Improve parse error message on indentation mistake

Consider the following code fragment:

                   (ExecGhc, args) -> execCompiler "" args
                    -- NOTE: this won't currently work for GHCJS, because it doesn't have
                    -- a runghcjs binary. It probably will someday, though.
                    (ExecRunGhc, args) ->

The last line is indented by one space but it is hard to notice esp when we looking at this along with a screenful of code and there are many comments between the two lines. When this code is compiled ghc throws the following error:

Main.hs:745:40: error: parse error on input ‘->’

I look at the error and then the code and wonder what is wrong with this code, there is nothing noticeably wrong. If ghc can provide some context I can easily make out where the problem is. ghc can perhaps tell me how the current expression is being parsed e.g. (ExecRunGhc, args) -> is being considered as a continuation of the expression on the previous line.

I have run into this many times. The first time it was really frustrating.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

Indeed these are frustratingly poor errors. I remember encountering this exact thing when I was learning the language.

Improving this would require that someone take some time to think about how you can robustly catch this issue in the parser without introducing too much complexity.

Trac metadata

Trac field	Value
Type	Bug → FeatureRequest
TypeOfFailure	OtherFailure → IncorrectWarning

added Pnormal label

added error messages label

mentioned in issue #22173