Add warning when cargo tree -i <spec> can not find packages

src/cargo/ops/tree/mod.rs

@@ -213,7 +213,15 @@ pub fn build_and_print(ws: &Workspace<'_>, opts: &TreeOptions) -> CargoResult<()
         })
         .collect::<CargoResult<Vec<PackageIdSpec>>>()?;
 
-    print(ws.config(), opts, root_indexes, &pkgs_to_prune, &graph)?;
+    if root_indexes.len() == 0 {
+        ws.config().shell().warn(
+            "nothing to print.\n\n\
+        To find dependencies that require specific target platforms, \
+        try use option `--target all` first, and then narrow your search scope accordingly.",
+        )?;
+    } else {
+        print(ws.config(), opts, root_indexes, &pkgs_to_prune, &graph)?;
+    }
     Ok(())
 }
 

tests/testsuite/tree.rs

@@ -489,6 +489,62 @@ foo v0.1.0 ([..]/foo)
         .run();
 }
 
+#[cargo_test]
+fn no_selected_target_dependency() {
+    // --target flag
+    if cross_compile::disabled() {
+        return;
+    }
+    Package::new("targetdep", "1.0.0").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [target.'{alt}'.dependencies]
+                targetdep = "1.0"
+
+                "#,
+                alt = alternate(),
+            ),
+        )
+        .file("src/lib.rs", "")
+        .file("build.rs", "fn main() {}")
+        .build();
+
+    p.cargo("tree")
+        .with_stdout(
+            "\
+foo v0.1.0 ([..]/foo)
+",
+        )
+        .run();
+
+    p.cargo("tree -i targetdep")
+        .with_stderr(
+            "\
+[WARNING] nothing to print.
+
+To find dependencies that require specific target platforms, \
+try use option `--target all` first, and then narrow your search scope accordingly.
+",
+        )
+        .run();
+    p.cargo("tree -i targetdep --target all")
+        .with_stdout(
+            "\
+targetdep v1.0.0
+└── foo v0.1.0 ([..]/foo)
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn dep_kinds() {
     Package::new("inner-devdep", "1.0.0").publish();