Auto merge of rust-lang#78088 - fusion-engineering-forks:panic-fmt-lint, r=estebank

Add lint for panic!("{}")

This adds a lint that warns about `panic!("{}")`.

`panic!(msg)` invocations with a single argument use their argument as panic payload literally, without using it as a format string. The same holds for `assert!(expr, msg)`.

This lints checks if `msg` is a string literal (after expansion), and warns in case it contained braces. It suggests to insert `"{}", ` to use the message literally, or to add arguments to use it as a format string.

![image](https://user-images.githubusercontent.com/783247/96643867-79eb1080-1328-11eb-8d4e-a5586837c70a.png)

This lint is also a good starting point for adding warnings about `panic!(not_a_string)` later, once [`panic_any()`](rust-lang#74622) becomes a stable alternative.

Author: bors

core/src/macros/mod.rs

@@ -2,6 +2,7 @@
 #[macro_export]
 #[allow_internal_unstable(core_panic, const_caller_location)]
 #[stable(feature = "core", since = "1.6.0")]
+#[cfg_attr(not(bootstrap), rustc_diagnostic_item = "core_panic_macro")]
 macro_rules! panic {
     () => (
         $crate::panic!("explicit panic")
@@ -162,6 +163,7 @@ macro_rules! assert_ne {
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), rustc_diagnostic_item = "debug_assert_macro")]
 macro_rules! debug_assert {
     ($($arg:tt)*) => (if $crate::cfg!(debug_assertions) { $crate::assert!($($arg)*); })
 }
@@ -1215,6 +1217,8 @@ pub(crate) mod builtin {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_builtin_macro]
     #[macro_export]
+    #[cfg_attr(not(bootstrap), rustc_diagnostic_item = "assert_macro")]
+    #[allow_internal_unstable(core_panic)]
     macro_rules! assert {
         ($cond:expr $(,)?) => {{ /* compiler built-in */ }};
         ($cond:expr, $($arg:tt)+) => {{ /* compiler built-in */ }};

core/tests/nonzero.rs

@@ -85,7 +85,7 @@ fn test_match_option_string() {
     let five = "Five".to_string();
     match Some(five) {
         Some(s) => assert_eq!(s, "Five"),
-        None => panic!("unexpected None while matching on Some(String { ... })"),
+        None => panic!("{}", "unexpected None while matching on Some(String { ... })"),
     }
 }
 

std/src/macros.rs

@@ -8,6 +8,7 @@
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow_internal_unstable(libstd_sys_internals)]
+#[cfg_attr(not(any(bootstrap, test)), rustc_diagnostic_item = "std_panic_macro")]
 macro_rules! panic {
     () => ({ $crate::panic!("explicit panic") });
     ($msg:expr $(,)?) => ({ $crate::rt::begin_panic($msg) });