lang: ast: Give resource-build errors a source byline

Storing an incompatible value into a resource field (e.g. a negative int
into an unsigned field) surfaced from the interpret layer without any
position. Attach the offending StmtResField's byline at the types.Into
failure site so the user is pointed at the specific field, with a caret
snippet, rather than the whole resource block.

Author: purpleidea

lang/ast/structs.go

@@ -1161,7 +1161,9 @@ func (obj *StmtRes) resource(table interfaces.Table, resName, data string) (engi
 
 		// mutate the struct field f with the mcl data in fv
 		if err := types.Into(fv, f); err != nil {
-			return nil, err
+			// Point at the specific resource field in the source,
+			// e.g. storing a negative int into an unsigned field.
+			return nil, interfaces.HighlightHelper(x, x.data.Logf, err)
 		}
 	}
 

lang/interpret_test/TestAstFunc2/uint64-negative-wrap.txtar

@@ -9,4 +9,4 @@ test "t1" {
 	uint64 => $x,
 }
 -- OUTPUT --
-# err: errInterpret: error building resource: can't store negative int `-1` into unsigned `uint64` field
+# err: errInterpret: error building resource: can't store negative int `-1` into unsigned `uint64` field: /main.mcl @ 8:2-8:15