Qualify panic! as core::panic! in non-built-in core macros

Otherwise code like this

    #![no_implicit_prelude]

    fn main() {
        ::std::todo!();
        ::std::unimplemented!();
    }

will fail to compile, which is unfortunate and presumably unintended.

This changes many invocations of `panic!` in a `macro_rules!` definition
to invocations of `$crate::panic!`, which makes the invocations hygienic.

Note that this does not make the built-in macro `assert!` hygienic.

Author: camelid

core/src/macros/mod.rs

@@ -45,7 +45,7 @@ macro_rules! assert_eq {
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    panic!(r#"assertion failed: `(left == right)`
+                    $crate::panic!(r#"assertion failed: `(left == right)`
   left: `{:?}`,
  right: `{:?}`"#, &*left_val, &*right_val)
                 }
@@ -59,7 +59,7 @@ macro_rules! assert_eq {
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    panic!(r#"assertion failed: `(left == right)`
+                    $crate::panic!(r#"assertion failed: `(left == right)`
   left: `{:?}`,
  right: `{:?}`: {}"#, &*left_val, &*right_val,
                            $crate::format_args!($($arg)+))
@@ -96,7 +96,7 @@ macro_rules! assert_ne {
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    panic!(r#"assertion failed: `(left != right)`
+                    $crate::panic!(r#"assertion failed: `(left != right)`
   left: `{:?}`,
  right: `{:?}`"#, &*left_val, &*right_val)
                 }
@@ -110,7 +110,7 @@ macro_rules! assert_ne {
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    panic!(r#"assertion failed: `(left != right)`
+                    $crate::panic!(r#"assertion failed: `(left != right)`
   left: `{:?}`,
  right: `{:?}`: {}"#, &*left_val, &*right_val,
                            $crate::format_args!($($arg)+))
@@ -468,7 +468,7 @@ macro_rules! writeln {
 ///
 /// # Panics
 ///
-/// This will always [`panic!`]
+/// This will always [`panic!`].
 ///
 /// # Examples
 ///
@@ -502,13 +502,13 @@ macro_rules! writeln {
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! unreachable {
     () => ({
-        panic!("internal error: entered unreachable code")
+        $crate::panic!("internal error: entered unreachable code")
     });
     ($msg:expr $(,)?) => ({
         $crate::unreachable!("{}", $msg)
     });
     ($fmt:expr, $($arg:tt)*) => ({
-        panic!($crate::concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
+        $crate::panic!($crate::concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
     });
 }
 
@@ -586,8 +586,8 @@ macro_rules! unreachable {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! unimplemented {
-    () => (panic!("not implemented"));
-    ($($arg:tt)+) => (panic!("not implemented: {}", $crate::format_args!($($arg)+)));
+    () => ($crate::panic!("not implemented"));
+    ($($arg:tt)+) => ($crate::panic!("not implemented: {}", $crate::format_args!($($arg)+)));
 }
 
 /// Indicates unfinished code.
@@ -647,8 +647,8 @@ macro_rules! unimplemented {
 #[macro_export]
 #[stable(feature = "todo_macro", since = "1.40.0")]
 macro_rules! todo {
-    () => (panic!("not yet implemented"));
-    ($($arg:tt)+) => (panic!("not yet implemented: {}", $crate::format_args!($($arg)+)));
+    () => ($crate::panic!("not yet implemented"));
+    ($($arg:tt)+) => ($crate::panic!("not yet implemented: {}", $crate::format_args!($($arg)+)));
 }
 
 /// Definitions of built-in macros.