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.

10 files changed, 286 insertions, 42 deletions

diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index eeb7e56e2b1..9c1dfeb1a61 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/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;
diff --git a/compiler/rustc_ast/src/token.rs b/compiler/rustc_ast/src/token.rs
index 83b10d906e2..f6aac0b55f1 100644
--- a/compiler/rustc_ast/src/token.rs
+++ b/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)
     }
diff --git a/compiler/rustc_ast/src/util/case.rs b/compiler/rustc_ast/src/util/case.rs
new file mode 100644
index 00000000000..1afd7dea740
--- /dev/null
+++ b/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,
+}
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index da2d20e47ee..7355730c9eb 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -33,6 +33,7 @@ use core::mem;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::Spacing;
+use rustc_ast::util::case::Case;
 use rustc_ast::util::classify;
 use rustc_ast::util::literal::LitError;
 use rustc_ast::util::parser::{prec_let_scrutinee_needs_par, AssocOp, Fixity};
@@ -2090,7 +2091,7 @@ impl<'a> Parser<'a> {
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };
 
         let asyncness = if self.token.uninterpolated_span().rust_2018() {
-            self.parse_asyncness()
+            self.parse_asyncness(Case::Sensitive)
         } else {
             Async::No
         };
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index d657a289117..494f0cf56a8 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -8,6 +8,7 @@ use rustc_ast::ast::*;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, TokenKind};
 use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
+use rustc_ast::util::case::Case;
 use rustc_ast::{self as ast, AttrVec, Attribute, DUMMY_NODE_ID};
 use rustc_ast::{Async, Const, Defaultness, IsAuto, Mutability, Unsafe, UseTree, UseTreeKind};
 use rustc_ast::{BindingAnnotation, Block, FnDecl, FnSig, Param, SelfKind};
@@ -34,7 +35,7 @@ impl<'a> Parser<'a> {
 
     /// Parses a `mod <foo> { ... }` or `mod <foo>;` item.
     fn parse_item_mod(&mut self, attrs: &mut AttrVec) -> PResult<'a, ItemInfo> {
-        let unsafety = self.parse_unsafety();
+        let unsafety = self.parse_unsafety(Case::Sensitive);
         self.expect_keyword(kw::Mod)?;
         let id = self.parse_ident()?;
         let mod_kind = if self.eat(&token::Semi) {
@@ -143,8 +144,15 @@ impl<'a> Parser<'a> {
         let lo = self.token.span;
         let vis = self.parse_visibility(FollowedByType::No)?;
         let mut def = self.parse_defaultness();
-        let kind =
-            self.parse_item_kind(&mut attrs, mac_allowed, lo, &vis, &mut def, fn_parse_mode)?;
+        let kind = self.parse_item_kind(
+            &mut attrs,
+            mac_allowed,
+            lo,
+            &vis,
+            &mut def,
+            fn_parse_mode,
+            Case::Sensitive,
+        )?;
         if let Some((ident, kind)) = kind {
             self.error_on_unconsumed_default(def, &kind);
             let span = lo.to(self.prev_token.span);
@@ -205,16 +213,17 @@ impl<'a> Parser<'a> {
         vis: &Visibility,
         def: &mut Defaultness,
         fn_parse_mode: FnParseMode,
+        case: Case,
     ) -> PResult<'a, Option<ItemInfo>> {
         let def_final = def == &Defaultness::Final;
-        let mut def = || mem::replace(def, Defaultness::Final);
+        let mut def_ = || mem::replace(def, Defaultness::Final);
 
