Merge pull request #2668 from ShoyuVanilla/ra-solver
Mention sharing the solver with rust-analyzer
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- [Proof trees](./solve/proof-trees.md)
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- [Opaque types](./solve/opaque-types.md)
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- [Significant changes and quirks](./solve/significant-changes.md)
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- [Sharing the trait solver with rust-analyzer](./solve/sharing-crates-with-rust-analyzer.md)
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- [`Unsize` and `CoerceUnsized` traits](./traits/unsize.md)
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- [Variance](./variance.md)
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- [Coherence checking](./coherence.md)
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# Sharing the trait solver with rust-analyzer
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rust-analyzer can be viewed as a compiler frontend: it performs tasks similar to the parts of rustc
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that run before code generation, such as parsing, lexing, AST construction and lowering, HIR
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lowering, and even limited MIR building and const evaluation.
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However, because rust-analyzer is primarily a language server, its architecture differs in several
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important ways from that of rustc.
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Despite these differences, a substantial portion of its responsibilities—most notably type
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inference and trait solving—overlap with the compiler.
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To avoid duplication and to maintain consistency between the two implementations, rust-analyzer
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reuses several crates from rustc, relying on shared abstractions wherever possible.
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## Shared Crates
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Currently, rust-analyzer depends on several `rustc_*` crates from the compiler:
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- `rustc_abi`
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- `rustc_ast_ir`
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- `rustc_index`
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- `rustc_lexer`
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- `rustc_next_trait_solver`
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- `rustc_parse_format`
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- `rustc_pattern_analysis`
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- `rustc_type_ir`
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Since these crates are not published on `crates.io` as part of the compiler's normal distribution
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process, rust-analyzer maintains its own publishing pipeline.
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It uses the [rustc-auto-publish script][rustc-auto-publish] to publish these crates to `crates.io`
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with the prefix `ra-ap-rustc_*`
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(for example: https://crates.io/crates/ra-ap-rustc_next_trait_solver).
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rust-analyzer then depends on these re-published crates in its own build.
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For trait solving specifically, the primary shared crates are `rustc_type_ir` and
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`rustc_next_trait_solver`, which provide the core IR and solver logic used by both compiler
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frontends.
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## The Abstraction Layer
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Because rust-analyzer is a language server, it must handle frequently changing source code and
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partially invalid or incomplete source codes.
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This requires an infrastructure quite different from rustc's, especially in the layers between
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the source code and the HIR—for example, `Ty` and its backing interner.
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To bridge these differences, the compiler provides `rustc_type_ir` as an abstraction layer shared
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between rustc and rust-analyzer.
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This crate defines the fundamental interfaces used to represent types, predicates, and the context
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required by the trait solver.
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Both rustc and rust-analyzer implement these traits for their own concrete type representations,
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and `rustc_next_trait_solver` is written to be generic over these abstractions.
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In addition to these interfaces, `rustc_type_ir` also includes several non-trivial components built
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on top of the abstraction layer—such as elaboration logic and the search graph machinery used by the
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solver.
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## Design Concepts
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`rustc_next_trait_solver` is intended to depend only on the abstract interfaces defined in
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`rustc_type_ir`.
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To support this, the type-system traits in `rustc_type_ir` must expose every interface the solver
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requires—for example, [creating a new inference type variable][ir new_infer]
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([rustc][rustc new_infer], [rust-analyzer][r-a new_infer]).
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For items that do not need compiler-specific representations, `rustc_type_ir` defines them directly
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as structs or enums parameterized over these traits—for example, [`TraitRef`][ir tr].
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The following are some notable items from the `rustc_type_ir` crate.
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### `trait Interner`
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The central trait in this design is [`Interner`][ir interner], which specifies all
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implementation-specific details for both rustc and rust-analyzer.
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Among its essential responsibilities:
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- it **specifies** the concrete types used by the implementation via its
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[associated types][ir interner assocs]; these form the backbone of how each compiler frontend
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instantiates the shared IR,
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- it provides the context required by the solver (e.g., querying [lang items][ir require_lang_item],
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enumerating [all blanket impls for a trait][ir for_each_blanket_impl]);
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- and it must implement [`IrPrint`][ir irprint] for formatting and tracing.
