Auto merge of #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()`](https://github.com/rust-lang/rust/pull/74622) becomes a stable alternative.

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -1173,7 +1173,8 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
         mbe::TokenTree::MetaVar(_, name) => format!("${}", name),
         mbe::TokenTree::MetaVarDecl(_, name, kind) => format!("${}:{}", name, kind),
         _ => panic!(
-            "unexpected mbe::TokenTree::{{Sequence or Delimited}} \
+            "{}",
+            "unexpected mbe::TokenTree::{Sequence or Delimited} \
              in follow set checker"
         ),
     }