-        let info = if self.eat_keyword(kw::Use) {
+        let info = if self.eat_keyword_case(kw::Use, case) {
             self.parse_use_item()?
-        } else if self.check_fn_front_matter(def_final) {
+        } else if self.check_fn_front_matter(def_final, case) {
             // FUNCTION ITEM
             let (ident, sig, generics, body) = self.parse_fn(attrs, fn_parse_mode, lo, vis)?;
-            (ident, ItemKind::Fn(Box::new(Fn { defaultness: def(), sig, generics, body })))
+            (ident, ItemKind::Fn(Box::new(Fn { defaultness: def_(), sig, generics, body })))
         } else if self.eat_keyword(kw::Extern) {
             if self.eat_keyword(kw::Crate) {
                 // EXTERN CRATE
@@ -225,7 +234,7 @@ impl<'a> Parser<'a> {
             }
         } else if self.is_unsafe_foreign_mod() {
             // EXTERN BLOCK
-            let unsafety = self.parse_unsafety();
+            let unsafety = self.parse_unsafety(Case::Sensitive);
             self.expect_keyword(kw::Extern)?;
             self.parse_item_foreign_mod(attrs, unsafety)?
         } else if self.is_static_global() {
@@ -234,15 +243,15 @@ impl<'a> Parser<'a> {
             let m = self.parse_mutability();
             let (ident, ty, expr) = self.parse_item_global(Some(m))?;
             (ident, ItemKind::Static(ty, m, expr))
-        } else if let Const::Yes(const_span) = self.parse_constness() {
+        } else if let Const::Yes(const_span) = self.parse_constness(Case::Sensitive) {
             // CONST ITEM
             if self.token.is_keyword(kw::Impl) {
                 // recover from `const impl`, suggest `impl const`
-                self.recover_const_impl(const_span, attrs, def())?
+                self.recover_const_impl(const_span, attrs, def_())?
             } else {
                 self.recover_const_mut(const_span);
                 let (ident, ty, expr) = self.parse_item_global(None)?;
-                (ident, ItemKind::Const(def(), ty, expr))
+                (ident, ItemKind::Const(def_(), ty, expr))
             }
         } else if self.check_keyword(kw::Trait) || self.check_auto_or_unsafe_trait_item() {
             // TRAIT ITEM
@@ -251,7 +260,7 @@ impl<'a> Parser<'a> {
             || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Impl])
         {
             // IMPL ITEM
-            self.parse_item_impl(attrs, def())?
+            self.parse_item_impl(attrs, def_())?
         } else if self.check_keyword(kw::Mod)
             || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Mod])
         {
@@ -259,7 +268,7 @@ impl<'a> Parser<'a> {
             self.parse_item_mod(attrs)?
         } else if self.eat_keyword(kw::Type) {
             // TYPE ITEM
-            self.parse_type_alias(def())?
+            self.parse_type_alias(def_())?
         } else if self.eat_keyword(kw::Enum) {
             // ENUM ITEM
             self.parse_item_enum()?
@@ -286,6 +295,19 @@ impl<'a> Parser<'a> {
         } else if self.isnt_macro_invocation() && vis.kind.is_pub() {
             self.recover_missing_kw_before_item()?;
             return Ok(None);
+        } else if self.isnt_macro_invocation() && case == Case::Sensitive {
+            _ = def_;
+
+            // Recover wrong cased keywords
+            return self.parse_item_kind(
+                attrs,
+                macros_allowed,
+                lo,
+                vis,
+                def,
+                fn_parse_mode,
+                Case::Insensitive,
+            );
         } else if macros_allowed && self.check_path() {
             // MACRO INVOCATION ITEM
             (Ident::empty(), ItemKind::MacCall(P(self.parse_item_macro(vis)?)))
@@ -538,7 +560,7 @@ impl<'a> Parser<'a> {
         attrs: &mut AttrVec,
         defaultness: Defaultness,
     ) -> PResult<'a, ItemInfo> {
-        let unsafety = self.parse_unsafety();
+        let unsafety = self.parse_unsafety(Case::Sensitive);
         self.expect_keyword(kw::Impl)?;
 
         // First, parse generic parameters if necessary.
@@ -552,7 +574,7 @@ impl<'a> Parser<'a> {
             generics
         };
 
-        let constness = self.parse_constness();
+        let constness = self.parse_constness(Case::Sensitive);
         if let Const::Yes(span) = constness {
             self.sess.gated_spans.gate(sym::const_trait_impl, span);
         }
@@ -796,7 +818,7 @@ impl<'a> Parser<'a> {
 
