Formally implement let chains

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -90,17 +90,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 };
 
                 let join_block = this.cfg.start_new_block();
-                this.cfg.terminate(
-                    then_blk,
-                    source_info,
-                    TerminatorKind::Goto { target: join_block },
-                );
-                this.cfg.terminate(
-                    else_blk,
-                    source_info,
-                    TerminatorKind::Goto { target: join_block },
-                );
-
+                this.cfg.goto(then_blk, source_info, join_block);
+                this.cfg.goto(else_blk, source_info, join_block);
                 join_block.unit()
             }
             ExprKind::Let { expr, ref pat } => {
@@ -109,8 +100,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     this.lower_let_expr(block, &this.thir[expr], pat, scope, expr_span)
                 });
 
-                let join_block = this.cfg.start_new_block();
-
                 this.cfg.push_assign_constant(
                     true_block,
                     source_info,
@@ -133,6 +122,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     },
                 );
 
+                let join_block = this.cfg.start_new_block();
                 this.cfg.goto(true_block, source_info, join_block);
                 this.cfg.goto(false_block, source_info, join_block);
                 join_block.unit()

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -47,6 +47,25 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let expr_span = expr.span;
 
         match expr.kind {
+            ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
+                let lhs_then_block = unpack!(this.then_else_break(
+                    block,
+                    &this.thir[lhs],
+                    temp_scope_override,
+                    break_scope,
+                    variable_scope_span,
+                ));
+
+                let rhs_then_block = unpack!(this.then_else_break(
+                    lhs_then_block,
+                    &this.thir[rhs],
+                    temp_scope_override,
+                    break_scope,
+                    variable_scope_span,
+                ));
+
+                rhs_then_block.unit()
+            }
             ExprKind::Scope { region_scope, lint_level, value } => {
                 let region_scope = (region_scope, this.source_info(expr_span));
                 this.in_scope(region_scope, lint_level, |this| {

compiler/rustc_mir_build/src/build/scope.rs

@@ -498,7 +498,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ///
     /// if let Some(x) = a && let Some(y) = b && let Some(z) = c { ... }
     ///
-    /// there are three possible ways the condition can be false and we may have
+    /// There are three possible ways the condition can be false and we may have
     /// to drop `x`, `x` and `y`, or neither depending on which binding fails.
     /// To handle this correctly we use a `DropTree` in a similar way to a
     /// `loop` expression and 'break' out on all of the 'else' paths.

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -315,7 +315,6 @@ impl<'tcx> Cx<'tcx> {
                             lhs: self.mirror_expr(lhs),
                             rhs: self.mirror_expr(rhs),
                         },
-
                         _ => {
                             let op = bin_op(op.node);
                             ExprKind::Binary {

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -17,6 +17,7 @@ use rustc_session::lint::builtin::{
     BINDINGS_WITH_VARIANT_NAME, IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS,
 };
 use rustc_session::Session;
+use rustc_span::source_map::Spanned;
 use rustc_span::{DesugaringKind, ExpnKind, Span};
 
 crate fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
@@ -445,6 +446,10 @@ fn check_let_reachability<'p, 'tcx>(
     pat: &'p DeconstructedPat<'p, 'tcx>,
     span: Span,
 ) {
+    if is_let_chain(cx.tcx, pat_id) {
+        return;
+    }
+
     let arms = [MatchArm { pat, hir_id: pat_id, has_guard: false }];
     let report = compute_match_usefulness(&cx, &arms, pat_id, pat.ty());
 
@@ -764,8 +769,11 @@ pub enum LetSource {
 
 fn let_source(tcx: TyCtxt<'_>, pat_id: HirId) -> LetSource {
     let hir = tcx.hir();
+
     let parent = hir.get_parent_node(pat_id);
-    match hir.get(parent) {
+    let parent_node = hir.get(parent);
+
+    match parent_node {
         hir::Node::Arm(hir::Arm {
             guard: Some(hir::Guard::IfLet(&hir::Pat { hir_id, .. }, _)),
             ..
@@ -780,6 +788,7 @@ fn let_source(tcx: TyCtxt<'_>, pat_id: HirId) -> LetSource {
         }
         _ => {}
     }
+
     let parent_parent = hir.get_parent_node(parent);
     let parent_parent_node = hir.get(parent_parent);
 
@@ -792,12 +801,30 @@ fn let_source(tcx: TyCtxt<'_>, pat_id: HirId) -> LetSource {
         ..
     }) = parent_parent_parent_parent_node
     {
-        LetSource::WhileLet
-    } else if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::If { .. }, .. }) =
-        parent_parent_node
-    {
-        LetSource::IfLet
-    } else {
-        LetSource::GenericLet
+        return LetSource::WhileLet;
     }
+
+    if let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::If(..), .. }) = parent_parent_node {
+        return LetSource::IfLet;
+    }
+
+    LetSource::GenericLet
+}
+
+// Since this function is called within a let context, it is reasonable to assume that any parent
+// `&&` infers a let chain
+fn is_let_chain(tcx: TyCtxt<'_>, pat_id: HirId) -> bool {
+    let hir = tcx.hir();
+    let parent = hir.get_parent_node(pat_id);
+    let parent_parent = hir.get_parent_node(parent);
+    matches!(
+        hir.get(parent_parent),
+        hir::Node::Expr(
+            hir::Expr {
+                kind: hir::ExprKind::Binary(Spanned { node: hir::BinOpKind::And, .. }, ..),
+                ..
+            },
+            ..
+        )
+    )
 }