Rollup merge of #94091 - GuillaumeGomez:rustdoc-const-computed-value, r=oli-obk

Fix rustdoc const computed value

Fixes #85088.

It looks like this now (instead of hexadecimal):

![Screenshot from 2022-02-17 17-55-39](https://user-images.githubusercontent.com/3050060/154532115-0f9861a0-406f-4c9c-957f-32bedd8aca7d.png)

r? ````@oli-obk````

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -456,6 +456,11 @@ impl<'tcx, Tag: Provenance> Scalar<Tag> {
         // Going through `u64` to check size and truncation.
         Ok(Double::from_bits(self.to_u64()?.into()))
     }
+
+    // FIXME: Replace current `impl Display for Scalar` with `impl LowerHex`.
+    pub fn rustdoc_display(&self) -> String {
+        if let Scalar::Int(int) = self { int.to_string() } else { self.to_string() }
+    }
 }
 
 #[derive(Clone, Copy, Eq, PartialEq, TyEncodable, TyDecodable, HashStable, Hash)]

compiler/rustc_middle/src/ty/consts/int.rs

@@ -384,3 +384,10 @@ impl fmt::UpperHex for ScalarInt {
         write!(f, "{:01$X}", { self.data }, self.size as usize * 2)
     }
 }
+
+impl fmt::Display for ScalarInt {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.check_data();
+        write!(f, "{}", { self.data })
+    }
+}