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Auto merge of #6951 - Jarcho:implicit_return_fp, r=flip1995
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`implicit_return` improvements

fixes: #6940

changelog: Fix `implicit_return` suggestion for async functions
changelog: Improve `implicit_return` suggestions when returning the result of a macro
changelog: Check for `break` expressions inside a loop which are then implicitly returned
changelog: Allow all diverging functions in `implicit_return`, not just panic functions
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@bors
bors committed Apr 30, 2021
2 parents 14f1551 + 3d793f3 commit 5e49c4b
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Showing 7 changed files with 450 additions and 140 deletions.
251 changes: 169 additions & 82 deletions clippy_lints/src/implicit_return.rs
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Original file line number Diff line number Diff line change
@@ -1,13 +1,16 @@
use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::match_panic_def_id;
use clippy_utils::source::snippet_opt;
use if_chain::if_chain;
use clippy_utils::{
diagnostics::span_lint_and_sugg,
get_async_fn_body, is_async_fn,
source::{snippet_with_applicability, snippet_with_context, walk_span_to_context},
visitors::visit_break_exprs,
};
use rustc_errors::Applicability;
use rustc_hir::intravisit::FnKind;
use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, MatchSource, StmtKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, FnRetTy, HirId};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::source_map::Span;
use rustc_span::{Span, SyntaxContext};

declare_clippy_lint! {
/// **What it does:** Checks for missing return statements at the end of a block.
Expand Down Expand Up @@ -39,109 +42,193 @@ declare_clippy_lint! {

declare_lint_pass!(ImplicitReturn => [IMPLICIT_RETURN]);

static LINT_BREAK: &str = "change `break` to `return` as shown";
static LINT_RETURN: &str = "add `return` as shown";

fn lint(cx: &LateContext<'_>, outer_span: Span, inner_span: Span, msg: &str) {
let outer_span = outer_span.source_callsite();
let inner_span = inner_span.source_callsite();

span_lint_and_then(cx, IMPLICIT_RETURN, outer_span, "missing `return` statement", |diag| {
if let Some(snippet) = snippet_opt(cx, inner_span) {
diag.span_suggestion(
outer_span,
msg,
format!("return {}", snippet),
Applicability::MachineApplicable,
);
}
});
fn lint_return(cx: &LateContext<'_>, span: Span) {
let mut app = Applicability::MachineApplicable;
let snip = snippet_with_applicability(cx, span, "..", &mut app);
span_lint_and_sugg(
cx,
IMPLICIT_RETURN,
span,
"missing `return` statement",
"add `return` as shown",
format!("return {}", snip),
app,
);
}

fn lint_break(cx: &LateContext<'_>, break_span: Span, expr_span: Span) {
let mut app = Applicability::MachineApplicable;
let snip = snippet_with_context(cx, expr_span, break_span.ctxt(), "..", &mut app).0;
span_lint_and_sugg(
cx,
IMPLICIT_RETURN,
break_span,
"missing `return` statement",
"change `break` to `return` as shown",
format!("return {}", snip),
app,
)
}

#[derive(Clone, Copy, PartialEq, Eq)]
enum LintLocation {
/// The lint was applied to a parent expression.
Parent,
/// The lint was applied to this expression, a child, or not applied.
Inner,
}
impl LintLocation {
fn still_parent(self, b: bool) -> Self {
if b { self } else { Self::Inner }
}

fn is_parent(self) -> bool {
self == Self::Parent
}
}

// Gets the call site if the span is in a child context. Otherwise returns `None`.
fn get_call_site(span: Span, ctxt: SyntaxContext) -> Option<Span> {
(span.ctxt() != ctxt).then(|| walk_span_to_context(span, ctxt).unwrap_or(span))
}

fn expr_match(cx: &LateContext<'_>, expr: &Expr<'_>) {
fn lint_implicit_returns(
cx: &LateContext<'tcx>,
expr: &'tcx Expr<'_>,
// The context of the function body.
ctxt: SyntaxContext,
// Whether the expression is from a macro expansion.
call_site_span: Option<Span>,
) -> LintLocation {
match expr.kind {
// loops could be using `break` instead of `return`
ExprKind::Block(block, ..) | ExprKind::Loop(block, ..) => {
if let Some(expr) = &block.expr {
expr_match(cx, expr);
}
// only needed in the case of `break` with `;` at the end
else if let Some(stmt) = block.stmts.last() {
if_chain! {
if let StmtKind::Semi(expr, ..) = &stmt.kind;
// make sure it's a break, otherwise we want to skip
if let ExprKind::Break(.., Some(break_expr)) = &expr.kind;
then {
lint(cx, expr.span, break_expr.span, LINT_BREAK);
}
}
}
},
// use `return` instead of `break`
ExprKind::Break(.., break_expr) => {
if let Some(break_expr) = break_expr {
lint(cx, expr.span, break_expr.span, LINT_BREAK);
ExprKind::Block(
Block {
expr: Some(block_expr), ..
},
_,
) => lint_implicit_returns(
cx,
block_expr,
ctxt,
call_site_span.or_else(|| get_call_site(block_expr.span, ctxt)),
)
.still_parent(call_site_span.is_some()),

