Small fix for incremental compilation (#1930)

* Small fix for incremental compilation

In incremental mode we need to add the arity of a compiled definition to
the hash, because if that changes, we need to rebuild the dependencies
too to make sure the arity is correct at the call site

* Fix indentation in CHANGELOG

CHANGELOG.md

@@ -55,6 +55,9 @@ filter p (x :: xs) with (p x)
   and reverts to whole program compilation. Incremental compilation is currently
   supported only by the Chez Scheme back end.
   This is currently supported only on Unix-like platforms (not yet Windows)
+  - Note that you must set `IDRIS2_INC_CGS` when building and installing
+    all libraries you plan to link with an incremental build.
+  - Note also that this is experimental and not yet well tested!
 * The type checker now tries a lot harder to avoid reducing expressions where
   it is not needed. This gives a huge performance improvement in programs
   that potentially do a lot of compile time evaluation. However, sometimes

src/Compiler/Inline.idr

@@ -8,6 +8,7 @@ import Core.CompileExpr
 import Core.Context
 import Core.Context.Log
 import Core.FC
+import Core.Hash
 import Core.Options
 import Core.TT
 
@@ -514,6 +515,16 @@ mergeLamDef n
                              | Nothing => pure ()
                     setCompiled n !(mergeLam cexpr)
 
+export
+addArityHash : {auto c : Ref Ctxt Defs} ->
+               Name -> Core ()
+addArityHash n
+    = do defs <- get Ctxt
+         Just def <- lookupCtxtExact n (gamma defs) | Nothing => pure ()
+         let Just cexpr =  compexpr def             | Nothing => pure ()
+         let MkFun args _ = cexpr                   | _ => pure ()
+         addHash (n, length args)
+
 export
 compileAndInlineAll : {auto c : Ref Ctxt Defs} ->
                       Core ()
@@ -528,6 +539,11 @@ compileAndInlineAll
                          -- This seems to be the point where not much useful
                          -- happens any more.
          traverse_ updateCallGraph cns
+         -- in incremental mode, add the arity of the definitions to the hash,
+         -- because if these change we need to recompile dependencies
+         -- accordingly
+         when (not (isNil (incrementalCGs !getSession))) $
+           traverse_ addArityHash cns
   where
     transform : Nat -> List Name -> Core ()
     transform Z cns = pure ()