diff --git a/crates/ide-assists/src/handlers/convert_comment_from_or_to_doc.rs b/crates/ide-assists/src/handlers/convert_comment_from_or_to_doc.rs
new file mode 100644
index 0000000000..953119fd1f
--- /dev/null
+++ b/crates/ide-assists/src/handlers/convert_comment_from_or_to_doc.rs
@@ -0,0 +1,685 @@
+use itertools::Itertools;
+use syntax::{
+    ast::{self, edit::IndentLevel, Comment, CommentPlacement, Whitespace},
+    AstToken, Direction, SyntaxElement, TextRange,
+};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: comment_to_doc
+//
+// Converts comments to documentation.
+//
+// ```
+// // Wow what $0a nice module
+// // I sure hope this shows up when I hover over it
+// ```
+// ->
+// ```
+// //! Wow what a nice module
+// //! I sure hope this shows up when I hover over it
+// ```
+pub(crate) fn convert_comment_from_or_to_doc(
+    acc: &mut Assists,
+    ctx: &AssistContext<'_>,
+) -> Option<()> {
+    let comment = ctx.find_token_at_offset::<ast::Comment>()?;
+
+    match comment.kind().doc {
+        Some(_) => doc_to_comment(acc, comment),
+        None => can_be_doc_comment(&comment).and_then(|style| comment_to_doc(acc, comment, style)),
+    }
+}
+
+fn doc_to_comment(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
+    let target = if comment.kind().shape.is_line() {
+        line_comments_text_range(&comment)?
+    } else {
+        comment.syntax().text_range()
+    };
+
+    acc.add(
+        AssistId("doc_to_comment", AssistKind::RefactorRewrite),
+        "Replace comment with doc comment",
+        target,
+        |edit| {
+            // We need to either replace the first occurrence of /* with /***, or we need to replace
+            // the occurrences // at the start of each line with ///
+            let output = match comment.kind().shape {
+                ast::CommentShape::Line => {
+                    let indentation = IndentLevel::from_token(comment.syntax());
+                    let line_start = comment.prefix();
+                    let prefix = format!("{indentation}//");
+                    relevant_line_comments(&comment)
+                        .iter()
+                        .map(|comment| comment.text())
+                        .flat_map(|text| text.lines())
+                        .map(|line| line.replacen(line_start, &prefix, 1))
+                        .join("\n")
+                }
+                ast::CommentShape::Block => {
+                    let block_start = comment.prefix();
+                    comment
+                        .text()
+                        .lines()
+                        .enumerate()
+                        .map(|(idx, line)| {
+                            if idx == 0 {
+                                line.replacen(block_start, "/*", 1)
+                            } else {
+                                line.replacen("*  ", "* ", 1)
+                            }
+                        })
+                        .join("\n")
+                }
+            };
+            edit.replace(target, output)
+        },
+    )
+}
+
+fn comment_to_doc(acc: &mut Assists, comment: ast::Comment, style: CommentPlacement) -> Option<()> {
+    let target = if comment.kind().shape.is_line() {
+        line_comments_text_range(&comment)?
+    } else {
+        comment.syntax().text_range()
+    };
+
+    acc.add(
+        AssistId("comment_to_doc", AssistKind::RefactorRewrite),
+        "Replace doc comment with comment",
+        target,
+        |edit| {
+            // We need to either replace the first occurrence of /* with /***, or we need to replace
+            // the occurrences // at the start of each line with ///
+            let output = match comment.kind().shape {
+                ast::CommentShape::Line => {
+                    let indentation = IndentLevel::from_token(comment.syntax());
+                    let line_start = match style {
+                        CommentPlacement::Inner => format!("{indentation}//!"),
+                        CommentPlacement::Outer => format!("{indentation}///"),
+                    };
+                    relevant_line_comments(&comment)
+                        .iter()
+                        .map(|comment| comment.text())
+                        .flat_map(|text| text.lines())
+                        .map(|line| line.replacen("//", &line_start, 1))
+                        .join("\n")
+                }
+                ast::CommentShape::Block => {
+                    let block_start = match style {
+                        CommentPlacement::Inner => "/*!",
+                        CommentPlacement::Outer => "/**",
+                    };
+                    comment
+                        .text()
+                        .lines()
+                        .enumerate()
+                        .map(|(idx, line)| {
+                            if idx == 0 {
+                                // On the first line we replace the comment start with a doc comment
+                                // start.
+                                line.replacen("/*", block_start, 1)
+                            } else {
+                                // put one extra space after each * since we moved the first line to
+                                // the right by one column as well.
