Analyzer: Disallow including the same part in multiple libraries in the same program

[askeksa-google]

Motivated by the discussion in #32999 we want to make it a compile-time error if a file is used as a part in more than one library in the same program.

The implementation of the check for this in Fasta is covered by #32999 and the spec change by #33225 - this issue covers the implementation in the analyzer.

[bwilkerson]

I'm not certain that analyzer can implement this check. It doesn't have any notion of a "program" or assume that there is a single entry point.

[askeksa-google]

Neither does the spec. It gets around it by formulating the requirement thus (in 18.3):

"Let L be a library. [...] Furthermore, let L1 be L or any library which is directly
or indirectly imported by L, and let L2 be any library distinct from L1 which is
directly or indirectly imported by L (18.1). It is a compile-time error if L1 and
L2 both contain a part directive with the same URI."

Would such a check be implementable in the analyzer?

[bwilkerson]

Maybe.

• It requires that when L is analyzed we examine all of the libraries in the transitive closure of imports for a violation. I don't know how performant that would be.

• It requires that we add errors to compilation units other than the one we're currently analyzing. The design of the analyzer doesn't currently support that.

• It also requires that we keep track of whether we have already generated an error for a given library so that if the same pair of libraries are in the transitive closure of a library other than L that we don't generate duplicate errors.

None of this is conducive to modular analysis, which is critical to the way analyzer works.

[matanlurey]

@bwilkerson @askeksa-google

Might I suggest a simpler specification for analyzer, i.e. disallow a file being referenced as a part (i.e. part of 'file.dart'; or part of some_lib) more than once in a given package?

This should in reality catch almost every error (its nearly impossible with pub or in google3 to part import a file outside of your current package because of build system restrictions) - and should work better for analyzer's per-package modular analysis.