Improve doc of MIR queries & passes

Author: Jaic1

src/SUMMARY.md

@@ -94,7 +94,7 @@
 - [The MIR (Mid-level IR)](./mir/index.md)
     - [MIR construction](./mir/construction.md)
     - [MIR visitor and traversal](./mir/visitor.md)
-    - [MIR passes: getting the MIR for a function](./mir/passes.md)
+    - [MIR queries and passes](./mir/passes.md)
 - [Identifiers in the Compiler](./identifiers.md)
 - [Closure expansion](./closure.md)
 - [Inline assembly](./asm.md)

src/mir/passes.md

@@ -1,102 +1,169 @@
-# MIR passes
+# MIR queries and passes
 
-If you would like to get the MIR for a function (or constant, etc),
-you can use the `optimized_mir(def_id)` query. This will give you back
-the final, optimized MIR. For foreign def-ids, we simply read the MIR
+If you would like to get the MIR:
+
+- for a function - you can use the `optimized_mir(def_id)` query;
+- for a promoted - you can use the `promoted_mir(def_id)` query.
+
+These will give you back the final, optimized MIR. For foreign def-ids, we simply read the MIR
 from the other crate's metadata. But for local def-ids, the query will
-construct the MIR and then iteratively optimize it by applying a
-series of passes. This section describes how those passes work and how
-you can extend them.
-
-To produce the `optimized_mir(D)` for a given def-id `D`, the MIR
-passes through several suites of optimizations, each represented by a
-query. Each suite consists of multiple optimizations and
-transformations. These suites represent useful intermediate points
-where we want to access the MIR for type checking or other purposes:
-
-- `mir_build(D)` – not a query, but this constructs the initial MIR
-- `mir_const(D)` – applies some simple transformations to make MIR ready for
-  constant evaluation;
-- `mir_validated(D)` – applies some more transformations, making MIR ready for
-  borrow checking;
-- `optimized_mir(D)` – the final state, after all optimizations have been
-  performed.
-
-### Implementing and registering a pass
-
-A `MirPass` is some bit of code that processes the MIR, typically –
-but not always – transforming it along the way somehow. For example,
-it might perform an optimization. The `MirPass` trait itself is found
-in [the `rustc_mir_transform` crate][mirtransform], and it
-basically consists of one method, `run_pass`, that simply gets an
-`&mut Mir` (along with the tcx and some information about where it
-came from). The MIR is therefore modified in place (which helps to
-keep things efficient).
-
-A good example of a basic MIR pass is [`NoLandingPads`], which walks
-the MIR and removes all edges that are due to unwinding – this is
-used when configured with `panic=abort`, which never unwinds. As you
-can see from its source, a MIR pass is defined by first defining a
-dummy type, a struct with no fields, something like:
+construct the optimized MIR by requesting a pipeline of upstream queries[^query].
+Each query will contain a series of passes.
+This section describes how those queries and passes work and how you can extend them.
+
+To produce the optimized MIR for a given def-id `D`, `optimized_mir(D)`
+goes through several suites of passes, each grouped by a
+query. Each suite consists of passes which perform analysis, transformation or optimization.
+Each query represent a useful intermediate point
+where we can access the MIR dialect for type checking or other purposes:
+
+- `mir_built(D)` – it gives the initial MIR just after it's built;
+- `mir_const(D)` – it applies some simple transformation passes to make MIR ready for
+  const qualification;
+- `mir_promoted(D)` - it extracts promotable temps into separate MIR bodies, and also makes MIR
+  ready for borrow checking;
+- `mir_drops_elaborated_and_const_checked(D)` - it performs borrow checking, runs major
+  transformation passes (such as drop elaboration) and makes MIR ready for optimization;
+- `optimized_mir(D)` – it performs all enabled optimizations and reaches the final state.
+
+[^query]: See the [Queries](../query.md) chapter for the general concept of query.
+
+## Implementing and registering a pass
+
+A `MirPass` is some bit of code that processes the MIR, typically transforming it along the way
+somehow. But it may also do other things like analysis (e.g., [`CheckPackedRef`][lint1],
+[`CheckConstItemMutation`][lint2], [`FunctionItemReferences`][lint3], which implement `MirLint`) or
+optimization (e.g., [`SimplifyCfg`][opt1], [`RemoveUnneededDrops`][opt2]). While most MIR passes
+are defined in the [`rustc_mir_transform`][mirtransform] crate, the `MirPass` trait itself is
+[found][mirpass] in the `rustc_middle` crate, and it basically consists of one primary method,
+`run_pass`, that simply gets an `&mut Body` (along with the `tcx`).
+The MIR is therefore modified in place (which helps to keep things efficient).
+
+A good example of a simple MIR pass is [`CleanupNonCodegenStatements`][cleanup-pass], which walks
+the MIR and removes all statements that are not relevant to code generation. As you can see from
+its [source][cleanup-source], it is defined by first defining a dummy type, a struct with no
+fields:
 
 ```rust
-struct MyPass;
+pub struct CleanupNonCodegenStatements;
 ```
 
