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| <h1 id="the-ty-module-representing-types"><a class="header" href="#the-ty-module-representing-types">The <code>ty</code> module: representing types</a></h1> |
| <p>The <code>ty</code> module defines how the Rust compiler represents types internally. It also defines the |
| <em>typing context</em> (<code>tcx</code> or <code>TyCtxt</code>), which is the central data structure in the compiler.</p> |
| <h2 id="tyty"><a class="header" href="#tyty"><code>ty::Ty</code></a></h2> |
| <p>When we talk about how rustc represents types, we usually refer to a type called <code>Ty</code> . There are |
| quite a few modules and types for <code>Ty</code> in the compiler (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html">Ty documentation</a>).</p> |
| <p>The specific <code>Ty</code> we are referring to is <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html"><code>rustc_middle::ty::Ty</code></a> (and not |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Ty.html"><code>rustc_hir::Ty</code></a>). The distinction is important, so we will discuss it first before going |
| into the details of <code>ty::Ty</code>.</p> |
| <h2 id="rustc_hirty-vs-tyty"><a class="header" href="#rustc_hirty-vs-tyty"><code>rustc_hir::Ty</code> vs <code>ty::Ty</code></a></h2> |
| <p>The HIR in rustc can be thought of as the high-level intermediate representation. It is more or less |
| the AST (see <a href="hir.html">this chapter</a>) as it represents the |
| syntax that the user wrote, and is obtained after parsing and some <em>desugaring</em>. It has a |
| representation of types, but in reality it reflects more of what the user wrote, that is, what they |
| wrote so as to represent that type.</p> |
| <p>In contrast, <code>ty::Ty</code> represents the semantics of a type, that is, the <em>meaning</em> of what the user |
| wrote. For example, <code>rustc_hir::Ty</code> would record the fact that a user used the name <code>u32</code> twice |
| in their program, but the <code>ty::Ty</code> would record the fact that both usages refer to the same type.</p> |
| <p><strong>Example: <code>fn foo(x: u32) → u32 { x }</code></strong></p> |
| <p>In this function, we see that <code>u32</code> appears twice. We know |
| that that is the same type, |
| i.e. the function takes an argument and returns an argument of the same type, |
| but from the point of view of the HIR, |
| there would be two distinct type instances because these |
| are occurring in two different places in the program. |
| That is, they have two different <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html"><code>Span</code>s</a> (locations).</p> |
| <p><strong>Example: <code>fn foo(x: &u32) -> &u32</code></strong></p> |
| <p>In addition, HIR might have information left out. This type |
| <code>&u32</code> is incomplete, since in the full Rust type there is actually a lifetime, but we didn’t need |
| to write those lifetimes. There are also some elision rules that insert information. The result may |
| look like <code>fn foo<'a>(x: &'a u32) -> &'a u32</code>.</p> |
| <p>In the HIR level, these things are not spelled out and you can say the picture is rather incomplete. |
| However, at the <code>ty::Ty</code> level, these details are added and it is complete. Moreover, we will have |
| exactly one <code>ty::Ty</code> for a given type, like <code>u32</code>, and that <code>ty::Ty</code> is used for all <code>u32</code>s in the |
| whole program, not a specific usage, unlike <code>rustc_hir::Ty</code>.</p> |
| <p>Here is a summary:</p> |
| <div class="table-wrapper"> |
| <table> |
| <thead> |
| <tr><th><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Ty.html"><code>rustc_hir::Ty</code></a></th><th><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html"><code>ty::Ty</code></a></th></tr> |
| </thead> |
| <tbody> |
| <tr><td>Describe the <em>syntax</em> of a type: what the user wrote (with some desugaring).</td><td>Describe the <em>semantics</em> of a type: the meaning of what the user wrote.</td></tr> |
| <tr><td>Each <code>rustc_hir::Ty</code> has its own spans corresponding to the appropriate place in the program.</td><td>Doesn’t correspond to a single place in the user’s program.</td></tr> |
| <tr><td><code>rustc_hir::Ty</code> has generics and lifetimes; however, some of those lifetimes are special markers like <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.LifetimeKind.html#variant.Implicit"><code>LifetimeKind::Implicit</code></a>.</td><td><code>ty::Ty</code> has the full type, including generics and lifetimes, even if the user left them out</td></tr> |
