feat: suggest name in let_stmt and fn_param

crates/ide-completion/src/completions.rs

@@ -617,6 +617,16 @@ impl Completions {
         }
         self.add_opt(render_struct_pat(RenderContext::new(ctx), pattern_ctx, strukt, local_name));
     }
+
+    pub(crate) fn suggest_name(&mut self, ctx: &CompletionContext<'_>, name: &str) {
+        let item = CompletionItem::new(
+            CompletionItemKind::Binding,
+            ctx.source_range(),
+            SmolStr::from(name),
+            ctx.edition,
+        );
+        item.add_to(self, ctx.db);
+    }
 }
 
 /// Calls the callback for each variant of the provided enum with the path to the variant.

crates/ide-completion/src/completions/pattern.rs

@@ -1,6 +1,7 @@
 //! Completes constants and paths in unqualified patterns.
 
 use hir::{db::DefDatabase, AssocItem, ScopeDef};
+use ide_db::syntax_helpers::suggest_name;
 use syntax::ast::Pat;
 
 use crate::{
@@ -45,6 +46,18 @@ pub(crate) fn complete_pattern(
         return;
     }
 
+    // Suggest name only in let-stmt and fn param
+    if pattern_ctx.should_suggest_name {
+        if let Some(suggested) = ctx
+            .expected_type
+            .as_ref()
+            .map(|ty| ty.strip_references())
+            .and_then(|ty| suggest_name::for_type(&ty, ctx.db, ctx.edition))
+        {
+            acc.suggest_name(ctx, &suggested);
+        }
+    }
+
     let refutable = pattern_ctx.refutability == PatternRefutability::Refutable;
     let single_variant_enum = |enum_: hir::Enum| ctx.db.enum_data(enum_.into()).variants.len() == 1;
 

crates/ide-completion/src/context.rs

@@ -264,6 +264,7 @@ pub(crate) struct PatternContext {
     pub(crate) refutability: PatternRefutability,
     pub(crate) param_ctx: Option<ParamContext>,
     pub(crate) has_type_ascription: bool,
+    pub(crate) should_suggest_name: bool,
     pub(crate) parent_pat: Option<ast::Pat>,
     pub(crate) ref_token: Option<SyntaxToken>,
     pub(crate) mut_token: Option<SyntaxToken>,

crates/ide-completion/src/context/analysis.rs

@@ -1430,10 +1430,23 @@ fn pattern_context_for(
         _ => (None, None),
     };
 
+    // Only suggest name in let-stmt or fn param
+    let should_suggest_name = matches!(
+            &pat,
+            ast::Pat::IdentPat(it)
+                if it.syntax()
+                .parent()
+                .map_or(false, |node| {
+                    let kind = node.kind();
+                    ast::LetStmt::can_cast(kind) || ast::Param::can_cast(kind)
+                })
+    );
+
     PatternContext {
         refutability,
         param_ctx,
         has_type_ascription,
+        should_suggest_name,
         parent_pat: pat.syntax().parent().and_then(ast::Pat::cast),
         mut_token,
         ref_token,

crates/ide-db/src/syntax_helpers/suggest_name.rs

@@ -60,6 +60,21 @@ const USELESS_METHODS: &[&str] = &[
     "into_future",
 ];
 
+/// Suggest a name for given type.
+///
+/// The function will strip references first, and suggest name from the inner type.
+///
+/// - If `ty` is an ADT, it will suggest the name of the ADT.
+///   + If `ty` is wrapped in `Box`, `Option` or `Result`, it will suggest the name from the inner type.
+/// - If `ty` is a trait, it will suggest the name of the trait.
+/// - If `ty` is an `impl Trait`, it will suggest the name of the first trait.
+///
+/// If the suggested name conflicts with reserved keywords, it will return `None`.
+pub fn for_type(ty: &hir::Type, db: &RootDatabase, edition: Edition) -> Option<String> {
+    let ty = ty.strip_references();
+    name_of_type(&ty, db, edition)
+}
+
 /// Suggest a unique name for generic parameter.
 ///
 /// `existing_params` is used to check if the name conflicts with existing
@@ -269,10 +284,9 @@ fn var_name_from_pat(pat: &ast::Pat) -> Option<ast::Name> {
 
 fn from_type(expr: &ast::Expr, sema: &Semantics<'_, RootDatabase>) -> Option<String> {
     let ty = sema.type_of_expr(expr)?.adjusted();
-    let ty = ty.remove_ref().unwrap_or(ty);
     let edition = sema.scope(expr.syntax())?.krate().edition(sema.db);
 
-    name_of_type(&ty, sema.db, edition)
+    for_type(&ty, sema.db, edition)
 }
 
 fn name_of_type(ty: &hir::Type, db: &RootDatabase, edition: Edition) -> Option<String> {