Use correct article in help message for conversion or cast

compiler/rustc_middle/src/ty/sty.rs

@@ -210,6 +210,18 @@ impl TyKind<'tcx> {
             _ => false,
         }
     }
+
+    /// Get the article ("a" or "an") to use with this type.
+    pub fn article(&self) -> &'static str {
+        match self {
+            Int(_) | Float(_) | Array(_, _) => "an",
+            Adt(def, _) if def.is_enum() => "an",
+            // This should never happen, but ICEing and causing the user's code
+            // to not compile felt too harsh.
+            Error(_) => "a",
+            _ => "a",
+        }
+    }
 }
 
 // `TyKind` is used a lot. Make sure it doesn't unintentionally get bigger.

compiler/rustc_typeck/src/check/demand.rs

@@ -752,8 +752,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        let msg = format!("you can convert an `{}` to `{}`", checked_ty, expected_ty);
-        let cast_msg = format!("you can cast an `{} to `{}`", checked_ty, expected_ty);
+        let msg = format!(
+            "you can convert {} `{}` to {} `{}`",
+            checked_ty.kind().article(),
+            checked_ty,
+            expected_ty.kind().article(),
+            expected_ty,
+        );
+        let cast_msg = format!(
+            "you can cast {} `{}` to {} `{}`",
+            checked_ty.kind().article(),
+            checked_ty,
+            expected_ty.kind().article(),
+            expected_ty,
+        );
         let lit_msg = format!(
             "change the type of the numeric literal from `{}` to `{}`",
             checked_ty, expected_ty,
@@ -815,7 +827,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let suggestion = format!("{}::from({})", checked_ty, lhs_src);
                     (lhs_expr.span, msg, suggestion)
                 } else {
-                    let msg = format!("{} and panic if the converted value wouldn't fit", msg);
+                    let msg = format!("{} and panic if the converted value doesn't fit", msg);
                     let suggestion =
                         format!("{}{}.try_into().unwrap()", prefix, with_opt_paren(&src));
                     (expr.span, msg, suggestion)

src/test/ui/associated-types/associated-types-path-2.stderr

@@ -47,7 +47,7 @@ LL |     let _: i32 = f2(2i32);
    |            |
    |            expected due to this
    |
-help: you can convert an `u32` to `i32` and panic if the converted value wouldn't fit
+help: you can convert a `u32` to an `i32` and panic if the converted value doesn't fit
    |
 LL |     let _: i32 = f2(2i32).try_into().unwrap();
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/indexing-requires-a-uint.stderr

@@ -13,7 +13,7 @@ error[E0308]: mismatched types
 LL |     bar::<isize>(i);  // i should not be re-coerced back to an isize
    |                  ^ expected `isize`, found `usize`
    |
-help: you can convert an `usize` to `isize` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to an `isize` and panic if the converted value doesn't fit
    |
 LL |     bar::<isize>(i.try_into().unwrap());  // i should not be re-coerced back to an isize
    |                  ^^^^^^^^^^^^^^^^^^^^^