Spurious irrefutable_let_patterns warning

[obsgolem]

Code

use pubgrub::{OfflineDependencyProvider, PubGrubError, Range};

fn f() -> PubGrubError<OfflineDependencyProvider<String, Range<u32>>> {
    todo!();
}

fn main() {
    if let pubgrub::PubGrubError::NoSolution(der) = f() {
    } else {
    }
}

requires

pubgrub = { git = "https://github.com/pubgrub-rs/pubgrub.git" }

Current output

Compiling equivalent v1.0.2
   Compiling hashbrown v0.15.3
   Compiling smallvec v1.15.0
   Compiling rustc-hash v2.1.1
   Compiling log v0.4.27
   Compiling thiserror v2.0.12
   Compiling version-ranges v0.1.1 (https://github.com/pubgrub-rs/pubgrub.git#b90cd72e)
   Compiling indexmap v2.9.0
   Compiling priority-queue v2.3.1
   Compiling pubgrub v0.3.0 (https://github.com/pubgrub-rs/pubgrub.git#b90cd72e)
   Compiling tmp v0.1.0 (C:\Users\jbills\Document\Code\tmp)
warning: irrefutable `if let` pattern
 --> src\main.rs:8:8
  |
8 |     if let pubgrub::PubGrubError::NoSolution(der) = f() {
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this pattern will always match, so the `if let` is useless
  = help: consider replacing the `if let` with a `let`
  = note: `#[warn(irrefutable_let_patterns)]` on by default

warning: unused variable: `der`
 --> src\main.rs:8:46
  |
8 |     if let pubgrub::PubGrubError::NoSolution(der) = f() {
  |                                              ^^^ help: if this is intentional, prefix it with an underscore: `_der`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: `tmp` (bin "tmp") generated 2 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.63s

Desired output

No warning

Rationale and extra context

Distinct from #139369. No let chains in sight. The relevant enum has multiple cases, and the compiler correctly diagnoses an error if I remove the if.

Other cases

Rust Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-pc-windows-msvc
release: 1.87.0
LLVM version: 20.1.1

Also tested on 
rustc 1.89.0-nightly (bc8215286 2025-05-20)
binary: rustc
commit-hash: bc821528634632b4ff8dee5ac1ea4ad90d1b3eb5
commit-date: 2025-05-20
host: x86_64-pc-windows-msvc
release: 1.89.0-nightly
LLVM version: 20.1.5

Anything else?

No response

[compiler-errors]

The relevant enum has multiple cases, and the compiler correctly diagnoses an error if I remove the if.

This code works fine for me?

use pubgrub::{OfflineDependencyProvider, PubGrubError, Range};

fn f() -> PubGrubError<OfflineDependencyProvider<String, Range<u32>>> {
    todo!();
}

fn main() {
    let pubgrub::PubGrubError::NoSolution(der) = f();
}

The pattern is really irrefutable, since <OfflineDependencyProvider<..> as DependencyProvider>::Error = Infallible, so all the other variants of PubGrubError are impossible to construct.

[obsgolem]

Ah, I see, I am getting an error from RA, not from the compiler.