3 - Make more use of let_chains

Continuation of rust-lang#94376.

cc rust-lang#53667

Author: c410-f3r

Diff for: compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1921,17 +1921,16 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 err.span_label(assigned_span, format!("first assignment to {}", place_description));
             }
         }
-        if let Some(decl) = local_decl {
-            if let Some(name) = local_name {
-                if decl.can_be_made_mutable() {
-                    err.span_suggestion(
-                        decl.source_info.span,
-                        "consider making this binding mutable",
-                        format!("mut {}", name),
-                        Applicability::MachineApplicable,
-                    );
-                }
-            }
+        if let Some(decl) = local_decl
+            && let Some(name) = local_name
+            && decl.can_be_made_mutable()
+        {
+            err.span_suggestion(
+                decl.source_info.span,
+                "consider making this binding mutable",
+                format!("mut {}", name),
+                Applicability::MachineApplicable,
+            );
         }
         err.span_label(span, msg);
         self.buffer_error(err);

Diff for: compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -375,15 +375,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
             Some(Cause::DropVar(local, location)) => {
                 let mut should_note_order = false;
-                if self.local_names[local].is_some() {
-                    if let Some((WriteKind::StorageDeadOrDrop, place)) = kind_place {
-                        if let Some(borrowed_local) = place.as_local() {
-                            if self.local_names[borrowed_local].is_some() && local != borrowed_local
-                            {
-                                should_note_order = true;
-                            }
-                        }
-                    }
+                if self.local_names[local].is_some()
+                    && let Some((WriteKind::StorageDeadOrDrop, place)) = kind_place
+                    && let Some(borrowed_local) = place.as_local()
+                    && self.local_names[borrowed_local].is_some() && local != borrowed_local
+                {
+                    should_note_order = true;
                 }
 
                 BorrowExplanation::UsedLaterWhenDropped {

Diff for: compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1086,21 +1086,17 @@ fn get_mut_span_in_struct_field<'tcx>(
     field: &mir::Field,
 ) -> Option<Span> {
     // Expect our local to be a reference to a struct of some kind.
-    if let ty::Ref(_, ty, _) = ty.kind() {
-        if let ty::Adt(def, _) = ty.kind() {
-            let field = def.all_fields().nth(field.index())?;
-            // Use the HIR types to construct the diagnostic message.
-            let node = tcx.hir().find_by_def_id(field.did.as_local()?)?;
-            // Now we're dealing with the actual struct that we're going to suggest a change to,
-            // we can expect a field that is an immutable reference to a type.
-            if let hir::Node::Field(field) = node {
-                if let hir::TyKind::Rptr(lifetime, hir::MutTy { mutbl: hir::Mutability::Not, ty }) =
-                    field.ty.kind
-                {
-                    return Some(lifetime.span.between(ty.span));
-                }
-            }
-        }
+    if let ty::Ref(_, ty, _) = ty.kind()
+        && let ty::Adt(def, _) = ty.kind()
+        && let field = def.all_fields().nth(field.index())?
+        // Use the HIR types to construct the diagnostic message.
+        && let node = tcx.hir().find_by_def_id(field.did.as_local()?)?
+        // Now we're dealing with the actual struct that we're going to suggest a change to,
+        // we can expect a field that is an immutable reference to a type.
+        && let hir::Node::Field(field) = node
+        && let hir::TyKind::Rptr(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
+    {
+        return Some(lt.span.between(ty.span));
     }
 
     None

Diff for: compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -140,14 +140,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
-        if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref() {
-            if let ty::BoundRegionKind::BrEnv = free_region.bound_region {
-                if let DefiningTy::Closure(_, substs) =
-                    self.regioncx.universal_regions().defining_ty
-                {
-                    return substs.as_closure().kind() == ty::ClosureKind::FnMut;
-                }
-            }
+        if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref()
+            && let ty::BoundRegionKind::BrEnv = free_region.bound_region
+            && let DefiningTy::Closure(_, substs) = self.regioncx.universal_regions().defining_ty
+        {
+            return substs.as_closure().kind() == ty::ClosureKind::FnMut;
         }
 
         false

Diff for: compiler/rustc_borrowck/src/lib.rs

@@ -1,15 +1,16 @@
 //! This query borrow-checks the MIR to (further) ensure it is not broken.
 
+#![allow(rustc::potential_query_instability)]
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(crate_visibility_modifier)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 
 #[macro_use]
 extern crate rustc_middle;
@@ -159,16 +160,14 @@ fn do_mir_borrowck<'a, 'tcx>(
     for var_debug_info in &input_body.var_debug_info {
         if let VarDebugInfoContents::Place(place) = var_debug_info.value {
             if let Some(local) = place.as_local() {
-                if let Some(prev_name) = local_names[local] {
-                    if var_debug_info.name != prev_name {
-                        span_bug!(
-                            var_debug_info.source_info.span,
-                            "local {:?} has many names (`{}` vs `{}`)",
-                            local,
-                            prev_name,
-                            var_debug_info.name
-                        );
-                    }
+                if let Some(prev_name) = local_names[local] && var_debug_info.name != prev_name {
+                    span_bug!(
+                        var_debug_info.source_info.span,
+                        "local {:?} has many names (`{}` vs `{}`)",
+                        local,
+                        prev_name,
+                        var_debug_info.name
+                    );
                 }
                 local_names[local] = Some(var_debug_info.name);
             }

Diff for: compiler/rustc_borrowck/src/places_conflict.rs

@@ -60,10 +60,8 @@ pub(super) fn borrow_conflicts_with_place<'tcx>(
 
     // This Local/Local case is handled by the more general code below, but
     // it's so common that it's a speed win to check for it first.
-    if let Some(l1) = borrow_place.as_local() {
-        if let Some(l2) = access_place.as_local() {
-            return l1 == l2;
-        }
+    if let Some(l1) = borrow_place.as_local() && let Some(l2) = access_place.as_local() {
+        return l1 == l2;
     }
 
     place_components_conflict(tcx, body, borrow_place, borrow_kind, access_place, access, bias)