> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Rust Block Comment
A block comment in Rust is a lexical construct used to instruct the compiler to ignore a specific sequence of characters during the lexical analysis phase. It is delimited by `/*` at the beginning and `*/` at the end, allowing the enclosed text to span multiple lines or be embedded directly within a single statement.
**Standard Syntax**
The lexer strips the comment and treats everything between the opening and closing delimiters as whitespace before parsing begins.
```rust theme={"dark"}
/*
This text is ignored by the Rust compiler.
It can span multiple lines safely.
*/
```
**Inline Evaluation**
Because the Rust lexer evaluates block comments as whitespace, they can be placed inline within expressions without breaking the abstract syntax tree (AST).
```rust theme={"dark"}
let total = 100 + /* offset */ 50;
```
**Nesting Mechanics**
Unlike C and C++, Rust's lexical analyzer explicitly supports nested block comments. The compiler maintains a depth counter for the delimiters; every opening `/*` found within an existing block comment requires a corresponding closing `*/` before the outermost comment is considered terminated. This prevents premature termination when commenting out blocks of code that already contain block comments.
```rust theme={"dark"}
/* Level 1 block comment begins
/* Level 2 nested comment begins
Level 2 nested comment ends */
Level 1 block comment ends */
```
**Block Documentation Comments**
Rust differentiates standard block comments from block documentation comments via a specific character immediately following the opening delimiter. Instead of being entirely ignored, the compiler parses these into `#[doc]` attributes.
* **Outer block doc comment (`/** ... */`):** Applies to the item immediately following it. Parses as `#[doc = "..."]`.
* **Inner block doc comment (`/*! ... */`):** Applies to the *enclosing item* (such as a module, crate, or function). Parses as `#![doc = "..."]`.
```rust theme={"dark"}
/**
* This is an outer block documentation comment.
* It is parsed into a doc attribute for the function below.
*/
pub fn execute() {
/*!
* This is an inner block documentation comment.
* It applies to the enclosing item (the `execute` function itself),
* appending this text to the function's overall documentation.
*/
}
```
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