Auto merge of #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 https://github.com/dtolnay/syn/issues/1085.

library/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.

src/test/ui/proc-macro/auxiliary/api/parse.rs

@@ -1,3 +1,5 @@
+// ignore-tidy-linelength
+
 use proc_macro::Literal;
 
 pub fn test() {
@@ -8,6 +10,14 @@ pub fn test() {
 fn test_display_literal() {
     assert_eq!(Literal::isize_unsuffixed(-10).to_string(), "-10");
     assert_eq!(Literal::isize_suffixed(-10).to_string(), "-10isize");
+    assert_eq!(Literal::f32_unsuffixed(-10.0).to_string(), "-10.0");
+    assert_eq!(Literal::f32_suffixed(-10.0).to_string(), "-10f32");
+    assert_eq!(Literal::f64_unsuffixed(-10.0).to_string(), "-10.0");
+    assert_eq!(Literal::f64_suffixed(-10.0).to_string(), "-10f64");
+    assert_eq!(
+        Literal::f64_unsuffixed(1e100).to_string(),
+        "10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0",
+    );
 }
 
 fn test_parse_literal() {