diff --git a/tests/testsuite/build_script.rs b/tests/testsuite/build_script.rs
index ef7630923..65a004ee6 100644
--- a/tests/testsuite/build_script.rs
+++ b/tests/testsuite/build_script.rs
@@ -2177,61 +2177,6 @@ fn shared_dep_with_a_build_script() {
     p.cargo("build -v").run();
 }
 
-#[cargo_test]
-fn transitive_dep_host() {
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.5.0"
-                edition = "2015"
-                authors = []
-                build = "build.rs"
-
-                [build-dependencies.b]
-                path = "b"
-            "#,
-        )
-        .file("src/lib.rs", "")
-        .file("build.rs", "fn main() {}")
-        .file(
-            "a/Cargo.toml",
-            r#"
-                [package]
-                name = "a"
-                version = "0.5.0"
-                edition = "2015"
-                authors = []
-                links = "foo"
-                build = "build.rs"
-            "#,
-        )
-        .file("a/build.rs", "fn main() {}")
-        .file("a/src/lib.rs", "")
-        .file(
-            "b/Cargo.toml",
-            r#"
-                [package]
-                name = "b"
-                version = "0.5.0"
-                edition = "2015"
-                authors = []
-
-                [lib]
-                name = "b"
-                plugin = true
-
-                [dependencies.a]
-                path = "../a"
-            "#,
-        )
-        .file("b/src/lib.rs", "")
-        .build();
-    p.cargo("build").run();
-}
-
 #[cargo_test]
 fn test_a_lib_with_a_build_command() {
     let p = project()
diff --git a/tests/testsuite/cross_compile.rs b/tests/testsuite/cross_compile.rs
index bcb0a4190..85cc7174c 100644
--- a/tests/testsuite/cross_compile.rs
+++ b/tests/testsuite/cross_compile.rs
@@ -883,47 +883,6 @@ fn build_script_only_host() {
     p.cargo("build -v --target").arg(&target).run();
 }
 
-#[cargo_test]
-fn plugin_build_script_right_arch() {
-    if cross_compile::disabled() {
-        return;
-    }
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-                edition = "2015"
-                authors = []
-                build = "build.rs"
-
-                [lib]
-                name = "foo"
-                plugin = true
-            "#,
-        )
-        .file("build.rs", "fn main() {}")
-        .file("src/lib.rs", "")
-        .build();
-
-    p.cargo("build -v --target")
-        .arg(cross_compile::alternate())
-        .with_stderr(
-            "\
-[WARNING] support for rustc plugins has been removed from rustc. library `foo` should not specify `plugin = true`
-[WARNING] support for `plugin = true` will be removed from cargo in the future
-[COMPILING] foo v0.0.1 ([..])
-[RUNNING] `rustc [..] build.rs [..]`
-[RUNNING] `[..]/build-script-build`
-[RUNNING] `rustc [..] src/lib.rs [..]`
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [..]
-",
-        )
-        .run();
-}
-
 #[cargo_test]
 fn build_script_with_platform_specific_dependencies() {
     if cross_compile::disabled() {
diff --git a/tests/testsuite/proc_macro.rs b/tests/testsuite/proc_macro.rs
index 86bfe086c..0d793f10c 100644
--- a/tests/testsuite/proc_macro.rs
+++ b/tests/testsuite/proc_macro.rs
@@ -345,6 +345,28 @@ fn proc_macro_crate_type_warning() {
         .run();
 }
 
+#[cargo_test]
+fn lib_plugin_unused_key_warning() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2015"
+                [lib]
+                plugin = true
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("check")
+        .with_stderr_contains("[WARNING] unused manifest key: lib.plugin")
+        .run();
+}
+
 #[cargo_test]
 fn proc_macro_crate_type_warning_plugin() {
     let foo = project()
@@ -357,23 +379,15 @@ fn proc_macro_crate_type_warning_plugin() {
                 edition = "2015"
                 [lib]
                 crate-type = ["proc-macro"]
-                plugin = true
             "#,
         )
         .file("src/lib.rs", "")
         .build();
 
     foo.cargo("check")
-        .with_stderr_contains(
-            "[WARNING] support for rustc plugins has been removed from rustc. \
-            library `foo` should not specify `plugin = true`")
-        .with_stderr_contains(
-            "[WARNING] support for `plugin = true` will be removed from cargo in the future")
-        .with_stderr_contains(
-            "[WARNING] proc-macro library `foo` should not specify `plugin = true`")
-        .with_stderr_contains(
-            "[WARNING] library `foo` should only specify `proc-macro = true` instead of setting `crate-type`")
-        .run();
+    .with_stderr_contains(
+        "[WARNING] library `foo` should only specify `proc-macro = true` instead of setting `crate-type`")
+    .run();
 }
 