| <tr><td><code>fn foo(x: u32) -> u32 { }</code> - Two <code>rustc_hir::Ty</code> representing each usage of <code>u32</code>, each has its own <code>Span</code>s, and <code>rustc_hir::Ty</code> doesn’t tell us that both are the same type</td><td><code>fn foo(x: u32) -> u32 { }</code> - One <code>ty::Ty</code> for all instances of <code>u32</code> throughout the program, and <code>ty::Ty</code> tells us that both usages of <code>u32</code> mean the same type.</td></tr> |
| <tr><td><code>fn foo(x: &u32) -> &u32 { }</code> - Two <code>rustc_hir::Ty</code> again. Lifetimes for the references show up in the <code>rustc_hir::Ty</code>s using a special marker, <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.LifetimeKind.html#variant.Implicit"><code>LifetimeKind::Implicit</code></a>.</td><td><code>fn foo(x: &u32) -> &u32 { }</code>- A single <code>ty::Ty</code>. The <code>ty::Ty</code> has the hidden lifetime param.</td></tr> |
| </tbody> |
| </table> |
| </div> |
| <p><strong>Order</strong></p> |
| <p>HIR is built directly from the AST, so it happens before any <code>ty::Ty</code> is produced. After |
| HIR is built, some basic type inference and type checking is done. During the type inference, we |
| figure out what the <code>ty::Ty</code> of everything is and we also check if the type of something is |
| ambiguous. The <code>ty::Ty</code> is then used for type checking while making sure everything has the |
| expected type. The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/hir_ty_lowering/index.html"><code>hir_ty_lowering</code> module</a> is where the code responsible for |
| lowering a <code>rustc_hir::Ty</code> to a <code>ty::Ty</code> is located. The main routine used is <code>lower_ty</code>. |
| This occurs during the type-checking phase, but also in other parts of the compiler that want to ask |
| questions like “what argument types does this function expect?”</p> |
| <p><strong>How semantics drive the two instances of <code>Ty</code></strong></p> |
| <p>You can think of HIR as the perspective |
| of the type information that assumes the least. We assume two things are distinct until they are |
| proven to be the same thing. In other words, we know less about them, so we should assume less about |
| them.</p> |
| <p>They are syntactically two strings: <code>"u32"</code> at line N column 20 and <code>"u32"</code> at line N column 35. We |
| don’t know that they are the same yet. So, in the HIR we treat them as if they are different. Later, |
| we determine that they semantically are the same type and that’s the <code>ty::Ty</code> we use.</p> |
| <p>Consider another example: <code>fn foo<T>(x: T) -> u32</code>. Suppose that someone invokes <code>foo::<u32>(0)</code>. |
| This means that <code>T</code> and <code>u32</code> (in this invocation) actually turns out to be the same type, so we |
| would eventually end up with the same <code>ty::Ty</code> in the end, but we have distinct <code>rustc_hir::Ty</code>. |
| (This is a bit over-simplified, though, since during type checking, we would check the function |
| generically and would still have a <code>T</code> distinct from <code>u32</code>. Later, when doing code generation, |
| we would always be handling “monomorphized” (fully substituted) versions of each function, |
| and hence we would know what <code>T</code> represents (and specifically that it is <code>u32</code>).)</p> |
| <p>Here is one more example:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>mod a { |
| type X = u32; |
| pub fn foo(x: X) -> u32 { 22 } |
| } |
| mod b { |
| type X = i32; |
| pub fn foo(x: X) -> i32 { x } |
| } |
| <span class="boring">}</span></code></pre> |
| <p>Here the type <code>X</code> will vary depending on context, clearly. If you look at the <code>rustc_hir::Ty</code>, |
| you will get back that <code>X</code> is an alias in both cases (though it will be mapped via name resolution |
| to distinct aliases). But if you look at the <code>ty::Ty</code> signature, it will be either <code>fn(u32) -> u32</code> |
| or <code>fn(i32) -> i32</code> (with type aliases fully expanded).</p> |
| <h2 id="tyty-implementation"><a class="header" href="#tyty-implementation"><code>ty::Ty</code> implementation</a></h2> |
| <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html"><code>rustc_middle::ty::Ty</code></a> is actually a wrapper around |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html"><code>Interned<WithCachedTypeInfo<TyKind>></code></a>. |
| You can ignore <code>Interned</code> in general; you will basically never access it explicitly. |
| We always hide them within <code>Ty</code> and skip over it via <code>Deref</code> impls or methods. |
| <code>TyKind</code> is a big enum |
