Improve UI test coverage

With this commit I believe I've covered all `compile_error!`-based
errors.

test_suite/tests/ui/expected-string/boolean.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = true)]
+    boolean: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/boolean.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/boolean.rs:6:22
+  |
+6 |     #[serde(rename = true)]
+  |                      ^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/expected-string/byte_character.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = b'a')]
+    byte_character: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/byte_character.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/byte_character.rs:6:22
+  |
+6 |     #[serde(rename = b'a')]
+  |                      ^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/expected-string/byte_string.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = b"byte string")]
+    byte_string: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/byte_string.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/byte_string.rs:6:22
+  |
+6 |     #[serde(rename = b"byte string")]
+  |                      ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/expected-string/character.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = 'a')]
+    character: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/character.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/character.rs:6:22
+  |
+6 |     #[serde(rename = 'a')]
+  |                      ^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/expected-string/float.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = 3.14)]
+    float: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/float.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/float.rs:6:22
+  |
+6 |     #[serde(rename = 3.14)]
+  |                      ^^^^
+
+error: aborting due to previous error
+

test_suite/tests/ui/expected-string/integer.rs

@@ -0,0 +1,10 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+struct S {
+    #[serde(rename = 100)]
+    integer: (),
+}
+
+fn main() {}

test_suite/tests/ui/expected-string/integer.stderr

@@ -0,0 +1,8 @@
+error: expected serde rename attribute to be a string: `rename = "..."`
+ --> $DIR/integer.rs:6:22
+  |
+6 |     #[serde(rename = 100)]
+  |                      ^^^
+
+error: aborting due to previous error
+