feat: show parameter hint for missing arguments

crates/ide/src/inlay_hints.rs

@@ -307,6 +307,7 @@ pub struct InlayHintsConfig<'a> {
     pub sized_bound: bool,
     pub discriminant_hints: DiscriminantHints,
     pub parameter_hints: bool,
+    pub parameter_hints_for_missing_arguments: bool,
     pub generic_parameter_hints: GenericParameterHints,
     pub chaining_hints: bool,
     pub adjustment_hints: AdjustmentHints,
@@ -886,6 +887,7 @@ mod tests {
         render_colons: false,
         type_hints: false,
         parameter_hints: false,
+        parameter_hints_for_missing_arguments: false,
         sized_bound: false,
         generic_parameter_hints: GenericParameterHints {
             type_hints: false,

crates/ide/src/inlay_hints/param_name.rs

@@ -11,6 +11,7 @@ use hir::{EditionedFileId, Semantics};
 use ide_db::{RootDatabase, famous_defs::FamousDefs};
 
 use stdx::to_lower_snake_case;
+use syntax::T;
 use syntax::ast::{self, AstNode, HasArgList, HasName, UnaryOp};
 
 use crate::{InlayHint, InlayHintLabel, InlayHintPosition, InlayHintsConfig, InlayKind};
@@ -88,9 +89,75 @@ pub(super) fn hints(
         });
 
     acc.extend(hints);
+
+    // Show hint for the next expected (missing) argument if enabled
+    if config.parameter_hints_for_missing_arguments {
+        let provided_args_count = arg_list.args().count();
+        let params = callable.params();
+        let total_params = params.len();
+
+        if provided_args_count < total_params
+            && let Some(next_param) = params.get(provided_args_count)
+            && let Some(param_name) = next_param.name(sema.db)
+        {
+            // Apply heuristics to hide obvious parameter hints
+            if should_hide_missing_param_hint(unary_function, function_name, param_name.as_str()) {
+                return Some(());
+            }
+
+            // Determine the position for the hint
+            if let Some(hint_range) = missing_arg_hint_position(&arg_list) {
+                let colon = if config.render_colons { ":" } else { "" };
+                let label = InlayHintLabel::simple(
+                    format!("{}{}", param_name.display(sema.db, krate.edition(sema.db)), colon),
+                    None,
+                    config.lazy_location_opt(|| {
+                        let source = sema.source(next_param.clone())?;
+                        let name_syntax = match source.value.as_ref() {
+                            Either::Left(pat) => pat.name(),
+                            Either::Right(param) => match param.pat()? {
+                                ast::Pat::IdentPat(it) => it.name(),
+                                _ => None,
+                            },
+                        }?;
+                        sema.original_range_opt(name_syntax.syntax()).map(|frange| {
+                            ide_db::FileRange {
+                                file_id: frange.file_id.file_id(sema.db),
+                                range: frange.range,
+                            }
+                        })
+                    }),
+                );
+                acc.push(InlayHint {
+                    range: hint_range,
+                    kind: InlayKind::Parameter,
+                    label,
+                    text_edit: None,
+                    position: InlayHintPosition::Before,
+                    pad_left: true,
+                    pad_right: false,
+                    resolve_parent: Some(expr.syntax().text_range()),
+                });
+            }
+        }
+    }
+
     Some(())
 }
 
+/// Determines the position where the hint for a missing argument should be placed.
+/// Returns the range of the token where the hint should appear.
+fn missing_arg_hint_position(arg_list: &ast::ArgList) -> Option<syntax::TextRange> {
+    // Always place the hint on the closing paren, so it appears before `)`.
+    // This way `foo()` becomes `foo(a)` visually with the hint.
+    arg_list
+        .syntax()
+        .children_with_tokens()
+        .filter_map(|it| it.into_token())
+        .find(|t| t.kind() == T![')'])
+        .map(|t| t.text_range())
+}
+
 fn get_callable<'db>(
     sema: &Semantics<'db, RootDatabase>,
     expr: &ast::Expr,
@@ -153,6 +220,37 @@ fn should_hide_param_name_hint(
     is_argument_expr_similar_to_param_name(sema, argument, param_name)
 }
 
