Store dependencies as edges of the graph

[matklad]

r? @alexcrichton

[alexcrichton]

@bors: r+

Awesome, thanks!

Much nicer now that it's always Vec<T> which has an "empty" state :)

[bors]

📌 Commit 8164be2 has been approved by alexcrichton

[bors]

⌛ Testing commit 8164be2 with merge 76d3e1e2e87756071fe726ab6cf8d19773c2e556...

[bors]

💔 Test failed - status-travis

[alexcrichton]

@bors: retry

??

[bors]

⌛ Testing commit 8164be2 with merge 72a7e06c290dad6248d4c1ce8a424316256ec7ba...

[bors]

💔 Test failed - status-travis

[alexcrichton]

@bors: retry

[bors]

⌛ Testing commit 8164be2 with merge 0e6edcf...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 0e6edcf to master...

[Eh2406]

I think I am confused by this. What is being stored? Why a Graph<PackageId, Vec<Dependency>> I'd have thought that the data for an edge is A Dependency not a Vec of them?

For some background I was trying to use this to improve the resolver error messages. Currently they are like:

cargo/tests/testsuite/build.rs

Lines 1356 to 1358 in 0b530c3

	previously selected package `bad v1.0.0`
	... which is depended on by `bar v0.1.0`
	... which is depended on by `incompatible_dependencies v0.0.1 ([..])`

But if we had a link to the dependency that caused the relationship it could be improved to: (wording needs improvement)

  previously selected package `bad v1.0.0`
    ... which is depended on by `bar v0.1.0` which requires `bad: "=1.0.0"`
    ... which is depended on by `incompatible_dependencies v0.0.1 ([..])` which requires `bar: "0.1.0"`

[alexcrichton]

Oh that'd be awesome to include this info in error messages!

I originally though it was gonna be Dependency as well and was surprised myself when it had to be Vec, the tl;dr; is basically:

[dependencies]
foo = "0.1"

[target.foo.dependencies]
foo = "0.1"

[build-dependencies]
foo = "0.1"

# ... etc

[Eh2406]

Ah, so it is a Vec of all the ways that parent depends on child. They should all apply, at least as resolver sees the world, so child must fulfill each of them. Thus the error messages can reasonably use any of them.

[alexcrichton]

@Eh2406 indeed!