checker: disallow mut for blank idents (#18114)

vlib/v/parser/parser.v

@@ -2077,6 +2077,10 @@ fn (mut p Parser) parse_multi_expr(is_top_level bool) ast.Stmt {
 	p.defer_vars = defer_vars
 
 	left0 := left[0]
+	if tok.kind in [.key_mut, .key_shared, .key_atomic] && left0.is_blank_ident() {
+		return p.error_with_pos('cannot use `${tok.kind}` on `_`', tok.pos())
+	}
+
 	if tok.kind == .key_mut && p.tok.kind != .decl_assign {
 		return p.error('expecting `:=` (e.g. `mut x :=`)')
 	}

vlib/v/parser/tests/atomic_blank_ident_err.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/atomic_blank_ident_err.vv:1:1: error: cannot use `atomic` on `_`
+    1 | atomic _ = 3
+      | ~~~~~~
+    2 |

vlib/v/parser/tests/atomic_blank_ident_err.vv

@@ -0,0 +1,2 @@
+atomic _ = 3
+

vlib/v/parser/tests/mut_blank_ident_err.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/mut_blank_ident_err.vv:1:1: error: cannot use `mut` on `_`
+    1 | mut _ = 3
+      | ~~~

vlib/v/parser/tests/mut_blank_ident_err.vv

@@ -0,0 +1 @@
+mut _ = 3

vlib/v/parser/tests/shared_blank_ident_err.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/shared_blank_ident_err.vv:1:1: error: cannot use `shared` on `_`
+    1 | shared _ = 3
+      | ~~~~~~

vlib/v/parser/tests/shared_blank_ident_err.vv

@@ -0,0 +1 @@
+shared _ = 3