| with variants to represent many different Rust types |
| (e.g. primitives, references, algebraic data types, generics, lifetimes, etc). |
| <code>WithCachedTypeInfo</code> has a few cached values like <code>flags</code> and <code>outer_exclusive_binder</code>. They |
| are convenient hacks for efficiency and summarize information about the type that we may want to |
| know, but they don’t come into the picture as much here. Finally, <a href="./memory.html"><code>Interned</code></a> allows |
| the <code>ty::Ty</code> to be a thin pointer-like |
| type. This allows us to do cheap comparisons for equality, along with the other |
| benefits of interning.</p> |
| <h2 id="allocating-and-working-with-types"><a class="header" href="#allocating-and-working-with-types">Allocating and working with types</a></h2> |
| <p>To allocate a new type, you can use the various <code>new_*</code> methods defined on |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html"><code>Ty</code></a>. |
| These have names |
| that correspond mostly to the various kinds of types. For example:</p> |
| <pre><code class="language-rust ignore">let array_ty = Ty::new_array_with_const_len(tcx, ty, count);</code></pre> |
| <p>These methods all return a <code>Ty<'tcx></code> – note that the lifetime you get back is the lifetime of the |
| arena that this <code>tcx</code> has access to. Types are always canonicalized and interned (so we never |
| allocate exactly the same type twice).</p> |
| <p>You can also find various common types in the <code>tcx</code> itself by accessing its fields: |
| <code>tcx.types.bool</code>, <code>tcx.types.char</code>, etc. (See <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.CommonTypes.html"><code>CommonTypes</code></a> for more.)</p> |
| <!-- N.B: This section is linked from the type comparison internal lint. --> |
| <h2 id="comparing-types"><a class="header" href="#comparing-types">Comparing types</a></h2> |
| <p>Because types are interned, it is possible to compare them for equality efficiently using <code>==</code> |
| – however, this is almost never what you want to do unless you happen to be hashing and looking |
| for duplicates. This is because often in Rust there are multiple ways to represent the same type, |
| particularly once inference is involved.</p> |
| <p>For example, the type <code>{integer}</code> (<code>ty::Infer(ty::IntVar(..))</code> an integer inference variable, |
| the type of an integer literal like <code>0</code>) and <code>u8</code> (<code>ty::UInt(..)</code>) should often be treated as |
| equal when testing whether they can be assigned to each other (which is a common operation in |
| diagnostics code). <code>==</code> on them will return <code>false</code> though, since they are different types.</p> |
| <p>The simplest way to compare two types correctly requires an inference context (<code>infcx</code>). |
| If you have one, you can use <code>infcx.can_eq(param_env, ty1, ty2)</code> |
| to check whether the types can be made equal. |
| This is typically what you want to check during diagnostics, which is concerned with questions such |
| as whether two types can be assigned to each other, not whether they’re represented identically in |
| the compiler’s type-checking layer.</p> |
| <p>When working with an inference context, you have to be careful to ensure that potential inference |
| variables inside the types actually belong to that inference context. If you are in a function |
| that has access to an inference context already, this should be the case. Specifically, this is the |
| case during HIR type checking or MIR borrow checking.</p> |
| <p>Another consideration is normalization. Two types may actually be the same, but one is behind an |
| associated type. To compare them correctly, you have to normalize the types first. This is |
| primarily a concern during HIR type checking and with all types from a <code>TyCtxt</code> query |
| (for example from <code>tcx.type_of()</code>).</p> |
| <p>When a <code>FnCtxt</code> or an <code>ObligationCtxt</code> is available during type checking, <code>.normalize(ty)</code> |
| should be used on them to normalize the type. After type checking, diagnostics code can use |
| <code>tcx.normalize_erasing_regions(ty)</code>.</p> |
| <p>There are also cases where using <code>==</code> on <code>Ty</code> is fine. This is for example the case in late lints |
| or after monomorphization, since type checking has been completed, meaning all inference variables |
| are resolved and all regions have been erased. In these cases, if you know that inference variables |
| or normalization won’t be a concern, <code>#[allow]</code> or <code>#[expect]</code>ing the lint is recommended.</p> |
