Auto merge of #30681 - Toby-S:master, r=bluss

Make `".".parse::<f32>()` and `".".parse::<f64>()` return Err

This fixes #30344.

This is a [breaking-change], which the libs team have classified as a
bug fix.

src/libcore/num/dec2flt/parse.rs

@@ -56,27 +56,28 @@ pub enum ParseResult<'a> {
 /// Check if the input string is a valid floating point number and if so, locate the integral
 /// part, the fractional part, and the exponent in it. Does not handle signs.
 pub fn parse_decimal(s: &str) -> ParseResult {
+    if s.is_empty() {
+        return Invalid;
+    }
+
     let s = s.as_bytes();
     let (integral, s) = eat_digits(s);
+
     match s.first() {
-        None => {
-            if integral.is_empty() {
-                return Invalid; // No digits at all
-            }
-            Valid(Decimal::new(integral, b"", 0))
-        }
+        None => Valid(Decimal::new(integral, b"", 0)),
         Some(&b'e') | Some(&b'E') => {
             if integral.is_empty() {
                 return Invalid; // No digits before 'e'
             }
+
             parse_exp(integral, b"", &s[1..])
         }
         Some(&b'.') => {
             let (fractional, s) = eat_digits(&s[1..]);
             if integral.is_empty() && fractional.is_empty() && s.is_empty() {
-                // For historic reasons "." is a valid input.
-                return Valid(Decimal::new(b"", b"", 0));
+                return Invalid;
             }
+
             match s.first() {
                 None => Valid(Decimal::new(integral, fractional, 0)),
                 Some(&b'e') | Some(&b'E') => parse_exp(integral, fractional, &s[1..]),

src/libcoretest/num/dec2flt/mod.rs

@@ -98,7 +98,8 @@ fn fast_path_correct() {
 
 #[test]
 fn lonely_dot() {
-    assert_eq!(".".parse(), Ok(0.0));
+    assert!(".".parse::<f32>().is_err());
+    assert!(".".parse::<f64>().is_err());
 }
 
 #[test]