Use correct article in help message for conversion or cast

Before it always used `an`; now it uses the correct article for the type.

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.
+    ///
+    /// **Panics if `self` is [`TyKind::Error`].**
+    pub fn article(&self) -> &'static str {
+        match self {
+            Int(_) | Float(_) | Array(_, _) => "an",
+            Adt(def, _) if def.is_enum() => "an",
+            Error(_) => panic!(),
+            _ => "a",
+        }
+    }
 }
 
 // `TyKind` is used a lot. Make sure it doesn't unintentionally get bigger.

compiler/rustc_typeck/src/check/demand.rs

@@ -752,8 +752,18 @@ 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
+        );
+        let cast_msg = format!(
+            "you can cast {} `{} to `{}`",
+            checked_ty.kind().article(),
+            checked_ty,
+            expected_ty
+        );
         let lit_msg = format!(
             "change the type of the numeric literal from `{}` to `{}`",
             checked_ty, expected_ty,

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 `i32` and panic if the converted value wouldn'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 `isize` and panic if the converted value wouldn't fit
    |
 LL |     bar::<isize>(i.try_into().unwrap());  // i should not be re-coerced back to an isize
    |                  ^^^^^^^^^^^^^^^^^^^^^

src/test/ui/integer-literal-suffix-inference.stderr

@@ -328,7 +328,7 @@ error[E0308]: mismatched types
 LL |     id_u8(b16);
    |           ^^^ expected `u8`, found `u16`
    |
-help: you can convert an `u16` to `u8` and panic if the converted value wouldn't fit
+help: you can convert a `u16` to `u8` and panic if the converted value wouldn't fit
    |
 LL |     id_u8(b16.try_into().unwrap());
    |           ^^^^^^^^^^^^^^^^^^^^^^^
@@ -339,7 +339,7 @@ error[E0308]: mismatched types
 LL |     id_u8(b32);
    |           ^^^ expected `u8`, found `u32`
    |
-help: you can convert an `u32` to `u8` and panic if the converted value wouldn't fit
+help: you can convert a `u32` to `u8` and panic if the converted value wouldn't fit
    |
 LL |     id_u8(b32.try_into().unwrap());
    |           ^^^^^^^^^^^^^^^^^^^^^^^
@@ -350,7 +350,7 @@ error[E0308]: mismatched types
 LL |     id_u8(b64);
    |           ^^^ expected `u8`, found `u64`
    |
-help: you can convert an `u64` to `u8` and panic if the converted value wouldn't fit
+help: you can convert a `u64` to `u8` and panic if the converted value wouldn't fit
    |
 LL |     id_u8(b64.try_into().unwrap());
    |           ^^^^^^^^^^^^^^^^^^^^^^^
@@ -361,7 +361,7 @@ error[E0308]: mismatched types
 LL |     id_u8(bsize);
    |           ^^^^^ expected `u8`, found `usize`
    |
-help: you can convert an `usize` to `u8` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u8` and panic if the converted value wouldn't fit
    |
 LL |     id_u8(bsize.try_into().unwrap());
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -373,15 +373,15 @@ LL |     id_u16(b8);
    |            ^^
    |            |
    |            expected `u16`, found `u8`
-   |            help: you can convert an `u8` to `u16`: `b8.into()`
+   |            help: you can convert a `u8` to `u16`: `b8.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:169:12
    |
 LL |     id_u16(b32);
    |            ^^^ expected `u16`, found `u32`
    |
-help: you can convert an `u32` to `u16` and panic if the converted value wouldn't fit
+help: you can convert a `u32` to `u16` and panic if the converted value wouldn't fit
    |
 LL |     id_u16(b32.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^
@@ -392,7 +392,7 @@ error[E0308]: mismatched types
 LL |     id_u16(b64);
    |            ^^^ expected `u16`, found `u64`
    |
-help: you can convert an `u64` to `u16` and panic if the converted value wouldn't fit
+help: you can convert a `u64` to `u16` and panic if the converted value wouldn't fit
    |
 LL |     id_u16(b64.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^
@@ -403,7 +403,7 @@ error[E0308]: mismatched types
 LL |     id_u16(bsize);
    |            ^^^^^ expected `u16`, found `usize`
    |
-help: you can convert an `usize` to `u16` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u16` and panic if the converted value wouldn't fit
    |
 LL |     id_u16(bsize.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -415,7 +415,7 @@ LL |     id_u32(b8);
    |            ^^
    |            |
    |            expected `u32`, found `u8`
-   |            help: you can convert an `u8` to `u32`: `b8.into()`
+   |            help: you can convert a `u8` to `u32`: `b8.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:182:12
@@ -424,15 +424,15 @@ LL |     id_u32(b16);
    |            ^^^
    |            |
    |            expected `u32`, found `u16`
-   |            help: you can convert an `u16` to `u32`: `b16.into()`
+   |            help: you can convert a `u16` to `u32`: `b16.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:186:12
    |
 LL |     id_u32(b64);
    |            ^^^ expected `u32`, found `u64`
    |
-help: you can convert an `u64` to `u32` and panic if the converted value wouldn't fit
+help: you can convert a `u64` to `u32` and panic if the converted value wouldn't fit
    |
 LL |     id_u32(b64.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^
@@ -443,7 +443,7 @@ error[E0308]: mismatched types
 LL |     id_u32(bsize);
    |            ^^^^^ expected `u32`, found `usize`
    |
-help: you can convert an `usize` to `u32` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u32` and panic if the converted value wouldn't fit
    |
 LL |     id_u32(bsize.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -455,7 +455,7 @@ LL |     id_u64(b8);
    |            ^^
    |            |
    |            expected `u64`, found `u8`
-   |            help: you can convert an `u8` to `u64`: `b8.into()`
+   |            help: you can convert a `u8` to `u64`: `b8.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:196:12
@@ -464,7 +464,7 @@ LL |     id_u64(b16);
    |            ^^^
    |            |
    |            expected `u64`, found `u16`
-   |            help: you can convert an `u16` to `u64`: `b16.into()`
+   |            help: you can convert a `u16` to `u64`: `b16.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:199:12
@@ -473,15 +473,15 @@ LL |     id_u64(b32);
    |            ^^^
    |            |
    |            expected `u64`, found `u32`
-   |            help: you can convert an `u32` to `u64`: `b32.into()`
+   |            help: you can convert a `u32` to `u64`: `b32.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:203:12
    |
 LL |     id_u64(bsize);
    |            ^^^^^ expected `u64`, found `usize`
    |
-help: you can convert an `usize` to `u64` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u64` and panic if the converted value wouldn't fit
    |
 LL |     id_u64(bsize.try_into().unwrap());
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -493,7 +493,7 @@ LL |     id_usize(b8);
    |              ^^
    |              |
    |              expected `usize`, found `u8`
-   |              help: you can convert an `u8` to `usize`: `b8.into()`
+   |              help: you can convert a `u8` to `usize`: `b8.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:210:14
@@ -502,15 +502,15 @@ LL |     id_usize(b16);
    |              ^^^
    |              |
    |              expected `usize`, found `u16`
-   |              help: you can convert an `u16` to `usize`: `b16.into()`
+   |              help: you can convert a `u16` to `usize`: `b16.into()`
 
