add unstable -Zroot-path 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_path: Option<PathBuf> = ("Invoke rustc from the given root rather than the workspace root; affects diagnostics"),
     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 +1289,11 @@ 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-path" => self.root_path = v.map(|v| {
+                // Make the path absolute, if we can.
+                let v = PathBuf::from(v);
+                v.canonicalize().unwrap_or(v)
+            }),
             "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

@@ -109,15 +109,22 @@ 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 = ws
+            .gctx()
+            .cli_unstable()
+            .root_path
+            .as_deref()
+            .unwrap_or(ws.root());
+        if let Ok(path) = src.strip_prefix(root) {
+            return (path.to_path_buf(), root.to_path_buf());
         }
     }
     (src, unit.pkg.root().to_path_buf())

tests/testsuite/directory.rs

@@ -784,3 +784,41 @@ Caused by:
         .with_status(101)
         .run();
 }
+
+#[cargo_test]
+fn custom_root_path() {
+    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-path=.")
+        .arg("--manifest-path=ws_root/Cargo.toml")
+        .masquerade_as_nightly_cargo(&["-Zroot-path"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.1.0 ([ROOT]/ws_root)
+[ERROR] expected one of `!` or `::`, found `;`
+ --> ws_root/src/lib.rs:1:8
+  |
+1 | invalid;
+  |        ^ expected one of `!` or `::`
+
+[ERROR] could not compile `foo` (lib) due to 1 previous error
+
+"#]])
+        .run();
+}