     /// Parses `unsafe? auto? trait Foo { ... }` or `trait Foo = Bar;`.
     fn parse_item_trait(&mut self, attrs: &mut AttrVec, lo: Span) -> PResult<'a, ItemInfo> {
-        let unsafety = self.parse_unsafety();
+        let unsafety = self.parse_unsafety(Case::Sensitive);
         // Parse optional `auto` prefix.
         let is_auto = if self.eat_keyword(kw::Auto) { IsAuto::Yes } else { IsAuto::No };
 
@@ -1762,7 +1784,7 @@ impl<'a> Parser<'a> {
         let (ident, is_raw) = self.ident_or_err()?;
         if !is_raw && ident.is_reserved() {
             let snapshot = self.create_snapshot_for_diagnostic();
-            let err = if self.check_fn_front_matter(false) {
+            let err = if self.check_fn_front_matter(false, Case::Sensitive) {
                 let inherited_vis = Visibility {
                     span: rustc_span::DUMMY_SP,
                     kind: VisibilityKind::Inherited,
@@ -2172,7 +2194,7 @@ impl<'a> Parser<'a> {
     ///
     /// `check_pub` adds additional `pub` to the checks in case users place it
     /// wrongly, can be used to ensure `pub` never comes after `default`.
-    pub(super) fn check_fn_front_matter(&mut self, check_pub: bool) -> bool {
+    pub(super) fn check_fn_front_matter(&mut self, check_pub: bool, case: Case) -> bool {
         // We use an over-approximation here.
         // `const const`, `fn const` won't parse, but we're not stepping over other syntax either.
         // `pub` is added in case users got confused with the ordering like `async pub fn`,
@@ -2182,23 +2204,30 @@ impl<'a> Parser<'a> {
         } else {
             &[kw::Const, kw::Async, kw::Unsafe, kw::Extern]
         };
-        self.check_keyword(kw::Fn) // Definitely an `fn`.
+        self.check_keyword_case(kw::Fn, case) // Definitely an `fn`.
             // `$qual fn` or `$qual $qual`:
-            || quals.iter().any(|&kw| self.check_keyword(kw))
+            || quals.iter().any(|&kw| self.check_keyword_case(kw, case))
                 && self.look_ahead(1, |t| {
                     // `$qual fn`, e.g. `const fn` or `async fn`.
-                    t.is_keyword(kw::Fn)
+                    t.is_keyword_case(kw::Fn, case)
                     // Two qualifiers `$qual $qual` is enough, e.g. `async unsafe`.
-                    || t.is_non_raw_ident_where(|i| quals.contains(&i.name)
-                        // Rule out 2015 `const async: T = val`.
-                        && i.is_reserved()
+                    || (
+                        (
+                            t.is_non_raw_ident_where(|i|
+                                quals.contains(&i.name)
+                                    // Rule out 2015 `const async: T = val`.
+                                    && i.is_reserved()
+                            )
+                            || case == Case::Insensitive
+                                && t.is_non_raw_ident_where(|i| quals.iter().any(|qual| qual.as_str() == i.name.as_str().to_lowercase()))
+                        )
                         // Rule out unsafe extern block.
                         && !self.is_unsafe_foreign_mod())
                 })
             // `extern ABI fn`
-            || self.check_keyword(kw::Extern)
+            || self.check_keyword_case(kw::Extern, case)
                 && self.look_ahead(1, |t| t.can_begin_literal_maybe_minus())
-                && self.look_ahead(2, |t| t.is_keyword(kw::Fn))
+                && self.look_ahead(2, |t| t.is_keyword_case(kw::Fn, case))
     }
 
