Support bare unit structs in destructuring assignments

compiler/rustc_ast_lowering/src/expr.rs

@@ -1084,6 +1084,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 | ExprKind::Struct(..)
                 | ExprKind::Tup(..)
                 | ExprKind::Underscore => false,
+                // Check for unit struct constructor.
+                ExprKind::Path(..) => lower_ctx.extract_unit_struct_path(lhs).is_none(),
                 // Check for tuple struct constructor.
                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
                 ExprKind::Paren(e) => {

tests/ui/destructuring-assignment/bad-expr-lhs.rs

@@ -4,6 +4,4 @@ fn main() {
     (1, 2) = (3, 4);
     //~^ ERROR invalid left-hand side of assignment
     //~| ERROR invalid left-hand side of assignment
-
-    None = Some(3); //~ ERROR invalid left-hand side of assignment
 }

tests/ui/destructuring-assignment/bad-expr-lhs.stderr

@@ -30,15 +30,7 @@ LL |     (1, 2) = (3, 4);
    |         |
    |         cannot assign to this expression
 
-error[E0070]: invalid left-hand side of assignment
-  --> $DIR/bad-expr-lhs.rs:8:10
-   |
-LL |     None = Some(3);
-   |     ---- ^
-   |     |
-   |     cannot assign to this expression
-
-error: aborting due to 5 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0067, E0070.
 For more information about an error, try `rustc --explain E0067`.

tests/ui/destructuring-assignment/non-exhaustive-destructure.rs

@@ -0,0 +1,4 @@
+fn main() {
+    None = Some(3);
+    //~^ ERROR refutable pattern in local binding
+}

tests/ui/destructuring-assignment/non-exhaustive-destructure.stderr

@@ -0,0 +1,17 @@
+error[E0005]: refutable pattern in local binding
+  --> $DIR/non-exhaustive-destructure.rs:2:5
+   |
+LL |     None = Some(3);
+   |     ^^^^ pattern `Some(_)` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+   = note: the matched value is of type `Option<i32>`
+help: you might want to use `if let` to ignore the variant that isn't matched
+   |
+LL |     if None = Some(3) { todo!() };
+   |     ++                +++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0005`.

tests/ui/destructuring-assignment/struct-or-enum-variant-path.rs

@@ -11,24 +11,30 @@ type A = E;
 fn main() {
     let mut a;
 
+    S = S;
     (S, a) = (S, ());
 
+    E::V = E::V;
     (E::V, a) = (E::V, ());
 
+    <E>::V = E::V;
     (<E>::V, a) = (E::V, ());
+    A::V = A::V;
     (A::V, a) = (E::V, ());
 }
 
 impl S {
     fn check() {
         let a;
+        Self = S;
         (Self, a) = (S, ());
     }
 }
 
 impl E {
     fn check() {
         let a;
+        Self::V = E::V;
         (Self::V, a) = (E::V, ());
     }
 }

tests/ui/inference/issue-103587.stderr

@@ -26,14 +26,10 @@ error[E0308]: mismatched types
 LL |     if None = x { }
    |        ^^^^^^^^ expected `bool`, found `()`
    |
-help: you might have meant to use pattern matching
+help: consider adding `let`
    |
 LL |     if let None = x { }
    |        +++
-help: you might have meant to compare for equality
-   |
-LL |     if None == x { }
-   |              +
 
 error: aborting due to 3 previous errors
 

tests/ui/issues/issue-13407.rs

@@ -4,6 +4,6 @@ mod A {
 
 fn main() {
     A::C = 1;
-    //~^ ERROR: invalid left-hand side of assignment
-    //~| ERROR: struct `C` is private
+    //~^ ERROR: mismatched types
+    //~| ERROR: unit struct `C` is private
 }

tests/ui/issues/issue-13407.stderr

@@ -10,15 +10,18 @@ note: the unit struct `C` is defined here
 LL |     struct C;
    |     ^^^^^^^^^
 
-error[E0070]: invalid left-hand side of assignment
-  --> $DIR/issue-13407.rs:6:10
+error[E0308]: mismatched types
+  --> $DIR/issue-13407.rs:6:5
    |
+LL |     struct C;
+   |     -------- unit struct defined here
+...
 LL |     A::C = 1;
-   |     ---- ^
+   |     ^^^^   - this expression has type `{integer}`
    |     |
-   |     cannot assign to this expression
+   |     expected integer, found `C`
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0070, E0603.
-For more information about an error, try `rustc --explain E0070`.
+Some errors have detailed explanations: E0308, E0603.
+For more information about an error, try `rustc --explain E0308`.