Revert part to #5393

Commit 0b530c3 (which this commit mostly reverts) did some refactoring around the `target_dir` function. However, it introduced a bug because it meant that where `CARGO_TARGET_DIR` was specified but `--target-dir` was not, the value from `CARGO_TARGET_DIR` was ignored.

src/cargo/core/workspace.rs

@@ -131,7 +131,7 @@ impl<'cfg> Workspace<'cfg> {
     /// root and all member packages. It will then validate the workspace
     /// before returning it, so `Ok` is only returned for valid workspaces.
     pub fn new(manifest_path: &Path, config: &'cfg Config) -> CargoResult<Workspace<'cfg>> {
-        let target_dir = config.target_dir();
+        let target_dir = config.target_dir()?;
 
         let mut ws = Workspace {
             config,
@@ -191,7 +191,7 @@ impl<'cfg> Workspace<'cfg> {
             ws.target_dir = if let Some(dir) = target_dir {
                 Some(dir)
             } else {
-                ws.config.target_dir()
+                ws.config.target_dir()?
             };
             ws.members.push(ws.current_manifest.clone());
             ws.default_members.push(ws.current_manifest.clone());

src/cargo/ops/cargo_install.rs

@@ -213,7 +213,7 @@ fn install_one(
     let mut needs_cleanup = false;
     let overidden_target_dir = if source_id.is_path() {
         None
-    } else if let Some(dir) = config.target_dir() {
+    } else if let Some(dir) = config.target_dir()? {
         Some(dir)
     } else if let Ok(td) = TempFileBuilder::new().prefix("cargo-install").tempdir() {
         let p = td.path().to_owned();

src/cargo/util/config.rs

@@ -242,8 +242,17 @@ impl Config {
         &self.cwd
     }
 
-    pub fn target_dir(&self) -> Option<Filesystem> {
-        self.target_dir.clone()
+    pub fn target_dir(&self) -> CargoResult<Option<Filesystem>> {
+        if let Some(ref dir) = self.target_dir {
+            Ok(Some(dir.clone()))
+        } else if let Some(dir) = env::var_os("CARGO_TARGET_DIR") {
+            Ok(Some(Filesystem::new(self.cwd.join(dir))))
+        } else if let Some(val) = self.get_path("build.target-dir")? {
+            let val = self.cwd.join(val.val);
+            Ok(Some(Filesystem::new(val)))
+        } else {
+            Ok(None)
+        }
     }
 
     fn get(&self, key: &str) -> CargoResult<Option<ConfigValue>> {
@@ -491,23 +500,17 @@ impl Config {
             | (None, None, None) => Verbosity::Normal,
         };
 
-        let target_dir = if let Some(dir) = target_dir.as_ref() {
-            Some(Filesystem::new(self.cwd.join(dir)))
-        } else if let Some(dir) = env::var_os("CARGO_TARGET_DIR") {
-            Some(Filesystem::new(self.cwd.join(dir)))
-        } else if let Ok(Some(val)) = self.get_path("build.target-dir") {
-            let val = self.cwd.join(val.val);
-            Some(Filesystem::new(val))
-        } else {
-            None
+        let cli_target_dir = match target_dir.as_ref() {
+            Some(dir) => Some(Filesystem::new(dir.clone())),
+            None => None,
         };
 
         self.shell().set_verbosity(verbosity);
         self.shell().set_color_choice(color.map(|s| &s[..]))?;
         self.extra_verbose = extra_verbose;
         self.frozen = frozen;
         self.locked = locked;
-        self.target_dir = target_dir;
+        self.target_dir = cli_target_dir;
         self.cli_flags.parse(unstable_flags)?;
 
         Ok(())