Support passing values into dbg with the pipe operator

In order to desugar `dbg` in a pipeline we need to allow a bare `dbg`
node in desugaring and only report it as an error if the bare node
survives to the next step of canonicalization. This means we move the
error code out of `desugar_expr` and into `canonicalize_expr`. This is
much simpler to do now that these functions use the same `env` struct,
since previously we would have had to pass down extra args to
`canonicalize_expr`. Sharing the `env` struct means that we also don't
have to worry about calculating `line_info` more than once.

crates/compiler/can/src/expr.rs

@@ -1209,8 +1209,20 @@ pub fn canonicalize_expr<'a>(
                 output,
             )
         }
-        ast::Expr::Dbg | ast::Expr::DbgStmt(_, _) => {
-            internal_error!("Dbg should have been desugared by now")
+        ast::Expr::Dbg => {
+            // Dbg was not desugared as either part of an `Apply` or a `Pizza` binop, so it's
+            // invalid.
+            env.problem(Problem::UnappliedDbg { region });
+
+            let invalid_dbg_expr = crate::desugar::desugar_invalid_dbg_expr(env, scope, region);
+
+            let (loc_expr, output) =
+                canonicalize_expr(env, var_store, scope, region, invalid_dbg_expr);
+
+            (loc_expr.value, output)
+        }
+        ast::Expr::DbgStmt(_, _) => {
+            internal_error!("DbgStmt should have been desugared by now")
         }
         ast::Expr::LowLevelDbg((source_location, source), message, continuation) => {
             let mut output = Output::default();