Auto merge of rust-lang#90297 - dtolnay:dotzero, r=petrochenkov

Append .0 to unsuffixed float if it would otherwise become int token

Previously the unsuffixed f32/f64 constructors of `proc_macro::Literal` would create literal tokens that are definitely not a float:

```rust
Literal::f32_unsuffixed(10.0)  // 10
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10
Literal::f64_suffixed(10.0)    // 10f64
```

Notice that the `10` are actually integer tokens if you were to reparse them, not float tokens.

This diff updates `Literal::f32_unsuffixed` and `Literal::f64_unsuffixed` to produce tokens that unambiguously parse as a float. This matches longstanding behavior of the proc-macro2 crate's implementation of these APIs dating back at least 3.5 years, so it's likely an unobjectionable behavior.

```rust
Literal::f32_unsuffixed(10.0)  // 10.0
Literal::f32_suffixed(10.0)    // 10f32
Literal::f64_unsuffixed(10.0)  // 10.0
Literal::f64_suffixed(10.0)    // 10f64
```

Fixes dtolnay/syn#1085.

Author: bors

proc_macro/src/lib.rs

@@ -1074,7 +1074,11 @@ impl Literal {
         if !n.is_finite() {
             panic!("Invalid float literal {}", n);
         }
-        Literal(bridge::client::Literal::float(&n.to_string()))
+        let mut repr = n.to_string();
+        if !repr.contains('.') {
+            repr.push_str(".0");
+        }
+        Literal(bridge::client::Literal::float(&repr))
     }
 
     /// Creates a new suffixed floating-point literal.
@@ -1115,7 +1119,11 @@ impl Literal {
         if !n.is_finite() {
             panic!("Invalid float literal {}", n);
         }
-        Literal(bridge::client::Literal::float(&n.to_string()))
+        let mut repr = n.to_string();
+        if !repr.contains('.') {
+            repr.push_str(".0");
+        }
+        Literal(bridge::client::Literal::float(&repr))
     }
 
     /// Creates a new suffixed floating-point literal.