Format Unexpected::Float with decimal point

serde/src/de/mod.rs

@@ -402,7 +402,7 @@ impl<'a> fmt::Display for Unexpected<'a> {
             Bool(b) => write!(formatter, "boolean `{}`", b),
             Unsigned(i) => write!(formatter, "integer `{}`", i),
             Signed(i) => write!(formatter, "integer `{}`", i),
-            Float(f) => write!(formatter, "floating point `{}`", f),
+            Float(f) => write!(formatter, "floating point `{}`", WithDecimalPoint(f)),
             Char(c) => write!(formatter, "character `{}`", c),
             Str(s) => write!(formatter, "string {:?}", s),
             Bytes(_) => write!(formatter, "byte array"),
@@ -2290,3 +2290,36 @@ impl Display for OneOf {
         }
     }
 }
+
+struct WithDecimalPoint(f64);
+
+impl Display for WithDecimalPoint {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        struct LookForDecimalPoint<'f, 'a> {
+            formatter: &'f mut fmt::Formatter<'a>,
+            has_decimal_point: bool,
+        }
+
+        impl<'f, 'a> fmt::Write for LookForDecimalPoint<'f, 'a> {
+            fn write_str(&mut self, fragment: &str) -> fmt::Result {
+                self.has_decimal_point |= fragment.contains('.');
+                self.formatter.write_str(fragment)
+            }
+
+            fn write_char(&mut self, ch: char) -> fmt::Result {
+                self.has_decimal_point |= ch == '.';
+                self.formatter.write_char(ch)
+            }
+        }
+
+        let mut writer = LookForDecimalPoint {
+            formatter,
+            has_decimal_point: false,
+        };
+        tri!(write!(writer, "{}", self.0));
+        if !writer.has_decimal_point {
+            tri!(formatter.write_str(".0"));
+        }
+        Ok(())
+    }
+}

serde/src/lib.rs

@@ -182,7 +182,7 @@ mod lib {
     pub use self::core::cmp::Reverse;
     pub use self::core::convert::{self, From, Into};
     pub use self::core::default::{self, Default};
-    pub use self::core::fmt::{self, Debug, Display};
+    pub use self::core::fmt::{self, Debug, Display, Write as FmtWrite};
     pub use self::core::marker::{self, PhantomData};
     pub use self::core::num::Wrapping;
     pub use self::core::ops::{Bound, Range, RangeFrom, RangeInclusive, RangeTo};

test_suite/tests/test_de_error.rs

@@ -1434,7 +1434,7 @@ fn test_number_from_string() {
 fn test_integer_from_float() {
     assert_de_tokens_error::<isize>(
         &[Token::F32(0.0)],
-        "invalid type: floating point `0`, expected isize",
+        "invalid type: floating point `0.0`, expected isize",
     );
 }