> ## 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 Line Comment
A line comment in Rust is a lexical construct that instructs the compiler to ignore all text following a specific token sequence up to the end of the current line. It is initiated with a double forward slash (`//`) and terminates automatically at the next newline character (`\n` or `\r\n`).
During the lexical analysis phase, the Rust compiler's lexer discards the `//` token and all subsequent characters on that line, preventing them from being parsed as executable code or converted into Abstract Syntax Tree (AST) nodes.
**Syntax and Parsing Rules:**
* **Initiation:** Must begin with exactly two forward slashes (`//`). Modifying this sequence by adding a third slash (`///`) or an exclamation mark (`//!`) alters the lexical semantics. These variations transform the sequence into an outer or inner documentation comment, respectively, which the compiler parses into `#[doc]` attributes rather than discarding. However, adding four or more slashes (e.g., `////`) reverts the sequence back to being parsed as a standard, non-documentation line comment.
* **Placement:** Can be positioned at the beginning of a line or trailing immediately after any valid Rust tokens. This includes statements, expressions, items (such as `struct` or `fn` declarations), attributes, and macros.
* **Evaluation:** Line comments have no concept of nesting. Any subsequent comment tokens (such as `//` or `/*`) that appear after the initial `//` on the same line are treated strictly as ignored plain text.
```rust theme={"dark"}
// This is a standard line comment occupying the entire line.
fn main() {
let _x = 5; // This is a trailing line comment following a statement.
//// This is parsed as a standard line comment, not a documentation comment.
// The lexer treats the following as ignored plain text, not as nested comments:
// // Double slashes inside a line comment
// /* Block comment syntax inside a line comment */
}
```
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