fix(depinfo): prevent invalid trailing backslash on Windows

Joining an empty path adds a trailing path separator.
On Windows, this causes cargo to emit an invalid depfile,
where a dependency ends in a backslash, which is invalid.

This patch fixes the two known cases where this issue can happen
_without_ the user passing in any path with a trailing backslash.

Author: seritools

src/cargo/core/compiler/fingerprint/dep_info.rs

@@ -503,11 +503,19 @@ fn make_absolute_path(
     build_root: &Path,
     path: PathBuf,
 ) -> PathBuf {
-    match ty {
-        DepInfoPathType::PackageRootRelative => pkg_root.join(path),
-        // N.B. path might be absolute here in which case the join will have no effect
-        DepInfoPathType::BuildRootRelative => build_root.join(path),
+    let relative_to = match ty {
+        DepInfoPathType::PackageRootRelative => pkg_root,
+        // N.B. path might be absolute here in which case the join below will have no effect
+        DepInfoPathType::BuildRootRelative => build_root,
+    };
+
+    if path.as_os_str().is_empty() {
+        // Joining with an empty path causes Rust to add a trailing path separator. On Windows, this
+        // would add an invalid trailing backslash to the .d file.
+        return relative_to.to_path_buf();
     }
+
+    relative_to.join(path)
 }
 
 /// Some algorithms are here to ensure compatibility with possible rustc outputs.

src/cargo/core/compiler/output_depinfo.rs

@@ -84,9 +84,18 @@ fn add_deps_for_unit(
                 .get(metadata)
             {
                 for path in &output.rerun_if_changed {
-                    // The paths we have saved from the unit are of arbitrary relativeness and may be
-                    // relative to the crate root of the dependency.
-                    let path = unit.pkg.root().join(path);
+                    let package_root = unit.pkg.root();
+
+                    let path = if path.as_os_str().is_empty() {
+                        // Joining with an empty path causes Rust to add a trailing path separator.
+                        // On Windows, this would add an invalid trailing backslash to the .d file.
+                        package_root.to_path_buf()
+                    } else {
+                        // The paths we have saved from the unit are of arbitrary relativeness and
+                        // may be relative to the crate root of the dependency.
+                        package_root.join(path)
+                    };
+
                     deps.insert(path);
                 }
             }

tests/testsuite/dep_info.rs

@@ -554,7 +554,7 @@ fn non_local_build_script() {
 }
 
 #[cargo_test]
-fn trailing_separator_after_package_root_build_script() {
+fn no_trailing_separator_after_package_root_build_script() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -582,14 +582,14 @@ fn trailing_separator_after_package_root_build_script() {
     assert_e2e().eq(
         &contents,
         str![[r#"
-[ROOT]/foo/target/debug/foo[EXE]: [ROOT]/foo/ [ROOT]/foo/build.rs [ROOT]/foo/src/main.rs
+[ROOT]/foo/target/debug/foo[EXE]: [ROOT]/foo [ROOT]/foo/build.rs [ROOT]/foo/src/main.rs
 
 "#]],
     );
 }
 
 #[cargo_test(nightly, reason = "proc_macro::tracked_path is unstable")]
-fn trailing_separator_after_package_root_proc_macro() {
+fn no_trailing_separator_after_package_root_proc_macro() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -647,7 +647,7 @@ fn trailing_separator_after_package_root_proc_macro() {
     assert_e2e().eq(
         &contents,
         str![[r#"
-[ROOT]/foo/target/debug/foo[EXE]: [ROOT]/foo/ [ROOT]/foo/pm/src/lib.rs [ROOT]/foo/src/main.rs
+[ROOT]/foo/target/debug/foo[EXE]: [ROOT]/foo [ROOT]/foo/pm/src/lib.rs [ROOT]/foo/src/main.rs
 
 "#]],
     );