rustdoc: Catch fatal errors when syntax highlighting

For some errors the lexer will unwind so we need to handle that in addition to handling `token::Unknown`.

src/test/rustdoc-ui/invalid-syntax.rs

@@ -93,3 +93,9 @@ pub fn empty_rust_with_whitespace() {}
 ///
 pub fn indent_after_fenced() {}
 //~^^^ WARNING could not parse code block as Rust code
+
+/// ```
+/// "invalid
+/// ```
+pub fn invalid() {}
+//~^^^^ WARNING could not parse code block as Rust code

src/test/rustdoc-ui/invalid-syntax.stderr

@@ -132,3 +132,18 @@ LL | ///     \____/
    |
    = note: error from rustc: unknown start of token: \
 
+warning: could not parse code block as Rust code
+  --> $DIR/invalid-syntax.rs:97:5
+   |
+LL |   /// ```
+   |  _____^
+LL | | /// "invalid
+LL | | /// ```
+   | |_______^
+   |
+   = note: error from rustc: unterminated double quote string
+help: mark blocks that do not contain Rust code as text
+   |
+LL | /// ```text
+   |     ^^^^^^^
+

src/test/rustdoc/bad-codeblock-syntax.rs

@@ -33,3 +33,10 @@ pub fn ok() {}
 /// <script>alert("not valid Rust");</script>
 /// ```
 pub fn escape() {}
+
+// @has bad_codeblock_syntax/fn.unterminated.html
+// @has - '//*[@class="docblock"]/pre/code' '"unterminated'
+/// ```
+/// "unterminated
+/// ```
+pub fn unterminated() {}