Fix unit_arg suggests wrongly for Default::default (#14881)

Closes rust-lang/rust-clippy#14857

changelog: [`unit_arg`] fix wrong suggestion for `Default::default`

Author: samueltardieu

clippy_lints/src/default.rs

@@ -1,10 +1,9 @@
 use clippy_utils::diagnostics::{span_lint_and_note, span_lint_and_sugg};
 use clippy_utils::source::snippet_with_context;
 use clippy_utils::ty::{has_drop, is_copy};
-use clippy_utils::{contains_name, get_parent_expr, in_automatically_derived, is_from_proc_macro};
+use clippy_utils::{contains_name, get_parent_expr, in_automatically_derived, is_expr_default, is_from_proc_macro};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
-use rustc_hir::def::Res;
 use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
@@ -129,7 +128,7 @@ impl<'tcx> LateLintPass<'tcx> for Default {
                 // only take bindings to identifiers
                 && let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind
                 // only when assigning `... = Default::default()`
-                && is_expr_default(expr, cx)
+                && is_expr_default(cx, expr)
                 && let binding_type = cx.typeck_results().node_type(binding_id)
                 && let ty::Adt(adt, args) = *binding_type.kind()
                 && adt.is_struct()
@@ -251,19 +250,6 @@ impl<'tcx> LateLintPass<'tcx> for Default {
     }
 }
 
-/// Checks if the given expression is the `default` method belonging to the `Default` trait.
-fn is_expr_default<'tcx>(expr: &'tcx Expr<'tcx>, cx: &LateContext<'tcx>) -> bool {
-    if let ExprKind::Call(fn_expr, []) = &expr.kind
-        && let ExprKind::Path(qpath) = &fn_expr.kind
-        && let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id)
-    {
-        // right hand side of assignment is `Default::default`
-        cx.tcx.is_diagnostic_item(sym::default_fn, def_id)
-    } else {
-        false
-    }
-}
-
 /// Returns the reassigned field and the assigning expression (right-hand side of assign).
 fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Option<(Ident, &'tcx Expr<'tcx>)> {
     if let StmtKind::Semi(later_expr) = this.kind

clippy_lints/src/unit_types/unit_arg.rs

@@ -1,9 +1,14 @@
+use std::iter;
+
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::is_from_proc_macro;
-use clippy_utils::source::{SourceText, SpanRangeExt, indent_of, reindent_multiline};
+use clippy_utils::source::{SpanRangeExt, indent_of, reindent_multiline};
+use clippy_utils::sugg::Sugg;
+use clippy_utils::ty::expr_type_is_certain;
+use clippy_utils::{is_expr_default, is_from_proc_macro};
 use rustc_errors::Applicability;
 use rustc_hir::{Block, Expr, ExprKind, MatchSource, Node, StmtKind};
 use rustc_lint::LateContext;
+use rustc_span::SyntaxContext;
 
 use super::{UNIT_ARG, utils};
 
@@ -59,7 +64,7 @@ fn is_questionmark_desugar_marked_call(expr: &Expr<'_>) -> bool {
     }
 }
 
-fn lint_unit_args(cx: &LateContext<'_>, expr: &Expr<'_>, args_to_recover: &[&Expr<'_>]) {
+fn lint_unit_args<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, args_to_recover: &[&'tcx Expr<'tcx>]) {
     let mut applicability = Applicability::MachineApplicable;
     let (singular, plural) = if args_to_recover.len() > 1 {
         ("", "s")
@@ -100,34 +105,41 @@ fn lint_unit_args(cx: &LateContext<'_>, expr: &Expr<'_>, args_to_recover: &[&Exp
 
