Only use non-absolute paths for path dependencies

Previously Cargo would use a non-absolute path for any dependency contained
within the workspace root but this switches Cargo to only using relative paths
for `path` dependencies. In practice this shouldn't make much difference, but
for vendored crates and moving around `CARGO_HOME` it can produce more
consistent results when target directories are shared.

Closes #5923

src/cargo/core/compiler/mod.rs

@@ -680,8 +680,10 @@ fn path_args(bcx: &BuildContext, unit: &Unit) -> (PathBuf, PathBuf) {
         unit.target.src_path().path().to_path_buf()
     };
     assert!(src.is_absolute());
-    if let Ok(path) = src.strip_prefix(ws_root) {
-        return (path.to_path_buf(), ws_root.to_path_buf());
+    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());
+        }
     }
     (src, unit.pkg.root().to_path_buf())
 }

tests/testsuite/directory.rs

@@ -16,12 +16,12 @@ fn setup() {
     t!(fs::create_dir(&root.join(".cargo")));
     t!(t!(File::create(root.join(".cargo/config"))).write_all(
         br#"
-        [source.crates-io]
-        replace-with = 'my-awesome-local-registry'
+            [source.crates-io]
+            replace-with = 'my-awesome-local-registry'
 
-        [source.my-awesome-local-registry]
-        directory = 'index'
-    "#
+            [source.my-awesome-local-registry]
+            directory = 'index'
+        "#
     ));
 }
 
@@ -643,3 +643,61 @@ restore the source replacement configuration to continue the build
         ),
     );
 }
+
+#[test]
+fn workspace_different_locations() {
+    let p = project()
+        .no_manifest()
+        .file(
+            "foo/Cargo.toml",
+            r#"
+                [package]
+                name = 'foo'
+                version = '0.1.0'
+
+                [dependencies]
+                baz = "*"
+            "#,
+        )
+        .file("foo/src/lib.rs", "")
+        .file("foo/vendor/baz/Cargo.toml", &basic_manifest("baz", "0.1.0"))
+        .file("foo/vendor/baz/src/lib.rs", "")
+        .file("foo/vendor/baz/.cargo-checksum.json", "{\"files\":{}}")
+        .file(
+            "bar/Cargo.toml",
+            r#"
+                [package]
+                name = 'bar'
+                version = '0.1.0'
+
+                [dependencies]
+                baz = "*"
+            "#,
+        )
+        .file("bar/src/lib.rs", "")
+        .file(
+            ".cargo/config",
+            r#"
+                [build]
+                target-dir = './target'
+
+                [source.crates-io]
+                replace-with = 'my-awesome-local-registry'
+
+                [source.my-awesome-local-registry]
+                directory = 'foo/vendor'
+            "#,
+        )
+        .build();
+
+    assert_that(p.cargo("build").cwd(p.root().join("foo")), execs());
+    assert_that(
+        p.cargo("build").cwd(p.root().join("bar")),
+        execs().with_status(0).with_stderr(
+            "\
+[COMPILING] bar [..]
+[FINISHED] [..]
+",
+        ),
+    );
+}