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The anonymous lifetime, denoted by '_, is a reserved lifetime identifier in Rust that explicitly instructs the compiler to infer a lifetime based on standard lifetime elision rules. It serves as a syntactic placeholder indicating that a type contains a borrowed reference, but the specific name of that lifetime parameter is irrelevant to the current scope or can be unambiguously deduced by the compiler.

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Mechanics of '_

The '_ marker does not introduce new borrowing rules; rather, it makes implicit lifetime elision explicit. When the compiler encounters '_, it applies the standard rules of lifetime elision to resolve the placeholder to a concrete, albeit unnamed, lifetime.

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1. Struct and Enum Instantiation in Signatures

When a function accepts or returns a custom type (like a struct or enum) that requires a lifetime parameter, Rust mandates that the lifetime be acknowledged. Using '_ satisfies the compiler’s requirement for explicit parameterization while deferring to elision rules. 
struct Context<'a> {
    data: &'a str,
}

// The compiler infers that the return type's lifetime is bound 
// to the single input reference `data`.
fn build_context(data: &str) -> Context<'_> {
    Context { data }
}
Equivalent desugared syntax: 
fn build_context<'a>(data: &'a str) -> Context<'a> { ... }

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2. Opaque Types (impl Trait)

When returning an opaque type that captures a reference, '_ can be used to explicitly declare that the returned impl Trait captures a lifetime from the function’s input parameters. This satisfies the compiler’s requirement for lifetime bounds on opaque return types without needing to name the lifetime. 
trait Processor {}

struct DataProcessor<'a> { 
    data: &'a [u8] 
}

impl<'a> Processor for DataProcessor<'a> {}

// `'_` binds the lifetime of the returned opaque type to the elided lifetime of `data`.
fn create_processor(data: &[u8]) -> impl Processor + '_ {
    DataProcessor { data }
}

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3. Trait Object Lifetime Bounds

By default, trait objects (dyn Trait) have an implicit 'static lifetime bound when used in certain contexts (like Box<dyn Trait>). The '_ marker overrides this default, instructing the compiler to infer the trait object’s lifetime bound from the surrounding function signature (e.g., an input parameter). 
trait Formatter {}

struct Context;

// Without `'_`, `Box<dyn Formatter>` defaults to `Box<dyn Formatter + 'static>`.
// With `'_`, it binds to the elided lifetime of `context`.
fn get_formatter(context: &Context) -> Box<dyn Formatter + '_> {
    // ...
    unimplemented!()
}

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Compiler Restrictions

The anonymous lifetime is subject to strict compiler constraints to prevent ambiguity:
  1. Implementing Types in impl Blocks: '_ is explicitly forbidden in the implementing type of an impl block (yielding compiler error E0637). Explicit, named lifetimes are strictly required. For example, attempting to write impl Drop for Parser<'_> is syntactically invalid and must be explicitly parameterized as impl<'a> Drop for Parser<'a>.
  2. Multiple Input Lifetimes: If a function signature contains multiple input lifetimes and no &self or &mut self parameter, the compiler cannot unambiguously resolve '_ in the return type. This will result in a compilation error requiring explicit, named lifetimes.
  3. In-Struct Definitions: '_ cannot be used when defining the fields of a struct or enum. Struct definitions require explicit, named lifetime parameters to establish the structural bounds of the data.
  4. Binding Multiple Outputs: You cannot use '_ to tie two distinct output types to the same input lifetime if the elision rules do not naturally map them. Named lifetimes must be used to enforce identical bounds across multiple return values.
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