Auto merge of #115872 - ferrocene:pa-remap-cargo-home, r=clubby789

Remap Cargo dependencies to /rust/deps

⚠️ **This doesn't affect user-compiled programs, it only affects building the Rust compiler itself.** ⚠️

Right now, `rust.remap-debuginfo = true` doesn't completely remap all paths: while LLVM and rustc sources are properly remapped (respectively to `/rust/llvm` and `/rust/$commit`), Cargo dependencies still use absolute paths from the Cargo home.

This never affected builds from CI much, because `CARGO_HOME=/cargo` in CI, so users see paths like this included in the precompiled binaries and libraries:

```
/cargo/registry/src/index.crates.io-6f17d22bba15001f/gimli-0.26.2/src/read/line.rs
```

Builds outside CI don't have remapping though, and it's confusing that the config flag doesn't fully do what it advertises.

This PR fixes it by adding remapping for dependencies too. *All registries's* source directory are remapped to `/rust/deps`, to account for multiple registries being able to contain crates.io crates (sparse index vs git, and source replacement mirrors). This results in paths like this being included:

```
/rust/deps/gimli-0.26.2/src/read/line.rs
```

src/bootstrap/Cargo.lock

@@ -50,6 +50,7 @@ dependencies = [
  "fd-lock",
  "filetime",
  "hex",
+ "home",
  "ignore",
  "junction",
  "libc",
@@ -350,6 +351,15 @@ version = "0.4.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
 
+[[package]]
+name = "home"
+version = "0.5.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "747309b4b440c06d57b0b25f2aee03ee9b5e5397d288c60e21fc709bb98a7408"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "ignore"
 version = "0.4.18"

src/bootstrap/Cargo.toml

@@ -40,6 +40,7 @@ clap_complete = "4.4.3"
 cmake = "0.1.38"
 filetime = "0.2"
 hex = "0.4"
+home = "0.5.4"
 ignore = "0.4.10"
 libc = "0.2"
 object = { version = "0.32.0", default-features = false, features = ["archive", "coff", "read_core", "unaligned"] }

src/bootstrap/src/bin/rustc.rs

@@ -124,6 +124,13 @@ fn main() {
     if let Ok(map) = env::var("RUSTC_DEBUGINFO_MAP") {
         cmd.arg("--remap-path-prefix").arg(&map);
     }
+    // The remap flags for Cargo registry sources need to be passed after the remapping for the
+    // Rust source code directory, to handle cases when $CARGO_HOME is inside the source directory.
+    if let Ok(maps) = env::var("RUSTC_CARGO_REGISTRY_SRC_TO_REMAP") {
+        for map in maps.split('\t') {
+            cmd.arg("--remap-path-prefix").arg(map);
+        }
+    }
 
     // Force all crates compiled by this compiler to (a) be unstable and (b)
     // allow the `rustc_private` feature to link to other unstable crates

src/bootstrap/src/core/builder.rs

@@ -2,7 +2,7 @@
 use std::cell::{Cell, RefCell};
 use std::collections::BTreeSet;
 use std::env;
-use std::ffi::OsStr;
+use std::ffi::{OsStr, OsString};
 use std::fmt::{Debug, Write};
 use std::fs::{self, File};
 use std::hash::Hash;
@@ -1767,6 +1767,20 @@ pub fn cargo(
             cargo.env("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR", map_to);
         }
 
+        if self.config.rust_remap_debuginfo {
+            // FIXME: handle vendored sources
+            let registry_src = t!(home::cargo_home()).join("registry").join("src");
+            let mut env_var = OsString::new();
+            for entry in t!(std::fs::read_dir(registry_src)) {
+                if !env_var.is_empty() {
+                    env_var.push("\t");
+                }
+                env_var.push(t!(entry).path());
+                env_var.push("=/rust/deps");
+            }
+            cargo.env("RUSTC_CARGO_REGISTRY_SRC_TO_REMAP", env_var);
+        }
+
         // Enable usage of unstable features
         cargo.env("RUSTC_BOOTSTRAP", "1");
         self.add_rust_test_threads(&mut cargo);