add unstable -Zroot-dir flag to configure the path from which rustc should be invoked

src/cargo/core/features.rs

@@ -121,6 +121,7 @@
 use std::collections::BTreeSet;
 use std::env;
 use std::fmt::{self, Write};
+use std::path::PathBuf;
 use std::str::FromStr;
 
 use anyhow::{bail, Error};
@@ -783,6 +784,7 @@ unstable_cli_options!(
     profile_rustflags: bool = ("Enable the `rustflags` option in profiles in .cargo/config.toml file"),
     public_dependency: bool = ("Respect a dependency's `public` field in Cargo.toml to control public/private dependencies"),
     publish_timeout: bool = ("Enable the `publish.timeout` key in .cargo/config.toml file"),
+    root_dir: Option<PathBuf> = ("Set the root directory relative to which paths are printed (defaults to workspace root)"),
     rustdoc_map: bool = ("Allow passing external documentation mappings to rustdoc"),
     rustdoc_scrape_examples: bool = ("Allows Rustdoc to scrape code examples from reverse-dependencies"),
     script: bool = ("Enable support for single-file, `.rs` packages"),
@@ -1287,6 +1289,7 @@ impl CliUnstable {
             "profile-rustflags" => self.profile_rustflags = parse_empty(k, v)?,
             "trim-paths" => self.trim_paths = parse_empty(k, v)?,
             "publish-timeout" => self.publish_timeout = parse_empty(k, v)?,
+            "root-dir" => self.root_dir = v.map(|v| v.into()),
             "rustdoc-map" => self.rustdoc_map = parse_empty(k, v)?,
             "rustdoc-scrape-examples" => self.rustdoc_scrape_examples = parse_empty(k, v)?,
             "separate-nightlies" => self.separate_nightlies = parse_empty(k, v)?,

src/cargo/util/workspace.rs

@@ -4,6 +4,7 @@ use crate::core::{Target, Workspace};
 use crate::ops::CompileOptions;
 use crate::util::CargoResult;
 use anyhow::bail;
+use cargo_util::paths::normalize_path;
 use cargo_util::ProcessBuilder;
 use std::fmt::Write;
 use std::path::PathBuf;
@@ -109,15 +110,20 @@ pub fn print_available_tests(ws: &Workspace<'_>, options: &CompileOptions) -> Ca
 /// The first returned value here is the argument to pass to rustc, and the
 /// second is the cwd that rustc should operate in.
 pub fn path_args(ws: &Workspace<'_>, unit: &Unit) -> (PathBuf, PathBuf) {
-    let ws_root = ws.root();
     let src = match unit.target.src_path() {
         TargetSourcePath::Path(path) => path.to_path_buf(),
         TargetSourcePath::Metabuild => unit.pkg.manifest().metabuild_path(ws.target_dir()),
     };
     assert!(src.is_absolute());
     if unit.pkg.package_id().source_id().is_path() {
-        if let Ok(path) = src.strip_prefix(ws_root) {
-            return (path.to_path_buf(), ws_root.to_path_buf());
+        // Determine which path we make this relative to: usually it's the workspace root,
+        // but this can be overwritten with a `-Z` flag.
+        let root = match &ws.gctx().cli_unstable().root_dir {
+            None => ws.root().to_owned(),
+            Some(root_dir) => normalize_path(&ws.gctx().cwd().join(root_dir)),
+        };
+        if let Ok(path) = src.strip_prefix(&root) {
+            return (path.to_path_buf(), root);
         }
     }
     (src, unit.pkg.root().to_path_buf())

tests/testsuite/directory.rs

@@ -802,13 +802,17 @@ fn root_dir_diagnostics() {
         .file("ws_root/src/lib.rs", "invalid;")
         .build();
 
+    // Crucially, the rustc error message below says `ws_root/...`, i.e.
+    // it is relative to our fake home, not to the workspace root.
     p.cargo("check")
+        .arg("-Zroot-dir=.")
         .arg("--manifest-path=ws_root/Cargo.toml")
+        .masquerade_as_nightly_cargo(&["-Zroot-dir"])
         .with_status(101)
         .with_stderr_data(str![[r#"
 [CHECKING] foo v0.1.0 ([ROOT]/ws_root)
 [ERROR] [..]
- --> src/lib.rs:1:8
+ --> ws_root/src/lib.rs:1:8
   |
 1 | invalid;
   | [..]
@@ -839,10 +843,23 @@ fn root_dir_file_macro() {
         )
         .build();
 
+    // Crucially, the path is relative to our fake home, not to the workspace root.
     p.cargo("run")
+        .arg("-Zroot-dir=.")
         .arg("--manifest-path=ws_root/Cargo.toml")
+        .masquerade_as_nightly_cargo(&["-Zroot-dir"])
         .with_stdout_data(str![[r#"
-src/main.rs
+ws_root/src/main.rs
+
+"#]])
+        .run();
+    // Try again with an absolute path for `root-dir`.
+    p.cargo("run")
+        .arg(format!("-Zroot-dir={}", p.root().display()))
+        .arg("--manifest-path=ws_root/Cargo.toml")
+        .masquerade_as_nightly_cargo(&["-Zroot-dir"])
+        .with_stdout_data(str![[r#"
+ws_root/src/main.rs
 
 "#]])
         .run();