             let arg_snippets: Vec<_> = args_to_recover
                 .iter()
-                .filter_map(|arg| arg.span.get_source_text(cx))
+                // If the argument is from an expansion and is a `Default::default()`, we skip it
+                .filter(|arg| !arg.span.from_expansion() || !is_expr_default_nested(cx, arg))
+                .filter_map(|arg| get_expr_snippet(cx, arg))
                 .collect();
-            let arg_snippets_without_empty_blocks: Vec<_> = args_to_recover
+
+            // If the argument is an empty block or `Default::default()`, we can replace it with `()`.
+            let arg_snippets_without_redundant_exprs: Vec<_> = args_to_recover
                 .iter()
-                .filter(|arg| !is_empty_block(arg))
-                .filter_map(|arg| arg.span.get_source_text(cx))
+                .filter(|arg| !is_expr_default_nested(cx, arg) && (arg.span.from_expansion() || !is_empty_block(arg)))
+                .filter_map(|arg| get_expr_snippet_with_type_certainty(cx, arg))
                 .collect();
 
             if let Some(call_snippet) = expr.span.get_source_text(cx) {
-                let sugg = fmt_stmts_and_call(
-                    cx,
-                    expr,
-                    &call_snippet,
-                    &arg_snippets,
-                    &arg_snippets_without_empty_blocks,
-                );
-
-                if arg_snippets_without_empty_blocks.is_empty() {
+                if arg_snippets_without_redundant_exprs.is_empty()
+                    && let suggestions = args_to_recover
+                        .iter()
+                        .filter(|arg| !arg.span.from_expansion() || !is_expr_default_nested(cx, arg))
+                        .map(|arg| (arg.span.parent_callsite().unwrap_or(arg.span), "()".to_string()))
+                        .collect::<Vec<_>>()
+                    && !suggestions.is_empty()
+                {
                     db.multipart_suggestion(
                         format!("use {singular}unit literal{plural} instead"),
-                        args_to_recover
-                            .iter()
-                            .map(|arg| (arg.span, "()".to_string()))
-                            .collect::<Vec<_>>(),
+                        suggestions,
                         applicability,
                     );
                 } else {
-                    let plural = arg_snippets_without_empty_blocks.len() > 1;
+                    let plural = arg_snippets_without_redundant_exprs.len() > 1;
+                    let sugg = fmt_stmts_and_call(
+                        cx,
+                        expr,
+                        &call_snippet,
+                        arg_snippets,
+                        arg_snippets_without_redundant_exprs,
+                    );
                     let empty_or_s = if plural { "s" } else { "" };
                     let it_or_them = if plural { "them" } else { "it" };
                     db.span_suggestion(
@@ -144,6 +156,55 @@ fn lint_unit_args(cx: &LateContext<'_>, expr: &Expr<'_>, args_to_recover: &[&Exp
     );
 }
 
+fn is_expr_default_nested<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+    is_expr_default(cx, expr)
+        || matches!(expr.kind, ExprKind::Block(block, _)
+        if block.expr.is_some() && is_expr_default_nested(cx, block.expr.unwrap()))
+}
+
+enum MaybeTypeUncertain<'tcx> {
+    Certain(Sugg<'tcx>),
+    Uncertain(Sugg<'tcx>),
+}
+
+impl From<MaybeTypeUncertain<'_>> for String {
+    fn from(value: MaybeTypeUncertain<'_>) -> Self {
+        match value {
+            MaybeTypeUncertain::Certain(sugg) => sugg.to_string(),
+            MaybeTypeUncertain::Uncertain(sugg) => format!("let _: () = {sugg}"),
+        }
+    }
+}
+
+fn get_expr_snippet<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> Option<Sugg<'tcx>> {
+    let mut app = Applicability::MachineApplicable;
+    let snip = Sugg::hir_with_context(cx, expr, SyntaxContext::root(), "..", &mut app);
+    if app != Applicability::MachineApplicable {
+        return None;
+    }
+
+    Some(snip)
+}
+
+fn get_expr_snippet_with_type_certainty<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+) -> Option<MaybeTypeUncertain<'tcx>> {
+    get_expr_snippet(cx, expr).map(|snip| {
+        // If the type of the expression is certain, we can use it directly.
+        // Otherwise, we wrap it in a `let _: () = ...` to ensure the type is correct.
+        if !expr_type_is_certain(cx, expr) && !is_block_with_no_expr(expr) {
+            MaybeTypeUncertain::Uncertain(snip)
+        } else {
+            MaybeTypeUncertain::Certain(snip)
+        }
+    })
+}
+
+fn is_block_with_no_expr(expr: &Expr<'_>) -> bool {
+    matches!(expr.kind, ExprKind::Block(Block { expr: None, .. }, _))
+}
+
 fn is_empty_block(expr: &Expr<'_>) -> bool {
     matches!(
         expr.kind,
@@ -162,23 +223,20 @@ fn fmt_stmts_and_call(
     cx: &LateContext<'_>,
     call_expr: &Expr<'_>,
     call_snippet: &str,
-    args_snippets: &[SourceText],
-    non_empty_block_args_snippets: &[SourceText],
+    args_snippets: Vec<Sugg<'_>>,
+    non_empty_block_args_snippets: Vec<MaybeTypeUncertain<'_>>,
 ) -> String {
     let call_expr_indent = indent_of(cx, call_expr.span).unwrap_or(0);
-    let call_snippet_with_replacements = args_snippets
-        .iter()
-        .fold(call_snippet.to_owned(), |acc, arg| acc.replacen(arg.as_ref(), "()", 1));
+    let call_snippet_with_replacements = args_snippets.into_iter().fold(call_snippet.to_owned(), |acc, arg| {
+        acc.replacen(&arg.to_string(), "()", 1)
+    });
 
