Add "Pattern match on argument" code action

CHANGELOG.md

@@ -39,6 +39,63 @@
 
   ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
 
+- The language server now suggests a code action to pattern match on a
+  function's argument. For example:
+
+  ```gleam
+  pub type Pokemon {
+    Pokemon(pokedex_number: Int, name: String)
+  }
+
+  pub fn to_string(pokemon: Pokemon) {
+    //             ^ If you put your cursor over the argument
+    todo
+  }
+  ```
+
+  Triggering the code action on the `pokemon` argument will generate the
+  following code for you:
+
+  ```gleam
+  pub type Pokemon {
+    Pokemon(pokedex_number: Int, name: String)
+  }
+
+  pub fn to_string(pokemon: Pokemon) {
+    let Pokemon(pokedex_number:, name:) = pokemon
+    todo
+  }
+  ```
+
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
+- The language server now suggests a code action to pattern match on a variable.
+  For example:
+
+  ```gleam
+  pub fn main() {
+    let result = list.first(a_list)
+    //  ^ If you put your cursor over the variable
+    todo
+  }
+  ```
+
+  Triggering the code action on the `result` variable will generate the
+  following code for you:
+
+  ```gleam
+  pub fn main() {
+    let result = list.first(a_list)
+    case result {
+      Ok(value) -> todo
+      Error(value) -> todo
+    }
+    todo
+  }
+  ```
+
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
 ### Formatter
 
 ### Bug fixes

compiler-core/src/ast.rs

@@ -2180,7 +2180,7 @@ impl<A> BitArrayOption<A> {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum TodoKind {
     Keyword,
-    EmptyFunction,
+    EmptyFunction { function_location: SrcSpan },
     IncompleteUse,
     EmptyBlock,
 }

compiler-core/src/ast/typed.rs

@@ -200,7 +200,9 @@ impl TypedExpr {
                 // We don't want to match on todos that were implicitly inserted
                 // by the compiler as it would result in confusing suggestions
                 // from the LSP.
-                TodoKind::EmptyFunction | TodoKind::EmptyBlock | TodoKind::IncompleteUse => None,
+                TodoKind::EmptyFunction { .. } | TodoKind::EmptyBlock | TodoKind::IncompleteUse => {
+                    None
+                }
             },
 
             Self::Pipeline {

compiler-core/src/ast/visit.rs

@@ -48,6 +48,7 @@ use crate::{
 use std::sync::Arc;
 
 use ecow::EcoString;
+use vec1::Vec1;
 
 use crate::type_::Type;
 
@@ -143,7 +144,7 @@ pub trait Visit<'ast> {
         type_: &'ast Arc<Type>,
         kind: &'ast FunctionLiteralKind,
         args: &'ast [TypedArg],
-        body: &'ast [TypedStatement],
+        body: &'ast Vec1<TypedStatement>,
         return_annotation: &'ast Option<TypeAst>,
     ) {
         visit_typed_expr_fn(self, location, type_, kind, args, body, return_annotation);
@@ -824,7 +825,7 @@ pub fn visit_typed_expr_fn<'a, V>(
     _typ: &'a Arc<Type>,
     _kind: &'a FunctionLiteralKind,
     _args: &'a [TypedArg],
-    body: &'a [TypedStatement],
+    body: &'a Vec1<TypedStatement>,
     _return_annotation: &'a Option<TypeAst>,
 ) where
     V: Visit<'a> + ?Sized,