[PATCH] D53354: [WIP][NOT FOR COMMIT][PROTOTYPE] clang-scan-deps: dependency scanning tool rough prototype

[Duncan P. N. Exon Smith via Phabricator via cfe-commits]

dexonsmith added inline comments.


================
Comment at: lib/Lex/FilterToIncludes.cpp:628
+  First = Id.Last;
+  auto Kind = llvm::StringSwitch<TokenKind>(Id.Name)
+                  .Case("include", pp_include)
----------------
arphaman wrote:
> arphaman wrote:
> > arphaman wrote:
> > > ddunbar wrote:
> > > > What is our feeling w.r.t. _Pragma, which can in theory influence the preprocessor. I'm not sure this model can sanely support it?
> > > We need to look into that. In the worst case we can always avoid minimizing the file if it has a _Pragma anywhere in it.
> > We also have to handle cases like this one:
> > 
> > foo.h:
> > ```
> > #define PP _Pragma
> > ```
> > 
> > bar.h:
> > ```
> > PP ("foo")
> > ```
> > 
> > Unfortunately this can't be handled by just disabling minimization for `foo.h` as PP will be stripped out from `bar.h`. Furthermore, this pattern is quite reasonable, so we should expect it in the code we receive. Right now I can't think of a reasonable solution for this problem.
> > 
> > There's also a more "evil" case:
> > 
> > ```
> > #define P(A, B) A##B
> > 
> > P(_Pra,gma)("foo")
> > ```
> > 
> > It would be reasonable to introduce a warning for the use of `_Pragma` token that was created using PP token concatenation and just hope that our users won't use this pattern.
> One possible solution to the first issue is to simply fallback to the regular -Eonly invocation if we detect an include of a file that has a macro with a `_Pragma` in it. Then we could emit a remark to the user saying that their build could be faster if they rewrite their code so that this pattern no longer occurs.
Hmm.  Our plan for `@import` is to stop supporting code such as:
```
#define IMPORT(M) @import M
IMPORT(LLVMSupport);
```
where the @import is buried.  I.e., start erroring out in normal Clang builds.  Perhaps it would be reasonable to similarly disallow `_Pragma` usage that buries import/include statements.  I.e., do not support (even in normal builds) the following:
```
#define IMPORT(M) _Pragma("clang module import " #M)
IMPORT(LLVMSupport)
```

Possibly, this could be a warning that is error-by-default.


Repository:
  rC Clang

https://reviews.llvm.org/D53354