Make the capitalization explicit on keyword misspell error

compiler/rustc_errors/src/emitter.rs

@@ -3544,6 +3544,8 @@ pub fn detect_confusion_type(sm: &SourceMap, suggested: &str, sp: Span) -> Confu
         let mut has_digit_letter_confusable = false;
         let mut has_other_diff = false;
 
+        // Letters whose lowercase version is very similar to the uppercase
+        // version.
         let ascii_confusables = &['c', 'f', 'i', 'k', 'o', 's', 'u', 'v', 'w', 'x', 'y', 'z'];
 
         let digit_letter_confusables = [('0', 'O'), ('1', 'l'), ('5', 'S'), ('8', 'B'), ('9', 'g')];

compiler/rustc_parse/messages.ftl

@@ -512,7 +512,7 @@ parse_keyword_lifetime =
     lifetimes cannot use keyword names
 
 parse_kw_bad_case = keyword `{$kw}` is written in the wrong case
-    .suggestion = write it in the correct case
+    .suggestion = write it in {$case}
 
 parse_label_inner_attr_does_not_annotate_this = the inner attribute doesn't annotate this {$item}
 parse_label_unexpected_token = unexpected token

compiler/rustc_parse/src/errors.rs

@@ -1,14 +1,15 @@
 // ignore-tidy-filelength
 
 use std::borrow::Cow;
+use std::path::PathBuf;
 
 use rustc_ast::token::Token;
 use rustc_ast::util::parser::ExprPrecedence;
 use rustc_ast::{Path, Visibility};
 use rustc_errors::codes::*;
 use rustc_errors::{
-    Applicability, Diag, DiagCtxtHandle, Diagnostic, EmissionGuarantee, Level, Subdiagnostic,
-    SuggestionStyle,
+    Applicability, Diag, DiagArgValue, DiagCtxtHandle, Diagnostic, EmissionGuarantee, IntoDiagArg,
+    Level, Subdiagnostic, SuggestionStyle,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_session::errors::ExprParenthesesNeeded;
@@ -3335,6 +3336,24 @@ pub(crate) struct KwBadCase<'a> {
     #[suggestion(code = "{kw}", style = "verbose", applicability = "machine-applicable")]
     pub span: Span,
     pub kw: &'a str,
+    pub case: Case,
+}
+
+pub(crate) enum Case {
+    Upper,
+    Lower,
+    Mixed,
+}
+
+impl IntoDiagArg for Case {
+    fn into_diag_arg(self, path: &mut Option<PathBuf>) -> DiagArgValue {
+        match self {
+            Case::Upper => "uppercase",
+            Case::Lower => "lowercase",
+            Case::Mixed => "the correct case",
+        }
+        .into_diag_arg(path)
+    }
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/mod.rs

@@ -606,7 +606,20 @@ impl<'a> Parser<'a> {
             // Do an ASCII case-insensitive match, because all keywords are ASCII.
             && ident.as_str().eq_ignore_ascii_case(exp.kw.as_str())
         {
-            self.dcx().emit_err(errors::KwBadCase { span: ident.span, kw: exp.kw.as_str() });
+            let kw = exp.kw.as_str();
+            let is_upper = kw.chars().all(char::is_uppercase);
+            let is_lower = kw.chars().all(char::is_lowercase);
+
+            let case = match (is_upper, is_lower) {
+                (true, true) => {
+                    unreachable!("keyword that is both fully upper- and fully lowercase")
+                }
+                (true, false) => errors::Case::Upper,
+                (false, true) => errors::Case::Lower,
+                (false, false) => errors::Case::Mixed,
+            };
+
+            self.dcx().emit_err(errors::KwBadCase { span: ident.span, kw, case });
             self.bump();
             true
         } else {

tests/ui/parser/item-kw-case-mismatch.stderr

@@ -4,7 +4,7 @@ error: keyword `use` is written in the wrong case
 LL | Use std::ptr::read;
    | ^^^
    |
-help: write it in the correct case (notice the capitalization)
+help: write it in lowercase (notice the capitalization)
    |
 LL - Use std::ptr::read;
 LL + use std::ptr::read;
@@ -16,7 +16,7 @@ error: keyword `use` is written in the wrong case
 LL | USE std::ptr::write;
    | ^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - USE std::ptr::write;
 LL + use std::ptr::write;
@@ -28,7 +28,7 @@ error: keyword `fn` is written in the wrong case
 LL | async Fn _a() {}
    |       ^^
    |
-help: write it in the correct case (notice the capitalization)
+help: write it in lowercase (notice the capitalization)
    |
 LL - async Fn _a() {}
 LL + async fn _a() {}
@@ -40,7 +40,7 @@ error: keyword `fn` is written in the wrong case
 LL | Fn _b() {}
    | ^^
    |
-help: write it in the correct case (notice the capitalization)
+help: write it in lowercase (notice the capitalization)
    |
 LL - Fn _b() {}
 LL + fn _b() {}
@@ -52,7 +52,7 @@ error: keyword `async` is written in the wrong case
 LL | aSYNC fN _c() {}
    | ^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - aSYNC fN _c() {}
 LL + async fN _c() {}
@@ -64,7 +64,7 @@ error: keyword `fn` is written in the wrong case
 LL | aSYNC fN _c() {}
    |       ^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - aSYNC fN _c() {}
 LL + aSYNC fn _c() {}
@@ -76,7 +76,7 @@ error: keyword `async` is written in the wrong case
 LL | Async fn _d() {}
    | ^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - Async fn _d() {}
 LL + async fn _d() {}
@@ -88,7 +88,7 @@ error: keyword `const` is written in the wrong case
 LL | CONST UNSAFE FN _e() {}
    | ^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - CONST UNSAFE FN _e() {}
 LL + const UNSAFE FN _e() {}
@@ -100,7 +100,7 @@ error: keyword `unsafe` is written in the wrong case
 LL | CONST UNSAFE FN _e() {}
    |       ^^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - CONST UNSAFE FN _e() {}
 LL + CONST unsafe FN _e() {}
@@ -112,7 +112,7 @@ error: keyword `fn` is written in the wrong case
 LL | CONST UNSAFE FN _e() {}
    |              ^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - CONST UNSAFE FN _e() {}
 LL + CONST UNSAFE fn _e() {}
@@ -124,7 +124,7 @@ error: keyword `unsafe` is written in the wrong case
 LL | unSAFE EXTern "C" fn _f() {}
    | ^^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - unSAFE EXTern "C" fn _f() {}
 LL + unsafe EXTern "C" fn _f() {}
@@ -136,7 +136,7 @@ error: keyword `extern` is written in the wrong case
 LL | unSAFE EXTern "C" fn _f() {}
    |        ^^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - unSAFE EXTern "C" fn _f() {}
 LL + unSAFE extern "C" fn _f() {}
@@ -148,7 +148,7 @@ error: keyword `extern` is written in the wrong case
 LL | EXTERN "C" FN _g() {}
    | ^^^^^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - EXTERN "C" FN _g() {}
 LL + extern "C" FN _g() {}
@@ -160,7 +160,7 @@ error: keyword `fn` is written in the wrong case
 LL | EXTERN "C" FN _g() {}
    |            ^^
    |
-help: write it in the correct case
+help: write it in lowercase
    |
 LL - EXTERN "C" FN _g() {}
 LL + EXTERN "C" fn _g() {}