1 files changed, 196 insertions, 0 deletions
diff --git a/src/tools/rust-analyzer/crates/ide-completion/src/completions/fn_param.rs b/src/tools/rust-analyzer/crates/ide-completion/src/completions/fn_param.rs
new file mode 100644
index 000000000..f0ecc595a
--- /dev/null
+++ b/src/tools/rust-analyzer/crates/ide-completion/src/completions/fn_param.rs
@@ -0,0 +1,196 @@
+//! See [`complete_fn_param`].
+
+use hir::HirDisplay;
+use ide_db::FxHashMap;
+use syntax::{
+    algo,
+    ast::{self, HasModuleItem},
+    match_ast, AstNode, Direction, SyntaxKind, TextRange, TextSize,
+};
+
+use crate::{
+    context::{ParamContext, ParamKind, PatternContext},
+    CompletionContext, CompletionItem, CompletionItemKind, Completions,
+};
+
+// FIXME: Make this a submodule of [`pattern`]
+/// Complete repeated parameters, both name and type. For example, if all
+/// functions in a file have a `spam: &mut Spam` parameter, a completion with
+/// `spam: &mut Spam` insert text/label will be suggested.
+///
+/// Also complete parameters for closure or local functions from the surrounding defined locals.
+pub(crate) fn complete_fn_param(
+    acc: &mut Completions,
+    ctx: &CompletionContext<'_>,
+    pattern_ctx: &PatternContext,
+) -> Option<()> {
+    let (ParamContext { param_list, kind, .. }, impl_) = match pattern_ctx {
+        PatternContext { param_ctx: Some(kind), impl_, .. } => (kind, impl_),
+        _ => return None,
+    };
+
+    let comma_wrapper = comma_wrapper(ctx);
+    let mut add_new_item_to_acc = |label: &str| {
+        let mk_item = |label: &str, range: TextRange| {
+            CompletionItem::new(CompletionItemKind::Binding, range, label)
+        };
+        let item = match &comma_wrapper {
+            Some((fmt, range)) => mk_item(&fmt(label), *range),
+            None => mk_item(label, ctx.source_range()),
+        };
+        // Completion lookup is omitted intentionally here.
+        // See the full discussion: https://github.com/rust-lang/rust-analyzer/issues/12073
+        item.add_to(acc)
+    };
+
+    match kind {
+        ParamKind::Function(function) => {
+            fill_fn_params(ctx, function, param_list, impl_, add_new_item_to_acc);
+        }
+        ParamKind::Closure(closure) => {
+            let stmt_list = closure.syntax().ancestors().find_map(ast::StmtList::cast)?;
+            params_from_stmt_list_scope(ctx, stmt_list, |name, ty| {
+                add_new_item_to_acc(&format!("{name}: {ty}"));
+            });
+        }
+    }
+
+    Some(())
+}
+
+fn fill_fn_params(
+    ctx: &CompletionContext<'_>,
+    function: &ast::Fn,
+    param_list: &ast::ParamList,
+    impl_: &Option<ast::Impl>,
+    mut add_new_item_to_acc: impl FnMut(&str),
+) {
+    let mut file_params = FxHashMap::default();
+
+    let mut extract_params = |f: ast::Fn| {
+        f.param_list().into_iter().flat_map(|it| it.params()).for_each(|param| {
+            if let Some(pat) = param.pat() {
+                // FIXME: We should be able to turn these into SmolStr without having to allocate a String
+                let whole_param = param.syntax().text().to_string();
+                let binding = pat.syntax().text().to_string();
+                file_params.entry(whole_param).or_insert(binding);
+            }
+        });
+    };
+
+    for node in ctx.token.parent_ancestors() {
+        match_ast! {
+            match node {
+                ast::SourceFile(it) => it.items().filter_map(|item| match item {
+                    ast::Item::Fn(it) => Some(it),
+                    _ => None,
+                }).for_each(&mut extract_params),
+                ast::ItemList(it) => it.items().filter_map(|item| match item {
+                    ast::Item::Fn(it) => Some(it),
+                    _ => None,
+                }).for_each(&mut extract_params),
