3 files changed, 191 insertions, 1 deletions

diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index 0d848629d..03eec73ad 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -93,6 +93,7 @@ mod flip_comma;
 mod flip_binexpr;
 mod change_visibility;
 mod fill_match_arms;
+mod merge_match_arms;
 mod introduce_variable;
 mod inline_local_variable;
 mod replace_if_let_with_match;
@@ -109,6 +110,7 @@ fn all_assists<DB: HirDatabase>() -> &'static [fn(AssistCtx<DB>) -> Option<Assis
         add_impl::add_impl,
         change_visibility::change_visibility,
         fill_match_arms::fill_match_arms,
+        merge_match_arms::merge_match_arms,
         flip_comma::flip_comma,
         flip_binexpr::flip_binexpr,
         introduce_variable::introduce_variable,
diff --git a/crates/ra_assists/src/merge_match_arms.rs b/crates/ra_assists/src/merge_match_arms.rs
new file mode 100644
index 000000000..bc5f6f17c
--- /dev/null
+++ b/crates/ra_assists/src/merge_match_arms.rs
@@ -0,0 +1,188 @@
+use crate::{Assist, AssistCtx, AssistId, TextRange, TextUnit};
+use hir::db::HirDatabase;
+use ra_syntax::ast::{AstNode, MatchArm};
+
+pub(crate) fn merge_match_arms(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let current_arm = ctx.node_at_offset::<MatchArm>()?;
+
+    // We check if the following match arm matches this one. We could, but don't,
+    // compare to the previous match arm as well.
+    let next = current_arm.syntax().next_sibling();
+    let next_arm = MatchArm::cast(next?.clone())?;
+
+    // Don't try to handle arms with guards for now - can add support for this later
+    if current_arm.guard().is_some() || next_arm.guard().is_some() {
+        return None;
+    }
+
+    let current_expr = current_arm.expr()?;
+    let next_expr = next_arm.expr()?;
+
+    // Check for match arm equality by comparing lengths and then string contents
+    if current_expr.syntax().text_range().len() != next_expr.syntax().text_range().len() {
+        return None;
+    }
+    if current_expr.syntax().text() != next_expr.syntax().text() {
+        return None;
+    }
+
+    let cursor_to_end = current_arm.syntax().text_range().end() - ctx.frange.range.start();
+
+    ctx.add_action(AssistId("merge_match_arms"), "merge match arms", |edit| {
+        fn contains_placeholder(a: &MatchArm) -> bool {
+            a.pats().any(|x| match x.kind() {
+                ra_syntax::ast::PatKind::PlaceholderPat(..) => true,
+                _ => false,
+            })
+        }
+
+        let pats = if contains_placeholder(&current_arm) || contains_placeholder(&next_arm) {
+            "_".into()
+        } else {
+            let ps: Vec<String> = current_arm
+                .pats()
+                .map(|x| x.syntax().to_string())
+                .chain(next_arm.pats().map(|x| x.syntax().to_string()))
+                .collect();
+            ps.join(" | ")
+        };
+
+        let arm = format!("{} => {}", pats, current_expr.syntax().text());
+        let offset = TextUnit::from_usize(arm.len()) - cursor_to_end;
+
+        let start = current_arm.syntax().text_range().start();
+        let end = next_arm.syntax().text_range().end();
+
+        edit.target(current_arm.syntax().text_range());
+        edit.replace(TextRange::from_to(start, end), arm);
+        edit.set_cursor(start + offset);
+    });
+
+    ctx.build()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::merge_match_arms;
+    use crate::helpers::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn merge_match_arms_single_patterns() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32<|> }
+                    X::B => { 1i32 }
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B => { 1i32<|> }
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merge_match_arms_multiple_patterns() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B => {<|> 1i32 },
+                    X::C | X::D => { 1i32 },
+                    X::E => { 2i32 },
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A | X::B | X::C | X::D => {<|> 1i32 },
+                    X::E => { 2i32 },
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merge_match_arms_placeholder_pattern() {
+        check_assist(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32 },
+                    X::B => { 2i<|>32 },
+                    _ => { 2i32 }
+                }
+            }
+            "#,
+            r#"
+            #[derive(Debug)]
+            enum X { A, B, C, D, E }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A => { 1i32 },
+                    _ => { 2i<|>32 }
+                }
+            }
+            "#,
+        );
+    }
+
+    #[test]
+    fn merge_match_arms_rejects_guards() {
+        check_assist_not_applicable(
+            merge_match_arms,
+            r#"
+            #[derive(Debug)]
+            enum X {
+                A(i32),
+                B,
+                C
+            }
+
+            fn main() {
+                let x = X::A;
+                let y = match x {
+                    X::A(a) if a > 5 => { <|>1i32 },
+                    X::B => { 1i32 },
+                    X::C => { 2i32 }
+                }
+            }
+            "#,
+        );
+    }
+}
diff --git a/crates/ra_syntax/Cargo.toml b/crates/ra_syntax/Cargo.toml
index 40d63ef7a..bc1c88070 100644
--- a/crates/ra_syntax/Cargo.toml
+++ b/crates/ra_syntax/Cargo.toml
@@ -10,7 +10,7 @@ repository = "https://github.com/rust-analyzer/rust-analyzer"
 [dependencies]
 unicode-xid = "0.1.0"
 itertools = "0.8.0"
-rowan = "0.6.0"
+rowan = "0.6.1"
 ra_rustc_lexer = { version = "0.1.0-pre.2" }
 
 # ideally, `serde` should be enabled by `ra_lsp_server`, but we enable it here