-    let mut stmts_and_call = non_empty_block_args_snippets
-        .iter()
-        .map(|it| it.as_ref().to_owned())
-        .collect::<Vec<_>>();
-    stmts_and_call.push(call_snippet_with_replacements);
-    stmts_and_call = stmts_and_call
+    let stmts_and_call = non_empty_block_args_snippets
         .into_iter()
+        .map(Into::into)
+        .chain(iter::once(call_snippet_with_replacements))
         .map(|v| reindent_multiline(&v, true, Some(call_expr_indent)))
-        .collect();
+        .collect::<Vec<_>>();
 
     let mut stmts_and_call_snippet = stmts_and_call.join(&format!("{}{}", ";\n", " ".repeat(call_expr_indent)));
     // expr is not in a block statement or result expression position, wrap in a block

clippy_utils/src/lib.rs

@@ -3473,3 +3473,15 @@ pub fn desugar_await<'tcx>(expr: &'tcx Expr<'_>) -> Option<&'tcx Expr<'tcx>> {
         None
     }
 }
+
+/// Checks if the given expression is a call to `Default::default()`.
+pub fn is_expr_default<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
+    if let ExprKind::Call(fn_expr, []) = &expr.kind
+        && let ExprKind::Path(qpath) = &fn_expr.kind
+        && let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id)
+    {
+        cx.tcx.is_diagnostic_item(sym::default_fn, def_id)
+    } else {
+        false
+    }
+}

tests/ui/unit_arg.rs

@@ -151,3 +151,27 @@ fn main() {
     bad();
     ok();
 }
+
+fn issue14857() {
+    let fn_take_unit = |_: ()| {};
+    fn some_other_fn(_: &i32) {}
+
+    macro_rules! mac {
+        (def) => {
+            Default::default()
+        };
+        (func $f:expr) => {
+            $f()
+        };
+        (nonempty_block $e:expr) => {{
+            some_other_fn(&$e);
+            $e
+        }};
+    }
+    fn_take_unit(mac!(def));
+    //~^ unit_arg
+    fn_take_unit(mac!(func Default::default));
+    //~^ unit_arg
+    fn_take_unit(mac!(nonempty_block Default::default()));
+    //~^ unit_arg
+}

tests/ui/unit_arg.stderr

@@ -199,5 +199,26 @@ LL ~     foo(1);
 LL +     Some(())
    |
 
-error: aborting due to 10 previous errors
+error: passing a unit value to a function
+  --> tests/ui/unit_arg.rs:171:5
+   |
+LL |     fn_take_unit(mac!(def));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+
+error: passing a unit value to a function
+  --> tests/ui/unit_arg.rs:173:5
+   |
+LL |     fn_take_unit(mac!(func Default::default));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+
+error: passing a unit value to a function
+  --> tests/ui/unit_arg.rs:175:5
+   |
+LL |     fn_take_unit(mac!(nonempty_block Default::default()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+
+error: aborting due to 13 previous errors