Specific error message for missplaced doc comments

Identify when documetation comments have been missplaced in the
following places:

 * After a struct element:

    ```rust
    // file.rs:
    struct X {
        a: u8 /** document a */,
    }
    ```

    ```bash
    $ rustc file.rs
    file.rs:2:11: 2:28 error: found documentation comment that doesn't
    document anything
    file.rs:2     a: u8 /** document a */,
                        ^~~~~~~~~~~~~~~~~
    file.rs:2:11: 2:28 help: doc comments must come before what they document,
    maybe a comment was intended with `//`?
    ```

 * As the last line of a struct:

    ```rust
    // file.rs:
    struct X {
        a: u8,
        /// incorrect documentation
    }
    ```

    ```bash
    $ rustc file.rs
    file.rs:3:5: 3:27 error: found a documentation comment that doesn't
    document anything
    file.rs:3     /// incorrect documentation
                  ^~~~~~~~~~~~~~~~~~~~~~
    file.rs:3:5: 3:27 help: doc comments must come before what they document,
    maybe a comment was intended with `//`?
    ```

 * As the last line of a `fn`:

    ```rust
    // file.rs:
    fn main() {
        let x = 1;
        /// incorrect documentation
    }
    ```

    ```bash
    $ rustc file.rs
    file.rs:3:5: 3:27 error: found a documentation comment that doesn't
    document anything
    file.rs:3     /// incorrect documentation
                  ^~~~~~~~~~~~~~~~~~~~~~
    file.rs:3:5: 3:27 help: doc comments must come before what they document,
    maybe a comment was intended with `//`?
    ```

Fix #27429, #30322

1 files changed, 9 insertions, 0 deletions

diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index 18fc826f9aa..1d0f185c60a 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -564,6 +564,15 @@ impl Handler {
         self.bump_err_count();
         self.panic_if_treat_err_as_bug();
     }
+    pub fn mut_span_err<'a, S: Into<MultiSpan>>(&'a self,
+                                                sp: S,
+                                                msg: &str)
+                                                -> DiagnosticBuilder<'a> {
+        let mut result = DiagnosticBuilder::new(self, Level::Error, msg);
+        result.set_span(sp);
+        self.bump_err_count();
+        result
+    }
     pub fn span_err_with_code<S: Into<MultiSpan>>(&self, sp: S, msg: &str, code: &str) {
         self.emit_with_code(&sp.into(), msg, code, Error);
         self.bump_err_count();