Removing a private ambiguous import can break downstream crates also relying on ambiguous imports

I tried this code:

I fixed up an ambiguous (private) import in rustc_ast:
https://github.com/rust-lang/rust/compare/main...mejrs:rust:globby

I expected to see this happen: rustc_ast compiles, so I expected the compiler to compile.

rustc_ast_pretty now fails to compile:

 error[E0433]: cannot find module `Ident` in this scope                                                                                                                                                                                                                                                    
   --> compiler\rustc_ast_pretty\src\pprust\state.rs:367:17
    |
367 |             if !Ident::new(*sym, *span).is_reserved() || matches!(is_raw, IdentIsRaw::Yes) =>
    |                 ^^^^^ `Ident` is a variant, not a module

error[E0433]: cannot find module `Ident` in this scope
   --> compiler\rustc_ast_pretty\src\pprust\state.rs:375:17
    |
375 |             if !Ident::new(*sym, *span).is_reserved()
    |                 ^^^^^ `Ident` is a variant, not a module

For more information about this error, try `rustc --explain E0433`. 

Code:

// allowed, no diagnostic emitted
use rustc_span::Ident;

fn space_between...
         use token::*; // also exports an `Ident`

          match .. {
                  // IDENT + `!`: `println!()`, but `if !x { ... }` needs a space after the `if`
                  (Tok(Token { kind: Ident(sym, is_raw), span }, _), Tok(Token { kind: Bang, .. }, _))
                      if !Ident::new(*sym, *span).is_reserved() || matches!(is_raw, IdentIsRaw::Yes) =>
                  {
                      false
                  }

once fixed, it later breaks rustc_expand:

error[E0559]: variant `rustc_ast::token::TokenKind::Ident` has no field named `sym`                                                                                                                                                                                                                       
   --> compiler\rustc_expand\src\proc_macro_server.rs:235:21
    |
235 |                     sym,
    |                     ^^^ field does not exist
    |
   ::: compiler\rustc_ast\src\token.rs:502:5
    |
502 |     Ident(Symbol, IdentIsRaw),
    |     ----- `rustc_ast::token::TokenKind::Ident` defined here

Notably, rustc_ast_pretty doesn't allow any lints. This should at least emit a (fcw) lint.

Meta

rustc --version --verbose:

current main

<backtrace>

Start by reproducing the ambiguous-import change across rustc_ast, rustc_ast_pretty, and rustc_expand. Read compiler/rustc_ast_pretty/src/pprust/state.rs and compiler/rustc_expand/src/proc_macro_server.rs, then trace the relevant import and name-resolution diagnostics. Done means the downstream crates no longer fail silently after the private ambiguous import is changed, and an appropriate lint is emitted.