[AST] Update the comments of the various Expr::Ignore* + Related cleanups

The description of what the various Expr::Ignore* do has drifted from the
actual implementation.

Inspection reveals that IgnoreParenImpCasts() is not equivalent to doing
IgnoreParens() + IgnoreImpCasts() until reaching a fixed point, but
IgnoreParenCasts() is equivalent to doing IgnoreParens() + IgnoreCasts()
until reaching a fixed point. There is also a fair amount of duplication
in the various Expr::Ignore* functions which increase the chance of further
future inconsistencies. In preparation for the next patch which will factor
out the implementation of the various Expr::Ignore*, do the following cleanups:

Remove Stmt::IgnoreImplicit, in favor of Expr::IgnoreImplicit. IgnoreImplicit
is the only function among all of the Expr::Ignore* which is available in Stmt.
There are only a few users of Stmt::IgnoreImplicit. They can just use instead
Expr::IgnoreImplicit like they have to do for the other Ignore*.

Move Expr::IgnoreImpCasts() from Expr.h to Expr.cpp. This made no difference
in the run-time with my usual benchmark (-fsyntax-only on all of Boost).

While we are at it, make IgnoreParenNoopCasts take a const reference to the
ASTContext for const correctness.

Update the comments to match what the Expr::Ignore* are actually doing.
I am not sure that listing exactly what each Expr::Ignore* do is optimal,
but it certainly looks better than the current state which is in my opinion
between misleading and just plain wrong.

The whole patch is NFC (if you count removing Stmt::IgnoreImplicit as NFC).

Differential Revision: https://reviews.llvm.org/D57266

Reviewed By: aaron.ballman

llvm-svn: 353006

3 files changed, 11 insertions, 8 deletions

diff --git a/clang/lib/ARCMigrate/TransRetainReleaseDealloc.cpp b/clang/lib/ARCMigrate/TransRetainReleaseDealloc.cpp
index 347c198f68f..b76fc65574d 100644
--- a/clang/lib/ARCMigrate/TransRetainReleaseDealloc.cpp
+++ b/clang/lib/ARCMigrate/TransRetainReleaseDealloc.cpp
@@ -269,8 +269,8 @@ private:
 
     if (prevChildS != childE) {
       prevStmt = *prevChildS;
-      if (prevStmt)
-        prevStmt = prevStmt->IgnoreImplicit();
+      if (auto *E = dyn_cast_or_null<Expr>(prevStmt))
+        prevStmt = E->IgnoreImplicit();
     }
 
     if (currChildS == childE)
@@ -280,8 +280,8 @@ private:
       return std::make_pair(prevStmt, nextStmt);
 
     nextStmt = *currChildS;
-    if (nextStmt)
-      nextStmt = nextStmt->IgnoreImplicit();
+    if (auto *E = dyn_cast_or_null<Expr>(nextStmt))
+      nextStmt = E->IgnoreImplicit();
 
     return std::make_pair(prevStmt, nextStmt);
   }
diff --git a/clang/lib/ARCMigrate/TransformActions.cpp b/clang/lib/ARCMigrate/TransformActions.cpp
index aa1a2c50c5a..f76cc2c130f 100644
--- a/clang/lib/ARCMigrate/TransformActions.cpp
+++ b/clang/lib/ARCMigrate/TransformActions.cpp
@@ -313,7 +313,9 @@ void TransformActionsImpl::removeStmt(Stmt *S) {
   assert(IsInTransaction && "Actions only allowed during a transaction");
   ActionData data;
   data.Kind = Act_RemoveStmt;
-  data.S = S->IgnoreImplicit(); // important for uniquing
+  if (auto *E = dyn_cast<Expr>(S))
+    S = E->IgnoreImplicit(); // important for uniquing
+  data.S = S;
   CachedActions.push_back(data);
 }
 
diff --git a/clang/lib/ARCMigrate/Transforms.cpp b/clang/lib/ARCMigrate/Transforms.cpp
index 6ca748bcdd1..59b80a917e5 100644
--- a/clang/lib/ARCMigrate/Transforms.cpp
+++ b/clang/lib/ARCMigrate/Transforms.cpp
@@ -286,10 +286,11 @@ private:
   void mark(Stmt *S) {
     if (!S) return;
 
-    while (LabelStmt *Label = dyn_cast<LabelStmt>(S))
+    while (auto *Label = dyn_cast<LabelStmt>(S))
       S = Label->getSubStmt();
-    S = S->IgnoreImplicit();
-    if (Expr *E = dyn_cast<Expr>(S))
+    if (auto *E = dyn_cast<Expr>(S))
+      S = E->IgnoreImplicit();
+    if (auto *E = dyn_cast<Expr>(S))
       Removables.insert(E);
   }
 };