Ignoring `collapsible_match` is extremely confusing

[ZaneErebos]

This lint does not include the place at which you can apply an attribute to ignore the lint, which can leave users very confused.

Take the following code

match e.downcast::<RustlsError>() {
  Ok(rustls_err) => match rustls_err {
    RustlsError::InvalidMessage(e) => match e {
      // non https request is received
      InvalidMessage::InvalidContentType => (),

      _ => eprintln!(
        "Error during tls handshake from {addr}: {rustls_err:?}",
      ),
    },

    RustlsError::AlertReceived(e) => match e {
      // self-signed certs
      AlertDescription::CertificateUnknown => (),

      _ => eprintln!(
        "Error during tls handshake from {addr}: {rustls_err:?}",
      ),
    },

    _ => eprintln!(
      "Error during tls handshake from {addr}: {rustls_err:?}",
    ),
  },

  Err(e) => match e.kind() {
    // https://docs.rs/rustls/latest/rustls/manual/_03_howto/index.html#unexpected-eof
    ErrorKind::UnexpectedEof => (),

    _ => {
      eprintln!("Error during tls handshake from {addr}: {e:?}");
    }
  },
};

$ cargo clippy
warning: this `match` can be collapsed into the outer `match`
   --> server/src/main.rs:137:51
    |
137 |                   RustlsError::InvalidMessage(e) => match e {
    |  ___________________________________________________^
138 | |                   // non https request is received
139 | |                   InvalidMessage::InvalidContentType => (),
...   |
143 | |                   ),
144 | |                 },
    | |_________________^
    |
help: the outer pattern can be modified to include the inner pattern
   --> server/src/main.rs:137:45
    |
137 |                 RustlsError::InvalidMessage(e) => match e {
    |                                             ^ replace this binding
138 |                   // non https request is received
139 |                   InvalidMessage::InvalidContentType => (),
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match
    = note: `#[warn(clippy::collapsible_match)]` on by default

warning: this `match` can be collapsed into the outer `match`
   --> server/src/main.rs:146:50
    |
146 |                   RustlsError::AlertReceived(e) => match e {
    |  __________________________________________________^
147 | |                   // self-signed certs
148 | |                   AlertDescription::CertificateUnknown => (),
...   |
152 | |                   ),
153 | |                 },
    | |_________________^
    |
help: the outer pattern can be modified to include the inner pattern
   --> server/src/main.rs:146:44
    |
146 |                 RustlsError::AlertReceived(e) => match e {
    |                                            ^ replace this binding
147 |                   // self-signed certs
148 |                   AlertDescription::CertificateUnknown => (),
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match

Ok, whatever. I like my code the way it is, I will ignore the warnings.

match e.downcast::<RustlsError>() {
  Ok(rustls_err) => match rustls_err {
    RustlsError::InvalidMessage(e) => {
+      #[expect(clippy::collapsible_match)]
      match e {
        // non https request is received
        InvalidMessage::InvalidContentType => (),
...
    }

    RustlsError::AlertReceived(e) => {
+      #[expect(clippy::collapsible_match)]
      match e {
        // self-signed certs
        AlertDescription::CertificateUnknown => (),
...

(& formatting changes)

$ cargo clippy
warning: this `match` can be collapsed into the outer `match`
   --> server/src/main.rs:139:19
    |
139 | /                   match e {
140 | |                     // non https request is received
141 | |                     InvalidMessage::InvalidContentType => (),
...   |
145 | |                     ),
146 | |                   }
    | |___________________^
    |
help: the outer pattern can be modified to include the inner pattern
   --> server/src/main.rs:137:45
    |
137 |                 RustlsError::InvalidMessage(e) => {
    |                                             ^ replace this binding
...
141 |                     InvalidMessage::InvalidContentType => (),
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match
    = note: `#[warn(clippy::collapsible_match)]` on by default

warning: this `match` can be collapsed into the outer `match`
   --> server/src/main.rs:151:19
    |
151 | /                   match e {
152 | |                     // self-signed certs
153 | |                     AlertDescription::CertificateUnknown => (),
...   |
157 | |                     ),
158 | |                   }
    | |___________________^
    |
help: the outer pattern can be modified to include the inner pattern
   --> server/src/main.rs:149:44
    |
149 |                 RustlsError::AlertReceived(e) => {
    |                                            ^ replace this binding
...
153 |                     AlertDescription::CertificateUnknown => (),
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match

warning: this lint expectation is unfulfilled
   --> server/src/main.rs:138:28
    |
138 |                   #[expect(clippy::collapsible_match)]
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unfulfilled_lint_expectations)]` on by default

warning: this lint expectation is unfulfilled
   --> server/src/main.rs:150:28
    |
150 |                   #[expect(clippy::collapsible_match)]
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^

Very confused, I tried to undo my changes and reapply but the warnings still persisted. I then tried to apply the attributes to the e bindings but got syntax errors.

Now even more confused, I glanced back at the code and saw that the 2 branches looked very similar. Out of curiosity I added a character to one of the strings, and the warning for that block disappeared. Maybe clippy detected that I was doing the same thing as the parent match in both cases and thus issued the warnings? To test it, I added the same character at the same position in the string in the parent block (match rustls_err) and suddenly the warning switched from the block without the character to the one that did. I undid all changes and applied the attribute to the parent block, and the warnings disappeared.