Improve documentation for <integer>::from_str_radix

Two improvements to the documentation:
- Document `-` as a valid character for signed integer destinations
- Make the documentation even more clear that extra whitespace and non-digit characters is invalid. Many other
  languages, e.g. c++, are very permissive in string to integer routines and simply try to consume as much as they can,
  ignoring the rest. This is trying to make the transition for developers who are used to the conversion semantics in
  these languages a bit easier.

library/core/src/num/mod.rs

@@ -23,6 +23,16 @@ macro_rules! unlikely {
     };
 }
 
+// Use this when the generated code should differ between signed and unsigned types.
+macro_rules! sign_dependent_expr {
+    (signed ? if signed { $signed_case:expr } if unsigned { $unsigned_case:expr } ) => {
+        $signed_case
+    };
+    (unsigned ? if signed { $signed_case:expr } if unsigned { $unsigned_case:expr } ) => {
+        $unsigned_case
+    };
+}
+
 // All these modules are technically private and only exposed for coretests:
 #[cfg(not(no_fp_fmt_parse))]
 pub mod bignum;
@@ -1410,15 +1420,25 @@ const fn from_str_radix_panic(radix: u32) {
 }
 
 macro_rules! from_str_radix {
-    ($($int_ty:ty)+) => {$(
+    ($signedness:ident $($int_ty:ty)+) => {$(
         impl $int_ty {
             /// Converts a string slice in a given base to an integer.
             ///
-            /// The string is expected to be an optional `+` sign
+            /// The string is expected to be an optional
-            /// followed by digits.
+            #[doc = sign_dependent_expr!{
-            /// Leading and trailing whitespace represent an error.
+                $signedness ?
-            /// Digits are a subset of these characters, depending on `radix`:
+                if signed {
+                    " `+` or `-` "
+                }
+                if unsigned {
+                    " `+` "
+                }
+            }]
+            /// sign followed by only digits. Leading and trailing non-digit characters (including
+            /// whitespace) represent an error. Underscores (which are accepted in rust literals)
+            /// also represent an error.
             ///
+            /// Digits are a subset of these characters, depending on `radix`:
             /// * `0-9`
             /// * `a-z`
             /// * `A-Z`
@@ -1430,10 +1450,13 @@ impl $int_ty {
             /// # Examples
             ///
             /// Basic usage:
-            ///
             /// ```
             #[doc = concat!("assert_eq!(", stringify!($int_ty), "::from_str_radix(\"A\", 16), Ok(10));")]
             /// ```
+            /// Trailing space returns error:
+            /// ```
+            #[doc = concat!("assert!(", stringify!($int_ty), "::from_str_radix(\"1 \", 10).is_err());")]
+            /// ```
             #[stable(feature = "rust1", since = "1.0.0")]
             #[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
             pub const fn from_str_radix(src: &str, radix: u32) -> Result<$int_ty, ParseIntError> {
@@ -1535,20 +1558,31 @@ macro_rules! run_checked_loop {
     )+}
 }
 
-from_str_radix! { i8 u8 i16 u16 i32 u32 i64 u64 i128 u128 }
+from_str_radix! { unsigned u8 u16 u32 u64 u128 }
+from_str_radix! { signed i8 i16 i32 i64 i128 }
 
 // Re-use the relevant implementation of from_str_radix for isize and usize to avoid outputting two
 // identical functions.
 macro_rules! from_str_radix_size_impl {
-    ($($t:ident $size:ty),*) => {$(
+    ($($signedness:ident $t:ident $size:ty),*) => {$(
     impl $size {
         /// Converts a string slice in a given base to an integer.
         ///
-        /// The string is expected to be an optional `+` sign
+        /// The string is expected to be an optional
-        /// followed by digits.
+        #[doc = sign_dependent_expr!{
-        /// Leading and trailing whitespace represent an error.
+            $signedness ?
-        /// Digits are a subset of these characters, depending on `radix`:
+            if signed {
+                " `+` or `-` "
+            }
+            if unsigned {
+                " `+` "
+            }
+        }]
+        /// sign followed by only digits. Leading and trailing non-digit characters (including
+        /// whitespace) represent an error. Underscores (which are accepted in rust literals)
+        /// also represent an error.
         ///
+        /// Digits are a subset of these characters, depending on `radix`:
         /// * `0-9`
         /// * `a-z`
         /// * `A-Z`
@@ -1560,10 +1594,13 @@ impl $size {
         /// # Examples
         ///
         /// Basic usage:
-        ///
         /// ```
         #[doc = concat!("assert_eq!(", stringify!($size), "::from_str_radix(\"A\", 16), Ok(10));")]
         /// ```
+        /// Trailing space returns error:
+        /// ```
+        #[doc = concat!("assert!(", stringify!($size), "::from_str_radix(\"1 \", 10).is_err());")]
+        /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
         #[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
         pub const fn from_str_radix(src: &str, radix: u32) -> Result<$size, ParseIntError> {
@@ -1576,8 +1613,8 @@ pub const fn from_str_radix(src: &str, radix: u32) -> Result<$size, ParseIntErro
 }
 
 #[cfg(target_pointer_width = "16")]
-from_str_radix_size_impl! { i16 isize, u16 usize }
+from_str_radix_size_impl! { signed i16 isize, unsigned u16 usize }
 #[cfg(target_pointer_width = "32")]
-from_str_radix_size_impl! { i32 isize, u32 usize }
+from_str_radix_size_impl! { signed i32 isize, unsigned u32 usize }
 #[cfg(target_pointer_width = "64")]
-from_str_radix_size_impl! { i64 isize, u64 usize }
+from_str_radix_size_impl! { signed i64 isize, unsigned u64 usize }

library/core/src/num/nonzero.rs

@@ -1972,16 +1972,6 @@ pub const fn wrapping_neg(self) -> Self {
     };
 }
 
-// Use this when the generated code should differ between signed and unsigned types.
-macro_rules! sign_dependent_expr {
-    (signed ? if signed { $signed_case:expr } if unsigned { $unsigned_case:expr } ) => {
-        $signed_case
-    };
-    (unsigned ? if signed { $signed_case:expr } if unsigned { $unsigned_case:expr } ) => {
-        $unsigned_case
-    };
-}
-
 nonzero_integer! {
     Self = NonZeroU8,
     Primitive = unsigned u8,

library/core/tests/num/int_macros.rs

@@ -244,6 +244,8 @@ fn test_from_str_radix() {
 
             assert_eq!($T::from_str_radix("Z", 35).ok(), None::<$T>);
             assert_eq!($T::from_str_radix("-9", 2).ok(), None::<$T>);
+            assert_eq!($T::from_str_radix("10_0", 10).ok(), None::<$T>);
+            assert_eq!(u32::from_str_radix("-9", 10).ok(), None::<u32>);
         }
 
         #[test]