| <p>When diagnostics code does not have access to an inference context, it should be threaded through |
| the function calls if one is available in some place (like during type checking).</p> |
| <p>If no inference context is available at all, then one can be created as described in |
| <a href="./type-inference.html#creating-an-inference-context">type-inference</a>. But this is only useful when the involved types (for example, if |
| they came from a query like <code>tcx.type_of()</code>) are actually substituted with fresh |
| inference variables using <a href="https://doc.rust-lang.org/beta/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.fresh_substs_for_item"><code>fresh_args_for_item</code></a>. This can be used to answer questions |
| like “can <code>Vec<T></code> for any <code>T</code> be unified with <code>Vec<u32></code>?”.</p> |
| <h2 id="tytykind-variants"><a class="header" href="#tytykind-variants"><code>ty::TyKind</code> Variants</a></h2> |
| <p>Note: <code>TyKind</code> is <strong>NOT</strong> the functional programming concept of <em>Kind</em>.</p> |
| <p>Whenever working with a <code>Ty</code> in the compiler, it is common to match on the kind of type:</p> |
| <pre><code class="language-rust ignore">fn foo(x: Ty<'tcx>) { |
| match x.kind { |
| ... |
| } |
| }</code></pre> |
| <p>The <code>kind</code> field is of type <code>TyKind<'tcx></code>, which is an enum defining all of the different kinds of |
| types in the compiler.</p> |
| <blockquote> |
| <p>N.B. inspecting the <code>kind</code> field on types during type inference can be risky, as there may be |
| inference variables and other things to consider, or sometimes types are not yet known and will |
| become known later.</p> |
| </blockquote> |
| <p>There are a lot of related types, and we’ll cover them in time (e.g regions/lifetimes, |
| “substitutions”, etc).</p> |
| <p>There are many variants on the <code>TyKind</code> enum, which you can see by looking at its |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html">documentation</a>. Here is a sampling:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Adt"><strong>Algebraic Data Types (ADTs)</strong></a> An <a href="https://en.wikipedia.org/wiki/Algebraic_data_type"><em>algebraic data type</em></a> is a <code>struct</code>, |
| <code>enum</code> or <code>union</code>. Under the hood, <code>struct</code>, <code>enum</code> and <code>union</code> are actually implemented |
| the same way: they are all <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Adt"><code>ty::TyKind::Adt</code></a>. It’s basically a user defined type. |
| We will talk more about these later.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Foreign"><strong>Foreign</strong></a> Corresponds to <code>extern type T</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Str"><strong>Str</strong></a> Is the type str. When the user writes <code>&str</code>, <code>Str</code> is the how we represent the |
| <code>str</code> part of that type.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Slice"><strong>Slice</strong></a> Corresponds to <code>[T]</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Array"><strong>Array</strong></a> Corresponds to <code>[T; n]</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.RawPtr"><strong>RawPtr</strong></a> Corresponds to <code>*mut T</code> or <code>*const T</code>.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Ref"><strong>Ref</strong></a> <code>Ref</code> stands for safe references, <code>&'a mut T</code> or <code>&'a T</code>. <code>Ref</code> has some |
| associated parts, like <code>Ty<'tcx></code> which is the type that the reference references. |
| <code>Region<'tcx></code> is the lifetime or region of the reference and <code>Mutability</code> if the reference |
| is mutable or not.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Param"><strong>Param</strong></a> Represents a type parameter (e.g. the <code>T</code> in <code>Vec<T></code>).</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variant.Error"><strong>Error</strong></a> Represents a type error somewhere so that we can print better diagnostics. We |
| will discuss this more later.</li> |
| <li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_type_ir/ty_kind/enum.TyKind.html#variants"><strong>And many more</strong>…</a></li> |
| </ul> |
| <h2 id="import-conventions"><a class="header" href="#import-conventions">Import conventions</a></h2> |
| <p>Although there is no hard and fast rule, the <code>ty</code> module tends to be used like so:</p> |
| <pre><code class="language-rust ignore">use ty::{self, Ty, TyCtxt};</code></pre> |
