path: Option<FileName>,
     /// The complete source code.
     pub src: Option<Lrc<String>>,
     /// The source code's hash.
@@ -1162,7 +1237,6 @@ impl<S: Encoder> Encodable<S> for SourceFile {
     fn encode(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_struct("SourceFile", 8, |s| {
             s.emit_struct_field("name", 0, |s| self.name.encode(s))?;
-            s.emit_struct_field("name_was_remapped", 1, |s| self.name_was_remapped.encode(s))?;
             s.emit_struct_field("src_hash", 2, |s| self.src_hash.encode(s))?;
             s.emit_struct_field("start_pos", 3, |s| self.start_pos.encode(s))?;
             s.emit_struct_field("end_pos", 4, |s| self.end_pos.encode(s))?;
@@ -1237,8 +1311,6 @@ impl<D: Decoder> Decodable<D> for SourceFile {
     fn decode(d: &mut D) -> Result<SourceFile, D::Error> {
         d.read_struct("SourceFile", 8, |d| {
             let name: FileName = d.read_struct_field("name", 0, |d| Decodable::decode(d))?;
-            let name_was_remapped: bool =
-                d.read_struct_field("name_was_remapped", 1, |d| Decodable::decode(d))?;
             let src_hash: SourceFileHash =
                 d.read_struct_field("src_hash", 2, |d| Decodable::decode(d))?;
             let start_pos: BytePos =
@@ -1282,8 +1354,6 @@ impl<D: Decoder> Decodable<D> for SourceFile {
             let cnum: CrateNum = d.read_struct_field("cnum", 10, |d| Decodable::decode(d))?;
             Ok(SourceFile {
                 name,
-                name_was_remapped,
-                unmapped_path: None,
                 start_pos,
                 end_pos,
                 src: None,
@@ -1304,15 +1374,13 @@ impl<D: Decoder> Decodable<D> for SourceFile {
 
 impl fmt::Debug for SourceFile {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(fmt, "SourceFile({})", self.name)
+        write!(fmt, "SourceFile({:?})", self.name)
     }
 }
 
 impl SourceFile {
     pub fn new(
         name: FileName,
-        name_was_remapped: bool,
-        unmapped_path: FileName,
         mut src: String,
         start_pos: BytePos,
         hash_kind: SourceFileHashAlgorithm,
@@ -1334,8 +1402,6 @@ impl SourceFile {
 
         SourceFile {
             name,
-            name_was_remapped,
-            unmapped_path: Some(unmapped_path),
             src: Some(Lrc::new(src)),
             src_hash,
             external_src: Lock::new(ExternalSource::Unneeded),
diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index e365844980b..0dadd419200 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -15,11 +15,11 @@ pub use crate::*;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::StableHasher;
 use rustc_data_structures::sync::{AtomicU32, Lrc, MappedReadGuard, ReadGuard, RwLock};
-use std::cmp;
-use std::convert::TryFrom;
 use std::hash::Hash;
 use std::path::{Path, PathBuf};
 use std::sync::atomic::Ordering;
+use std::{clone::Clone, cmp};
+use std::{convert::TryFrom, unreachable};
 
 use std::fs;
 use std::io;
@@ -127,30 +127,13 @@ pub struct StableSourceFileId(u128);
 // StableSourceFileId, perhaps built atop source_file.name_hash.
 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(),
-        )
+        StableSourceFileId::new_from_name(&source_file.name)
     }
 
-    fn new_from_pieces(
-        name: &FileName,
-        name_was_remapped: bool,
-        unmapped_path: Option<&FileName>,
-    ) -> StableSourceFileId {
+    fn new_from_name(name: &FileName) -> StableSourceFileId {
         let mut hasher = StableHasher::new();
 
-        if let FileName::Real(real_name) = name {
-            // rust-lang/rust#70924: Use the stable (virtualized) name when
-            // available. (We do not want artifacts from transient file system
-            // paths for libstd to leak into our build artifacts.)
-            real_name.stable_name().hash(&mut hasher)
-        } else {
-            name.hash(&mut hasher);
-        }
-        name_was_remapped.hash(&mut hasher);
-        unmapped_path.hash(&mut hasher);
+        name.hash(&mut hasher);
 
         StableSourceFileId(hasher.finish())
     }
@@ -283,35 +266,15 @@ impl SourceMap {
 
     fn try_new_source_file(
         &self,
-        mut filename: FileName,
+        filename: FileName,
         src: String,
     ) -> Result<Lrc<SourceFile>, OffsetOverflowError> {
-        // 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 was_remapped;
-        if let FileName::Real(real_filename) = &mut filename {
-            match real_filename {
-                RealFileName::Named(path_to_be_remapped)
-                | RealFileName::Devirtualized {
-                    local_path: path_to_be_remapped,
-                    virtual_name: _,
-                } => {
-                    let mapped = self.path_mapping.map_prefix(path_to_be_remapped.clone());
-                    was_remapped = mapped.1;
-                    *path_to_be_remapped = mapped.0;
-                }
-            }
-        } else {
-            was_remapped = false;
-        }
+        let (filename, _) = self.path_mapping.map_filename_prefix(&filename);
 
