bootstrap: remap compiler vs non-compiler sources differently

- Map compiler sources (corresponding to `rustc-dev` dist component)
  with `/rustc-dev/{hash}`.
- Map non-compiler sources (corresponding to `rust-src` dist component
  or other non-compiler sources) with `/rustc/{hash}`.

This allows the compiler to have the possibility of opportunistically
reverse the mapping. This is because

- `rust-src` unpacks sources to a path like
  `$sysroot/lib/rustlib/src/rust`, whereas
- `rustc-dev` unpacks sources to a path like
  `$sysroot/lib/rustlib/rustc-src/rust`

(Notice the `src` vs `rustc-src` difference.)

Author: jieyouxu

src/bootstrap/src/core/builder/cargo.rs

@@ -920,13 +920,28 @@ impl Builder<'_> {
             hostflags.arg(format!("-Ctarget-feature={sign}crt-static"));
         }
 
-        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 remapping scheme, in order to know how to reverse it (unremap)
+        // later. Two env vars are set and made available to the compiler
+        //
+        // - `CFG_VIRTUAL_RUST_SOURCE_BASE_DIR`: `rust-src` remap scheme (`NonCompiler`)
+        // - `CFG_VIRTUAL_RUSTC_DEV_SOURCE_BASE_DIR`: `rustc-dev` remap scheme (`Compiler`)
+        //
+        // Keep this scheme in sync with `rustc_metadata::rmeta::decoder`'s
+        // `try_to_translate_virtual_to_real`.
+        //
+        // `RUSTC_DEBUGINFO_MAP` is used to pass through to the underlying rustc
+        // `--remap-path-prefix`.
+        if let Some(map_to) = self.build.debuginfo_map_to(GitRepo::Rustc, Some(mode)) {
+            cargo.env("RUSTC_DEBUGINFO_MAP", format!("{}={}", self.build.src.display(), map_to));
 
-            // `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);
+            match mode {
+                Mode::Rustc | Mode::Codegen => {
+                    cargo.env("CFG_VIRTUAL_RUSTC_DEV_SOURCE_BASE_DIR", map_to);
+                }
+                Mode::Std | Mode::ToolBootstrap | Mode::ToolRustc | Mode::ToolStd => {
+                    cargo.env("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR", map_to);
+                }
+            }
         }
 
         if self.config.rust_remap_debuginfo {

src/bootstrap/src/lib.rs

@@ -272,6 +272,16 @@ impl Mode {
     }
 }
 
+/// When `rust.rust_remap_debuginfo` is requested, the compiler needs to know how to
+/// opportunistically unremap compiler vs non-compiler sources. We use two schemes,
+/// [`RemapScheme::Compiler`] and [`RemapScheme::NonCompiler`].
+pub enum RemapScheme {
+    /// The [`RemapScheme::Compiler`] scheme will remap to `/rustc-dev/{hash}`.
+    Compiler,
+    /// The [`RemapScheme::NonCompiler`] scheme will remap to `/rustc/{hash}`.
+    NonCompiler,
+}
+
 #[derive(Debug, Hash, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub enum CLang {
     C,
@@ -1210,15 +1220,38 @@ Executed at: {executed_at}"#,
         })
     }
 
-    fn debuginfo_map_to(&self, which: GitRepo) -> Option<String> {
+    fn debuginfo_map_to(&self, which: GitRepo, mode: Option<Mode>) -> Option<String> {
         if !self.config.rust_remap_debuginfo {
             return None;
         }
 
         match which {
             GitRepo::Rustc => {
                 let sha = self.rust_sha().unwrap_or(&self.version);
-                Some(format!("/rustc/{sha}"))
+
+                let remap_scheme = match mode {
+                    Some(Mode::Rustc | Mode::Codegen) => RemapScheme::Compiler,
+                    Some(Mode::Std | Mode::ToolBootstrap | Mode::ToolRustc | Mode::ToolStd)
+                    | None => RemapScheme::NonCompiler,
+                };
+
+                match remap_scheme {
+                    RemapScheme::Compiler => {
+                        // For compiler sources, remap via `/rustc-dev/{sha}` to allow
+                        // distinguishing between compiler sources vs library sources, since
+                        // `rustc-dev` dist component places them under
+                        // `$sysroot/lib/rustlib/rustc-src/rust` as opposed to `rust-src`'s
+                        // `$sysroot/lib/rustlib/src/rust`.
+                        //
+                        // Keep this scheme in sync with `rustc_metadata::rmeta::decoder`'s
+                        // `try_to_translate_virtual_to_real`.
+                        Some(format!("/rustc-dev/{sha}"))
+                    }
+                    RemapScheme::NonCompiler => {
+                        // For non-compiler sources, use `/rustc/{sha}` remapping scheme.
+                        Some(format!("/rustc/{sha}"))
+                    }
+                }
             }
             GitRepo::Llvm => Some(String::from("/rustc/llvm")),
         }
@@ -1285,7 +1318,7 @@ Executed at: {executed_at}"#,
             base.push("-fno-omit-frame-pointer".into());
         }
 
-        if let Some(map_to) = self.debuginfo_map_to(which) {
+        if let Some(map_to) = self.debuginfo_map_to(which, None) {
             let map = format!("{}={}", self.src.display(), map_to);
             let cc = self.cc(target);
             if cc.ends_with("clang") || cc.ends_with("gcc") {