Adjusted some doctests in libcore to use should_panic.

Previously, some doctests were spawning new threads and joining them to
indicate that a particular call should panic; this hurt readability, so
the tests have been adjusted to simply call the method and use the
`should_panic` marker.

Author: Jake Degen

src/libcore/cell.rs

@@ -778,18 +778,13 @@ impl<T: ?Sized> RefCell<T> {
     ///
     /// An example of panic:
     ///
-    /// ```
+    /// ```should_panic
     /// use std::cell::RefCell;
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(move || {
-    ///    let c = RefCell::new(5);
-    ///    let m = c.borrow_mut();
     ///
-    ///    let b = c.borrow(); // this causes a panic
-    /// }).join();
+    /// let c = RefCell::new(5);
     ///
-    /// assert!(result.is_err());
+    /// let m = c.borrow_mut();
+    /// let b = c.borrow(); // this causes a panic
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -858,18 +853,13 @@ impl<T: ?Sized> RefCell<T> {
     ///
     /// An example of panic:
     ///
-    /// ```
+    /// ```should_panic
     /// use std::cell::RefCell;
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(move || {
-    ///    let c = RefCell::new(5);
-    ///    let m = c.borrow();
     ///
-    ///    let b = c.borrow_mut(); // this causes a panic
-    /// }).join();
+    /// let c = RefCell::new(5);
+    /// let m = c.borrow();
     ///
-    /// assert!(result.is_err());
+    /// let b = c.borrow_mut(); // this causes a panic
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]

src/libcore/char/convert.rs

@@ -278,16 +278,11 @@ impl fmt::Display for CharTryFromError {
 ///
 /// Passing a large radix, causing a panic:
 ///
-/// ```
-/// use std::thread;
+/// ```should_panic
 /// use std::char;
 ///
-/// let result = thread::spawn(|| {
-///     // this panics
-///     let c = char::from_digit(1, 37);
-/// }).join();
-///
-/// assert!(result.is_err());
+/// // this panics
+/// let c = char::from_digit(1, 37);
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/char/methods.rs

@@ -229,16 +229,11 @@ impl char {
     ///
     /// Passing a large radix, causing a panic:
     ///
-    /// ```
-    /// use std::thread;
+    /// ```should_panic
     /// use std::char;
     ///
-    /// let result = thread::spawn(|| {
-    ///     // this panics
-    ///     let c = char::from_digit(1, 37);
-    /// }).join();
-    ///
-    /// assert!(result.is_err());
+    /// // this panics
+    /// char::from_digit(1, 37);
     /// ```
     #[unstable(feature = "assoc_char_funcs", reason = "recently added", issue = "71763")]
     #[inline]
@@ -282,15 +277,9 @@ impl char {
     ///
     /// Passing a large radix, causing a panic:
     ///
-    /// ```
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(|| {
-    ///     // this panics
-    ///     '1'.is_digit(37);
-    /// }).join();
-    ///
-    /// assert!(result.is_err());
+    /// ```should_panic
+    /// // this panics
+    /// '1'.is_digit(37);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -337,14 +326,9 @@ impl char {
     ///
     /// Passing a large radix, causing a panic:
     ///
-    /// ```
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(|| {
-    ///     '1'.to_digit(37);
-    /// }).join();
-    ///
-    /// assert!(result.is_err());
+    /// ```should_panic
+    /// // this panics
+    /// '1'.to_digit(37);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -646,17 +630,11 @@ impl char {
     ///
     /// A buffer that's too small:
     ///
-    /// ```
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(|| {
-    ///     let mut b = [0; 1];
-    ///
-    ///     // this panics
-    ///    'ß'.encode_utf8(&mut b);
-    /// }).join();
+    /// ```should_panic
+    /// let mut b = [0; 1];
     ///
-    /// assert!(result.is_err());
+    /// // this panics
+    /// 'ß'.encode_utf8(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
     #[inline]
@@ -687,17 +665,11 @@ impl char {
     ///
     /// A buffer that's too small:
     ///
-    /// ```
-    /// use std::thread;
-    ///
-    /// let result = thread::spawn(|| {
-    ///     let mut b = [0; 1];
-    ///
-    ///     // this panics
-    ///     '𝕊'.encode_utf16(&mut b);
-    /// }).join();
+    /// ```should_panic
+    /// let mut b = [0; 1];
     ///
-    /// assert!(result.is_err());
+    /// // this panics
+    /// '𝕊'.encode_utf16(&mut b);
     /// ```
     #[stable(feature = "unicode_encode_char", since = "1.15.0")]
     #[inline]