审题

// The procedural macro API uses a type called Span to attach source location
// and hygiene information to every token. In order for compiler errors to
// appear underlining the right places, procedural macros are responsible for
// propagating and manipulating these spans correctly.
//
// The invocation below expands to code that mentions a value Missing0 which
// does not exist. When the compiler reports that it "cannot find value
// Missing0", we would like for the error to point directly to where the user
// wrote `Missing~N` in their macro input.
//
//     error[E0425]: cannot find value `Missing0` in this scope
//       |
//       |         let _ = Missing~N;
//       |                 ^^^^^^^ not found in this scope
//
// For this test to pass, ensure that the pasted-together identifier is created
// using the Span of the identifier written by the caller.
//
// If you are using a nightly toolchain, there is a nightly-only method called
// Span::join which would allow joining the three spans of `Missing`, `~`, `N`
// so that the resulting error is as follows, but I would recommend not
// bothering with this for the purpose of this project while it is unstable.
//
//     error[E0425]: cannot find value `Missing0` in this scope
//       |
//       |         let _ = Missing~N;
//       |                 ^^^^^^^^^ not found in this scope
//

use seq::seq;

seq!(N in 0..1 {
    fn main() {
        let _ = Missing~N;
    }
});

这里考察的还是报错位置的解析，也就是span。

error[E0425]: cannot find value `Missing0` in this scope
  --> tests/08-ident-span.rs:34:17
   |
34 |         let _ = Missing~N;
   |                 ^^^^^^^ not found in this scope

因为对于N in 0..1后续{}内部的解析，我们都是基于标准的rust语法。
类型未定义这种情况，是必定能够解析出来的。

从这里也能够看出来rust强大的一方面，静态检查能够规避大多数开发问题。
编译期间，只要有良好、准确的错误提示，即使是macro中的自定义语句，也能够进行错误的定位。

虽然span信息的处理让人觉得鸡肋，但是完整的检查，可以让使用者在开发过程中，在一无所知的宏定义中，得到正确的指引。

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