+/// Determines whether to hide the parameter hint for a missing argument.
+/// This is a simplified version of `should_hide_param_name_hint` that doesn't
+/// require an actual argument expression.
+fn should_hide_missing_param_hint(
+    unary_function: bool,
+    function_name: Option<&str>,
+    param_name: &str,
+) -> bool {
+    let param_name = param_name.trim_matches('_');
+    if param_name.is_empty() {
+        return true;
+    }
+
+    if param_name.starts_with("ra_fixture") {
+        return true;
+    }
+
+    if unary_function {
+        if let Some(function_name) = function_name
+            && is_param_name_suffix_of_fn_name(param_name, function_name)
+        {
+            return true;
+        }
+        if is_obvious_param(param_name) {
+            return true;
+        }
+    }
+
+    false
+}
+
 /// Hide the parameter name of a unary function if it is a `_` - prefixed suffix of the function's name, or equal.
 ///
 /// `fn strip_suffix(suffix)` will be hidden.
@@ -606,6 +704,103 @@ fn main() {
      // ^^^^^^ a_d_e
     baz(a.d.ec);
      // ^^^^^^ a_d_e
+}"#,
+        );
+    }
+
+    #[track_caller]
+    fn check_missing_params(#[rust_analyzer::rust_fixture] ra_fixture: &str) {
+        check_with_config(
+            InlayHintsConfig {
+                parameter_hints: true,
+                parameter_hints_for_missing_arguments: true,
+                ..DISABLED_CONFIG
+            },
+            ra_fixture,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_empty_call() {
+        // When calling foo() with no args, show hint for first param on the closing paren
+        check_missing_params(
+            r#"
+fn foo(a: i32, b: i32) -> i32 { a + b }
+fn main() {
+    foo();
+      //^ a
+}"#,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_after_first_arg() {
+        // foo(1,) - show hint for 'a' on '1', and 'b' on the trailing comma
+        check_missing_params(
+            r#"
+fn foo(a: i32, b: i32) -> i32 { a + b }
+fn main() {
+    foo(1,);
+      //^ a
+        //^ b
+}"#,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_partial_args() {
+        // foo(1, 2,) - show hints for a, b on args, and c on trailing comma
+        check_missing_params(
+            r#"
+fn foo(a: i32, b: i32, c: i32) -> i32 { a + b + c }
+fn main() {
+    foo(1, 2,);
+      //^ a
+         //^ b
+           //^ c
+}"#,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_method_call() {
+        // S.foo(1,) - show hint for 'a' on '1', and 'b' on trailing comma
+        check_missing_params(
+            r#"
+struct S;
+impl S {
+    fn foo(&self, a: i32, b: i32) -> i32 { a + b }
+}
+fn main() {
+    S.foo(1,);
+        //^ a
+          //^ b
+}"#,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_no_hint_when_complete() {
+        // When all args provided, no missing hint - just regular param hints
+        check_missing_params(
+            r#"
+fn foo(a: i32, b: i32) -> i32 { a + b }
+fn main() {
+    foo(1, 2);
+      //^ a
+         //^ b
+}"#,
+        );
+    }
+
+    #[test]
+    fn missing_param_hint_respects_heuristics() {
+        // The hint should be hidden if it matches heuristics (e.g., single param unary fn with same name)
+        check_missing_params(
+            r#"
+fn foo(foo: i32) -> i32 { foo }
+fn main() {
+    foo();
 }"#,
         );
     }

crates/ide/src/static_index.rs

@@ -169,6 +169,7 @@ impl StaticIndex<'_> {
                     type_hints: true,
                     sized_bound: false,
                     parameter_hints: true,
+                    parameter_hints_for_missing_arguments: false,
                     generic_parameter_hints: crate::GenericParameterHints {
                         type_hints: false,
                         lifetime_hints: false,

crates/rust-analyzer/src/cli/analysis_stats.rs

@@ -1205,6 +1205,7 @@ impl flags::AnalysisStats {
                     sized_bound: false,
                     discriminant_hints: ide::DiscriminantHints::Always,
                     parameter_hints: true,
+                    parameter_hints_for_missing_arguments: false,
                     generic_parameter_hints: ide::GenericParameterHints {
                         type_hints: true,
                         lifetime_hints: true,

crates/rust-analyzer/src/config.rs

@@ -280,6 +280,9 @@ config_data! {
         /// Show function parameter name inlay hints at the call site.
         inlayHints_parameterHints_enable: bool = true,
 
+        /// Show parameter name inlay hints for missing arguments at the call site.
+        inlayHints_parameterHints_missingArguments_enable: bool = false,
+
         /// Show exclusive range inlay hints.
         inlayHints_rangeExclusiveHints_enable: bool = false,
 
@@ -1916,6 +1919,9 @@ impl Config {
             type_hints: self.inlayHints_typeHints_enable().to_owned(),
             sized_bound: self.inlayHints_implicitSizedBoundHints_enable().to_owned(),
             parameter_hints: self.inlayHints_parameterHints_enable().to_owned(),
+            parameter_hints_for_missing_arguments: self
+                .inlayHints_parameterHints_missingArguments_enable()
+                .to_owned(),
             generic_parameter_hints: GenericParameterHints {
                 type_hints: self.inlayHints_genericParameterHints_type_enable().to_owned(),
                 lifetime_hints: self.inlayHints_genericParameterHints_lifetime_enable().to_owned(),

docs/book/src/configuration_generated.md

@@ -1070,6 +1070,13 @@ Default: `true`
 Show function parameter name inlay hints at the call site.
 
 
+## rust-analyzer.inlayHints.parameterHints.missingArguments.enable {#inlayHints.parameterHints.missingArguments.enable}
+
+Default: `false`
+
+Show parameter name inlay hints for missing arguments at the call site.
+
+
 ## rust-analyzer.inlayHints.rangeExclusiveHints.enable {#inlayHints.rangeExclusiveHints.enable}
 
 Default: `false`

editors/code/package.json

@@ -2396,6 +2396,16 @@
                     }
                 }
             },
+            {
+                "title": "Inlay Hints",
+                "properties": {
+                    "rust-analyzer.inlayHints.parameterHints.missingArguments.enable": {
+                        "markdownDescription": "Show parameter name inlay hints for missing arguments at the call site.",
+                        "default": false,
+                        "type": "boolean"
+                    }
+                }
+            },
             {
                 "title": "Inlay Hints",
                 "properties": {