[PATCH] Add gnu::diagnose_as attribute

[Matthias Kretz]

How can we make progress here? I could try to produce some "Tony Tables" of 
diagnostic output of my modified stdx::simd. I believe it's a major 
productivity boost to see abbreviated / "obfuscated" diagnostics *out-of-the 
box* (with the possibility to opt-out). Actually, it already *is* a 
productivity boost to me. Understanding diagnostics has improved from 

"1. ooof, I'm not going to read this, let me rather guess what the issue was
2. sh** I have to read it
3. several minutes later: I finally found the five words to understand the 
problem; I could use a break"

to

"1. right, let me check that"

For reference I'll attach my stdx::simd diagnose_as patch.

We could also talk about extending the feature to provide more information 
about the diagnose_as substition. E.g. print a list of all diagnose_as 
substitutions, which were used, at the end of the output stream. Or simpler, 
print "note: some identifiers were simplified, use -fno-diagnostics-use-
aliases to see their real names".

On Tuesday, 1 June 2021 21:12:18 CEST Jason Merrill wrote:
> > Right, but then two of my design goals can't be met:
> > 
> > 1. Diagnostics have an improved signal-to-noise ratio out of the box.
> > 
> > 2. We can use replacement names that are not valid identifiers.
> 
> This is the basic disconnect: I think that these goals are
> contradictory, and that replacement names that are not valid identifiers
> will just confuse users that don't know about them.
> 
> If a user sees stdx::foo in a diagnostic and then tries to refer to
> stdx::foo and gets an error, the diagnostic is not more helpful than one
> that uses the fully qualified name.
> 
> Jonathan, David, any thoughts on this issue?

-- 
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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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