Move FatalError to syntax_pos

This is a bit unfortunate, but code needs to be able to fatally error
early on (in particular, syntax_pos after we move SourceMap there). It's
also a tiny bit of code, which means it's ultimately not that bad.

src/librustc_errors/lib.rs

@@ -259,36 +259,7 @@ impl CodeSuggestion {
     }
 }
 
-/// Used as a return value to signify a fatal error occurred. (It is also
-/// used as the argument to panic at the moment, but that will eventually
-/// not be true.)
-#[derive(Copy, Clone, Debug)]
-#[must_use]
-pub struct FatalError;
-
-pub struct FatalErrorMarker;
-
-// Don't implement Send on FatalError. This makes it impossible to panic!(FatalError).
-// We don't want to invoke the panic handler and print a backtrace for fatal errors.
-impl !Send for FatalError {}
-
-impl FatalError {
-    pub fn raise(self) -> ! {
-        panic::resume_unwind(Box::new(FatalErrorMarker))
-    }
-}
-
-impl fmt::Display for FatalError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "parser fatal error")
-    }
-}
-
-impl error::Error for FatalError {
-    fn description(&self) -> &str {
-        "The parser has encountered a fatal error"
-    }
-}
+pub use syntax_pos::fatal_error::{FatalError, FatalErrorMarker};
 
 /// Signifies that the compiler died with an explicit call to `.bug`
 /// or `.span_bug` rather than a failed assertion, etc.

src/libsyntax_pos/fatal_error.rs

@@ -0,0 +1,30 @@
+/// Used as a return value to signify a fatal error occurred. (It is also
+/// used as the argument to panic at the moment, but that will eventually
+/// not be true.)
+#[derive(Copy, Clone, Debug)]
+#[must_use]
+pub struct FatalError;
+
+pub struct FatalErrorMarker;
+
+// Don't implement Send on FatalError. This makes it impossible to panic!(FatalError).
+// We don't want to invoke the panic handler and print a backtrace for fatal errors.
+impl !Send for FatalError {}
+
+impl FatalError {
+    pub fn raise(self) -> ! {
+        std::panic::resume_unwind(Box::new(FatalErrorMarker))
+    }
+}
+
+impl std::fmt::Display for FatalError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "parser fatal error")
+    }
+}
+
+impl std::error::Error for FatalError {
+    fn description(&self) -> &str {
+        "The parser has encountered a fatal error"
+    }
+}

src/libsyntax_pos/lib.rs

@@ -29,6 +29,7 @@ pub mod symbol;
 pub use symbol::{Symbol, sym};
 
 mod analyze_source_file;
+pub mod fatal_error;
 
 use rustc_data_structures::stable_hasher::StableHasher;
 use rustc_data_structures::sync::{Lrc, Lock};