     /// Parses all the "front matter" (or "qualifiers") for a `fn` declaration,
@@ -2214,22 +2243,22 @@ impl<'a> Parser<'a> {
     /// `Visibility::Inherited` when no visibility is known.
     pub(super) fn parse_fn_front_matter(&mut self, orig_vis: &Visibility) -> PResult<'a, FnHeader> {
         let sp_start = self.token.span;
-        let constness = self.parse_constness();
+        let constness = self.parse_constness(Case::Insensitive);
 
         let async_start_sp = self.token.span;
-        let asyncness = self.parse_asyncness();
+        let asyncness = self.parse_asyncness(Case::Insensitive);
 
         let unsafe_start_sp = self.token.span;
-        let unsafety = self.parse_unsafety();
+        let unsafety = self.parse_unsafety(Case::Insensitive);
 
         let ext_start_sp = self.token.span;
-        let ext = self.parse_extern();
+        let ext = self.parse_extern(Case::Insensitive);
 
         if let Async::Yes { span, .. } = asyncness {
             self.ban_async_in_2015(span);
         }
 
-        if !self.eat_keyword(kw::Fn) {
+        if !self.eat_keyword_case(kw::Fn, Case::Insensitive) {
             // It is possible for `expect_one_of` to recover given the contents of
             // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
             // account for this.
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index 2e59c005e31..14dc490fb02 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -22,6 +22,7 @@ use rustc_ast::token::{self, Delimiter, Nonterminal, Token, TokenKind};
 use rustc_ast::tokenstream::AttributesData;
 use rustc_ast::tokenstream::{self, DelimSpan, Spacing};
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::util::case::Case;
 use rustc_ast::AttrId;
 use rustc_ast::DUMMY_NODE_ID;
 use rustc_ast::{self as ast, AnonConst, AttrStyle, AttrVec, Const, Extern};
@@ -636,6 +637,20 @@ impl<'a> Parser<'a> {
         self.token.is_keyword(kw)
     }
 
