Disallow elements in attributes (redux) (#460)

* Rename control flow nodes to avoid conflict with `syn` tokens

* Remove calls to `Token!`

* Disallow elements in attributes

* Add test

* Split `into_element` out of `MaybeElement`

* Fix Clippy type complexity

Author: lambda-fairy

maud/tests/warnings/attribute-missing-value.stderr

@@ -1,5 +1,5 @@
-error: unexpected end of input, expected one of: curly braces, literal, parentheses, identifier, `.`, `#`, `@`, `;`
- --> $DIR/attribute-missing-value.rs:4:5
+error: unexpected end of input, expected one of: curly braces, literal, parentheses, `@`, `;`
+ --> tests/warnings/attribute-missing-value.rs:4:5
   |
 4 | /     html! {
 5 | |         a href=

maud/tests/warnings/class-shorthand-missing-value.stderr

@@ -1,5 +1,5 @@
-error: unexpected end of input, expected one of: curly braces, literal, parentheses, identifier, `.`, `#`, `@`, `;`
- --> $DIR/class-shorthand-missing-value.rs:4:5
+error: unexpected end of input, expected one of: curly braces, literal, parentheses, `@`, `;`
+ --> tests/warnings/class-shorthand-missing-value.rs:4:5
   |
 4 | /     html! {
 5 | |         p.

maud/tests/warnings/elements-in-attributes.rs

@@ -0,0 +1,45 @@
+use maud::html;
+
+fn main() {
+    html! {
+        a href={ b {} } {}
+    };
+
+    html! {
+        a href=.pinkie-pie {} {}
+    };
+
+    html! {
+        a .{ b {} } {}
+    };
+
+    html! {
+        a #{ b {} } {}
+    };
+
+    html! {
+        @if true {
+        } @else if true {
+        } @else {
+            a href={ b #if-else {} } {}
+        }
+    };
+
+    html! {
+        @for _ in 0..10 {
+            a href={ b #for {} } {}
+        }
+    };
+
+    html! {
+        @while false {
+            a href={ b #while {} } {}
+        }
+    };
+
+    html! {
+        @match () {
+            () => a href={ b #match {} } {}
+        }
+    };
+}

maud/tests/warnings/elements-in-attributes.stderr

@@ -0,0 +1,47 @@
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+ --> tests/warnings/elements-in-attributes.rs:5:18
+  |
+5 |         a href={ b {} } {}
+  |                  ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+ --> tests/warnings/elements-in-attributes.rs:9:16
+  |
+9 |         a href=.pinkie-pie {} {}
+  |                ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:13:14
+   |
+13 |         a .{ b {} } {}
+   |              ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:17:14
+   |
+17 |         a #{ b {} } {}
+   |              ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:24:22
+   |
+24 |             a href={ b #if-else {} } {}
+   |                      ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:30:22
+   |
+30 |             a href={ b #for {} } {}
+   |                      ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:36:22
+   |
+36 |             a href={ b #while {} } {}
+   |                      ^
+
+error: expected one of: curly braces, literal, parentheses, `@`, `;`
+  --> tests/warnings/elements-in-attributes.rs:42:28
+   |
+42 |             () => a href={ b #match {} } {}
+   |                            ^