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Link to docs and address some review comments

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Oliver Scherer 2018-11-05 07:37:14 +01:00 committed by Who? Me?!
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src/mir/construction.md
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@ -11,16 +11,22 @@ list of items:
* Drop code (the `Drop::drop` function is not called directly) * Drop code (the `Drop::drop` function is not called directly)
* Drop implementations of types without an explicit `Drop` implementation * Drop implementations of types without an explicit `Drop` implementation
The lowering is triggered by calling the `mir_built` query. The entire lowering The lowering is triggered by calling the [`mir_built`] query.
code lives in `src/librustc_mir/build`. There is an intermediate representation There is an intermediate representation
between [HIR] and [MIR] called the `HAIR` that is only used during the lowering. between [HIR] and [MIR] called the [HAIR] that is only used during the lowering.
The `HAIR` has datatypes that mirror the [HIR] datatypes, but instead of e.g. `-x` The [HAIR]'s most important feature is that the various adjustments that happen
being a `hair::ExprKind::Neg(hair::Expr)` it is a `hair::ExprKind::Neg(hir::Expr)`. without explicit syntax (coercion, autoderef, autoref, ...) have become explicit
casts, deref operations or reference expressions.
The [HAIR] has datatypes that mirror the [HIR] datatypes, but instead of e.g. `-x`
being a `hair::ExprKind::Neg(hair::Expr)` it is a `hair::ExprKind::Neg(hir::Expr)`.
This shallowness enables the `HAIR` to represent all datatypes that [HIR] has, but This shallowness enables the `HAIR` to represent all datatypes that [HIR] has, but
without having to create an in-memory copy of the entire [HIR]. The `HAIR` also without having to create an in-memory copy of the entire [HIR]. The [HAIR] also
does a few simplifications, e.g. method calls and function calls have been merged does a few simplifications, e.g. method calls and function calls have been merged
into a single variant. into a single variant. [MIR] lowering will first convert the topmost expression from
[HIR] to [HAIR] (in
[https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/cx/expr/index.html])
and then process the [HAIR] expressions recursively.
The lowering creates local variables for every argument as specified in the signature. The lowering creates local variables for every argument as specified in the signature.
Next it creates local variables for every binding specified (e.g. `(a, b): (i32, String)`) Next it creates local variables for every binding specified (e.g. `(a, b): (i32, String)`)
@ -35,7 +41,7 @@ writes the result into the `RETURN_PLACE`.
## `unpack!` all the things ## `unpack!` all the things
One important thing of note is the `unpack!` macro, which accompanies all recursive One important thing of note is the `unpack!` macro, which accompanies all recursive
calls. The macro ensures, that you get the result of the recursive call while updating calls. The macro ensures that you get the result of the recursive call while updating
the basic block that you are now in. As an example: lowering `a + b` will need to do the basic block that you are now in. As an example: lowering `a + b` will need to do
three somewhat independent things: three somewhat independent things:
@ -109,3 +115,5 @@ case of `enum`s.
[MIR]: ./index.html [MIR]: ./index.html
[HIR]: ../hir.html [HIR]: ../hir.html
[HAIR]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/index.html
[`mir_built`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/fn.mir_built.html