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In practice, these `IrPrint` impls simply route to existing formatting logic inside rustc or
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rust-analyzer.
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In rustc, [`TyCtxt` implements `Interner`][rustc interner impl]: it exposes the rustc's query
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methods, and the required `Interner` trait methods are implemented by invoking those queries.
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In rust-analyzer, the implementing type is named [`DbInterner`][r-a interner impl] (as it performs
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most interning through the [salsa] database), and most of its methods are backed by salsa queries
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rather than rustc queries.
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### `mod inherent`
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Another notable item in `rustc_type_ir` is the [`inherent` module][ir inherent].
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This module provides *forward definitions* of inherent methods—expressed as traits—corresponding to
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methods that exist on compiler-specific types such as `Ty` or `GenericArg`.
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These definitions allow the generic crates (such as `rustc_next_trait_solver`) to call methods that
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are implemented differently in rustc and rust-analyzer.
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Code in generic crates should import these definitions with:
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```rust
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use inherent::*;
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```
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These forward definitions **must never be used inside the concrete implementations themselves**.
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Crates that implement the traits from `mod inherent` should call the actual inherent methods on
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their concrete types once those types are nameable.
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You can find rustc’s implementations of these traits in the
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[rustc_middle::ty::inherent][rustc inherent impl] module.
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For rust-analyzer, the corresponding implementations are located across several modules under
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`hir_ty::next_solver`, such as [hir_ty::next_solver::region][r-a inherent impl].
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### `trait InferCtxtLike` and `trait SolverDelegate`
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These two traits correspond to the role of [`InferCtxt`][rustc inferctxt] in rustc.
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[`InferCtxtLike`][ir inferctxtlike] must be defined in `rustc_infer` due to coherence
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constraints(orphan rules).
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As a result, it cannot provide functionality that lives in `rustc_trait_selection`.
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Instead, behavior that depends on trait-solving logic is abstracted into a separate trait,
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[`SolverDelegate`][ir solverdelegate].
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Its implementator in rustc is [simply a newtype struct over `InferCtxt`][rustc solverdelegate impl]
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in `rustc_trait_selection`.
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(In rust-analyzer, it is also implemented for a newtype wrapper over its own
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[`InferCtxt`][r-a inferctxtlike impl], primarily to mirror rustc’s structure, although this is not
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strictly necessary because all solver-related logic already resides in the `hir-ty` crate.)
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In the long term, the ideal design is to move all of the logic currently expressed through
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`SolverDelegate` into `rustc_next_trait_solver`, with any required core operations added directly to
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`InferCtxtLike`.
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This would allow more of the solver’s behavior to live entirely inside the shared solver crate.
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### `rustc_type_ir::search_graph::{Cx, Delegate}`
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The abstraction traits [`Cx`][ir searchgraph cx impl] and [`Delegate`][ir searchgraph delegate impl]
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are already implemented within `rustc_next_trait_solver` itself.
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Therefore, users of the shared crates—both rustc and rust-analyzer—do not need to provide their own
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implementations.
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These traits exist primarily to support fuzzing of the search graph independently of the full trait
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solver.
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This infrastructure is used by the external fuzzing project:
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<https://github.com/lcnr/search_graph_fuzz>.
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## Long-term plans for supporting rust-analyzer
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In general, we aim to support rust-analyzer just as well as rustc in these shared crates—provided
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doing so does not substantially harm rustc's performance or maintainability.
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(e.g., [#145377][pr 145377], [#146111][pr 146111], [#146182][pr 146182] and [#147723][pr 147723])
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Shared crates that require nightly-only features must guard such code behind a `nightly` feature
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flag, since rust-analyzer is built with the stable toolchain.
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Looking forward, we plan to uplift more shared logic into `rustc_type_ir`.
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There are still duplicated implementations between rustc and rust-analyzer—such as `ObligationCtxt`
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([rustc][rustc oblctxt], [rust-analyzer][r-a oblctxt]) and type coercion logic
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([rustc][rustc coerce], [rust-analyzer][r-a coerce])—that we would like to unify over time.