 #[cargo_test]
diff --git a/tests/testsuite/rustflags.rs b/tests/testsuite/rustflags.rs
index 1f7ca1bd7..564f70c33 100644
--- a/tests/testsuite/rustflags.rs
+++ b/tests/testsuite/rustflags.rs
@@ -1,9 +1,7 @@
 //! Tests for setting custom rustc flags.
 
 use cargo_test_support::registry::Package;
-use cargo_test_support::{
-    basic_lib_manifest, basic_manifest, paths, project, project_in_home, rustc_host,
-};
+use cargo_test_support::{basic_manifest, paths, project, project_in_home, rustc_host};
 use std::fs;
 
 #[cargo_test]
@@ -114,76 +112,6 @@ fn env_rustflags_build_script_dep() {
     foo.cargo("check").env("RUSTFLAGS", "--cfg foo").run();
 }
 
-#[cargo_test]
-fn env_rustflags_plugin() {
-    // RUSTFLAGS should be passed to rustc for plugins
-    // when --target is not specified.
-    // In this test if --cfg foo is not passed the build will fail.
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-            "#,
-        )
-        .file(
-            "src/lib.rs",
-            r#"
-                fn main() { }
-                #[cfg(not(foo))]
-                fn main() { }
-            "#,
-        )
-        .build();
-
-    p.cargo("check").env("RUSTFLAGS", "--cfg foo").run();
-}
-
-#[cargo_test]
-fn env_rustflags_plugin_dep() {
-    // RUSTFLAGS should be passed to rustc for plugins
-    // when --target is not specified.
-    // In this test if --cfg foo is not passed the build will fail.
-    let foo = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-
-                [dependencies.bar]
-                path = "../bar"
-            "#,
-        )
-        .file("src/lib.rs", "fn foo() {}")
-        .build();
-    let _bar = project()
-        .at("bar")
-        .file("Cargo.toml", &basic_lib_manifest("bar"))
-        .file(
-            "src/lib.rs",
-            r#"
-                fn bar() { }
-                #[cfg(not(foo))]
-                fn bar() { }
-            "#,
-        )
-        .build();
-
-    foo.cargo("check").env("RUSTFLAGS", "--cfg foo").run();
-}
-
 #[cargo_test]
 fn env_rustflags_normal_source_with_target() {
     let p = project()
@@ -345,84 +273,6 @@ fn env_rustflags_build_script_dep_with_target() {
         .run();
 }
 
-#[cargo_test]
-fn env_rustflags_plugin_with_target() {
-    // RUSTFLAGS should not be passed to rustc for plugins
-    // when --target is specified.
-    // In this test if --cfg foo is passed the build will fail.
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-            "#,
-        )
-        .file(
-            "src/lib.rs",
-            r#"
-                fn main() { }
-                #[cfg(foo)]
-                fn main() { }
-            "#,
-        )
-        .build();
-
-    let host = rustc_host();
-    p.cargo("check --target")
-        .arg(host)
-        .env("RUSTFLAGS", "--cfg foo")
-        .run();
-}
-
-#[cargo_test]
-fn env_rustflags_plugin_dep_with_target() {
-    // RUSTFLAGS should not be passed to rustc for plugins
-    // when --target is specified.
-    // In this test if --cfg foo is passed the build will fail.
-    let foo = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-
-                [dependencies.bar]
-                path = "../bar"
-            "#,
-        )
-        .file("src/lib.rs", "fn foo() {}")
-        .build();
-    let _bar = project()
-        .at("bar")
-        .file("Cargo.toml", &basic_lib_manifest("bar"))
-        .file(
-            "src/lib.rs",
-            r#"
-                fn bar() { }
-                #[cfg(foo)]
-                fn bar() { }
-            "#,
-        )
-        .build();
-
-    let host = rustc_host();
-    foo.cargo("check --target")
-        .arg(host)
-        .env("RUSTFLAGS", "--cfg foo")
-        .run();
-}
-
 #[cargo_test]
 fn env_rustflags_recompile() {
     let p = project().file("src/lib.rs", "").build();
@@ -583,90 +433,6 @@ fn build_rustflags_build_script_dep() {
     foo.cargo("check").run();
 }
 
