Simplify future breakage control flow.

`emit_future_breakage` calls
`self.dcx().take_future_breakage_diagnostics()` and then passes the
result to `self.dcx().emit_future_breakage_report(diags)`. This commit
removes the first of these and lets `emit_future_breakage_report` do the
taking.

It also inlines and removes what is left of `emit_future_breakage`,
which has a single call site.

compiler/rustc_errors/src/lib.rs

@@ -1038,10 +1038,6 @@ impl DiagCtxt {
         }
     }
 
-    pub fn take_future_breakage_diagnostics(&self) -> Vec<Diagnostic> {
-        std::mem::take(&mut self.inner.borrow_mut().future_breakage_diagnostics)
-    }
-
     pub fn abort_if_errors(&self) {
         let mut inner = self.inner.borrow_mut();
         inner.emit_stashed_diagnostics();
@@ -1149,8 +1145,12 @@ impl DiagCtxt {
         self.inner.borrow_mut().emitter.emit_artifact_notification(path, artifact_type);
     }
 
-    pub fn emit_future_breakage_report(&self, diags: Vec<Diagnostic>) {
-        self.inner.borrow_mut().emitter.emit_future_breakage_report(diags)
+    pub fn emit_future_breakage_report(&self) {
+        let mut inner = self.inner.borrow_mut();
+        let diags = std::mem::take(&mut inner.future_breakage_diagnostics);
+        if !diags.is_empty() {
+            inner.emitter.emit_future_breakage_report(diags);
+        }
     }
 
     pub fn emit_unused_externs(

compiler/rustc_session/src/session.rs

@@ -288,19 +288,9 @@ impl Session {
     pub fn finish_diagnostics(&self, registry: &Registry) {
         self.check_miri_unleashed_features();
         self.dcx().print_error_count(registry);
-        self.emit_future_breakage();
-    }
-
-    fn emit_future_breakage(&self) {
-        if !self.opts.json_future_incompat {
-            return;
-        }
-
-        let diags = self.dcx().take_future_breakage_diagnostics();
-        if diags.is_empty() {
-            return;
+        if self.opts.json_future_incompat {
+            self.dcx().emit_future_breakage_report();
         }
-        self.dcx().emit_future_breakage_report(diags);
     }
 
     /// Returns true if the crate is a testing one.