Rustdoc search

Typing in the search bar instantly searches the available documentation, matching either the name and path of an item, or a function's approximate type signature.

Search By Name

To search by the name of an item (items include modules, types, traits, functions, and macros), write its name or path. As a special case, the parts of a path that normally get divided by :: double colons can instead be separated by spaces. For example:

• vec new and vec::new both show the function std::vec::Vec::new as a result.
• vec, vec vec, std::vec, and std::vec::Vec all include the struct std::vec::Vec itself in the results (and all but the last one also include the module in the results).

As a quick way to trim down the list of results, there's a drop-down selector below the search input, labeled "Results in [std]". Clicking it can change which crate is being searched.

Rustdoc uses a fuzzy matching function that can tolerate typos for this, though it's based on the length of the name that's typed in, so a good example of how this works would be HahsMap. To avoid this, wrap the item in quotes, searching for "HahsMap" (in this example, no results will be returned).

Tabs in the Search By Name interface

In fact, using HahsMap again as the example, it tells you that you're using "In Names" by default, but also lists two other tabs below the crate drop-down: "In Parameters" and "In Return Types".

These two tabs are lists of functions, defined on the closest matching type to the search (for HahsMap, it loudly auto-corrects to hashmap). This auto-correct only kicks in if nothing is found that matches the literal.

These tabs are not just methods. For example, searching the alloc crate for Layout also lists functions that accept layouts even though they're methods on the allocator or free functions.

Searching By Type Signature for functions

If you know more specifically what the function you want to look at does, Rustdoc can search by more than one type at once in the parameters and return value. Multiple parameters are separated by , commas, and the return value is written with after a -> arrow.

Before describing the syntax in more detail, here's a few sample searches of the standard library and functions that are included in the results list:

How type-based search works

In a complex type-based search, Rustdoc always treats every item's name as literal. If a name is used and nothing in the docs matches the individual item, such as a typo-ed uize -> vec search, the item uize is treated as a generic type parameter (resulting in vec::from and other generic vec constructors).

After deciding which items are type parameters and which are actual types, it then searches by matching up the function parameters (written before the ->) and the return types (written after the ->). Type matching is order-agnostic, and allows items to be left out of the query, but items that are present in the query must be present in the function for it to match. The self parameter is treated the same as any other parameter, and Self is resolved to the underlying type's name.

Function signature searches can query generics, wrapped in angle brackets, and traits will be normalized like types in the search engine if no type parameters match them. For example, a function with the signature fn my_function<I: Iterator<Item=u32>>(input: I) -> usize can be matched with the following queries:

• Iterator<Item=u32> -> usize
• Iterator<u32> -> usize (you can leave out the Item= part)
• Iterator -> usize (you can leave out iterator's generic entirely)
• T -> usize (you can match with a generic parameter)

Each of the above queries is progressively looser, except the last one would not match dyn Iterator, since that's not a type parameter.

If a bound has multiple associated types, specifying the name allows you to pick which one gets matched. If no name is specified, then the query will match of any of them. For example,

#![allow(unused)]
fn main() {
pub trait MyTrait {
    type First;
    type Second;
}

/// This function can be found using the following search queries:
///
///     MyTrait<First=u8, Second=u32> -> bool
///     MyTrait<Second=u32> -> bool
///
/// The following queries, however, will *not* match it:
///
///     MyTrait<First=u32> -> bool
///     MyTrait<u32, u32> -> bool
///     MyTrait<u32, First=u8> -> bool
///     MyTrait<u32, u8> -> bool
pub fn my_fn(x: impl MyTrait<First=u8, Second=u32>) -> bool { true }
}

Function parameters are order-agnostic, but sensitive to nesting and number of matches. For example, a function with the signature fn read_all(&mut self: impl Read) -> Result<Vec<u8>, Error> will match these queries:

• &mut Read -> Result<Vec<u8>, Error>
• Read -> Result<Vec<u8>, Error>
• Read -> Result<Vec<u8>>
• Read -> u8

But it does not match Result<Vec, u8> or Result<u8<Vec>>, because those are nested incorrectly, and it does not match Result<Error, Vec<u8>> or Result<Error>, because those are in the wrong order.

To search for a function that accepts a function as a parameter, like Iterator::all, wrap the nested signature in parenthesis, as in Iterator<T>, (T -> bool) -> bool. You can also search for a specific closure trait, such as Iterator<T>, (FnMut(T) -> bool) -> bool, but you need to know which one you want.

Primitives with Special Syntax

When searching for [], Rustdoc will return search results with either slices or arrays. If you know which one you want, you can force it to return results for primitive:slice or primitive:array using the explicit name syntax. Empty square brackets, [], will match any slice or array regardless of what it contains, or an item type can be provided, such as [u8] or [T], to explicitly find functions that operate on byte slices or generic slices, respectively.