+                ast::AssocItemList(it) => it.assoc_items().filter_map(|item| match item {
+                    ast::AssocItem::Fn(it) => Some(it),
+                    _ => None,
+                }).for_each(&mut extract_params),
+                _ => continue,
+            }
+        };
+    }
+
+    if let Some(stmt_list) = function.syntax().parent().and_then(ast::StmtList::cast) {
+        params_from_stmt_list_scope(ctx, stmt_list, |name, ty| {
+            file_params.entry(format!("{name}: {ty}")).or_insert(name.to_string());
+        });
+    }
+    remove_duplicated(&mut file_params, param_list.params());
+    let self_completion_items = ["self", "&self", "mut self", "&mut self"];
+    if should_add_self_completions(ctx.token.text_range().start(), param_list, impl_) {
+        self_completion_items.into_iter().for_each(|self_item| add_new_item_to_acc(self_item));
+    }
+
+    file_params.keys().for_each(|whole_param| add_new_item_to_acc(whole_param));
+}
+
+fn params_from_stmt_list_scope(
+    ctx: &CompletionContext<'_>,
+    stmt_list: ast::StmtList,
+    mut cb: impl FnMut(hir::Name, String),
+) {
+    let syntax_node = match stmt_list.syntax().last_child() {
+        Some(it) => it,
+        None => return,
+    };
+    if let Some(scope) =
+        ctx.sema.scope_at_offset(stmt_list.syntax(), syntax_node.text_range().end())
+    {
+        let module = scope.module().into();
+        scope.process_all_names(&mut |name, def| {
+            if let hir::ScopeDef::Local(local) = def {
+                if let Ok(ty) = local.ty(ctx.db).display_source_code(ctx.db, module) {
+                    cb(name, ty);
+                }
+            }
+        });
+    }
+}
+
+fn remove_duplicated(
+    file_params: &mut FxHashMap<String, String>,
+    fn_params: ast::AstChildren<ast::Param>,
+) {
+    fn_params.for_each(|param| {
+        let whole_param = param.syntax().text().to_string();
+        file_params.remove(&whole_param);
+
+        match param.pat() {
+            // remove suggestions for patterns that already exist
+            // if the type is missing we are checking the current param to be completed
+            // in which case this would find itself removing the suggestions due to itself
+            Some(pattern) if param.ty().is_some() => {
+                let binding = pattern.syntax().text().to_string();
+                file_params.retain(|_, v| v != &binding);
+            }
+            _ => (),
+        }
+    })
+}
+
+fn should_add_self_completions(
+    cursor: TextSize,
+    param_list: &ast::ParamList,
+    impl_: &Option<ast::Impl>,
+) -> bool {
+    if impl_.is_none() || param_list.self_param().is_some() {
+        return false;
+    }
+    match param_list.params().next() {
+        Some(first) => first.pat().map_or(false, |pat| pat.syntax().text_range().contains(cursor)),
+        None => true,
+    }
+}
+
+fn comma_wrapper(ctx: &CompletionContext<'_>) -> Option<(impl Fn(&str) -> String, TextRange)> {
+    let param = ctx.token.parent_ancestors().find(|node| node.kind() == SyntaxKind::PARAM)?;
+
+    let next_token_kind = {
+        let t = param.last_token()?.next_token()?;
+        let t = algo::skip_whitespace_token(t, Direction::Next)?;
+        t.kind()
+    };
+    let prev_token_kind = {
+        let t = param.first_token()?.prev_token()?;
+        let t = algo::skip_whitespace_token(t, Direction::Prev)?;
+        t.kind()
+    };
+
+    let has_trailing_comma =
+        matches!(next_token_kind, SyntaxKind::COMMA | SyntaxKind::R_PAREN | SyntaxKind::PIPE);
+    let trailing = if has_trailing_comma { "" } else { "," };
+
+    let has_leading_comma =
+        matches!(prev_token_kind, SyntaxKind::COMMA | SyntaxKind::L_PAREN | SyntaxKind::PIPE);
+    let leading = if has_leading_comma { "" } else { ", " };
+
+    Some((move |label: &_| (format!("{}{}{}", leading, label, trailing)), param.text_range()))
+}