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source map pattern

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Aleksey Kladov 2019-01-19 22:31:28 +03:00
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@ -426,8 +426,65 @@ actually written by the user.
[`HirFileId`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/ids.rs#L18-L125
Now that we understand how to identify a definition, in a source or in a
macro-generated file, we can discuss name resolution a bit.
## Name resolution
Name resolution faces the same problem as the module tree: if we look at the
syntax tree directly, we'll have to recompute name resolution after every
modification. The solution to the problem is the same: we [lower] source code of
each module into a position-independent representation which does not change if
we modify bodies of the items. After that we [loop] resolving all imports until
we've reached a fixed point.
[lower]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L113-L117
[loop]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L113-L117
And, given all our preparation with ids and position-independent representation,
it is satisfying to [test] that typing inside function body does not invalidate
name resolution results.
[test]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/tests.rs#L376
An interesting fact about name resolution is that it "erases" all of
intermediate paths from the imports: in the end, we know which items are defined
and which items are imported in each module, but, if the import was `use
foo::bar::baz`, we deliberately forget what modules `foo` and `bar` resolve to.
To serve "goto definition" requests on intermediate segments we need this info
in IDE. Luckily, we need it only for a tiny fraction of imports, so we just ask
the module explicitly, "where does `foo::bar` path resolve to?". This is a
general pattern: we try to compute the minimal possible amount of information
during analysis while allowing IDE to ask for additional specific bits.
Name resolution is also a good place to introduce another salsa pattern used
throughout the analyzer:
## Source Map pattern
Due to an obscure edge case in completion, IDE needs to know the syntax node of
an use statement which imported the given completion candidate. We can't just
store the syntax node as a part of name resolution: this will break
incrementality, due to the fact that syntax changes after every file
modification.
We solve this problem during the lowering step of name resolution. Lowering
query actually produces a *pair* of outputs: `LoweredModule` and [`SourceMap`].
`LoweredModule` module contains [imports], but in a position-independent form.
The `SourceMap` contains a mapping from position-independent imports to
(position-dependent) syntax nodes.
The result of this basic lowering query changes after every modification. But
there's an intermediate [projection query] which returns only the first
position-independent part of the lowering. The result of this query is stable.
Naturally, name resolution [uses] this stable projection query.
[imports]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L52-L59
[`SourceMap`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L52-L59
[projection query]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/nameres/lower.rs#L97-L103
[uses]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/query_definitions.rs#L49
## Type inference
## Tying it all together: completion