Merge #10459

10459: feat: Add generate constant assist r=Veykril a=longfangsong

Close #10330.
![demo(1)](https://user-images.githubusercontent.com/13777628/135885262-c80de86f-5555-4f84-9508-822243f8a876.gif)



Co-authored-by: longfangsong <longfangsong@icloud.com>
Co-authored-by: 龙方淞 <longfangsong@icloud.com>

crates/ide_assists/src/handlers/generate_constant.rs

@@ -0,0 +1,119 @@
+use crate::assist_context::{AssistContext, Assists};
+use hir::HirDisplay;
+use ide_db::{
+    assists::{AssistId, AssistKind},
+    defs::NameRefClass,
+};
+use syntax::{
+    ast::{self, edit::IndentLevel},
+    AstNode,
+};
+
+// Assist: generate_constant
+//
+// Generate a named constant.
+//
+// ```
+// struct S { i: usize }
+// impl S { pub fn new(n: usize) {} }
+// fn main() {
+//     let v = S::new(CAPA$0CITY);
+// }
+// ```
+// ->
+// ```
+// struct S { i: usize }
+// impl S { pub fn new(n: usize) {} }
+// fn main() {
+//     const CAPACITY: usize = $0;
+//     let v = S::new(CAPACITY);
+// }
+// ```
+
+pub(crate) fn generate_constant(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let constant_token = ctx.find_node_at_offset::<ast::NameRef>()?;
+    let expr = constant_token.syntax().ancestors().find_map(ast::Expr::cast)?;
+    let statement = expr.syntax().ancestors().find_map(ast::Stmt::cast)?;
+    let ty = ctx.sema.type_of_expr(&expr)?;
+    let scope = ctx.sema.scope(statement.syntax());
+    let module = scope.module()?;
+    let type_name = ty.original().display_source_code(ctx.db(), module.into()).ok()?;
+    let indent = IndentLevel::from_node(statement.syntax());
+    if constant_token.to_string().chars().any(|it| !(it.is_uppercase() || it == '_')) {
+        cov_mark::hit!(not_constant_name);
+        return None;
+    }
+    if NameRefClass::classify(&ctx.sema, &constant_token).is_some() {
+        cov_mark::hit!(already_defined);
+        return None;
+    }
+    let target = statement.syntax().parent()?.text_range();
+    acc.add(
+        AssistId("generate_constant", AssistKind::QuickFix),
+        "Generate constant",
+        target,
+        |builder| {
+            builder.insert(
+                statement.syntax().text_range().start(),
+                format!("const {}: {} = $0;\n{}", constant_token, type_name, indent),
+            );
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn test_trivial() {
+        check_assist(
+            generate_constant,
+            r#"struct S { i: usize }
+impl S {
+    pub fn new(n: usize) {}
+}
+fn main() {
+    let v = S::new(CAPA$0CITY);
+}"#,
+            r#"struct S { i: usize }
+impl S {
+    pub fn new(n: usize) {}
+}
+fn main() {
+    const CAPACITY: usize = $0;
+    let v = S::new(CAPACITY);
+}"#,
+        );
+    }
+    #[test]
+    fn test_wont_apply_when_defined() {
+        cov_mark::check!(already_defined);
+        check_assist_not_applicable(
+            generate_constant,
+            r#"struct S { i: usize }
+impl S {
+    pub fn new(n: usize) {}
+}
+fn main() {
+    const CAPACITY: usize = 10;
+    let v = S::new(CAPAC$0ITY);
+}"#,
+        );
+    }
+    #[test]
+    fn test_wont_apply_when_maybe_not_constant() {
+        cov_mark::check!(not_constant_name);
+        check_assist_not_applicable(
+            generate_constant,
+            r#"struct S { i: usize }
+impl S {
+    pub fn new(n: usize) {}
+}
+fn main() {
+    let v = S::new(capa$0city);
+}"#,
+        );
+    }
+}

crates/ide_assists/src/lib.rs

@@ -130,6 +130,7 @@ mod handlers {
     mod flip_binexpr;
     mod flip_comma;
     mod flip_trait_bound;
+    mod generate_constant;
     mod generate_default_from_enum_variant;
     mod generate_default_from_new;
     mod generate_deref;
@@ -205,6 +206,7 @@ mod handlers {
             flip_binexpr::flip_binexpr,
             flip_comma::flip_comma,
             flip_trait_bound::flip_trait_bound,
+            generate_constant::generate_constant,
             generate_default_from_enum_variant::generate_default_from_enum_variant,
             generate_default_from_new::generate_default_from_new,
             generate_deref::generate_deref,

crates/ide_assists/src/tests/generated.rs

@@ -648,6 +648,28 @@ fn foo<T: Copy + Clone>() { }
     )
 }
 
+#[test]
+fn doctest_generate_constant() {
+    check_doc_test(
+        "generate_constant",
+        r#####"
+struct S { i: usize }
+impl S { pub fn new(n: usize) {} }
+fn main() {
+    let v = S::new(CAPA$0CITY);
+}
+"#####,
+        r#####"
+struct S { i: usize }
+impl S { pub fn new(n: usize) {} }
+fn main() {
+    const CAPACITY: usize = $0;
+    let v = S::new(CAPACITY);
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_generate_default_from_enum_variant() {
     check_doc_test(