+                                line.replacen("* ", "*  ", 1)
+                            }
+                        })
+                        .join("\n")
+                }
+            };
+            edit.replace(target, output)
+        },
+    )
+}
+
+/// Not all comments are valid candidates for conversion into doc comments. For example, the
+/// comments in the code:
+/// ```rust
+/// // Brilliant module right here
+///
+/// // Really good right
+/// fn good_function(foo: Foo) -> Bar {
+///     foo.into_bar()
+/// }
+///
+/// // So nice
+/// mod nice_module {}
+/// ```
+/// can be converted to doc comments. However, the comments in this example:
+/// ```rust
+/// fn foo_bar(foo: Foo /* not bar yet */) -> Bar {
+///     foo.into_bar()
+///     // Nicely done
+/// }
+/// // end of function
+///
+/// struct S {
+///     // The S struct
+/// }
+/// ```
+/// are not allowed to become doc comments. Moreover, some comments _are_ allowed, but aren't common
+/// style in Rust. For example, the following comments are allowed to be doc comments, but it is not
+/// common style for them to be:
+/// ```rust
+/// fn foo_bar(foo: Foo) -> Bar {
+///     // this could be an inner comment with //!
+///     foo.into_bar()
+/// }
+///
+/// trait T {
+///     // The T struct could also be documented from within
+/// }
+///
+/// mod mymod {
+///     // Modules only normally get inner documentation when they are defined as a separate file.
+/// }
+/// ```
+fn can_be_doc_comment(comment: &ast::Comment) -> Option<CommentPlacement> {
+    use syntax::SyntaxKind::*;
+
+    // if the comment is not on its own line, then we do not propose anything.
+    match comment.syntax().prev_token() {
+        Some(prev) => {
+            // There was a previous token, now check if it was a newline
+            Whitespace::cast(prev).filter(|w| w.text().contains('\n'))?;
+        }
+        // There is no previous token, this is the start of the file.
+        None => return Some(CommentPlacement::Inner),
+    }
+
+    // check if comment is followed by: `struct`, `trait`, `mod`, `fn`, `type`, `extern crate`,
+    // `use` or `const`.
+    let parent = comment.syntax().parent();
+    let par_kind = parent.as_ref().map(|parent| parent.kind());
+    matches!(par_kind, Some(STRUCT | TRAIT | MODULE | FN | TYPE_ALIAS | EXTERN_CRATE | USE | CONST))
+        .then_some(CommentPlacement::Outer)
+}
+
+/// The line -> block assist can  be invoked from anywhere within a sequence of line comments.
+/// relevant_line_comments crawls backwards and forwards finding the complete sequence of comments that will
+/// be joined.
+pub(crate) fn relevant_line_comments(comment: &ast::Comment) -> Vec<Comment> {
+    // The prefix identifies the kind of comment we're dealing with
+    let prefix = comment.prefix();
+    let same_prefix = |c: &ast::Comment| c.prefix() == prefix;
+
+    // These tokens are allowed to exist between comments
+    let skippable = |not: &SyntaxElement| {
+        not.clone()
+            .into_token()
+            .and_then(Whitespace::cast)
+            .map(|w| !w.spans_multiple_lines())
+            .unwrap_or(false)
+    };
+
+    // Find all preceding comments (in reverse order) that have the same prefix
+    let prev_comments = comment
+        .syntax()
+        .siblings_with_tokens(Direction::Prev)
+        .filter(|s| !skippable(s))
+        .map(|not| not.into_token().and_then(Comment::cast).filter(same_prefix))
+        .take_while(|opt_com| opt_com.is_some())
+        .flatten()
+        .skip(1); // skip the first element so we don't duplicate it in next_comments
+
+    let next_comments = comment
+        .syntax()
+        .siblings_with_tokens(Direction::Next)
+        .filter(|s| !skippable(s))
+        .map(|not| not.into_token().and_then(Comment::cast).filter(same_prefix))
+        .take_while(|opt_com| opt_com.is_some())
+        .flatten();
+
+    let mut comments: Vec<_> = prev_comments.collect();
+    comments.reverse();
+    comments.extend(next_comments);
+    comments
+}
+
+fn line_comments_text_range(comment: &ast::Comment) -> Option<TextRange> {
+    let comments = relevant_line_comments(comment);
+    let first = comments.first()?;
+    let indentation = IndentLevel::from_token(first.syntax());
+    let start =
+        first.syntax().text_range().start().checked_sub((indentation.0 as u32 * 4).into())?;
+    let end = comments.last()?.syntax().text_range().end();
+    Some(TextRange::new(start, end))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn module_comment_to_doc() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            // such a nice module$0
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+            //! such a nice module
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_line_comment_to_doc() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            // unseen$0 docs
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /// unseen docs
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_line_comment_to_doc() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            // unseen$0 docs
+            // make me seen!
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /// unseen docs
+            /// make me seen!
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_line_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /// visible$0 docs
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            // visible docs
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_line_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /// visible$0 docs
+            /// Hide me!
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            // visible docs
+            // Hide me!
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_line_block_comment_to_doc() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /* unseen$0 docs */
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /** unseen docs */
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_line_block_comment_to_doc() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /* unseen$0 docs
+            *  make me seen!
+            */
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /** unseen docs
+            *   make me seen!
+            */
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_line_block_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /** visible$0 docs */
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /* visible docs */
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_line_block_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+
+            /** visible$0 docs
+            *   Hide me!
+            */
+            fn main() {
+                foo();
+            }
+            "#,
+            r#"
+
+            /* visible docs
+            *  Hide me!
