diagnostics: mention the : token when struct fields fail to parse

compiler/rustc_parse/src/parser/expr.rs

@@ -3012,6 +3012,11 @@ impl<'a> Parser<'a> {
                 }
             };
 
+            let is_shorthand = parsed_field.as_ref().map_or(false, |f| f.ident.span == f.expr.span);
+            // A shorthand field can be turned into a full field with `:`.
+            // We should point this out.
+            self.check_or_expected(!is_shorthand, TokenType::Token(token::Colon));
+
             match self.expect_one_of(&[token::Comma], &[token::CloseDelim(close_delim)]) {
                 Ok(_) => {
                     if let Some(f) = parsed_field.or(recovery_field) {

src/test/ui/parser/issues/issue-52496.stderr

@@ -4,11 +4,11 @@ error: float literals must have an integer part
 LL |     let _ = Foo { bar: .5, baz: 42 };
    |                        ^^ help: must have an integer part: `0.5`
 
-error: expected one of `,` or `}`, found `.`
+error: expected one of `,`, `:`, or `}`, found `.`
   --> $DIR/issue-52496.rs:8:22
    |
 LL |     let _ = Foo { bar.into(), bat: -1, . };
-   |             ---      ^ expected one of `,` or `}`
+   |             ---      ^ expected one of `,`, `:`, or `}`
    |             |
    |             while parsing this struct
 

src/test/ui/parser/issues/issue-62973.stderr

@@ -20,11 +20,11 @@ LL |
 LL |
    |  ^
 
-error: expected one of `,` or `}`, found `{`
+error: expected one of `,`, `:`, or `}`, found `{`
   --> $DIR/issue-62973.rs:6:8
    |
 LL | fn p() { match s { v, E { [) {) }
-   |        ^       -       -^ expected one of `,` or `}`
+   |        ^       -       -^ expected one of `,`, `:`, or `}`
    |        |       |       |
    |        |       |       help: `}` may belong here
    |        |       while parsing this struct

src/test/ui/parser/removed-syntax-with-2.rs

@@ -6,6 +6,6 @@ fn main() {
 
     let a = S { foo: (), bar: () };
     let b = S { foo: (), with a };
-    //~^ ERROR expected one of `,` or `}`, found `a`
+    //~^ ERROR expected one of `,`, `:`, or `}`, found `a`
     //~| ERROR missing field `bar` in initializer of `S`
 }

src/test/ui/parser/removed-syntax-with-2.stderr

@@ -1,8 +1,8 @@
-error: expected one of `,` or `}`, found `a`
+error: expected one of `,`, `:`, or `}`, found `a`
   --> $DIR/removed-syntax-with-2.rs:8:31
    |
 LL |     let b = S { foo: (), with a };
-   |             -                 ^ expected one of `,` or `}`
+   |             -                 ^ expected one of `,`, `:`, or `}`
    |             |
    |             while parsing this struct