+    fn check_keyword_case(&mut self, kw: Symbol, case: Case) -> bool {
+        if self.check_keyword(kw) {
+            return true;
+        }
+
+        if case == Case::Insensitive
+        && let Some((ident, /* is_raw */ false)) = self.token.ident()
+        && ident.as_str().to_lowercase() == kw.as_str().to_lowercase() {
+            true
+        } else {
+            false
+        }
+    }
+
     /// If the next token is the given keyword, eats it and returns `true`.
     /// Otherwise, returns `false`. An expectation is also added for diagnostics purposes.
     // Public for rustfmt usage.
@@ -648,6 +663,33 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Eats a keyword, optionally ignoring the case.
+    /// If the case differs (and is ignored) an error is issued.
+    /// This is useful for recovery.
+    fn eat_keyword_case(&mut self, kw: Symbol, case: Case) -> bool {
+        if self.eat_keyword(kw) {
+            return true;
+        }
+
+        if case == Case::Insensitive
+        && let Some((ident, /* is_raw */ false)) = self.token.ident()
+        && ident.as_str().to_lowercase() == kw.as_str().to_lowercase() {
+            self
+                .struct_span_err(ident.span, format!("keyword `{kw}` is written in a wrong case"))
+                .span_suggestion(
+                    ident.span,
+                    "write it in the correct case",
+                    kw,
+                    Applicability::MachineApplicable
+                ).emit();
+
+            self.bump();
+            return true;
+        }
+
+        false
+    }
+
     fn eat_keyword_noexpect(&mut self, kw: Symbol) -> bool {
         if self.token.is_keyword(kw) {
             self.bump();
@@ -1127,8 +1169,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses asyncness: `async` or nothing.
-    fn parse_asyncness(&mut self) -> Async {
-        if self.eat_keyword(kw::Async) {
+    fn parse_asyncness(&mut self, case: Case) -> Async {
+        if self.eat_keyword_case(kw::Async, case) {
             let span = self.prev_token.uninterpolated_span();
             Async::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
         } else {
@@ -1137,8 +1179,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses unsafety: `unsafe` or nothing.
-    fn parse_unsafety(&mut self) -> Unsafe {
-        if self.eat_keyword(kw::Unsafe) {
+    fn parse_unsafety(&mut self, case: Case) -> Unsafe {
+        if self.eat_keyword_case(kw::Unsafe, case) {
             Unsafe::Yes(self.prev_token.uninterpolated_span())
         } else {
             Unsafe::No
@@ -1146,10 +1188,10 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses constness: `const` or nothing.
-    fn parse_constness(&mut self) -> Const {
+    fn parse_constness(&mut self, case: Case) -> Const {
         // Avoid const blocks to be parsed as const items
         if self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
-            && self.eat_keyword(kw::Const)
+            && self.eat_keyword_case(kw::Const, case)
         {
             Const::Yes(self.prev_token.uninterpolated_span())
         } else {
@@ -1404,8 +1446,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `extern string_literal?`.
-    fn parse_extern(&mut self) -> Extern {
-        if self.eat_keyword(kw::Extern) {
+    fn parse_extern(&mut self, case: Case) -> Extern {
+        if self.eat_keyword_case(kw::Extern, case) {
             let mut extern_span = self.prev_token.span;
             let abi = self.parse_abi();
             if let Some(abi) = abi {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 3a67c032b3b..4d78c5bd0e2 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -4,6 +4,7 @@ use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
+use rustc_ast::util::case::Case;
 use rustc_ast::{
     self as ast, BareFnTy, FnRetTy, GenericBound, GenericBounds, GenericParam, Generics, Lifetime,
     MacCall, MutTy, Mutability, PolyTraitRef, TraitBoundModifier, TraitObjectSyntax, Ty, TyKind,
@@ -267,7 +268,7 @@ impl<'a> Parser<'a> {
         } else if self.eat_keyword(kw::Underscore) {
             // A type to be inferred `_`
             TyKind::Infer
-        } else if self.check_fn_front_matter(false) {
+        } else if self.check_fn_front_matter(false, Case::Sensitive) {
             // Function pointer type
             self.parse_ty_bare_fn(lo, Vec::new(), recover_return_sign)?
         } else if self.check_keyword(kw::For) {
@@ -275,7 +276,7 @@ impl<'a> Parser<'a> {
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
             let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
-            if self.check_fn_front_matter(false) {
+            if self.check_fn_front_matter(false, Case::Sensitive) {
                 self.parse_ty_bare_fn(lo, lifetime_defs, recover_return_sign)?
             } else {
                 let path = self.parse_path(PathStyle::Type)?;
diff --git a/src/test/ui/parser/item-kw-case-mismatch.fixed b/src/test/ui/parser/item-kw-case-mismatch.fixed
new file mode 100644
index 00000000000..1794268f260