-for which you then implement the `MirPass` trait. You can then insert
-this pass into the appropriate list of passes found in a query like
-`optimized_mir`, `mir_validated`, etc. (If this is an optimization, it
-should go into the `optimized_mir` list.)
+for which we then implement the `MirPass` trait:
+
+```rust
+impl<'tcx> MirPass<'tcx> for CleanupNonCodegenStatements {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        ...
+    }
+}
+```
+
+We [register][pass-register] this pass inside the `mir_drops_elaborated_and_const_checked` query.
+(If this is an optimization, it should go into the `optimized_mir` list.)
 
 If you are writing a pass, there's a good chance that you are going to
 want to use a [MIR visitor]. MIR visitors are a handy way to walk all
 the parts of the MIR, either to search for something or to make small
 edits.
 
-### Stealing
+## Stealing
 
-The intermediate queries `mir_const()` and `mir_validated()` yield up
-a `&'tcx Steal<Mir<'tcx>>`, allocated using
-`tcx.alloc_steal_mir()`. This indicates that the result may be
-**stolen** by the next suite of optimizations – this is an
+The intermediate queries `mir_const()` and `mir_promoted()` yield up
+a `&'tcx Steal<Body<'tcx>>`, allocated using `tcx.alloc_steal_mir()`.
+This indicates that the result may be **stolen** by a subsequent query – this is an
 optimization to avoid cloning the MIR. Attempting to use a stolen
 result will cause a panic in the compiler. Therefore, it is important
-that you do not read directly from these intermediate queries except as
-part of the MIR processing pipeline.
+that you do not accidently read from these intermediate queries without
+the consideration of the dependency in the MIR processing pipeline.
 
-Because of this stealing mechanism, some care must also be taken to
+Because of this stealing mechanism, some care must be taken to
 ensure that, before the MIR at a particular phase in the processing
 pipeline is stolen, anyone who may want to read from it has already
-done so. Concretely, this means that if you have some query `foo(D)`
+done so.
+
+<!-- FIXME - What is force? Do we still have it in rustc? -->
+Concretely, this means that if you have a query `foo(D)`
 that wants to access the result of `mir_const(D)` or
-`mir_validated(D)`, you need to have the successor pass "force"
+`mir_promoted(D)`, you need to have the successor pass "force"
 `foo(D)` using `ty::queries::foo::force(...)`. This will force a query
 to execute even though you don't directly require its result.
 
