Add diagnostic when using public instead of pub

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -601,6 +601,17 @@ impl<'a> Parser<'a> {
         self.last_unexpected_token_span = Some(self.token.span);
         let mut err = self.struct_span_err(self.token.span, &msg_exp);
 
+        if let TokenKind::Ident(symbol, _) = &self.prev_token.kind {
+            if symbol.as_str() == "public" {
+                err.span_suggestion_short(
+                    self.prev_token.span,
+                    "write `pub` instead of `public` to make the item public",
+                    "pub",
+                    appl,
+                );
+            }
+        }
+
         // Add suggestion for a missing closing angle bracket if '>' is included in expected_tokens
         // there are unclosed angle brackets
         if self.unmatched_angle_bracket_count > 0

src/test/ui/parser/public-instead-of-pub.fixed

@@ -0,0 +1,8 @@
+// Checks what happens when `public` is used instead of the correct, `pub`
+// edition:2018
+// run-rustfix
+pub struct X;
+//~^ ERROR expected one of `!` or `::`, found keyword `struct`
+//~^^ HELP write `pub` instead of `public` to make the item public
+
+fn main() {}

src/test/ui/parser/public-instead-of-pub.rs

@@ -0,0 +1,8 @@
+// Checks what happens when `public` is used instead of the correct, `pub`
+// edition:2018
+// run-rustfix
+public struct X;
+//~^ ERROR expected one of `!` or `::`, found keyword `struct`
+//~^^ HELP write `pub` instead of `public` to make the item public
+
+fn main() {}

src/test/ui/parser/public-instead-of-pub.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found keyword `struct`
+  --> $DIR/public-instead-of-pub.rs:4:8
+   |
+LL | public struct X;
+   |        ^^^^^^ expected one of `!` or `::`
+   |
+help: write `pub` instead of `public` to make the item public
+   |
+LL | pub struct X;
+   | ~~~
+
+error: aborting due to previous error
+