Derived `DecidableEq` instance is not inlined

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
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Description

In the following example, the function f is measurably slower than f':

inductive Node where
  | done
  | epsilon (next : Nat)
  | split (next₁ : Nat) (next₂ : Nat)
deriving DecidableEq

@[inline, always_inline]
def Node.isDone (n : Node) : Bool :=
  match n with
  | .done => true
  | _ => false

set_option trace.Compiler.result true

def f (n : Node) : Nat :=
  -- This comparison was not inlined
  if n = .done then
    42
  else
    1

def f' (n : Node) : Nat :=
  -- This version seems faster in my actual example
  if n.isDone then
    42
  else
    1

Context

As reported on Zulip: #lean4 > Suboptimal codegen when comparing against a constant

Steps to Reproduce

Here is the above turned into a benchmark (adapted from the first comment below to prevent Lean from reusing the Node storage for the IO result):

import Lean.Meta.Basic

inductive Node where
  | done
  | char (c : Char) (next : Nat)
deriving DecidableEq

@[inline, always_inline]
def Node.isDone (n : Node) : Bool :=
  match n with
  | .done => true
  | _ => false

@[noinline]
def f (n : Node) : Nat :=
  -- This comparison was not inlined
  if n = .done then
    42
  else
    1

def fIO (n : Node) : IO Nat :=
  return f n

def n := 100000000

def loopF : IO Unit := do
  for _ in [0:n] do
    discard <| fIO <| .char 'a' 0

@[noinline]
def f' (n : Node) : Nat :=
  -- This version seems faster in my actual example
  if n.isDone then
    42
  else
    1

def f'IO (n : Node) : IO Nat :=
  return f' n

def loopF' : IO Unit := do
  for _ in [0:n] do
    discard <| f'IO <| .char 'a' 0

def main : IO Unit := do
  for _ in [0:5] do
    timeit "f" loopF
    timeit "f'" loopF'

Expected behavior: f and f' are both equally fast in all situations.

Actual behavior: On my machine, I get timing like this for compiled code:

f 1.36s
f' 1.21s
f 1.36s
f' 1.17s
f 1.34s
f' 1.2s
f 1.37s
f' 1.21s
f 1.38s
f' 1.22s

Versions

nightly-2025-08-07 on Linux

Impact

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Start with the minimal Node benchmark in the issue and enable trace.Compiler.result to compare generated code for f and f'. Investigate why the derived DecidableEq comparison is not inlined; done means compiled f and f' have equivalent performance in the reported cases.