Give Handler::fatal and Session::fatal the same return type.

Currently, `Handler::fatal` returns `FatalError`. But `Session::fatal`
returns `!`, because it calls `Handler::fatal` and then calls `raise` on
the result. This inconsistency is unfortunate.

This commit changes `Handler::fatal` to do the `raise` itself, changing
its return type to `!`. This is safe because there are only two calls to
`Handler::fatal`, one in `rustc_session` and one in
`rustc_codegen_cranelift`, and they both call `raise` on the result.

`HandlerInner::fatal` still returns `FatalError`, so I renamed it
`fatal_no_raise` to emphasise the return type difference.

compiler/rustc_codegen_cranelift/src/concurrency_limiter.rs

@@ -64,7 +64,7 @@ impl ConcurrencyLimiter {
                     // Make sure to drop the mutex guard first to prevent poisoning the mutex.
                     drop(state);
                     if let Some(err) = err {
-                        handler.fatal(err).raise();
+                        handler.fatal(err);
                     } else {
                         // The error was already emitted, but compilation continued. Raise a silent
                         // fatal error.

compiler/rustc_errors/src/lib.rs

@@ -1034,10 +1034,9 @@ impl Handler {
         db
     }
 
-    // NOTE: intentionally doesn't raise an error so rustc_codegen_ssa only reports fatal errors in the main thread
     #[rustc_lint_diagnostics]
-    pub fn fatal(&self, msg: impl Into<DiagnosticMessage>) -> FatalError {
-        self.inner.borrow_mut().fatal(msg)
+    pub fn fatal(&self, msg: impl Into<DiagnosticMessage>) -> ! {
+        self.inner.borrow_mut().fatal_no_raise(msg).raise()
     }
 
     #[rustc_lint_diagnostics]
@@ -1469,10 +1468,10 @@ impl HandlerInner {
                 DiagnosticMessage::Str(warnings),
             )),
             (_, 0) => {
-                let _ = self.fatal(errors);
+                let _ = self.fatal_no_raise(errors);
             }
             (_, _) => {
-                let _ = self.fatal(format!("{errors}; {warnings}"));
+                let _ = self.fatal_no_raise(format!("{errors}; {warnings}"));
             }
         }
 
@@ -1631,7 +1630,9 @@ impl HandlerInner {
         self.emit_diagnostic(&mut Diagnostic::new(FailureNote, msg));
     }
 
-    fn fatal(&mut self, msg: impl Into<DiagnosticMessage>) -> FatalError {
+    // Note: unlike `Handler::fatal`, this doesn't return `!`, because that is
+    // inappropriate for some of its call sites.
+    fn fatal_no_raise(&mut self, msg: impl Into<DiagnosticMessage>) -> FatalError {
         self.emit(Fatal, msg);
         FatalError
     }

compiler/rustc_session/src/session.rs

@@ -461,7 +461,7 @@ impl Session {
     }
     #[rustc_lint_diagnostics]
     pub fn fatal(&self, msg: impl Into<DiagnosticMessage>) -> ! {
-        self.diagnostic().fatal(msg).raise()
+        self.diagnostic().fatal(msg)
     }
     #[rustc_lint_diagnostics]
     #[track_caller]