rust-clippy/clippy_lints/src/default.rs

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use clippy_utils::diagnostics::{span_lint_and_note, span_lint_and_sugg};
use clippy_utils::source::snippet_with_macro_callsite;
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use clippy_utils::{any_parent_is_automatically_derived, contains_name, in_macro, match_def_path, paths};
use if_chain::if_chain;
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};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::symbol::{Ident, Symbol};
use rustc_span::Span;
declare_clippy_lint! {
/// **What it does:** Checks for literal calls to `Default::default()`.
///
/// **Why is this bad?** It's more clear to the reader to use the name of the type whose default is
/// being gotten than the generic `Default`.
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// // Bad
/// let s: String = Default::default();
///
/// // Good
/// let s = String::default();
/// ```
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:**
/// Bad:
/// ```
/// # #[derive(Default)]
/// # struct A { i: i32 }
/// let mut a: A = Default::default();
/// a.i = 42;
/// ```
/// Use instead:
/// ```
/// # #[derive(Default)]
/// # struct A { i: i32 }
/// let a = A {
/// i: 42,
/// .. Default::default()
/// };
/// ```
pub FIELD_REASSIGN_WITH_DEFAULT,
style,
"binding initialized with Default should have its fields set in the initializer"
}
#[derive(Default)]
pub struct Default {
// Spans linted by `field_reassign_with_default`.
reassigned_linted: FxHashSet<Span>,
}
impl_lint_pass!(Default => [DEFAULT_TRAIT_ACCESS, FIELD_REASSIGN_WITH_DEFAULT]);
impl LateLintPass<'_> for Default {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
if_chain! {
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if !in_macro(expr.span);
// Avoid cases already linted by `field_reassign_with_default`
if !self.reassigned_linted.contains(&expr.span);
if let ExprKind::Call(path, ..) = expr.kind;
if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
if let ExprKind::Path(ref qpath) = path.kind;
if let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id();
if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
// Detect and ignore <Foo as Default>::default() because these calls do explicitly name the type.
if let QPath::Resolved(None, _path) = qpath;
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let expr_ty = cx.typeck_results().expr_ty(expr);
if let ty::Adt(def, ..) = expr_ty.kind();
then {
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// TODO: Work out a way to put "whatever the imported way of referencing
// this type in this file" rather than a fully-qualified type.
let replacement = format!("{}::default()", cx.tcx.def_path_str(def.did));
span_lint_and_sugg(
cx,
DEFAULT_TRAIT_ACCESS,
expr.span,
&format!("calling `{}` is more clear than this expression", replacement),
"try",
replacement,
Applicability::Unspecified, // First resolve the TODO above
);
}
}
}
#[allow(clippy::too_many_lines)]
fn check_block<'tcx>(&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 {
// Skip the last statement since there cannot possibly be any following statements that re-assign fields.
[head @ .., _] if !head.is_empty() => head,
_ => return,
};
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_chain! {
// only take `let ...` statements
if let StmtKind::Local(local) = stmt.kind;
if let Some(expr) = local.init;
if !any_parent_is_automatically_derived(cx.tcx, expr.hir_id);
if !in_macro(expr.span);
// only take bindings to identifiers
if let PatKind::Binding(_, binding_id, ident, _) = local.pat.kind;
// only when assigning `... = Default::default()`
if is_expr_default(expr, cx);
let binding_type = cx.typeck_results().node_type(binding_id);
if let Some(adt) = binding_type.ty_adt_def();
if adt.is_struct();
let variant = adt.non_enum_variant();
if adt.did.is_local() || !variant.is_field_list_non_exhaustive();
let module_did = cx.tcx.parent_module(stmt.hir_id).to_def_id();
if variant
.fields
.iter()
.all(|field| field.vis.is_accessible_from(module_did, cx.tcx));
then {
(local, variant, ident.name, binding_type, expr.span)
} else {
continue;
}
};
// 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) {
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.ident.name));
let field_list = assigned_fields
.into_iter()
.map(|(field, rhs)| {
// extract and store the assigned value for help message
let value_snippet = snippet_with_macro_callsite(cx, rhs.span, "..");
format!("{}: {}", field, value_snippet)
})
.collect::<Vec<String>>()
.join(", ");
// give correct suggestion if generics are involved (see #6944)
let binding_type = if_chain! {
if let ty::Adt(adt_def, substs) = binding_type.kind();
if !substs.is_empty();
then {
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let adt_def_ty_name = cx.tcx.item_name(adt_def.did);
let generic_args = substs.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!("{}::default()", binding_type)
} else {
format!("{} {{ {}, ..Default::default() }}", binding_type, field_list)
}
} 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 `{}` and removing relevant reassignments",
sugg
),
);
self.reassigned_linted.insert(span);
}
}
}
}
/// 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_chain! {
if let ExprKind::Call(fn_expr, _) = &expr.kind;
if let ExprKind::Path(qpath) = &fn_expr.kind;
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if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
then {
// right hand side of assignment is `Default::default`
match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD)
} 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_chain! {
// only take assignments
if let StmtKind::Semi(later_expr) = this.kind;
if 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
if let ExprKind::Field(binding, field_ident) = assign_lhs.kind;
if let ExprKind::Path(QPath::Resolved(_, path)) = binding.kind;
if let Some(second_binding_name) = path.segments.last();
if second_binding_name.ident.name == binding_name;
then {
Some((field_ident, assign_rhs))
} else {
None
}
}
}