fix(lexer): Allow '-' in the frontmatter infostring continue set

compiler/rustc_lexer/src/lib.rs

@@ -540,11 +540,11 @@ impl Cursor<'_> {
         // whitespace between the opening and the infostring.
         self.eat_while(|ch| ch != '\n' && is_whitespace(ch));
 
-        // copied from `eat_identifier`, but allows `.` in infostring to allow something like
+        // copied from `eat_identifier`, but allows `-` and `.` in infostring to allow something like
         // `---Cargo.toml` as a valid opener
         if is_id_start(self.first()) {
             self.bump();
-            self.eat_while(|c| is_id_continue(c) || c == '.');
+            self.eat_while(|c| is_id_continue(c) || c == '-' || c == '.');
         }
 
         self.eat_while(|ch| ch != '\n' && is_whitespace(ch));

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
+---
+
+// infostrings can contain hyphens as long as a hyphen isn't the first character.
+//@ check-pass
+
+#![feature(frontmatter)]
+
+fn main() {}