clippy_lints/src/default.rs

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_expr_default, is_from_proc_macro};
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::Applicability;
use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind, StructTailExpr};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_middle::ty::print::with_forced_trimmed_paths;
use rustc_session::impl_lint_pass;
use rustc_span::symbol::{Ident, Symbol};
use rustc_span::{Span, sym};

declare_clippy_lint! {
    /// ### What it does
    /// Checks for literal calls to `Default::default()`.
    ///
    /// ### Why is this bad?
    /// It's easier for the reader if the name of the type is used, rather than the
    /// generic `Default`.
    ///
    /// ### Example
    /// ```no_run
    /// let s: String = Default::default();
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// let s = String::default();
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub DEFAULT_TRAIT_ACCESS,
    pedantic,
    "checks for literal calls to `Default::default()`"
}

declare_clippy_lint! {
    /// ### What it does
    /// Checks for immediate reassignment of fields initialized
    /// with Default::default().
    ///
    /// ### Why is this bad?
    ///It's more idiomatic to use the [functional update syntax](https://doc.rust-lang.org/reference/expressions/struct-expr.html#functional-update-syntax).
    ///
    /// ### Known problems
    /// Assignments to patterns that are of tuple type are not linted.
    ///
    /// ### Example
    /// ```no_run
    /// # #[derive(Default)]
    /// # struct A { i: i32 }
    /// let mut a: A = Default::default();
    /// a.i = 42;
    /// ```
    ///
    /// Use instead:
    /// ```no_run
    /// # #[derive(Default)]
    /// # struct A { i: i32 }
    /// let a = A {
    ///     i: 42,
    ///     .. Default::default()
    /// };
    /// ```
    #[clippy::version = "1.49.0"]
    pub FIELD_REASSIGN_WITH_DEFAULT,
    style,
    "binding initialized with Default should have its fields set in the initializer"
}

impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);

#[derive(Default)]
pub struct Default {
    // Spans linted by `field_reassign_with_default`.
    reassigned_linted: FxHashSet<Span>,
}

impl<'tcx> LateLintPass<'tcx> for Default {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if !expr.span.from_expansion()
            // Avoid cases already linted by `field_reassign_with_default`
            && !self.reassigned_linted.contains(&expr.span)
            && let ExprKind::Call(path, []) = expr.kind
            && !in_automatically_derived(cx.tcx, expr.hir_id)
            && let ExprKind::Path(ref qpath) = path.kind
            && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
            && cx.tcx.is_diagnostic_item(sym::default_fn, def_id)
            && !is_update_syntax_base(cx, expr)
            // Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
            && let QPath::Resolved(None, _path) = qpath
            && let expr_ty = cx.typeck_results().expr_ty(expr)
            && let ty::Adt(def, ..) = expr_ty.kind()
            && !is_from_proc_macro(cx, expr)
        {
            let replacement = with_forced_trimmed_paths!(format!("{}::default()", cx.tcx.def_path_str(def.did())));
            span_lint_and_sugg(
                cx,
                DEFAULT_TRAIT_ACCESS,
                expr.span,
                format!("calling `{replacement}` is more clear than this expression"),
                "try",
                replacement,
                Applicability::Unspecified, // First resolve the TODO above
            );
        }
    }

    #[expect(clippy::too_many_lines)]
    fn check_block(&mut self, cx: &LateContext<'tcx>, block: &Block<'tcx>) {
        // start from the `let mut _ = _::default();` and look at all the following
        // statements, see if they re-assign the fields of the binding
        let stmts_head = match block.stmts {
            [] | [_] => return,
            // Skip the last statement since there cannot possibly be any following statements that re-assign fields.
            [head @ .., _] => head,
        };
        for (stmt_idx, stmt) in stmts_head.iter().enumerate() {
            // find all binding statements like `let mut _ = T::default()` where `T::default()` is the
            // `default` method of the `Default` trait, and store statement index in current block being
            // checked and the name of the bound variable
            let (local, variant, binding_name, binding_type, span) = if let StmtKind::Let(local) = stmt.kind
                // only take `let ...` statements
                && let Some(expr) = local.init
                && !in_automatically_derived(cx.tcx, expr.hir_id)
                && !expr.span.from_expansion()
                // only take bindings to identifiers
                && let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind
                // only when assigning `... = Default::default()`
                && 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()
                && let variant = adt.non_enum_variant()
                && !variant.field_list_has_applicable_non_exhaustive()
                && let module_did = cx.tcx.parent_module(stmt.hir_id)
                && variant
                    .fields
                    .iter()
                    .all(|field| field.vis.is_accessible_from(module_did, cx.tcx))
                && let all_fields_are_copy = variant
                    .fields
                    .iter()
                    .all(|field| {
                        is_copy(cx, cx.tcx.type_of(field.did).instantiate(cx.tcx, args))
                    })
                && (!has_drop(cx, binding_type) || all_fields_are_copy)
            {
                (local, variant, ident.name, binding_type, expr.span)
            } else {
                continue;
            };

            let init_ctxt = local.span.ctxt();

