Allow _ on the LHS of an assignment to discard a value

Author: fanzier

compiler/rustc_ast_lowering/src/expr.rs

@@ -871,7 +871,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> hir::ExprKind<'hir> {
         // Return early in case of an ordinary assignment.
         let is_ordinary = match &lhs.kind {
-            ExprKind::Array(..) | ExprKind::Struct(_, _, None) | ExprKind::Tup(..) => false,
+            ExprKind::Array(..)
+            | ExprKind::Struct(_, _, None)
+            | ExprKind::Tup(..)
+            | ExprKind::Underscore => false,
             ExprKind::Call(callee, ..) => {
                 // Check for tuple struct constructor.
                 if let ExprKind::Path(qself, path) = &callee.kind {
@@ -931,8 +934,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         eq_sign_span: Span,
         assignments: &mut Vec<hir::Stmt<'hir>>,
     ) -> &'hir hir::Pat<'hir> {
-        // FIXME: Handle `_`, requires changes to the parser
         match &lhs.kind {
+            ExprKind::Underscore => {
+                return self.pat(lhs.span, hir::PatKind::Wild);
+            }
             // slices:
             ExprKind::Array(elements) => {
                 let (pats, rest) =

src/test/ui/destructuring-assignment/nested_destructure.rs

@@ -14,4 +14,7 @@ fn main() {
     Struct { a: TupleStruct((a, b), c), b: [d] } =
         Struct { a: TupleStruct((0, 1), 2), b: [3] };
     assert_eq!((a, b, c, d), (0, 1, 2, 3));
+
+    // unnested underscore: just discard
+    _ = 1;
 }

src/test/ui/destructuring-assignment/slice_destructure.rs

@@ -8,4 +8,6 @@ fn main() {
   assert_eq!((a,b), (0,1));
   [a, .., b] = [1,2];
   assert_eq!((a,b), (1,2));
+  [_, a] = [1,2];
+  assert_eq!((a,b), (2,2));
 }

src/test/ui/destructuring-assignment/slice_destructure_fail.rs

@@ -4,4 +4,5 @@ fn main() {
   let (mut a, mut b);
   [a, .., b, ..] = [0, 1]; //~ ERROR `..` can only be used once per slice pattern
   [a, a, b] = [1,2]; //~ ERROR pattern requires 3 elements but array has 2
+  [_] = [1,2]; //~ ERROR pattern requires 1 element but array has 2
 }

src/test/ui/destructuring-assignment/slice_destructure_fail.stderr

@@ -12,6 +12,12 @@ error[E0527]: pattern requires 3 elements but array has 2
 LL |   [a, a, b] = [1,2];
    |   ^^^^^^^^^ expected 2 elements
 
-error: aborting due to 2 previous errors
+error[E0527]: pattern requires 1 element but array has 2
+  --> $DIR/slice_destructure_fail.rs:7:3
+   |
+LL |   [_] = [1,2];
+   |   ^^^ expected 2 elements
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0527`.

src/test/ui/destructuring-assignment/struct_destructure.rs

@@ -7,7 +7,11 @@ struct Struct<S, T> {
 }
 
 fn main() {
-    let (a, b);
+    let (mut a, mut b);
     Struct { a, b } = Struct { a: 0, b: 1 };
     assert_eq!((a, b), (0, 1));
+    Struct { a: b, b: a }  = Struct { a: 1, b: 2};
+    assert_eq!((a,b), (2,1));
+    Struct { a: _, b } = Struct { a: 1, b: 2 };
+    assert_eq!((a,b), (2,2));
 }

src/test/ui/destructuring-assignment/struct_destructure_fail.rs

@@ -8,4 +8,6 @@ fn main() {
     let (mut a, b);
     let mut c;
     Struct { a, b, c } = Struct { a: 0, b: 1 }; //~ ERROR does not have a field named `c`
+    Struct { a, _ } = Struct { a: 1, b: 2 }; //~ ERROR pattern does not mention field `b`
+    //~| ERROR expected identifier, found reserved identifier `_`
 }

src/test/ui/destructuring-assignment/struct_destructure_fail.stderr

@@ -1,9 +1,24 @@
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/struct_destructure_fail.rs:11:17
+   |
+LL |     Struct { a, _ } = Struct { a: 1, b: 2 };
+   |     ------      ^ expected identifier, found reserved identifier
+   |     |
+   |     while parsing this struct
+
 error[E0026]: struct `Struct` does not have a field named `c`
   --> $DIR/struct_destructure_fail.rs:10:20
    |
 LL |     Struct { a, b, c } = Struct { a: 0, b: 1 };
    |                    ^ struct `Struct` does not have this field
 
-error: aborting due to previous error
+error[E0027]: pattern does not mention field `b`
+  --> $DIR/struct_destructure_fail.rs:11:5
+   |
+LL |     Struct { a, _ } = Struct { a: 1, b: 2 };
+   |     ^^^^^^^^^^^^^^^ missing field `b`
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0026`.
+Some errors have detailed explanations: E0026, E0027.
+For more information about an error, try `rustc --explain E0026`.

src/test/ui/destructuring-assignment/tuple_destructure.rs

@@ -8,4 +8,6 @@ fn main() {
     assert_eq!((a,b), (0,1));
     (a, .., b) = (1,2);
     assert_eq!((a,b), (1,2));
+    (_, a) = (1,2);
+    assert_eq!((a,b), (2,2));
 }

src/test/ui/destructuring-assignment/tuple_destructure_fail.rs

@@ -4,4 +4,5 @@ fn main() {
     let (mut a, mut b);
     (a, .., b, ..) = (0, 1); //~ ERROR `..` can only be used once per tuple pattern
     (a, a, b) = (1,2); //~ ERROR mismatched types
+    (_,) = (1,2); //~ ERROR mismatched types
 }