 error[E0308]: mismatched types
   --> $DIR/integer-literal-suffix-inference.rs:213:14
    |
 LL |     id_usize(b32);
    |              ^^^ expected `usize`, found `u32`
    |
-help: you can convert an `u32` to `usize` and panic if the converted value wouldn't fit
+help: you can convert a `u32` to `usize` and panic if the converted value wouldn't fit
    |
 LL |     id_usize(b32.try_into().unwrap());
    |              ^^^^^^^^^^^^^^^^^^^^^^^
@@ -521,7 +521,7 @@ error[E0308]: mismatched types
 LL |     id_usize(b64);
    |              ^^^ expected `usize`, found `u64`
    |
-help: you can convert an `u64` to `usize` and panic if the converted value wouldn't fit
+help: you can convert a `u64` to `usize` and panic if the converted value wouldn't fit
    |
 LL |     id_usize(b64.try_into().unwrap());
    |              ^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/issues/issue-13359.stderr

@@ -15,7 +15,7 @@ error[E0308]: mismatched types
 LL |     bar(1*(1 as usize));
    |         ^^^^^^^^^^^^^^ expected `u32`, found `usize`
    |
-help: you can convert an `usize` to `u32` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u32` and panic if the converted value wouldn't fit
    |
 LL |     bar((1*(1 as usize)).try_into().unwrap());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/mismatched_types/issue-26480.stderr

@@ -8,7 +8,7 @@ LL |     write!(hello);
    |     -------------- in this macro invocation
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-help: you can convert an `usize` to `u64` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u64` and panic if the converted value wouldn't fit
    |
 LL |                   ($arr.len() * size_of($arr[0])).try_into().unwrap());
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/numeric/len.stderr

@@ -4,7 +4,7 @@ error[E0308]: mismatched types
 LL |     test(array.len());
    |          ^^^^^^^^^^^ expected `u32`, found `usize`
    |
-help: you can convert an `usize` to `u32` and panic if the converted value wouldn't fit
+help: you can convert a `usize` to `u32` and panic if the converted value wouldn't fit
    |
 LL |     test(array.len().try_into().unwrap());
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/numeric/numeric-cast-2.stderr

@@ -18,7 +18,7 @@ LL |     let y: i64 = x + x;
    |            ---   ^^^^^
    |            |     |
    |            |     expected `i64`, found `u16`
-   |            |     help: you can convert an `u16` to `i64`: `(x + x).into()`
+   |            |     help: you can convert a `u16` to `i64`: `(x + x).into()`
    |            expected due to this
 
 error[E0308]: mismatched types
@@ -28,7 +28,7 @@ LL |     let z: i32 = x + x;
    |            ---   ^^^^^
    |            |     |
    |            |     expected `i32`, found `u16`
-   |            |     help: you can convert an `u16` to `i32`: `(x + x).into()`
+   |            |     help: you can convert a `u16` to `i32`: `(x + x).into()`
    |            expected due to this
 
 error: aborting due to 3 previous errors