Auto merge of #99918 - WaffleLapkin:fnFnfun, r=estebank

Recover wrong-cased keywords that start items

(_this pr was inspired by [this tweet](https://twitter.com/Azumanga/status/1552982326409367561)_)

r? `@estebank`

We've talked a bit about this recovery, but I just wanted to make sure that this is the right approach :)

For now I've only added the case insensitive recovery to `use`s, since most other items like `impl` blocks, modules, functions can start with multiple keywords which complicates the matter.

compiler/rustc_ast/src/lib.rs

@@ -29,6 +29,7 @@ extern crate rustc_macros;
 extern crate tracing;
 
 pub mod util {
+    pub mod case;
     pub mod classify;
     pub mod comments;
     pub mod literal;

compiler/rustc_ast/src/token.rs

@@ -5,6 +5,7 @@ pub use TokenKind::*;
 
 use crate::ast;
 use crate::ptr::P;
+use crate::util::case::Case;
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
@@ -615,6 +616,15 @@ impl Token {
         self.is_non_raw_ident_where(|id| id.name == kw)
     }
 
+    /// Returns `true` if the token is a given keyword, `kw` or if `case` is `Insensitive` and this token is an identifier equal to `kw` ignoring the case.
+    pub fn is_keyword_case(&self, kw: Symbol, case: Case) -> bool {
+        self.is_keyword(kw)
+            || (case == Case::Insensitive
+                && self.is_non_raw_ident_where(|id| {
+                    id.name.as_str().to_lowercase() == kw.as_str().to_lowercase()
+                }))
+    }
+
     pub fn is_path_segment_keyword(&self) -> bool {
         self.is_non_raw_ident_where(Ident::is_path_segment_keyword)
     }

compiler/rustc_ast/src/util/case.rs

@@ -0,0 +1,6 @@
+/// Whatever to ignore case (`fn` vs `Fn` vs `FN`) or not. Used for recovering.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum Case {
+    Sensitive,
+    Insensitive,
+}