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write about type inference

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Aleksey Kladov 2019-01-19 22:53:57 +03:00
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@ -259,7 +259,11 @@ Java) there isn't a one-to-one mapping between source code and semantic model. A
single function definition in the source code might result in several semantic
functions: for example, the same source file might be included as a module into
several crate, or a single "crate" might be present in the compilation DAG
several times, with different sets of `cfg`s enabled.
several times, with different sets of `cfg`s enabled. The IDE-specific task of
mapping source code position into semantic model is inherently imprecise for
this reason, and is handled by the [`source_binder`].
[`source_binder`]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/source_binder.rs
The semantic interface is declared in [`code_model_api`] module. Each entity is
identified by integer id and has a bunch of methods which take a salsa database
@ -487,4 +491,27 @@ Naturally, name resolution [uses] this stable projection query.
## Type inference
First of all, implementation of type inference in rust analyzer was spearheaded
by [@flodiebold]. [#327] was an awesome Christmas present, thank you, Florian!
Type inference runs on per-function granularity and uses the patterns we've
discussed previously.
First, we [lower ast] of function body into a position-independent
representation. In this representation, each expression is assigned a
[positional id]. Alongside the lowered expression, [a source map] is produced,
which maps between expression ids and original syntax. This lowering step also
deals with "incomplete" source trees by replacing missing expressions by an
explicit `Missing` expression.
Given the lower body of the function, we can now run [type inference] and
construct a mapping from `ExprId`s to types.
[@flodiebold]: https://github.com/flodiebold
[#327]: https://github.com/rust-analyzer/rust-analyzer/pull/327
[lower ast]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs
[positional id]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs#L13-L15
[a source map]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/expr.rs#L41-L44
[type-inference]: https://github.com/rust-analyzer/rust-analyzer/blob/guide-2019-01/crates/ra_hir/src/ty.rs#L1208-L1223
## Tying it all together: completion