--- /dev/null
+++ b/src/test/ui/parser/item-kw-case-mismatch.fixed
@@ -0,0 +1,34 @@
+// run-rustfix
+// edition:2018
+#![allow(unused_imports)]
+
+fn main() {}
+
+use std::ptr::read;  //~ ERROR keyword `use` is written in a wrong case
+use std::ptr::write; //~ ERROR keyword `use` is written in a wrong case
+
+async fn _a() {}
+//~^ ERROR keyword `fn` is written in a wrong case
+
+fn _b() {}
+//~^ ERROR keyword `fn` is written in a wrong case
+
+async fn _c() {}
+//~^ ERROR keyword `async` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
+
+async fn _d() {}
+//~^ ERROR keyword `async` is written in a wrong case
+
+const unsafe fn _e() {}
+//~^ ERROR keyword `const` is written in a wrong case
+//~| ERROR keyword `unsafe` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
+
+unsafe extern fn _f() {}
+//~^ ERROR keyword `unsafe` is written in a wrong case
+//~| ERROR keyword `extern` is written in a wrong case
+
+extern "C" fn _g() {}
+//~^ ERROR keyword `extern` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
diff --git a/src/test/ui/parser/item-kw-case-mismatch.rs b/src/test/ui/parser/item-kw-case-mismatch.rs
new file mode 100644
index 00000000000..ac8390efdb9
--- /dev/null
+++ b/src/test/ui/parser/item-kw-case-mismatch.rs
@@ -0,0 +1,34 @@
+// run-rustfix
+// edition:2018
+#![allow(unused_imports)]
+
+fn main() {}
+
+Use std::ptr::read;  //~ ERROR keyword `use` is written in a wrong case
+USE std::ptr::write; //~ ERROR keyword `use` is written in a wrong case
+
+async Fn _a() {}
+//~^ ERROR keyword `fn` is written in a wrong case
+
+Fn _b() {}
+//~^ ERROR keyword `fn` is written in a wrong case
+
+aSYNC fN _c() {}
+//~^ ERROR keyword `async` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
+
+Async fn _d() {}
+//~^ ERROR keyword `async` is written in a wrong case
+
+CONST UNSAFE FN _e() {}
+//~^ ERROR keyword `const` is written in a wrong case
+//~| ERROR keyword `unsafe` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
+
+unSAFE EXTern fn _f() {}
+//~^ ERROR keyword `unsafe` is written in a wrong case
+//~| ERROR keyword `extern` is written in a wrong case
+
+EXTERN "C" FN _g() {}
+//~^ ERROR keyword `extern` is written in a wrong case
+//~| ERROR keyword `fn` is written in a wrong case
diff --git a/src/test/ui/parser/item-kw-case-mismatch.stderr b/src/test/ui/parser/item-kw-case-mismatch.stderr
new file mode 100644
index 00000000000..e66dae825f9
--- /dev/null
+++ b/src/test/ui/parser/item-kw-case-mismatch.stderr
@@ -0,0 +1,86 @@
+error: keyword `use` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:7:1
+   |
+LL | Use std::ptr::read;
+   | ^^^ help: write it in the correct case (notice the capitalization): `use`
+
+error: keyword `use` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:8:1
+   |
+LL | USE std::ptr::write;
+   | ^^^ help: write it in the correct case: `use`
+
+error: keyword `fn` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:10:7
+   |
+LL | async Fn _a() {}
+   |       ^^ help: write it in the correct case (notice the capitalization): `fn`
+
+error: keyword `fn` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:13:1
+   |
+LL | Fn _b() {}
+   | ^^ help: write it in the correct case (notice the capitalization): `fn`
+
+error: keyword `async` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:16:1
+   |
+LL | aSYNC fN _c() {}
+   | ^^^^^ help: write it in the correct case: `async`
+
+error: keyword `fn` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:16:7
+   |
+LL | aSYNC fN _c() {}
+   |       ^^ help: write it in the correct case: `fn`
+
+error: keyword `async` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:20:1
+   |
+LL | Async fn _d() {}
+   | ^^^^^ help: write it in the correct case: `async`
+
+error: keyword `const` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:23:1
+   |
+LL | CONST UNSAFE FN _e() {}
+   | ^^^^^ help: write it in the correct case: `const`
+
+error: keyword `unsafe` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:23:7
+   |
+LL | CONST UNSAFE FN _e() {}
+   |       ^^^^^^ help: write it in the correct case: `unsafe`
+
+error: keyword `fn` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:23:14
+   |
+LL | CONST UNSAFE FN _e() {}
+   |              ^^ help: write it in the correct case: `fn`
+
+error: keyword `unsafe` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:28:1
+   |
+LL | unSAFE EXTern fn _f() {}
+   | ^^^^^^ help: write it in the correct case: `unsafe`
+
+error: keyword `extern` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:28:8
+   |
+LL | unSAFE EXTern fn _f() {}
+   |        ^^^^^^ help: write it in the correct case: `extern`
+
+error: keyword `extern` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:32:1
+   |
+LL | EXTERN "C" FN _g() {}
+   | ^^^^^^ help: write it in the correct case: `extern`
+
+error: keyword `fn` is written in a wrong case
+  --> $DIR/item-kw-case-mismatch.rs:32:12
+   |
+LL | EXTERN "C" FN _g() {}
+   |            ^^ help: write it in the correct case: `fn`
+
+error: aborting due to 14 previous errors
+