typeck: Don't suggest converting LHS exprs

compiler/rustc_middle/src/hir/map/mod.rs

@@ -564,6 +564,17 @@ impl<'hir> Map<'hir> {
         )
     }
 
+    /// Checks if the node is left-hand side of an assignment.
+    pub fn is_lhs(&self, id: HirId) -> bool {
+        match self.find(self.get_parent_node(id)) {
+            Some(Node::Expr(expr)) => match expr.kind {
+                ExprKind::Assign(lhs, _rhs, _span) => lhs.hir_id == id,
+                _ => false,
+            },
+            _ => false,
+        }
+    }
+
     /// Whether the expression pointed at by `hir_id` belongs to a `const` evaluation context.
     /// Used exclusively for diagnostics, to avoid suggestion function calls.
     pub fn is_inside_const_context(&self, hir_id: HirId) -> bool {

compiler/rustc_typeck/src/check/demand.rs

@@ -816,6 +816,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             |err: &mut DiagnosticBuilder<'_>,
              found_to_exp_is_fallible: bool,
              exp_to_found_is_fallible: bool| {
+                let exp_is_lhs =
+                    expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
+
+                if exp_is_lhs {
+                    return;
+                }
+
                 let always_fallible = found_to_exp_is_fallible
                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
                 let msg = if literal_is_ty_suffixed(expr) {

src/test/ui/typeck/issue-81293.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let a: u16;
+    let b: u16 = 42;
+    let c: usize = 5;
+
+    a = c + b * 5; //~ ERROR: mismatched types [E0308]
+                   //~| ERROR: mismatched types [E0308]
+                   //~| ERROR: cannot add `u16` to `usize` [E0277]
+}

src/test/ui/typeck/issue-81293.stderr

@@ -0,0 +1,24 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-81293.rs:6:13
+   |
+LL |     a = c + b * 5;
+   |             ^^^^^ expected `usize`, found `u16`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-81293.rs:6:9
+   |
+LL |     a = c + b * 5;
+   |         ^^^^^^^^^ expected `u16`, found `usize`
+
+error[E0277]: cannot add `u16` to `usize`
+  --> $DIR/issue-81293.rs:6:11
+   |
+LL |     a = c + b * 5;
+   |           ^ no implementation for `usize + u16`
+   |
+   = help: the trait `Add<u16>` is not implemented for `usize`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.