+            */
+            fn main() {
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_inner_line_comment_to_doc() {
+        check_assist_not_applicable(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                // unseen$0 docs
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_inner_line_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                //! visible$0 docs
+                foo();
+            }
+            "#,
+            r#"
+            mod mymod {
+                // visible docs
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_inner_line_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                //! visible$0 docs
+                //! Hide me!
+                foo();
+            }
+            "#,
+            r#"
+            mod mymod {
+                // visible docs
+                // Hide me!
+                foo();
+            }
+            "#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                /// visible$0 docs
+                /// Hide me!
+                foo();
+            }
+            "#,
+            r#"
+            mod mymod {
+                // visible docs
+                // Hide me!
+                foo();
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn single_inner_line_block_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                /*! visible$0 docs */
+                type Int = i32;
+            }
+            "#,
+            r#"
+            mod mymod {
+                /* visible docs */
+                type Int = i32;
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn multi_inner_line_block_doc_to_comment() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                /*! visible$0 docs
+                *   Hide me!
+                */
+                type Int = i32;
+            }
+            "#,
+            r#"
+            mod mymod {
+                /* visible docs
+                *  Hide me!
+                */
+                type Int = i32;
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn not_overeager() {
+        check_assist_not_applicable(
+            convert_comment_from_or_to_doc,
+            r#"
+            fn main() {
+                foo();
+                // $0well that settles main
+            }
+            // $1 nicely done
+            "#,
+        );
+    }
+
+    #[test]
+    fn all_possible_items() {
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice struct$0 */
+                struct S {}
+            }"#,
+            r#"mod m {
+                /** Nice struct */
+                struct S {}
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice trait$0 */
+                trait T {}
+            }"#,
+            r#"mod m {
+                /** Nice trait */
+                trait T {}
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice module$0 */
+                mod module {}
+            }"#,
+            r#"mod m {
+                /** Nice module */
+                mod module {}
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice function$0 */
+                fn function() {}
+            }"#,
+            r#"mod m {
+                /** Nice function */
+                fn function() {}
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice type$0 */
+                type Type Int = i32;
+            }"#,
+            r#"mod m {
+                /** Nice type */
+                type Type Int = i32;
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice crate$0 */
+                extern crate rust_analyzer;
+            }"#,
+            r#"mod m {
+                /** Nice crate */
+                extern crate rust_analyzer;
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice import$0 */
+                use ide_assists::convert_comment_from_or_to_doc::tests
+            }"#,
+            r#"mod m {
+                /** Nice import */
+                use ide_assists::convert_comment_from_or_to_doc::tests
+            }"#,
+        );
+        check_assist(
+            convert_comment_from_or_to_doc,
+            r#"mod m {
+                /* Nice constant$0 */
+                const CONST: &str = "very const";
+            }"#,
+            r#"mod m {
+                /** Nice constant */
+                const CONST: &str = "very const";
+            }"#,
+        );
+    }
+
+    #[test]
+    fn no_inner_comments() {
+        check_assist_not_applicable(
+            convert_comment_from_or_to_doc,
+            r#"
+            mod mymod {
+                // aaa$0aa
+            }
+            "#,
+        );
+    }
+}
diff --git a/crates/ide-assists/src/lib.rs b/crates/ide-assists/src/lib.rs
index 0df5e913a5..cbaf03e4d1 100644
--- a/crates/ide-assists/src/lib.rs
+++ b/crates/ide-assists/src/lib.rs
@@ -116,6 +116,7 @@ mod handlers {
     mod change_visibility;
     mod convert_bool_then;
     mod convert_comment_block;
+    mod convert_comment_from_or_to_doc;
     mod convert_from_to_tryfrom;
     mod convert_integer_literal;
     mod convert_into_to_from;
@@ -239,6 +240,7 @@ mod handlers {
             convert_bool_then::convert_bool_then_to_if,
             convert_bool_then::convert_if_to_bool_then,
             convert_comment_block::convert_comment_block,
+            convert_comment_from_or_to_doc::convert_comment_from_or_to_doc,
             convert_from_to_tryfrom::convert_from_to_tryfrom,
             convert_integer_literal::convert_integer_literal,
             convert_into_to_from::convert_into_to_from,
diff --git a/crates/ide-assists/src/tests/generated.rs b/crates/ide-assists/src/tests/generated.rs
index 937e78f8d7..5ecce3cbb6 100644
--- a/crates/ide-assists/src/tests/generated.rs
+++ b/crates/ide-assists/src/tests/generated.rs
@@ -345,6 +345,21 @@ pub(crate) fn frobnicate() {}
     )
 }
 
+#[test]
+fn doctest_comment_to_doc() {
+    check_doc_test(
+        "comment_to_doc",
+        r#####"
+// Wow what $0a nice module
+// I sure hope this shows up when I hover over it
+"#####,
+        r#####"
+//! Wow what a nice module
+//! I sure hope this shows up when I hover over it
+"#####,
+    )
+}
+
 #[test]
 fn doctest_convert_bool_then_to_if() {
     check_doc_test(