Rework precise capturing syntax

compiler/rustc_ast_passes/src/errors.rs

@@ -844,3 +844,11 @@ pub struct MatchArmWithNoBody {
     #[suggestion(code = " => todo!(),", applicability = "has-placeholders")]
     pub suggestion: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_precise_capturing_not_allowed_here)]
+pub struct PreciseCapturingNotAllowedHere {
+    #[primary_span]
+    pub span: Span,
+    pub loc: &'static str,
+}