Translate the virtual /rustc/$hash prefix back to a real directory.

src/bootstrap/builder.rs

@@ -1022,8 +1022,13 @@ pub fn cargo(
             cargo.env("RUSTC_HOST_CRT_STATIC", x.to_string());
         }
 
-        if let Some(map) = self.build.debuginfo_map(GitRepo::Rustc) {
+        if let Some(map_to) = self.build.debuginfo_map_to(GitRepo::Rustc) {
+            let map = format!("{}={}", self.build.src.display(), map_to);
             cargo.env("RUSTC_DEBUGINFO_MAP", map);
+
+            // `rustc` needs to know the virtual `/rustc/$hash` we're mapping to,
+            // in order to opportunistically reverse it later.
+            cargo.env("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR", map_to);
         }
 
         // Enable usage of unstable features

src/bootstrap/compile.rs

@@ -22,7 +22,7 @@
 use crate::builder::Cargo;
 use crate::dist;
 use crate::native;
-use crate::util::{exe, is_dylib};
+use crate::util::{exe, is_dylib, symlink_dir};
 use crate::{Compiler, GitRepo, Mode};
 
 use crate::builder::{Builder, Kind, RunConfig, ShouldRun, Step};
@@ -633,6 +633,30 @@ fn run(self, builder: &Builder<'_>) -> Interned<PathBuf> {
         };
         let _ = fs::remove_dir_all(&sysroot);
         t!(fs::create_dir_all(&sysroot));
+
+        // Symlink the source root into the same location inside the sysroot,
+        // where `rust-src` component would go (`$sysroot/lib/rustlib/src/rust`),
+        // so that any tools relying on `rust-src` also work for local builds,
+        // and also for translating the virtual `/rustc/$hash` back to the real
+        // directory (for running tests with `rust.remap-debuginfo = true`).
+        let sysroot_lib_rustlib_src = sysroot.join("lib/rustlib/src");
+        t!(fs::create_dir_all(&sysroot_lib_rustlib_src));
+        let sysroot_lib_rustlib_src_rust = sysroot_lib_rustlib_src.join("rust");
+        if let Err(e) = symlink_dir(&builder.config, &builder.src, &sysroot_lib_rustlib_src_rust) {
+            eprintln!(
+                "warning: creating symbolic link `{}` to `{}` failed with {}",
+                sysroot_lib_rustlib_src_rust.display(),
+                builder.src.display(),
+                e,
+            );
+            if builder.config.rust_remap_debuginfo {
+                eprintln!(
+                    "warning: some `src/test/ui` tests will fail when lacking `{}`",
+                    sysroot_lib_rustlib_src_rust.display(),
+                );
+            }
+        }
+
         INTERNER.intern_path(sysroot)
     }
 }

src/bootstrap/lib.rs

@@ -740,19 +740,18 @@ fn jobs(&self) -> u32 {
         self.config.jobs.unwrap_or_else(|| num_cpus::get() as u32)
     }
 
-    fn debuginfo_map(&self, which: GitRepo) -> Option<String> {
+    fn debuginfo_map_to(&self, which: GitRepo) -> Option<String> {
         if !self.config.rust_remap_debuginfo {
             return None;
         }
 
-        let path = match which {
+        match which {
             GitRepo::Rustc => {
                 let sha = self.rust_sha().unwrap_or(channel::CFG_RELEASE_NUM);
-                format!("/rustc/{}", sha)
+                Some(format!("/rustc/{}", sha))
             }
-            GitRepo::Llvm => String::from("/rustc/llvm"),
-        };
-        Some(format!("{}={}", self.src.display(), path))
+            GitRepo::Llvm => Some(String::from("/rustc/llvm")),
+        }
     }
 
     /// Returns the path to the C compiler for the target specified.
@@ -787,7 +786,8 @@ fn cflags(&self, target: Interned<String>, which: GitRepo) -> Vec<String> {
             base.push("-fno-omit-frame-pointer".into());
         }
 
-        if let Some(map) = self.debuginfo_map(which) {
+        if let Some(map_to) = self.debuginfo_map_to(which) {
+            let map = format!("{}={}", self.src.display(), map_to);
             let cc = self.cc(target);
             if cc.ends_with("clang") || cc.ends_with("gcc") {
                 base.push(format!("-fdebug-prefix-map={}", map));

src/librustc_metadata/build.rs

@@ -1,4 +1,5 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-env-changed=CFG_VERSION");
+    println!("cargo:rerun-if-env-changed=CFG_VIRTUAL_RUST_SOURCE_BASE_DIR");
 }

src/librustc_metadata/rmeta/decoder.rs

@@ -32,7 +32,7 @@
 use rustc_middle::util::common::record_time;
 use rustc_serialize::{opaque, Decodable, Decoder, SpecializedDecoder};
 use rustc_session::Session;
-use rustc_span::source_map::{self, respan, Spanned};
+use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{self, hygiene::MacroKind, BytePos, Pos, Span, DUMMY_SP};
 
