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())

src/doc/src/reference/unstable.md

@@ -78,6 +78,7 @@ Each new feature described below should explain how to use it.
 * Output behavior
     * [artifact-dir](#artifact-dir) --- Adds a directory where artifacts are copied to.
     * [Different binary name](#different-binary-name) --- Assign a name to the built binary that is separate from the crate name.
+    * [root-dir](#root-dir) --- Controls the root directory relative to which paths are printed
 * Compile behavior
     * [mtime-on-use](#mtime-on-use) --- Updates the last-modified timestamp on every dependency every time it is used, to provide a mechanism to delete unused artifacts.
     * [doctest-xcompile](#doctest-xcompile) --- Supports running doctests with the `--target` flag.
@@ -236,6 +237,13 @@ This can also be specified in `.cargo/config.toml` files.
 artifact-dir = "out"
 ```
 
+## root-dir
+* Original Issue: [#9887](https://github.com/rust-lang/cargo/issues/9887)
+* Tracking Issue: None (not currently slated for stabilization)
+
+The `-Zroot-dir` flag sets the root directory relative to which paths are printed.
+This affects both diagnostics and paths emitted by the `file!()` macro.
+
 ## doctest-xcompile
 * Tracking Issue: [#7040](https://github.com/rust-lang/cargo/issues/7040)
 * Tracking Rustc Issue: [#64245](https://github.com/rust-lang/rust/issues/64245)

tests/testsuite/directory.rs

@@ -784,3 +784,83 @@ Caused by:
         .with_status(101)
         .run();
 }
+
+#[cargo_test]
+fn root_dir_diagnostics() {
+    let p = ProjectBuilder::new(paths::root())
+        .no_manifest() // we are placing it in a different dir
+        .file(
+            "ws_root/Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2015"
+                authors = []
+            "#,
+        )
+        .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] [..]
+ --> ws_root/src/lib.rs:1:8
+  |
+1 | invalid;
+  | [..]
+
+[ERROR] could not compile `foo` (lib) due to 1 previous error
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn root_dir_file_macro() {
+    let p = ProjectBuilder::new(paths::root())
+        .no_manifest() // we are placing it in a different dir
+        .file(
+            "ws_root/Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2015"
+                authors = []
+            "#,
+        )
+        .file(
+            "ws_root/src/main.rs",
+            r#"fn main() { println!("{}", file!()); }"#,
+        )
+        .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#"
+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();
+}