Inline and remove LoweringContext::handler().

It has a single call site.

compiler/rustc_ast_lowering/src/lib.rs

@@ -54,7 +54,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{DiagnosticArgFromDisplay, Handler, StashKey};
+use rustc_errors::{DiagnosticArgFromDisplay, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
@@ -763,10 +763,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.resolver.get_import_res(id).present_items()
     }
 
-    fn diagnostic(&self) -> &Handler {
-        self.tcx.sess.diagnostic()
-    }
-
     /// Reuses the span but adds information like the kind of the desugaring and features that are
     /// allowed inside this span.
     fn mark_span_with_reason(

compiler/rustc_ast_lowering/src/path.rs

@@ -139,7 +139,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         // We should've returned in the for loop above.
 
-        self.diagnostic().span_bug(
+        self.tcx.sess.diagnostic().span_bug(
             p.span,
             format!(
                 "lower_qpath: no final extension segment in {}..{}",