diff --git a/crates/ide_assists/src/handlers/convert_comment_block.rs b/crates/ide_assists/src/handlers/convert_comment_block.rs
new file mode 100644
index 0000000000..cdc45fc42c
--- /dev/null
+++ b/crates/ide_assists/src/handlers/convert_comment_block.rs
@@ -0,0 +1,419 @@
+use itertools::Itertools;
+use std::convert::identity;
+use syntax::{
+    ast::{
+        self,
+        edit::IndentLevel,
+        Comment, CommentKind,
+        CommentPlacement::{Inner, Outer},
+        CommentShape::{self, Block, Line},
+        Whitespace,
+    },
+    AstToken, Direction, SyntaxElement, TextRange,
+};
+
+use crate::{AssistContext, AssistId, AssistKind, Assists};
+
+/// Assist: line_to_block
+///
+/// Converts comments between block and single-line form
+///
+/// ```
+///    // Multi-line
+///    // comment
+/// ```
+/// ->
+/// ```
+///   /**
+///   Multi-line
+///   comment
+///   */
+/// ```
+pub(crate) fn convert_comment_block(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    if let Some(comment) = ctx.find_token_at_offset::<ast::Comment>() {
+        // Only allow comments which are alone on their line
+        if let Some(prev) = comment.syntax().prev_token() {
+            if Whitespace::cast(prev).filter(|w| w.text().contains('\n')).is_none() {
+                return None;
+            }
+        }
+
+        return match comment.kind().shape {
+            ast::CommentShape::Block => block_to_line(acc, comment),
+            ast::CommentShape::Line => line_to_block(acc, comment),
+        };
+    }
+
+    return None;
+}
+
+fn block_to_line(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
+    let target = comment.syntax().text_range();
+
+    acc.add(
+        AssistId("block_to_line", AssistKind::RefactorRewrite),
+        "Replace block comment with line comments",
+        target,
+        |edit| {
+            let indentation = IndentLevel::from_token(comment.syntax());
+            let line_prefix =
+                comment_kind_prefix(CommentKind { shape: CommentShape::Line, ..comment.kind() });
+
+            let text = comment.text();
+            let text = &text[comment.prefix().len()..(text.len() - "*/".len())].trim();
+
+            let lines = text.lines().peekable();
+
+            let indent_spaces = indentation.to_string();
+            let output = lines
+                .map(|l| l.trim_start_matches(&indent_spaces))
+                .map(|l| {
+                    // Don't introduce trailing whitespace
+                    if l.is_empty() {
+                        line_prefix.to_string()
+                    } else {
+                        format!("{} {}", line_prefix, l.trim_start_matches(&indent_spaces))
+                    }
+                })
+                .join(&format!("\n{}", indent_spaces));
+
+            edit.replace(target, output)
+        },
+    )
+}
+
+fn line_to_block(acc: &mut Assists, comment: ast::Comment) -> Option<()> {
+    // Find all the comments we'll be collapsing into a block
+    let comments = relevant_line_comments(&comment);
+
+    // Establish the target of our edit based on the comments we found
+    let target = TextRange::new(
+        comments[0].syntax().text_range().start(),
+        comments.last().unwrap().syntax().text_range().end(),
+    );
+
+    acc.add(
+        AssistId("line_to_block", AssistKind::RefactorRewrite),
+        "Replace line comments with a single block comment",
+        target,
+        |edit| {
+            // We pick a single indentation level for the whole block comment based on the
+            // comment where the assist was invoked. This will be prepended to the
+            // contents of each line comment when they're put into the block comment.
+            let indentation = IndentLevel::from_token(&comment.syntax());
+
+            let block_comment_body =
+                comments.into_iter().map(|c| line_comment_text(indentation, c)).join("\n");
+
+            let block_prefix =
+                comment_kind_prefix(CommentKind { shape: CommentShape::Block, ..comment.kind() });
+
+            let output =
+                format!("{}\n{}\n{}*/", block_prefix, block_comment_body, indentation.to_string());
+
+            edit.replace(target, output)
+        },
+    )
+}
+
+/// 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.
+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())
+        .filter_map(identity)
+        .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())
+        .filter_map(identity);
+
+    let mut comments: Vec<_> = prev_comments.collect();
+    comments.reverse();
+    comments.extend(next_comments);
+    comments
+}
+
+// Line comments usually begin with a single space character following the prefix as seen here:
+//^
+// But comments can also include indented text:
+//    > Hello there
+//
+// We handle this by stripping *AT MOST* one space character from the start of the line
+// This has its own problems because it can cause alignment issues:
+//
+//              /*
+// a      ----> a
+//b       ----> b
+//              */
+//
+// But since such comments aren't idiomatic we're okay with this.