| <p>In particular, since they are so common, the <code>Ty</code> and <code>TyCtxt</code> types are imported directly. Other |
| types are often referenced with an explicit <code>ty::</code> prefix (e.g. <code>ty::TraitRef<'tcx></code>). But some |
| modules choose to import a larger or smaller set of names explicitly.</p> |
| <h2 id="type-errors"><a class="header" href="#type-errors">Type errors</a></h2> |
| <p>There is a <code>TyKind::Error</code> that is produced when the user makes a type error. The idea is that |
| we would propagate this type and suppress other errors that come up due to it so as not to overwhelm |
| the user with cascading compiler error messages.</p> |
| <p>There is an <strong>important invariant</strong> for <code>TyKind::Error</code>. The compiler should |
| <strong>never</strong> produce <code>Error</code> unless we <strong>know</strong> that an error has already been |
| reported to the user. This is usually |
| because (a) you just reported it right there or (b) you are propagating an existing Error type (in |
| which case the error should’ve been reported when that error type was produced).</p> |
| <p>It’s important to maintain this invariant because the whole point of the <code>Error</code> type is to suppress |
| other errors – i.e., we don’t report them. If we were to produce an <code>Error</code> type without actually |
| emitting an error to the user, then this could cause later errors to be suppressed, and the |
| compilation might inadvertently succeed!</p> |
| <p>Sometimes there is a third case. You believe that an error has been reported, but you believe it |
| would’ve been reported earlier in the compilation, not locally. In that case, you can create a |
| “delayed bug” with <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.DiagCtxt.html#method.delayed_bug"><code>delayed_bug</code></a> or <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.DiagCtxt.html#method.span_delayed_bug"><code>span_delayed_bug</code></a>. This will make a note that you expect |
| compilation to yield an error – if however compilation should succeed, then it will trigger a |
| compiler bug report.</p> |
| <p>For added safety, it’s not actually possible to produce a <code>TyKind::Error</code> value |
| outside of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html"><code>rustc_middle::ty</code></a>; there is a private member of |
| <code>TyKind::Error</code> that prevents it from being constructable elsewhere. Instead, |
| one should use the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.new_error"><code>Ty::new_error</code></a> or |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.new_error_with_message"><code>Ty::new_error_with_message</code></a> methods. These methods either take an <code>ErrorGuaranteed</code> |
| or call <code>span_delayed_bug</code> before returning an interned <code>Ty</code> of kind <code>Error</code>. If you |
| were already planning to use <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.DiagCtxt.html#method.span_delayed_bug"><code>span_delayed_bug</code></a>, then you can just pass the |
| span and message to <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#method.new_error_with_message"><code>ty_error_with_message</code></a> instead to avoid |
| a redundant delayed bug.</p> |
| <h2 id="tykind-variant-shorthand-syntax"><a class="header" href="#tykind-variant-shorthand-syntax"><code>TyKind</code> variant shorthand syntax</a></h2> |
| <p>When looking at the debug output of <code>Ty</code> or simply talking about different types in the compiler, you may encounter syntax that is not valid rust but is used to concisely represent internal information about types. Below is a quick reference cheat sheet to tell what the various syntax actually means, these should be covered in more depth in later chapters.</p> |
| <ul> |
| <li>Generic parameters: <code>{name}/#{index}</code> e.g. <code>T/#0</code>, where <code>index</code> corresponds to its position in the list of generic parameters</li> |
| <li>Inference variables: <code>?{id}</code> e.g. <code>?x</code>/<code>?0</code>, where <code>id</code> identifies the inference variable</li> |
| <li>Variables from binders: <code>^{binder}_{index}</code> e.g. <code>^0_x</code>/<code>^0_2</code>, where <code>binder</code> and <code>index</code> identify which variable from which binder is being referred to</li> |
| <li>Placeholders: <code>!{id}</code> or <code>!{id}_{universe}</code> e.g. <code>!x</code>/<code>!0</code>/<code>!x_2</code>/<code>!0_2</code>, representing some unique type in the specified universe. The universe is often elided when it is <code>0</code></li> |
| </ul> |
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