Merge #3706

3706: missing match arms diagnostic r=flodiebold a=JoshMcguigan

Following up on https://github.com/rust-analyzer/rust-analyzer/pull/3689#issuecomment-602718222, this PR creates a missing match arms diagnostic.

At the moment this is a very early draft, but I wanted to open it just to get some initial feedback.

Initial questions:

* Have I roughly created the correct boilerplate? 
* Inside the new `validate_match` function:
  * Am I correct in thinking I want to do validation by comparing the match arms against `match_expr`? And when analyzing `match_expr` I should be looking at it as a `hir_def::expr::Expr`?
  * I mostly copied the chained if-let statements from the struct validation. Shouldn't there be a non-failable way to get an AstPtr from the hir data structures? 

Thanks for all the guidance.

Co-authored-by: Josh Mcguigan <[email protected]>

1 files changed, 9 insertions, 1 deletions

diff --git a/crates/ra_ide/src/diagnostics.rs b/crates/ra_ide/src/diagnostics.rs
index c1d7ddaf2..901ad104c 100644
--- a/crates/ra_ide/src/diagnostics.rs
+++ b/crates/ra_ide/src/diagnostics.rs
@@ -101,6 +101,14 @@ pub(crate) fn diagnostics(db: &RootDatabase, file_id: FileId) -> Vec<Diagnostic>
             fix,
         })
     })
+    .on::<hir::diagnostics::MissingMatchArms, _>(|d| {
+        res.borrow_mut().push(Diagnostic {
+            range: d.highlight_range(),
+            message: d.message(),
+            severity: Severity::Error,
+            fix: None,
+        })
+    })
     .on::<hir::diagnostics::MissingOkInTailExpr, _>(|d| {
         let node = d.ast(db);
         let replacement = format!("Ok({})", node.syntax());
@@ -291,7 +299,7 @@ mod tests {
     fn check_no_diagnostic(content: &str) {
         let (analysis, file_id) = single_file(content);
         let diagnostics = analysis.diagnostics(file_id).unwrap();
-        assert_eq!(diagnostics.len(), 0);
+        assert_eq!(diagnostics.len(), 0, "expected no diagnostic, found one");
     }
 
     #[test]