+fn line_comment_text(indentation: IndentLevel, comm: ast::Comment) -> String {
+    let contents_without_prefix = comm.text().strip_prefix(comm.prefix()).unwrap();
+    let contents = contents_without_prefix.strip_prefix(' ').unwrap_or(contents_without_prefix);
+
+    // Don't add the indentation if the line is empty
+    if contents.is_empty() {
+        contents.to_owned()
+    } else {
+        indentation.to_string() + &contents
+    }
+}
+
+fn comment_kind_prefix(ck: ast::CommentKind) -> &'static str {
+    match (ck.shape, ck.doc) {
+        (Line, Some(Inner)) => "//!",
+        (Line, Some(Outer)) => "///",
+        (Line, None) => "//",
+        (Block, Some(Inner)) => "/*!",
+        (Block, Some(Outer)) => "/**",
+        (Block, None) => "/*",
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn single_line_to_block() {
+        check_assist(
+            convert_comment_block,
+            r#"
+// line$0 comment
+fn main() {
+    foo();
+}
+"#,
+            r#"
+/*
+line comment
+*/
+fn main() {
+    foo();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn single_line_to_block_indented() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // line$0 comment
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /*
+    line comment
+    */
+    foo();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn multiline_to_block() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // above
+    // line$0 comment
+    //
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /*
+    above
+    line comment
+
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn end_of_line_to_block() {
+        check_assist_not_applicable(
+            convert_comment_block,
+            r#"
+fn main() {
+    foo(); // end-of-line$0 comment
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn single_line_different_kinds() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    /// different prefix
+    // line$0 comment
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    /// different prefix
+    /*
+    line comment
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn single_line_separate_chunks() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    // different chunk
+
+    // line$0 comment
+    // below
+    foo();
+}
+"#,
+            r#"
+fn main() {
+    // different chunk
+
+    /*
+    line comment
+    below
+    */
+    foo();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn doc_block_comment_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/**
+ hi$0 there
+*/
+"#,
+            r#"
+/// hi there
+"#,
+        );
+    }
+
+    #[test]
+    fn block_comment_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/*
+ hi$0 there
+*/
+"#,
+            r#"
+// hi there
+"#,
+        );
+    }
+
+    #[test]
+    fn inner_doc_block_to_lines() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/*!
+ hi$0 there
+*/
+"#,
+            r#"
+//! hi there
+"#,
+        );
+    }
+
+    #[test]
+    fn block_to_lines_indent() {
+        check_assist(
+            convert_comment_block,
+            r#"
+fn main() {
+    /*!
+    hi$0 there
+
+    ```
+      code_sample
+    ```
+    */
+}
+"#,
+            r#"
+fn main() {
+    //! hi there
+    //!
+    //! ```
+    //!   code_sample
+    //! ```
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn end_of_line_block_to_line() {
+        check_assist_not_applicable(
+            convert_comment_block,
+            r#"
+fn main() {
+    foo(); /* end-of-line$0 comment */
+}
+"#,
+        );
+    }
+}
diff --git a/crates/ide_assists/src/lib.rs b/crates/ide_assists/src/lib.rs
index 53542d4334..9c8148462c 100644
--- a/crates/ide_assists/src/lib.rs
+++ b/crates/ide_assists/src/lib.rs
@@ -115,6 +115,7 @@ mod handlers {
     mod auto_import;
     mod change_visibility;
     mod convert_integer_literal;
+    mod convert_comment_block;
     mod early_return;
     mod expand_glob_import;
     mod extract_function;
@@ -178,6 +179,7 @@ mod handlers {
             auto_import::auto_import,
             change_visibility::change_visibility,
             convert_integer_literal::convert_integer_literal,
+            convert_comment_block::convert_comment_block,
             early_return::convert_to_guarded_return,
             expand_glob_import::expand_glob_import,
             extract_struct_from_enum_variant::extract_struct_from_enum_variant,
diff --git a/crates/syntax/src/ast/edit.rs b/crates/syntax/src/ast/edit.rs
index 824ebf41cf..0b3b76d4a7 100644
--- a/crates/syntax/src/ast/edit.rs
+++ b/crates/syntax/src/ast/edit.rs
@@ -595,11 +595,14 @@ impl ops::Add<u8> for IndentLevel {
 
 impl IndentLevel {
     pub fn from_node(node: &SyntaxNode) -> IndentLevel {
-        let first_token = match node.first_token() {
-            Some(it) => it,
+        match node.first_token() {
+            Some(it) => Self::from_token(&it),
             None => return IndentLevel(0),
-        };
-        for ws in prev_tokens(first_token).filter_map(ast::Whitespace::cast) {
+        }
+    }
+
+    pub fn from_token(token: &SyntaxToken) -> IndentLevel {
+        for ws in prev_tokens(token.clone()).filter_map(ast::Whitespace::cast) {
             let text = ws.syntax().text();
             if let Some(pos) = text.rfind('\n') {
                 let level = text[pos + 1..].chars().count() / 4;
diff --git a/crates/syntax/src/ast/token_ext.rs b/crates/syntax/src/ast/token_ext.rs
index 044e3e5e84..977eb81819 100644
--- a/crates/syntax/src/ast/token_ext.rs
+++ b/crates/syntax/src/ast/token_ext.rs
@@ -85,8 +85,9 @@ pub enum CommentPlacement {
 }
 
 impl CommentKind {
-    const BY_PREFIX: [(&'static str, CommentKind); 8] = [
+    const BY_PREFIX: [(&'static str, CommentKind); 9] = [
         ("/**/", CommentKind { shape: CommentShape::Block, doc: None }),
+        ("/***", CommentKind { shape: CommentShape::Block, doc: None }),
         ("////", CommentKind { shape: CommentShape::Line, doc: None }),
         ("///", CommentKind { shape: CommentShape::Line, doc: Some(CommentPlacement::Outer) }),
         ("//!", CommentKind { shape: CommentShape::Line, doc: Some(CommentPlacement::Inner) }),