test(frontmatter): Show current hyphen behavior

Author: epage

tests/ui/frontmatter/hyphen-in-infostring-leading.rs

@@ -0,0 +1,9 @@
+--- -toml
+//~^ ERROR: invalid infostring for frontmatter
+---
+
+// infostrings cannot have leading hyphens
+
+#![feature(frontmatter)]
+
+fn main() {}

tests/ui/frontmatter/hyphen-in-infostring-leading.stderr

@@ -0,0 +1,10 @@
+error: invalid infostring for frontmatter
+  --> $DIR/hyphen-in-infostring-leading.rs:1:4
+   |
+LL | --- -toml
+   |    ^^^^^^
+   |
+   = note: frontmatter infostrings must be a single identifier immediately following the opening
+
+error: aborting due to 1 previous error
+

tests/ui/frontmatter/hyphen-in-infostring-non-leading.rs

@@ -0,0 +1,9 @@
+--- Cargo-toml
+//~^ ERROR: invalid infostring for frontmatter
+---
+
+// infostrings cannot have hyphens
+
+#![feature(frontmatter)]
+
+fn main() {}

tests/ui/frontmatter/hyphen-in-infostring-non-leading.stderr

@@ -0,0 +1,10 @@
+error: invalid infostring for frontmatter
+  --> $DIR/hyphen-in-infostring-non-leading.rs:1:4
+   |
+LL | --- Cargo-toml
+   |    ^^^^^^^^^^^
+   |
+   = note: frontmatter infostrings must be a single identifier immediately following the opening
+
+error: aborting due to 1 previous error
+