Eliminate magic numbers from expression precedence

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -6,6 +6,7 @@ use std::path::{Path, PathBuf};
 use std::process::ExitStatus;
 
 use rustc_abi::TargetDataLayoutErrors;
+use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast_pretty::pprust;
 use rustc_macros::Subdiagnostic;
 use rustc_span::Span;
@@ -298,6 +299,12 @@ impl IntoDiagArg for hir::def::Namespace {
     }
 }
 
+impl IntoDiagArg for ExprPrecedence {
+    fn into_diag_arg(self) -> DiagArgValue {
+        DiagArgValue::Number(self as i32)
+    }
+}
+
 #[derive(Clone)]
 pub struct DiagSymbolList<S = Symbol>(Vec<S>);