-        let file_id =
-            StableSourceFileId::new_from_pieces(&filename, was_remapped, Some(&unmapped_path));
+        let file_id = StableSourceFileId::new_from_name(&filename);
 
         let lrc_sf = match self.source_file_by_stable_id(file_id) {
             Some(lrc_sf) => lrc_sf,
@@ -320,8 +283,6 @@ impl SourceMap {
 
                 let source_file = Lrc::new(SourceFile::new(
                     filename,
-                    was_remapped,
-                    unmapped_path,
                     src,
                     Pos::from_usize(start_pos),
                     self.hash_kind,
@@ -345,7 +306,6 @@ impl SourceMap {
     pub fn new_imported_source_file(
         &self,
         filename: FileName,
-        name_was_remapped: bool,
         src_hash: SourceFileHash,
         name_hash: u128,
         source_len: usize,
@@ -382,8 +342,6 @@ impl SourceMap {
 
         let source_file = Lrc::new(SourceFile {
             name: filename,
-            name_was_remapped,
-            unmapped_path: None,
             src: None,
             src_hash,
             external_src: Lock::new(ExternalSource::Foreign {
@@ -411,11 +369,6 @@ impl SourceMap {
         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 {
         match file {
@@ -453,7 +406,7 @@ impl SourceMap {
         }
     }
 
-    pub fn span_to_string(&self, sp: Span) -> String {
+    fn span_to_string(&self, sp: Span, prefer_local: bool) -> String {
         if self.files.borrow().source_files.is_empty() && sp.is_dummy() {
             return "no-location".to_string();
         }
@@ -462,7 +415,7 @@ impl SourceMap {
         let hi = self.lookup_char_pos(sp.hi());
         format!(
             "{}:{}:{}: {}:{}",
-            lo.file.name,
+            if prefer_local { lo.file.name.prefer_local() } else { lo.file.name.prefer_remapped() },
             lo.line,
             lo.col.to_usize() + 1,
             hi.line,
@@ -470,16 +423,20 @@ impl SourceMap {
         )
     }
 
-    pub fn span_to_filename(&self, sp: Span) -> FileName {
-        self.lookup_char_pos(sp.lo()).file.name.clone()
+    /// Format the span location suitable for embedding in build artifacts
+    pub fn span_to_embeddable_string(&self, sp: Span) -> String {
+        self.span_to_string(sp, false)
+    }
+
+    /// Format the span location to be printed in diagnostics. Must not be emitted
+    /// to build artifacts as this may leak local file paths. Use span_to_embeddable_string
+    /// for string suitable for embedding.
+    pub fn span_to_diagnostic_string(&self, sp: Span) -> String {
+        self.span_to_string(sp, true)
     }
 
-    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 span_to_filename(&self, sp: Span) -> FileName {
+        self.lookup_char_pos(sp.lo()).file.name.clone()
     }
 
     pub fn is_multiline(&self, sp: Span) -> bool {
@@ -1001,7 +958,13 @@ impl SourceMap {
     }
     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.local_path()).ok(),
+            FileName::Real(ref name) => {
+                if let Some(local_path) = name.local_path() {
+                    self.file_loader.read_file(local_path).ok()
+                } else {
+                    None
+                }
+            }
             _ => None,
         })
     }
@@ -1046,9 +1009,20 @@ impl FilePathMapping {
     fn map_filename_prefix(&self, file: &FileName) -> (FileName, bool) {
         match file {
             FileName::Real(realfile) => {
-                let path = realfile.local_path();
-                let (path, mapped) = self.map_prefix(path.to_path_buf());
-                (FileName::Real(RealFileName::Named(path)), mapped)
+                if let RealFileName::LocalPath(local_path) = realfile {
+                    let (mapped_path, mapped) = self.map_prefix(local_path.to_path_buf());
+                    let realfile = if mapped {
+                        RealFileName::Remapped {
+                            local_path: Some(local_path.clone()),
+                            virtual_name: mapped_path,
+                        }
+                    } else {
+                        realfile.clone()
+                    };
+                    (FileName::Real(realfile), mapped)
+                } else {
+                    unreachable!("attempted to remap an already remapped filename");
+                }
             }
             other => (other.clone(), false),
         }
diff --git a/compiler/rustc_span/src/source_map/tests.rs b/compiler/rustc_span/src/source_map/tests.rs
index 7d814f1d82c..f13979941ab 100644
--- a/compiler/rustc_span/src/source_map/tests.rs
+++ b/compiler/rustc_span/src/source_map/tests.rs
@@ -193,7 +193,7 @@ fn t8() {
 fn t9() {
     let sm = init_source_map();
     let span = Span::with_root_ctxt(BytePos(12), BytePos(23));
-    let sstr = sm.span_to_string(span);
+    let sstr = sm.span_to_diagnostic_string(span);
 
     assert_eq!(sstr, "blork.rs:2:1: 2:12");
 }
@@ -229,7 +229,6 @@ fn t10() {
 
     let SourceFile {
         name,
-        name_was_remapped,
         src_hash,
         start_pos,
         end_pos,
@@ -243,7 +242,6 @@ fn t10() {
 
     let imported_src_file = sm.new_imported_source_file(
         name,
-        name_was_remapped,
         src_hash,
         name_hash,
         (end_pos - start_pos).to_usize(),