Adopt let else in more places

compiler/rustc_mir_dataflow/src/drop_flag_effects.rs

@@ -243,9 +243,8 @@ pub(crate) fn on_all_inactive_variants<'tcx>(
     active_variant: VariantIdx,
     mut handle_inactive_variant: impl FnMut(MovePathIndex),
 ) {
-    let enum_mpi = match move_data.rev_lookup.find(enum_place.as_ref()) {
-        LookupResult::Exact(mpi) => mpi,
-        LookupResult::Parent(_) => return,
+    let LookupResult::Exact(enum_mpi) = move_data.rev_lookup.find(enum_place.as_ref()) else {
+        return;
     };
 
     let enum_path = &move_data.move_paths[enum_mpi];
@@ -256,9 +255,8 @@ pub(crate) fn on_all_inactive_variants<'tcx>(
         let (downcast, base_proj) = variant_path.place.projection.split_last().unwrap();
         assert_eq!(enum_place.projection.len(), base_proj.len());
 
-        let variant_idx = match *downcast {
-            mir::ProjectionElem::Downcast(_, idx) => idx,
-            _ => unreachable!(),
+        let mir::ProjectionElem::Downcast(_, variant_idx) = *downcast else {
+            unreachable!();
         };
 
         if variant_idx != active_variant {

compiler/rustc_mir_dataflow/src/framework/engine.rs

@@ -274,11 +274,9 @@ where
     use std::io::{self, Write};
 
     let def_id = body.source.def_id();
-    let attrs = match RustcMirAttrs::parse(tcx, def_id) {
-        Ok(attrs) => attrs,
-
+    let Ok(attrs) = RustcMirAttrs::parse(tcx, def_id) else {
         // Invalid `rustc_mir` attrs are reported in `RustcMirAttrs::parse`
-        Err(()) => return Ok(()),
+        return Ok(());
     };
 
     let mut file = match attrs.output_path(A::NAME) {

compiler/rustc_mir_dataflow/src/framework/graphviz.rs

@@ -628,9 +628,8 @@ where
         ret
     });
 
-    let mut html_diff = match html_diff {
-        Cow::Borrowed(_) => return raw_diff,
-        Cow::Owned(s) => s,
+    let Cow::Owned(mut html_diff) = html_diff else {
+        return raw_diff;
     };
 
     if inside_font_tag {

compiler/rustc_mir_dataflow/src/impls/mod.rs

@@ -385,16 +385,14 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
             switch_on_enum_discriminant(self.tcx, &self.body, &self.body[block], discr)
         });
 
-        let (enum_place, enum_def) = match enum_ {
-            Some(x) => x,
-            None => return,
+        let Some((enum_place, enum_def)) = enum_ else {
+            return;
         };
 
         let mut discriminants = enum_def.discriminants(self.tcx);
         edge_effects.apply(|trans, edge| {
-            let value = match edge.value {
-                Some(x) => x,
-                None => return,
+            let Some(value) = edge.value else {
+                return;
             };
 
             // MIR building adds discriminants to the `values` array in the same order as they
@@ -507,16 +505,14 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
             switch_on_enum_discriminant(self.tcx, &self.body, &self.body[block], discr)
         });
 
-        let (enum_place, enum_def) = match enum_ {
-            Some(x) => x,
-            None => return,
+        let Some((enum_place, enum_def)) = enum_ else {
+            return;
         };
 
         let mut discriminants = enum_def.discriminants(self.tcx);
         edge_effects.apply(|trans, edge| {
-            let value = match edge.value {
-                Some(x) => x,
-                None => return,
+            let Some(value) = edge.value else {
+                return;
             };
 
             // MIR building adds discriminants to the `values` array in the same order as they