            // find all "later statement"'s where the fields of the binding set as
            // Default::default() get reassigned, unless the reassignment refers to the original binding
            let mut first_assign = None;
            let mut assigned_fields = Vec::new();
            let mut cancel_lint = false;
            for consecutive_statement in &block.stmts[stmt_idx + 1..] {
                // find out if and which field was set by this `consecutive_statement`
                if let Some((field_ident, assign_rhs)) = field_reassigned_by_stmt(consecutive_statement, binding_name) {
                    // interrupt and cancel lint if assign_rhs references the original binding
                    if contains_name(binding_name, assign_rhs, cx) || init_ctxt != consecutive_statement.span.ctxt() {
                        cancel_lint = true;
                        break;
                    }

                    // if the field was previously assigned, replace the assignment, otherwise insert the assignment
                    if let Some(prev) = assigned_fields
                        .iter_mut()
                        .find(|(field_name, _)| field_name == &field_ident.name)
                    {
                        *prev = (field_ident.name, assign_rhs);
                    } else {
                        assigned_fields.push((field_ident.name, assign_rhs));
                    }

                    // also set first instance of error for help message
                    if first_assign.is_none() {
                        first_assign = Some(consecutive_statement);
                    }
                }
                // interrupt if no field was assigned, since we only want to look at consecutive statements
                else {
                    break;
                }
            }

            // if there are incorrectly assigned fields, do a span_lint_and_note to suggest
            // construction using `Ty { fields, ..Default::default() }`
            if !assigned_fields.is_empty() && !cancel_lint {
                // if all fields of the struct are not assigned, add `.. Default::default()` to the suggestion.
                let ext_with_default = !variant
                    .fields
                    .iter()
                    .all(|field| assigned_fields.iter().any(|(a, _)| a == &field.name));

                let mut app = Applicability::Unspecified;
                let field_list = assigned_fields
                    .into_iter()
                    .map(|(field, rhs)| {
                        // extract and store the assigned value for help message
                        let value_snippet = snippet_with_context(cx, rhs.span, init_ctxt, "..", &mut app).0;
                        format!("{field}: {value_snippet}")
                    })
                    .collect::<Vec<String>>()
                    .join(", ");

                // give correct suggestion if generics are involved (see #6944)
                let binding_type = if let ty::Adt(adt_def, args) = binding_type.kind()
                    && !args.is_empty()
                {
                    let adt_def_ty_name = cx.tcx.item_name(adt_def.did());
                    let generic_args = args.iter().collect::<Vec<_>>();
                    let tys_str = generic_args
                        .iter()
                        .map(ToString::to_string)
                        .collect::<Vec<_>>()
                        .join(", ");
                    format!("{adt_def_ty_name}::<{tys_str}>")
                } else {
                    binding_type.to_string()
                };

                let sugg = if ext_with_default {
                    if field_list.is_empty() {
                        format!("{binding_type}::default()")
                    } else {
                        format!("{binding_type} {{ {field_list}, ..Default::default() }}")
                    }
                } else {
                    format!("{binding_type} {{ {field_list} }}")
                };

                // span lint once per statement that binds default
                span_lint_and_note(
                    cx,
                    FIELD_REASSIGN_WITH_DEFAULT,
                    first_assign.unwrap().span,
                    "field assignment outside of initializer for an instance created with Default::default()",
                    Some(local.span),
                    format!("consider initializing the variable with `{sugg}` and removing relevant reassignments"),
                );
                self.reassigned_linted.insert(span);
            }
        }
    }
}

/// 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
        // only take assignments
        && let ExprKind::Assign(assign_lhs, assign_rhs, _) = later_expr.kind
        // only take assignments to fields where the left-hand side field is a field of
        // the same binding as the previous statement
        && let ExprKind::Field(binding, field_ident) = assign_lhs.kind
        && let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind
        && let Some(second_binding_name) = path.segments.last()
        && second_binding_name.ident.name == binding_name
    {
        Some((field_ident, assign_rhs))
    } else {
        None
    }
}

/// Returns whether `expr` is the update syntax base: `Foo { a: 1, .. base }`
fn is_update_syntax_base<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
    if let Some(parent) = get_parent_expr(cx, expr)
        && let ExprKind::Struct(_, _, StructTailExpr::Base(base)) = parent.kind
    {
        base.hir_id == expr.hir_id
    } else {
        false
    }
}