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[rustc-auto-publish]: https://github.com/rust-analyzer/rustc-auto-publish
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[ir new_infer]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/inherent/trait.Ty.html#tymethod.new_infer
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[rustc new_infer]: https://github.com/rust-lang/rust/blob/63b1db05801271e400954e41b8600a3cf1482363/compiler/rustc_middle/src/ty/sty.rs#L413-L420
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[r-a new_infer]: https://github.com/rust-lang/rust-analyzer/blob/34f47d9298c478c12c6c4c0348771d1b05706e09/crates/hir-ty/src/next_solver/ty.rs#L59-L92
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[ir tr]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/struct.TraitRef.html
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[ir interner]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/trait.Interner.html
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[ir interner assocs]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/trait.Interner.html#required-associated-types
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[ir require_lang_item]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/trait.Interner.html#tymethod.require_lang_item
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[ir for_each_blanket_impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/trait.Interner.html#tymethod.for_each_blanket_impl
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[ir irprint]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/ir_print/trait.IrPrint.html
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[rustc interner impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#impl-Interner-for-TyCtxt%3C'tcx%3E
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[r-a interner impl]: https://github.com/rust-lang/rust-analyzer/blob/a50c1ccc9cf3dab1afdc857a965a9992fbad7a53/crates/hir-ty/src/next_solver/interner.rs
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[salsa]: https://github.com/salsa-rs/salsa
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[ir inherent]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/inherent/index.html
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[rustc inherent impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_middle/ty/inherent/index.html
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[r-a inherent impl]: https://github.com/rust-lang/rust-analyzer/blob/a50c1ccc9cf3dab1afdc857a965a9992fbad7a53/crates/hir-ty/src/next_solver/region.rs
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[ir inferctxtlike]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_type_ir/trait.InferCtxtLike.html
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[rustc inferctxt]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html
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[rustc inferctxtlike impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/src/rustc_infer/infer/context.rs.html#14-332
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[r-a inferctxtlike impl]: https://github.com/rust-lang/rust-analyzer/blob/a50c1ccc9cf3dab1afdc857a965a9992fbad7a53/crates/hir-ty/src/next_solver/infer/context.rs
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[ir solverdelegate]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_next_trait_solver/delegate/trait.SolverDelegate.html
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[rustc solverdelegate impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/rustc_trait_selection/solve/delegate/struct.SolverDelegate.html
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[r-a solverdelegate impl]: https://github.com/rust-lang/rust-analyzer/blob/a50c1ccc9cf3dab1afdc857a965a9992fbad7a53/crates/hir-ty/src/next_solver/solver.rs#L27-L330
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[ir searchgraph cx impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/src/rustc_type_ir/interner.rs.html#550-575
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[ir searchgraph delegate impl]: https://doc.rust-lang.org/1.91.1/nightly-rustc/src/rustc_next_trait_solver/solve/search_graph.rs.html#20-123
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[pr 145377]: https://github.com/rust-lang/rust/pull/145377
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[pr 146111]: https://github.com/rust-lang/rust/pull/146111
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[pr 146182]: https://github.com/rust-lang/rust/pull/146182
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[pr 147723]: https://github.com/rust-lang/rust/pull/147723
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[rustc oblctxt]: https://github.com/rust-lang/rust/blob/63b1db05801271e400954e41b8600a3cf1482363/compiler/rustc_trait_selection/src/traits/engine.rs#L48-L386
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[r-a oblctxt]: https://github.com/rust-lang/rust-analyzer/blob/34f47d9298c478c12c6c4c0348771d1b05706e09/crates/hir-ty/src/next_solver/obligation_ctxt.rs
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[rustc coerce]: https://github.com/rust-lang/rust/blob/63b1db05801271e400954e41b8600a3cf1482363/compiler/rustc_hir_typeck/src/coercion.rs
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[r-a coerce]: https://github.com/rust-lang/rust-analyzer/blob/34f47d9298c478c12c6c4c0348771d1b05706e09/crates/hir-ty/src/infer/coerce.rs
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