fix: account for namespaces/open decls in inlineExpr (#9108)

This PR fixes an issue that may have caused inline expressions in
messages to be unnecessarily rendered on a separate line.

src/Lean/Message.lean

@@ -618,7 +618,9 @@ actually rendered. Consider using this function in lazy message data to avoid un
 computation for messages that are not displayed.
 -/
 private def MessageData.formatLength (ctx : PPContext) (msg : MessageData) : BaseIO Nat := do
-  let { env, mctx, lctx, opts, ..} := ctx
+  let { env, mctx, lctx, opts, currNamespace, openDecls } := ctx
+  -- Simulate the naming context that will be added to the actual message
+  let msg := MessageData.withNamingContext { currNamespace, openDecls } msg
   let fmt ← msg.format (some { env, mctx, lctx, opts })
   return fmt.pretty.length
 

tests/lean/run/inlineExpr.lean

@@ -16,23 +16,48 @@ def runTest (e : Expr) : MetaM Unit := do
   let msg := inlineExpr e (maxInlineLength := 30)
   logInfo m!"Before{msg}After"
 
-def testShort : MetaM Unit := do
-  runTest <| .app (.const ``shortFun []) (.const ``shortConst [])
-
 /-- info: Before `shortFun shortConst` After -/
 #guard_msgs in
-#eval testShort
+#eval runTest <| .app (.const ``shortFun []) (.const ``shortConst [])
 
 opaque functionWithLongName : Nat → Nat
 opaque constantWithLongName : Nat
 
-def testLong : MetaM Unit := do
-  runTest <| .app (.const ``functionWithLongName []) (.const ``constantWithLongName [])
-
 /--
 info: Before
   functionWithLongName constantWithLongName
 After
 -/
 #guard_msgs in
-#eval testLong
+#eval runTest <| .app (.const ``functionWithLongName []) (.const ``constantWithLongName [])
+
+/-! Ensure that length computation accounts for namespace occlusion: -/
+
+namespace ExceptionallyLongNamespaceThatWillNotBePrinted
+opaque Bar.Baz : Nat → Nat
+
+/--
+info: Before `Bar.Baz Nat.zero` After
+-/
+#guard_msgs in
+#eval runTest <| .app (.const ``Bar.Baz []) (.const ``Nat.zero [])
+
+end ExceptionallyLongNamespaceThatWillNotBePrinted
+
+/-! Test `trailing` variant: -/
+
+def runTestTrailing (e : Expr) : MetaM Unit := do
+  let msg := inlineExprTrailing e (maxInlineLength := 30)
+  logInfo m!"Before{msg}"
+
+
+/-- info: Before `shortFun shortConst` -/
+#guard_msgs in
+#eval runTestTrailing <| .app (.const ``shortFun []) (.const ``shortConst [])
+
+/--
+info: Before
+  functionWithLongName constantWithLongName
+-/
+#guard_msgs in
+#eval runTestTrailing <| .app (.const ``functionWithLongName []) (.const ``constantWithLongName [])