test: Show frontmatter behavior before impl

Author: epage

tests/ui/feature-gates/feature-gate-frontmatter.rs

@@ -0,0 +1,6 @@
+---
+//~^ ERROR expected item, found `-`
+---
+
+pub fn main() {
+}

tests/ui/feature-gates/feature-gate-frontmatter.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `-`
+  --> $DIR/feature-gate-frontmatter.rs:1:1
+   |
+LL | ---
+   | ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+

tests/ui/frontmatter/frontmatter-escaped.rs

@@ -0,0 +1,12 @@
+-----
+//~^ ERROR expected item, found `-`
+
+---
+---
+
+-----
+
+#![feature(frontmatter)]
+
+fn main() {
+}

tests/ui/frontmatter/frontmatter-escaped.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `-`
+  --> $DIR/frontmatter-escaped.rs:1:1
+   |
+LL | -----
+   | ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+

tests/ui/frontmatter/frontmatter-ignored-space.rs

@@ -0,0 +1,13 @@
+
+
+---  
+//~^ ERROR expected item, found `-`
+
+---  
+
+#![feature(frontmatter)]
+// ignore-tidy-end-whitespace
+// ignore-tidy-leading-newlines
+
+fn main() {
+}

tests/ui/frontmatter/frontmatter-ignored-space.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `-`
+  --> $DIR/frontmatter-ignored-space.rs:3:1
+   |
+LL | ---  
+   | ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+

tests/ui/frontmatter/frontmatter-invalid-after-attribute.rs

@@ -0,0 +1,8 @@
+#![feature(frontmatter)]
+
+---
+//~^ ERROR expected item, found `-`
+---
+
+fn main() {
+}

tests/ui/frontmatter/frontmatter-invalid-after-attribute.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `-`
+  --> $DIR/frontmatter-invalid-after-attribute.rs:3:1
+   |
+LL | ---
+   | ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+

tests/ui/frontmatter/frontmatter-invalid-after-comment.rs

@@ -0,0 +1,10 @@
+// Comment
+
+---
+//~^ ERROR expected item, found `-`
+---
+
+#![feature(frontmatter)]
+
+fn main() {
+}

tests/ui/frontmatter/frontmatter-invalid-after-comment.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `-`
+  --> $DIR/frontmatter-invalid-after-comment.rs:3:1
+   |
+LL | ---
+   | ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 1 previous error
+