A single type expression wrapped in parens is the same as that type expression, since parens act as the grouping operator. If they're empty, though, they will match both unit and tuple, and if there's more than one type (or a trailing or leading comma) it is the same as primitive:tuple<...>.

However, since items can be left out of the query, (T) will still return results for types that match tuples, even though it also matches the type on its own. That is, (u32) matches (u32,) for the exact same reason that it also matches Result<u32, Error>.

The -> operator has lower precedence than comma. If it's not wrapped in brackets, it delimits the return value for the function being searched for. To search for functions that take functions as parameters, use parenthesis.

Limitations and quirks of type-based search

Type-based search is still a buggy, experimental, work-in-progress feature. Most of these limitations should be addressed in future version of Rustdoc.

• There's no way to write trait constraints on generic parameters. You can name traits directly, and if there's a type parameter with that bound, it'll match, but option<T> -> T where T: Default cannot be precisely searched for (use option<Default> -> Default).

• Supertraits, type aliases, and Deref are all ignored. Search mostly operates on type signatures as written, and not as they are represented within the compiler.

• Type parameters match type parameters, such that Option<A> matches Option<T>, but never match concrete types in function signatures. A trait named as if it were a type, such as Option<Read>, will match a type parameter constrained by that trait, such as Option<T> where T: Read, as well as matching dyn Trait and impl Trait.

• impl Trait in argument position is treated exactly like a type parameter, but in return position it will not match type parameters.

• Any type named in a complex type-based search will be assumed to be a type parameter if nothing matching the name exactly is found. If you want to force a type parameter, write generic:T and it will be used as a type parameter even if a matching name is found. If you know that you don't want a type parameter, you can force it to match something else by giving it a different prefix like struct:T.

• It's impossible to search for references or pointers. The wrapped types can be searched for, so a function that takes &File can be found with File, but you'll get a parse error when typing an & into the search field.

• Searching for lifetimes is not supported.

• It's impossible to search based on the length of an array.

Item filtering

Names in the search interface can be prefixed with an item type followed by a colon (such as mod:) to restrict the results to just that kind of item. Also, searching for println! will search for a macro named println, just like searching for macro:println does. The complete list of available filters is given under the ? Help area, and in the detailed syntax below.

Item filters can be used in both name-based and type signature-based searches.

Search query syntax

ident = *(ALPHA / DIGIT / "_")
path = ident *(DOUBLE-COLON ident) [BANG]
slice-like = OPEN-SQUARE-BRACKET [ nonempty-arg-list ] CLOSE-SQUARE-BRACKET
tuple-like = OPEN-PAREN [ nonempty-arg-list ] CLOSE-PAREN
borrow-ref = AMP *WS [MUT] *WS [arg]
arg = [type-filter *WS COLON *WS] (path [generics] / slice-like / tuple-like / borrow-ref)
type-sep = COMMA/WS *(COMMA/WS)
nonempty-arg-list = *(type-sep) arg *(type-sep arg) *(type-sep) [ return-args ]
generic-arg-list = *(type-sep) arg [ EQUAL arg ] *(type-sep arg [ EQUAL arg ]) *(type-sep)
normal-generics = OPEN-ANGLE-BRACKET [ generic-arg-list ] *(type-sep)
            CLOSE-ANGLE-BRACKET
fn-like-generics = OPEN-PAREN [ nonempty-arg-list ] CLOSE-PAREN [ RETURN-ARROW arg ]
generics = normal-generics / fn-like-generics
return-args = RETURN-ARROW *(type-sep) nonempty-arg-list

exact-search = [type-filter *WS COLON] [ RETURN-ARROW ] *WS QUOTE ident QUOTE [ generics ]
type-search = [ nonempty-arg-list ]

query = *WS (exact-search / type-search) *WS

type-filter = (
    "mod" /
    "externcrate" /
    "import" /
    "struct" /
    "enum" /
    "fn" /
    "type" /
    "static" /
    "trait" /
    "impl" /
    "tymethod" /
    "method" /
    "structfield" /
    "variant" /
    "macro" /
    "primitive" /
    "associatedtype" /
    "constant" /
    "associatedconstant" /
    "union" /
    "foreigntype" /
    "keyword" /
    "existential" /
    "attr" /
    "derive" /
    "traitalias" /
    "generic")

OPEN-ANGLE-BRACKET = "<"
CLOSE-ANGLE-BRACKET = ">"
OPEN-SQUARE-BRACKET = "["
CLOSE-SQUARE-BRACKET = "]"
OPEN-PAREN = "("
CLOSE-PAREN = ")"
COLON = ":"
DOUBLE-COLON = "::"
QUOTE = %x22
COMMA = ","
RETURN-ARROW = "->"
EQUAL = "="
BANG = "!"
AMP = "&"
MUT = "mut"

ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39
WS = %x09 / " "