Add FakeRead::ForLetUnderscore

We use it for the RHS of statements like `let _ = x`.

Author: ecstatic-morse

src/librustc_middle/mir/mod.rs

@@ -1717,6 +1717,14 @@ pub enum FakeReadCause {
     /// appropriate errors.
     ForLet,
 
+    /// This denotes the existence of a statement like
+    ///
+    /// `let _ = <expr>;`
+    ///
+    /// which would not otherwise be recorded in the MIR. This is required for to
+    /// implement the unused variable lint on the MIR. It should be ignored elsewhere.
+    ForLetUnderscore,
+
     /// If we have an index expression like
     ///
     /// (*x)[1][{ x = y; 4}]

src/librustc_mir/borrow_check/mod.rs

@@ -12,7 +12,7 @@ use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_middle::mir::{
-    traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
+    self, traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
     PlaceRef,
 };
 use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
@@ -571,6 +571,10 @@ impl<'cx, 'tcx> dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tc
 
                 self.mutate_place(location, (*lhs, span), Shallow(None), JustWrite, flow_state);
             }
+
+            // `ForLetUnderscore` is only intended for the unused variables lint.
+            StatementKind::FakeRead(mir::FakeReadCause::ForLetUnderscore, _) => {}
+
             StatementKind::FakeRead(_, box ref place) => {
                 // Read for match doesn't access any memory and is used to
                 // assert that a place is safe and live. So we don't have to

src/librustc_mir/transform/check_consts/validation.rs

@@ -514,7 +514,10 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
                 self.check_op(ops::InlineAsm);
             }
 
-            StatementKind::FakeRead(FakeReadCause::ForLet | FakeReadCause::ForIndex, _)
+            StatementKind::FakeRead(
+                FakeReadCause::ForLet | FakeReadCause::ForIndex | FakeReadCause::ForLetUnderscore,
+                _,
+            )
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
             | StatementKind::Retag { .. }

src/librustc_mir_build/build/matches/mod.rs

@@ -439,6 +439,20 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 block.unit()
             }
 
+            // Handle bindings like `let _ = var;` by lowering them to a fake read of the RHS. This
+            // is needed by the unused variables lint pass.
+            PatKind::Wild => {
+                let place = unpack!(block = self.as_place(block, initializer));
+                debug!("expr_into_pattern(place = {:?}, PatKind::Wild)", place);
+
+                if place.as_local().is_some() {
+                    let source_info = self.source_info(irrefutable_pat.span);
+                    self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLetUnderscore, place);
+                }
+
+                block.unit()
+            }
+
             _ => {
                 let place = unpack!(block = self.as_place(block, initializer));
                 self.place_into_pattern(block, irrefutable_pat, place, true)