Add SrcSpans of Vars in the compilation trace (#7299)

Author: zliu41

plutus-tx-plugin/changelog.d/20250831_170659_unsafeFixIO_var_srcspan.md

@@ -0,0 +1,5 @@
+### Added
+
+- During compilation, if a `Var` carries `SrcSpan`, the `SrcSpan` is added to the
+  compilation trace. The effect can be seen both in the full compilation trace
+  (via `dump-compilation-trace`), or in the error message when the compilation fails.

plutus-tx-plugin/src/PlutusTx/Compiler/Expr.hs

@@ -637,24 +637,30 @@ hoistExpr var t = do
   -- See Note [Dependency tracking]
   modifyCurDeps (Set.insert lexName)
   maybeDef <- PIR.lookupTerm lexName
-  let addSpan = case getVarSourceSpan var of
+  let varSpan = getVarSourceSpan var
+      addSpan = case varSpan of
         Nothing  -> id
         Just src -> fmap . fmap . addSrcSpan $ src ^. srcSpanIso
+      varSpanMsg = case varSpan of
+        Nothing  -> ""
+        Just src -> ", located at" GHC.<+> GHC.ppr src
   case maybeDef of
     Just term -> pure term
     -- See Note [Dependency tracking]
-    Nothing -> withCurDef lexName . traceCompilation 1 ("Compiling definition of:" GHC.<+> GHC.ppr var) $ do
-      var' <- compileVarFresh ann var
-      -- See Note [Occurrences of recursive names]
-      PIR.defineTerm
-        lexName
-        (PIR.Def var' (PIR.mkVar var', PIR.Strict))
-        mempty
-
-      t' <- maybeProfileRhs var var' =<< addSpan (compileExpr t)
-      -- See Note [Non-strict let-bindings]
-      PIR.modifyTermDef lexName (const $ PIR.Def var' (t', PIR.NonStrict))
-      pure $ PIR.mkVar var'
+    Nothing -> withCurDef lexName
+      . traceCompilation 1 ("Compiling definition of:" GHC.<+> GHC.ppr var GHC.<> varSpanMsg)
+      $ do
+        var' <- compileVarFresh ann var
+        -- See Note [Occurrences of recursive names]
+        PIR.defineTerm
+          lexName
+          (PIR.Def var' (PIR.mkVar var', PIR.Strict))
+          mempty
+
+        t' <- maybeProfileRhs var var' =<< addSpan (compileExpr t)
+        -- See Note [Non-strict let-bindings]
+        PIR.modifyTermDef lexName (const $ PIR.Def var' (t', PIR.NonStrict))
+        pure $ PIR.mkVar var'
 
 -- 'GHC.Var' in argument is only for extracting srcspan and accurate name.
 maybeProfileRhs