Slightly improve error message for invalid identifer

[Rageking8]

Given the following code: link

fn main() {
    let 1x = 123;
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: invalid suffix `x` for number literal
 --> src/main.rs:2:9
  |
2 |     let 1x = 123;
  |         ^^ invalid suffix `x`
  |
  = help: the suffix must be one of the numeric types (`u32`, `isize`, `f32`, etc.)

error: could not compile `playground` due to previous error

Ideally the output should inform the user that an identifier cannot start with a digit and not that the proceeding characters is an invalid suffix (since a literal in that place is invalid in the first place). Thanks.

[chenyukang]

@rustbot claim

[chenyukang]

Which one is a more proper?

   --> $DIR/const-match-check.rs:19:26
    |
 LL |     const X: i32 = { let 0 = 0; 0 };
-   |                          ^ patterns `i32::MIN..=-1_i32` and `1_i32..=i32::MAX` not covered
+   |                          ^ identifiers cannot start with a number
+

[chenyukang]

And, this actually is a valid code

fn main() {
    let 3 = 1 + (match 1 { n => n }) else {
        return;
    };
}

We only report for identifer starts with digit and also contains some chars which is not digit, and it's not a valid number literal？