-#[cargo_test]
-fn build_rustflags_plugin() {
-    // RUSTFLAGS should be passed to rustc for plugins
-    // when --target is not specified.
-    // In this test if --cfg foo is not passed the build will fail.
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-            "#,
-        )
-        .file(
-            "src/lib.rs",
-            r#"
-                fn main() { }
-                #[cfg(not(foo))]
-                fn main() { }
-            "#,
-        )
-        .file(
-            ".cargo/config.toml",
-            r#"
-            [build]
-            rustflags = ["--cfg", "foo"]
-            "#,
-        )
-        .build();
-
-    p.cargo("check").run();
-}
-
-#[cargo_test]
-fn build_rustflags_plugin_dep() {
-    // RUSTFLAGS should be passed to rustc for plugins
-    // when --target is not specified.
-    // In this test if --cfg foo is not passed the build will fail.
-    let foo = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-
-                [dependencies.bar]
-                path = "../bar"
-            "#,
-        )
-        .file("src/lib.rs", "fn foo() {}")
-        .file(
-            ".cargo/config.toml",
-            r#"
-            [build]
-            rustflags = ["--cfg", "foo"]
-            "#,
-        )
-        .build();
-    let _bar = project()
-        .at("bar")
-        .file("Cargo.toml", &basic_lib_manifest("bar"))
-        .file(
-            "src/lib.rs",
-            r#"
-                fn bar() { }
-                #[cfg(not(foo))]
-                fn bar() { }
-            "#,
-        )
-        .build();
-
-    foo.cargo("check").run();
-}
-
 #[cargo_test]
 fn build_rustflags_normal_source_with_target() {
     let p = project()
@@ -800,92 +566,6 @@ fn build_rustflags_build_script_dep_with_target() {
     foo.cargo("check --target").arg(host).run();
 }
 
-#[cargo_test]
-fn build_rustflags_plugin_with_target() {
-    // RUSTFLAGS should not be passed to rustc for plugins
-    // when --target is specified.
-    // In this test if --cfg foo is passed the build will fail.
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-            "#,
-        )
-        .file(
-            "src/lib.rs",
-            r#"
-                fn main() { }
-                #[cfg(foo)]
-                fn main() { }
-            "#,
-        )
-        .file(
-            ".cargo/config.toml",
-            r#"
-            [build]
-            rustflags = ["--cfg", "foo"]
-            "#,
-        )
-        .build();
-
-    let host = rustc_host();
-    p.cargo("check --target").arg(host).run();
-}
-
-#[cargo_test]
-fn build_rustflags_plugin_dep_with_target() {
-    // RUSTFLAGS should not be passed to rustc for plugins
-    // when --target is specified.
-    // In this test if --cfg foo is passed the build will fail.
-    let foo = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-
-                [lib]
-                name = "foo"
-                plugin = true
-
-                [dependencies.bar]
-                path = "../bar"
-            "#,
-        )
-        .file("src/lib.rs", "fn foo() {}")
-        .file(
-            ".cargo/config.toml",
-            r#"
-            [build]
-            rustflags = ["--cfg", "foo"]
-            "#,
-        )
-        .build();
-    let _bar = project()
-        .at("bar")
-        .file("Cargo.toml", &basic_lib_manifest("bar"))
-        .file(
-            "src/lib.rs",
-            r#"
-                fn bar() { }
-                #[cfg(foo)]
-                fn bar() { }
-            "#,
-        )
-        .build();
-
-    let host = rustc_host();
-    foo.cargo("check --target").arg(host).run();
-}
-
 #[cargo_test]
 fn build_rustflags_recompile() {
     let p = project().file("src/lib.rs", "").build();