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In !1869 (closed) if I add these rules in Unsafe/Coerce.hs:

{-# RULES
"map/unsafeCoerce" [1] map unsafeCoerce = unsafeCoerce
"amap/unsafeCoerce" amap unsafeCoerce = unsafeCoerce
-}

I get these warnings:

libraries/base/Unsafe/Coerce.hs:181:1: error: [-Winline-rule-shadowing, -Werror=inline-rule-shadowing]
    Rule "map/unsafeCoerce" may never fire
      because ‘unsafeCoerce’ might inline first
    Probable fix: add an INLINE[n] or NOINLINE[n] pragma for ‘unsafeCoerce’
    |
181 | "map/unsafeCoerce" [1] map unsafeCoerce = unsafeCoerce
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

libraries/base/Unsafe/Coerce.hs:183:1: error: [-Winline-rule-shadowing, -Werror=inline-rule-shadowing]
    Rule "amap/unsafeCoerce" may never fire
      because ‘unsafeCoerce’ might inline first
    Probable fix: add an INLINE[n] or NOINLINE[n] pragma for ‘unsafeCoerce’
    |
183 | "amap/unsafeCoerce" amap unsafeCoerce = unsafeCoerce
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This suggests adding an explicit phase in INLINE annotation of unsafeCoerce. However the same rules exist for coerce, and coerce is has an "always active" inline annotation:

coerceId :: Id
coerceId = pcMiscPrelId coerceName ty info
  where
    info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
    ...

(alwaysInlinePragma is what I get when I use INLINE without an explicit phase)

So it seems like there's a bug in this warning: it fires when I have {-# INLINE ... #-} but now when I mark an Id as the same thing in MkId no warnings generated.