test: Add UI tests showing parts of diagnostic system

tests/testsuite/lints/error/mod.rs

@@ -0,0 +1,36 @@
+use cargo_test_support::prelude::*;
+use cargo_test_support::str;
+use cargo_test_support::{file, project};
+
+#[cargo_test]
+fn case() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+cargo-features = ["test-dummy-unstable"]
+
+[package]
+name = "foo"
+version = "0.0.1"
+edition = "2015"
+authors = []
+im-a-teapot = true
+
+[lints.cargo]
+im_a_teapot = "deny"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    snapbox::cmd::Command::cargo_ui()
+        .masquerade_as_nightly_cargo(&["cargo-lints", "test-dummy-unstable"])
+        .current_dir(p.root())
+        .arg("check")
+        .arg("-Zcargo-lints")
+        .assert()
+        .code(101)
+        .stdout_matches(str![""])
+        .stderr_matches(file!["stderr.term.svg"]);
+}

tests/testsuite/lints/inherited/mod.rs

@@ -0,0 +1,43 @@
+use cargo_test_support::prelude::*;
+use cargo_test_support::str;
+use cargo_test_support::{file, project};
+
+#[cargo_test]
+fn case() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+[workspace]
+members = ["foo"]
+
+[workspace.lints.cargo]
+im_a_teapot = { level = "warn", priority = 10 }
+"#,
+        )
+        .file(
+            "foo/Cargo.toml",
+            r#"
+[package]
+name = "foo"
+version = "0.0.1"
+edition = "2015"
+authors = []
+
+[lints]
+workspace = true
+"#,
+        )
+        .file("foo/src/lib.rs", "")
+        .build();
+
+    snapbox::cmd::Command::cargo_ui()
+        .masquerade_as_nightly_cargo(&["cargo-lints"])
+        .current_dir(p.root())
+        .arg("check")
+        .arg("-Zcargo-lints")
+        .assert()
+        .code(101)
+        .stdout_matches(str![""])
+        .stderr_matches(file!["stderr.term.svg"]);
+}

tests/testsuite/lints/mod.rs

@@ -1,9 +1,12 @@
 use cargo_test_support::project;
 use cargo_test_support::registry::Package;
 
+mod error;
 mod implicit_features;
+mod inherited;
 mod unknown_lints;
 mod unused_optional_dependencies;
+mod warning;
 
 #[cargo_test]
 fn dashes_dont_get_rewritten() {

tests/testsuite/lints/warning/mod.rs

@@ -0,0 +1,36 @@
+use cargo_test_support::prelude::*;
+use cargo_test_support::str;
+use cargo_test_support::{file, project};
+
+#[cargo_test]
+fn case() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+cargo-features = ["test-dummy-unstable"]
+
+[package]
+name = "foo"
+version = "0.0.1"
+edition = "2015"
+authors = []
+im-a-teapot = true
+
+[lints.cargo]
+im_a_teapot = "warn"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    snapbox::cmd::Command::cargo_ui()
+        .masquerade_as_nightly_cargo(&["cargo-lints", "test-dummy-unstable"])
+        .current_dir(p.root())
+        .arg("check")
+        .arg("-Zcargo-lints")
+        .assert()
+        .success()
+        .stdout_matches(str![""])
+        .stderr_matches(file!["stderr.term.svg"]);
+}