Fix diagnostics for non-exhaustive destructuring assignments (#157553)

Author: Maximilian Azendorf

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

@@ -436,7 +436,16 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         assert!(self.let_source != LetSource::None);
         let scrut = scrutinee.map(|id| &self.thir[id]);
         if let LetSource::PlainLet = self.let_source {
-            self.check_binding_is_irrefutable(pat, "local binding", scrut, Some(span));
+            // `lhs = rhs` destructuring assignments are lowered to a `let` tagged
+            // `AssignDesugar`; report them as assignments, not `let` bindings (#157553).
+            if let hir::Node::LetStmt(&hir::LetStmt {
+                source: hir::LocalSource::AssignDesugar, ..
+            }) = self.tcx.hir_node(self.hir_source)
+            {
+                self.check_binding_is_irrefutable(pat, "assignment", scrut, None);
+            } else {
+                self.check_binding_is_irrefutable(pat, "local binding", scrut, Some(span));
+            }
         } else if let Ok(Irrefutable) = self.is_let_irrefutable(pat, scrut) {
             if span.from_expansion() {
                 self.lint_single_let(span, None, None);

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

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

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

@@ -1,16 +1,10 @@
-error[E0005]: refutable pattern in local binding
+error[E0005]: refutable pattern in assignment
   --> $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/ch19-02-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
 

tests/ui/destructuring-assignment/refutable-enum-assignment.rs

@@ -0,0 +1,11 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/157553>.
+
+enum Foo {
+    One,
+    Two,
+}
+
+fn main() {
+    Foo::One = Foo::One;
+    //~^ ERROR refutable pattern in assignment
+}

tests/ui/destructuring-assignment/refutable-enum-assignment.stderr

@@ -0,0 +1,19 @@
+error[E0005]: refutable pattern in assignment
+  --> $DIR/refutable-enum-assignment.rs:9:5
+   |
+LL |     Foo::One = Foo::One;
+   |     ^^^^^^^^ pattern `Foo::Two` not covered
+   |
+note: `Foo` defined here
+  --> $DIR/refutable-enum-assignment.rs:3:6
+   |
+LL | enum Foo {
+   |      ^^^
+LL |     One,
+LL |     Two,
+   |     --- not covered
+   = note: the matched value is of type `Foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0005`.