-As an example, consider MIR const qualification. It wants to read the
-result produced by the `mir_const()` suite. However, that result will
-be **stolen** by the `mir_validated()` suite. If nothing was done,
-then `mir_const_qualif(D)` would succeed if it came before
-`mir_validated(D)`, but fail otherwise. Therefore, `mir_validated(D)`
-will **force** `mir_const_qualif` before it actually steals, thus
-ensuring that the reads have already happened (remember that
-[queries are memoized](../query.html), so executing a query twice
-simply loads from a cache the second time):
-
-```text
-mir_const(D) --read-by--> mir_const_qualif(D)
-     |                       ^
-  stolen-by                  |
-     |                    (forces)
-     v                       |
-mir_validated(D) ------------+
+> This mechanism is a bit dodgy. There is a discussion of more elegant
+alternatives in [rust-lang/rust#41710].
+
+### Overview
+
+Below is an overview of the stealing dependency in the MIR processing pipeline[^part]:
+
+```mermaid
+flowchart BT
+  mir_for_ctfe* --borrow--> id40
+  id5 --steal--> id40
+
+  mir_borrowck* --borrow--> id3
+  id41 --steal part 1--> id3
+  id40 --steal part 0--> id3
+
+  mir_const_qualif* -- borrow --> id2
+  id3 -- steal --> id2
+
+  id2 -- steal --> id1
+
+  id1([mir_built])
+  id2([mir_const])
+  id3([mir_promoted])
+  id40([mir_drops_elaborated_and_const_checked])
+  id41([promoted_mir])
+  id5([optimized_mir])
+
+  style id1 fill:#bbf
+  style id2 fill:#bbf
+  style id3 fill:#bbf
+  style id40 fill:#bbf
+  style id41 fill:#bbf
+  style id5 fill:#bbf
 ```
 
-This mechanism is a bit dodgy. There is a discussion of more elegant
-alternatives in [rust-lang/rust#41710].
+The stadium-shape queries (e.g., `mir_built`) with a deep color are the primary queries in the
+pipeline, while the rectangle-shape queries (e.g., `mir_const_qualif*`[^star]) with a shallow color
+are those subsequent queries that need to read the results from `&'tcx Steal<Body<'tcx>>`. With the
+stealing mechanism, the rectangle-shape queries must be performed before any stadium-shape queries,
+that have an equal or larger height in the dependency tree, ever do.
+
+[^part]: The `mir_promoted` query will yield up a tuple
+`(&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>)`, `promoted_mir` will steal
+part 1 (`&'tcx Steal<IndexVec<Promoted, Body<'tcx>>>`) and `mir_drops_elaborated_and_const_checked`
+will steal part 0 (`&'tcx Steal<Body<'tcx>>`). And their stealing is irrelevant to each other,
+i.e., can be performed separately.
+
+[^star]: Note that the `*` suffix in the queries represent a set of queries with the same prefix.
+For example, `mir_borrowck*` represents `mir_borrowck`, `mir_borrowck_const_arg` and
+`mir_borrowck_opt_const_arg`.
+
+### Example
+
+As an example, consider MIR const qualification. It wants to read the result produced by the
+`mir_const` query. However, that result will be **stolen** by the `mir_promoted` query at some
+time in the pipeline. Before `mir_promoted` is ever queried, calling the `mir_const_qualif` query
+will succeed since `mir_const` will produce (if queried the first time) or cache (if queried
+multiple times) the `Steal` result and the result is **not** stolen yet. After `mir_promoted` is
+queried, the result would be stolen and calling the `mir_const_qualif` query to read the result
+would cause a panic.
+
+Therefore, with this stealing mechanism, `mir_promoted` should guarantee any `mir_const_qualif*`
+queries are called before it actually steals, thus ensuring that the reads have already happened
+(remember that [queries are memoized](../query.html), so executing a query twice
+simply loads from a cache the second time).
 
 [rust-lang/rust#41710]: https://github.com/rust-lang/rust/issues/41710
+[mirpass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/trait.MirPass.html
+[lint1]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/check_packed_ref/struct.CheckPackedRef.html
+[lint2]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/check_const_item_mutation/struct.CheckConstItemMutation.html
+[lint3]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/function_item_references/struct.FunctionItemReferences.html
+[opt1]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/simplify/struct.SimplifyCfg.html
+[opt2]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/remove_unneeded_drops/struct.RemoveUnneededDrops.html
 [mirtransform]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/
-<!--- TODO: Change NoLandingPads. [#1232](https://github.com/rust-lang/rustc-dev-guide/issues/1232) -->
-[`NoLandingPads`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/no_landing_pads/struct.NoLandingPads.html
+[cleanup-pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/cleanup_post_borrowck/struct.CleanupNonCodegenStatements.html
+[cleanup-source]: https://github.com/rust-lang/rust/blob/e2b52ff73edc8b0b7c74bc28760d618187731fe8/compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs#L27
+[pass-register]: https://github.com/rust-lang/rust/blob/e2b52ff73edc8b0b7c74bc28760d618187731fe8/compiler/rustc_mir_transform/src/lib.rs#L413
 [MIR visitor]: ./visitor.html