@@ -41,6 +41,7 @@
 use std::io;
 use std::mem;
 use std::num::NonZeroUsize;
+use std::path::Path;
 use std::u32;
 
 pub use cstore_impl::{provide, provide_extern};
@@ -427,7 +428,7 @@ fn specialized_decode(&mut self) -> Result<Span, Self::Error> {
         // we can call `imported_source_files` for the proper crate, and binary search
         // through the returned slice using our span.
         let imported_source_files = if tag == TAG_VALID_SPAN_LOCAL {
-            self.cdata().imported_source_files(sess.source_map())
+            self.cdata().imported_source_files(sess)
         } else {
             // FIXME: We don't decode dependencies of proc-macros.
             // Remove this once #69976 is merged
@@ -457,7 +458,7 @@ fn specialized_decode(&mut self) -> Result<Span, Self::Error> {
             self.last_source_file_index = 0;
 
             let foreign_data = self.cdata().cstore.get_crate_data(cnum);
-            foreign_data.imported_source_files(sess.source_map())
+            foreign_data.imported_source_files(sess)
         };
 
         let source_file = {
@@ -1460,10 +1461,45 @@ fn def_path(&self, id: DefIndex) -> DefPath {
     ///
     /// Proc macro crates don't currently export spans, so this function does not have
     /// to work for them.
-    fn imported_source_files(
-        &self,
-        local_source_map: &source_map::SourceMap,
-    ) -> &'a [ImportedSourceFile] {
+    fn imported_source_files(&self, sess: &Session) -> &'a [ImportedSourceFile] {
+        // Translate the virtual `/rustc/$hash` prefix back to a real directory
+        // that should hold actual sources, where possible.
+        let virtual_rust_source_base_dir = option_env!("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR")
+            .map(Path::new)
+            .filter(|_| {
+                // Only spend time on further checks if we have what to translate *to*.
+                sess.real_rust_source_base_dir.is_some()
+            })
+            .filter(|virtual_dir| {
+                // Don't translate away `/rustc/$hash` if we're still remapping to it,
+                // since that means we're still building `std`/`rustc` that need it,
+                // and we don't want the real path to leak into codegen/debuginfo.
+                !sess.opts.remap_path_prefix.iter().any(|(_from, to)| to == virtual_dir)
+            });
+        let try_to_translate_virtual_to_real = |name: &mut rustc_span::FileName| {
+            debug!(
+                "try_to_translate_virtual_to_real(name={:?}): \
+                 virtual_rust_source_base_dir={:?}, real_rust_source_base_dir={:?}",
+                name, virtual_rust_source_base_dir, sess.real_rust_source_base_dir,
+            );
+
+            if let Some(virtual_dir) = virtual_rust_source_base_dir {
+                if let Some(real_dir) = &sess.real_rust_source_base_dir {
+                    if let rustc_span::FileName::Real(path) = name {
+                        if let Ok(rest) = path.strip_prefix(virtual_dir) {
+                            let new_path = real_dir.join(rest);
+                            debug!(
+                                "try_to_translate_virtual_to_real: `{}` -> `{}`",
+                                path.display(),
+                                new_path.display(),
+                            );
+                            *path = new_path;
+                        }
+                    }
+                }
+            }
+        };
+
         self.cdata.source_map_import_info.init_locking(|| {
             let external_source_map = self.root.source_map.decode(self);
 
@@ -1472,7 +1508,7 @@ fn imported_source_files(
                     // We can't reuse an existing SourceFile, so allocate a new one
                     // containing the information we need.
                     let rustc_span::SourceFile {
-                        name,
+                        mut name,
                         name_was_remapped,
                         src_hash,
                         start_pos,
@@ -1485,6 +1521,13 @@ fn imported_source_files(
                         ..
                     } = source_file_to_import;
 
+                    // 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();
 
                     // Translate line-start positions and multibyte character
@@ -1505,7 +1548,7 @@ fn imported_source_files(
                         np.pos = np.pos - start_pos;
                     }
 
-                    let local_version = local_source_map.new_imported_source_file(
+                    let local_version = sess.source_map().new_imported_source_file(
                         name,
                         name_was_remapped,
                         src_hash,

src/librustc_session/session.rs

@@ -140,6 +140,15 @@ pub struct Session {
     /// Options range from returning the error without a backtrace to returning an error
     /// and immediately printing the backtrace to stderr.
     pub ctfe_backtrace: Lock<CtfeBacktrace>,
+
+    /// Base directory containing the `src/` for the Rust standard library, and
+    /// potentially `rustc` as well, if we can can find it. Right now it's always
+    /// `$sysroot/lib/rustlib/src/rust` (i.e. the `rustup` `rust-src` component).
+    ///
+    /// This directory is what the virtual `/rustc/$hash` is translated back to,
+    /// if Rust was built with path remapping to `/rustc/$hash` enabled
+    /// (the `rust.remap-debuginfo` option in `config.toml`).
+    pub real_rust_source_base_dir: Option<PathBuf>,
 }
 
 pub struct PerfStats {
@@ -1056,6 +1065,26 @@ fn build_session_(
         _ => CtfeBacktrace::Disabled,
     });
 
+    // Try to find a directory containing the Rust `src`, for more details see
+    // the doc comment on the `real_rust_source_base_dir` field.
+    let real_rust_source_base_dir = {
+        // This is the location used by the `rust-src` `rustup` component.
+        let mut candidate = sysroot.join("lib/rustlib/src/rust");
+        if let Ok(metadata) = candidate.symlink_metadata() {
+            // Replace the symlink rustbuild creates, with its destination.
+            // We could try to use `fs::canonicalize` instead, but that might
+            // produce unnecessarily verbose path.
+            if metadata.file_type().is_symlink() {
+                if let Ok(symlink_dest) = std::fs::read_link(&candidate) {
+                    candidate = symlink_dest;
+                }
+            }
+        }
+
+        // Only use this directory if it has a file we can expect to always find.
+        if candidate.join("src/libstd/lib.rs").is_file() { Some(candidate) } else { None }
+    };
+
     let sess = Session {
         target: target_cfg,
         host,
@@ -1094,6 +1123,7 @@ fn build_session_(
         confused_type_with_std_module: Lock::new(Default::default()),
         system_library_path: OneThread::new(RefCell::new(Default::default())),
         ctfe_backtrace,
+        real_rust_source_base_dir,
     };
 
     validate_commandline_args_with_session_available(&sess);