Auto merge of #13359 - feniljain:feat-must-use-option, r=Veykril

feat: add config for inserting must_use in `generate_enum_as_method`

Should fix #13312

Didn't add a test because I was not sure on how to add test for a specific configuration option, tried to look for the usages for other `AssistConfig` variants but couldn't find any in `tests`. If there is a way to test this, do point me towards it.

I tried to extract the formatting string as a common `template_string` and only have if-else for that, but it didn't compile :(

Also it seems these tests are failing:

```
test config::tests::generate_config_documentation ... FAILED
test config::tests::generate_package_json_config ... FAILED
```

Can you also point me to how to correct these 😅  ( I guess there is some command to automatically generate these? )

6 files changed, 32 insertions, 4 deletions

diff --git a/crates/ide-assists/src/assist_config.rs b/crates/ide-assists/src/assist_config.rs
index 60d1588a44e..b273ebc85a5 100644
--- a/crates/ide-assists/src/assist_config.rs
+++ b/crates/ide-assists/src/assist_config.rs
@@ -14,4 +14,5 @@ pub struct AssistConfig {
     pub allowed: Option<Vec<AssistKind>>,
     pub insert_use: InsertUseConfig,
     pub prefer_no_std: bool,
+    pub assist_emit_must_use: bool,
 }
diff --git a/crates/ide-assists/src/handlers/generate_enum_projection_method.rs b/crates/ide-assists/src/handlers/generate_enum_projection_method.rs
index bdd3cf4f06c..c9aa41c845a 100644
--- a/crates/ide-assists/src/handlers/generate_enum_projection_method.rs
+++ b/crates/ide-assists/src/handlers/generate_enum_projection_method.rs
@@ -124,6 +124,7 @@ fn generate_enum_projection_method(
         happy_case,
         sad_case,
     } = props;
+
     let variant = ctx.find_node_at_offset::<ast::Variant>()?;
     let variant_name = variant.name()?;
     let parent_enum = ast::Adt::Enum(variant.parent_enum());
@@ -144,7 +145,7 @@ fn generate_enum_projection_method(
         ast::StructKind::Unit => return None,
     };
 
-    let fn_name = format!("{}_{}", fn_name_prefix, &to_lower_snake_case(&variant_name.text()));
+    let fn_name = format!("{fn_name_prefix}_{}", &to_lower_snake_case(&variant_name.text()));
 
     // Return early if we've found an existing new fn
     let impl_def = find_struct_impl(ctx, &parent_enum, &[fn_name.clone()])?;
@@ -156,15 +157,25 @@ fn generate_enum_projection_method(
         assist_description,
         target,
         |builder| {
-            let vis = parent_enum.visibility().map_or(String::new(), |v| format!("{v} "));
+            let vis = parent_enum.visibility().map_or(String::new(), |v| format!("{} ", v));
+
+            let field_type_syntax = field_type.syntax();
+
+            let must_use = if ctx.config.assist_emit_must_use {
+                "#[must_use]\n    "
+            } else {
+                ""
+            };
+
             let method = format!(
-                "    {vis}fn {fn_name}({self_param}) -> {return_prefix}{field_type}{return_suffix} {{
+                "    {must_use}{vis}fn {fn_name}({self_param}) -> {return_prefix}{field_type_syntax}{return_suffix} {{
         if let Self::{variant_name}{pattern_suffix} = self {{
             {happy_case}({bound_name})
         }} else {{
             {sad_case}
         }}
-    }}");
+    }}"
+            );
 
             add_method_to_adt(builder, &parent_enum, impl_def, &method);
         },
diff --git a/crates/ide-assists/src/tests.rs b/crates/ide-assists/src/tests.rs
index f7f2417d074..92ced27c78a 100644
--- a/crates/ide-assists/src/tests.rs
+++ b/crates/ide-assists/src/tests.rs
@@ -30,6 +30,7 @@ pub(crate) const TEST_CONFIG: AssistConfig = AssistConfig {
         skip_glob_imports: true,
     },
     prefer_no_std: false,
+    assist_emit_must_use: false,
 };
 
 pub(crate) fn with_single_file(text: &str) -> (RootDatabase, FileId) {
diff --git a/crates/rust-analyzer/src/config.rs b/crates/rust-analyzer/src/config.rs
index 1ed8f2bb5f3..4072ae585db 100644
--- a/crates/rust-analyzer/src/config.rs
+++ b/crates/rust-analyzer/src/config.rs
@@ -56,6 +56,9 @@ mod patch_old_style;
 // parsing the old name.
 config_data! {
     struct ConfigData {
+        /// Whether to insert #[must_use] when generating `as_` methods
+        /// for enum variants.
+        assist_emitMustUse: bool               = "false",
         /// Placeholder expression to use for missing expressions in assists.
         assist_expressionFillDefault: ExprFillDefaultDef              = "\"todo\"",
 
@@ -1276,6 +1279,7 @@ impl Config {
             allowed: None,
             insert_use: self.insert_use_config(),
             prefer_no_std: self.data.imports_prefer_no_std,
+            assist_emit_must_use: self.data.assist_emitMustUse,
         }
     }
 
diff --git a/docs/user/generated_config.adoc b/docs/user/generated_config.adoc
index 502833de72c..36794efe427 100644
--- a/docs/user/generated_config.adoc
+++ b/docs/user/generated_config.adoc
@@ -1,3 +1,9 @@
+[[rust-analyzer.assist.emitMustUse]]rust-analyzer.assist.emitMustUse (default: `false`)::
++
+--
+Whether to insert #[must_use] when generating `as_` methods
+for enum variants.
+--
 [[rust-analyzer.assist.expressionFillDefault]]rust-analyzer.assist.expressionFillDefault (default: `"todo"`)::
 +
 --
diff --git a/editors/code/package.json b/editors/code/package.json
index 6771cad28a7..0ff66527451 100644
--- a/editors/code/package.json
+++ b/editors/code/package.json
@@ -397,6 +397,11 @@
                     "type": "boolean"
                 },
                 "$generated-start": {},
+                "rust-analyzer.assist.emitMustUse": {
+                    "markdownDescription": "Whether to insert #[must_use] when generating `as_` methods\nfor enum variants.",
+                    "default": false,
+                    "type": "boolean"
+                },
                 "rust-analyzer.assist.expressionFillDefault": {
                     "markdownDescription": "Placeholder expression to use for missing expressions in assists.",
                     "default": "todo",