From 90e6ddc8e77eadf5cc83bc94a1d81dfb51c64371 Mon Sep 17 00:00:00 2001 From: blyxyas Date: Fri, 7 Apr 2023 13:08:29 +0200 Subject: [PATCH] New chapter: "Type Checking" --- book/src/SUMMARY.md | 1 + book/src/development/type_checking.md | 126 ++++++++++++++++++++++++++ 2 files changed, 127 insertions(+) create mode 100644 book/src/development/type_checking.md diff --git a/book/src/SUMMARY.md b/book/src/SUMMARY.md index 0649f7a63..cbd73376d 100644 --- a/book/src/SUMMARY.md +++ b/book/src/SUMMARY.md @@ -13,6 +13,7 @@ - [Development](development/README.md) - [Basics](development/basics.md) - [Adding Lints](development/adding_lints.md) + - [Type Checking](development/type_checking.md) - [Common Tools](development/common_tools_writing_lints.md) - [Infrastructure](development/infrastructure/README.md) - [Syncing changes between Clippy and rust-lang/rust](development/infrastructure/sync.md) diff --git a/book/src/development/type_checking.md b/book/src/development/type_checking.md new file mode 100644 index 000000000..ff08cb1e5 --- /dev/null +++ b/book/src/development/type_checking.md @@ -0,0 +1,126 @@ +# Type Checking + +When we work on a new lint or improve an existing lint, we might want +to retrieve the type `Ty` of an expression `Expr` for a variety of +reasons. This can be achieved by utilizing the [`LateContext`][LateContext] +that is available for [`LateLintPass`][LateLintPass]. + +## `LateContext` and `TypeckResults` + +The lint context [`LateContext`][LateContext] and [`TypeckResults`][TypeckResults] +(returned by `LateContext::typeck_results`) are the two most useful data structures +in `LateLintPass`. They allow us to jump to type definitions and other compilation +stages such as HIR. + +> Note: `LateContext.typeck_results`'s return value is [`TypeckResults`][TypeckResults] +> and is created in the type checking step, it includes useful information such as types of +> expressions, ways to resolve methods and so on. + +`TypeckResults` contains useful methods such as [`expr_ty`][expr_ty], +which gives us access to the underlying structure [`Ty`][Ty] of a given expression. + +```rust +pub fn expr_ty(&self, expr: &Expr<'_>) -> Ty<'tcx> +``` + +As a side note, besides `expr_ty`, [`TypeckResults`][TypeckResults] contains a +[`pat_ty()`][pat_ty] method that is useful for retrieving a type from a pattern. + +## `Ty` + +`Ty` struct contains the type information of an expression. +Let's take a look at `rustc_middle`'s [`Ty`][Ty] struct to examine this struct: + +```rust +pub struct Ty<'tcx>(Interned<'tcx, WithStableHash>>); +``` + +At a first glance, this struct looks quite esoteric. But at a closer look, +we will see that this struct contains many useful methods for type checking. + +For instance, [`is_char`][is_char] checks if the given `Ty` struct corresponds +to the primitive character type. + +### `is_*` Usage + +In some scenarios, all we need to do is check if the `Ty` of an expression +is a specific type, such as `char` type, so we could write the following: + +```rust +impl LateLintPass<'_> for MyStructLint { + fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) { + // Get type of `expr` + let ty = cx.typeck_results().expr_ty(expr); + + // Check if the `Ty` of this expression is of character type + if ty.is_char() { + println!("Our expression is a char!"); + } + } +} +``` + +Furthermore, if we examine the [source code][is_char_source] for `is_char`, +we find something very interesting: + +```rust +#[inline] +pub fn is_char(self) -> bool { + matches!(self.kind(), Char) +} +``` + +Indeed, we just discovered `Ty`'s [`kind` method][kind], which provides us +with [`TyKind`][TyKind] of a `Ty`. + +## `TyKind` + +`TyKind` defines the kinds of types in Rust's type system. +Peeking into [`TyKind` documentation][TyKind], we will see that it is an +enum of 27 variants, including items such as `Bool`, `Int`, `Ref`, etc. + +### `kind` Usage + +The `TyKind` of `Ty` can be returned by calling [`Ty.kind` method][kind]. +We often use this method to perform pattern matching in Clippy. + +For instance, if we want to check for a `struct`, we could examine if the +`ty.kind` corresponds to an [`Adt`][Adt] (algebraic data type) and if its +[`AdtDef`][AdtDef] is a struct: + +```rust +impl LateLintPass<'_> for MyStructLint { + fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) { + // Get type of `expr` + let ty = cx.typeck_results().expr_ty(expr); + // Match its kind to enter the type + match ty.kind { + ty::Adt(adt_def, _) if adt_def.is_struct() => println!("Our `expr` is a struct!"), + _ => () + } + } +} +``` + +## Useful Links + +Below are some useful links to further explore the concepts covered +in this chapter: + +- [Stages of compilation](https://rustc-dev-guide.rust-lang.org/compiler-src.html#the-main-stages-of-compilation) +- [Diagnostic items](https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html) +- [Type checking](https://rustc-dev-guide.rust-lang.org/type-checking.html) +- [Ty module](https://rustc-dev-guide.rust-lang.org/ty.html) + +[Adt]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.TyKind.html#variant.Adt +[AdtDef]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/adt/struct.AdtDef.html +[expr_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckResults.html#method.expr_ty +[is_char]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.is_char +[is_char_source]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_middle/ty/sty.rs.html#1831-1834 +[kind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.kind +[LateContext]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/struct.LateContext.html +[LateLintPass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html +[pat_ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TypeckResults.html#method.pat_ty +[Ty]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html +[TyKind]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.TyKind.html +[TypeckResults]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TypeckResults.html