ExprKind::If(_, then_expr, Some(else_expr)) => {
// Both `then_expr` or `else_expr` are required to be blocks in the same context as the `if`. Don't
// bother checking.
let res = lint_implicit_returns(cx, then_expr, ctxt, call_site_span).still_parent(call_site_span.is_some());
if res.is_parent() {
// The return was added as a parent of this if expression.
return res;
}
lint_implicit_returns(cx, else_expr, ctxt, call_site_span).still_parent(call_site_span.is_some())
},
ExprKind::If(.., if_expr, else_expr) => {
expr_match(cx, if_expr);

if let Some(else_expr) = else_expr {
expr_match(cx, else_expr);
ExprKind::Match(_, arms, _) => {
for arm in arms {
let res = lint_implicit_returns(
cx,
arm.body,
ctxt,
call_site_span.or_else(|| get_call_site(arm.body.span, ctxt)),
)
.still_parent(call_site_span.is_some());
if res.is_parent() {
// The return was added as a parent of this match expression.
return res;
}
}
LintLocation::Inner
},
ExprKind::Match(.., arms, source) => {
let check_all_arms = match source {
MatchSource::IfLetDesugar {
contains_else_clause: has_else,
} => has_else,
_ => true,
};

if check_all_arms {
for arm in arms {
expr_match(cx, arm.body);

ExprKind::Loop(block, ..) => {
let mut add_return = false;
visit_break_exprs(block, |break_expr, dest, sub_expr| {
if dest.target_id.ok() == Some(expr.hir_id) {
if call_site_span.is_none() && break_expr.span.ctxt() == ctxt {
lint_break(cx, break_expr.span, sub_expr.unwrap().span);
} else {
// the break expression is from a macro call, add a return to the loop
add_return = true;
}
}
});
if add_return {
#[allow(clippy::option_if_let_else)]
if let Some(span) = call_site_span {
lint_return(cx, span);
LintLocation::Parent
} else {
lint_return(cx, expr.span);
LintLocation::Inner
}
} else {
expr_match(cx, arms.first().expect("`if let` doesn't have a single arm").body);
LintLocation::Inner
}
},
// skip if it already has a return statement
ExprKind::Ret(..) => (),
// make sure it's not a call that panics
ExprKind::Call(expr, ..) => {
if_chain! {
if let ExprKind::Path(qpath) = &expr.kind;
if let Some(path_def_id) = cx.qpath_res(qpath, expr.hir_id).opt_def_id();
if match_panic_def_id(cx, path_def_id);
then { }
else {
lint(cx, expr.span, expr.span, LINT_RETURN)
}

// If expressions without an else clause, and blocks without a final expression can only be the final expression
// if they are divergent, or return the unit type.
ExprKind::If(_, _, None) | ExprKind::Block(Block { expr: None, .. }, _) | ExprKind::Ret(_) => {
LintLocation::Inner
},

// Any divergent expression doesn't need a return statement.
ExprKind::MethodCall(..)
| ExprKind::Call(..)
| ExprKind::Binary(..)
| ExprKind::Unary(..)
| ExprKind::Index(..)
if cx.typeck_results().expr_ty(expr).is_never() =>
{
LintLocation::Inner
},

_ =>
{
#[allow(clippy::option_if_let_else)]
if let Some(span) = call_site_span {
lint_return(cx, span);
LintLocation::Parent
} else {
lint_return(cx, expr.span);
LintLocation::Inner
}
},
// everything else is missing `return`
_ => lint(cx, expr.span, expr.span, LINT_RETURN),
}
}

impl<'tcx> LateLintPass<'tcx> for ImplicitReturn {
fn check_fn(
&mut self,
cx: &LateContext<'tcx>,
_: FnKind<'tcx>,
_: &'tcx FnDecl<'_>,
kind: FnKind<'tcx>,
decl: &'tcx FnDecl<'_>,
body: &'tcx Body<'_>,
span: Span,
_: HirId,
) {
if span.from_expansion() {
if (!matches!(kind, FnKind::Closure) && matches!(decl.output, FnRetTy::DefaultReturn(_)))
|| span.ctxt() != body.value.span.ctxt()
|| in_external_macro(cx.sess(), span)
{
return;
}
let body = cx.tcx.hir().body(body.id());
if cx.typeck_results().expr_ty(&body.value).is_unit() {

let res_ty = cx.typeck_results().expr_ty(&body.value);
if res_ty.is_unit() || res_ty.is_never() {
return;
}
expr_match(cx, &body.value);

let expr = if is_async_fn(kind) {
match get_async_fn_body(cx.tcx, body) {
Some(e) => e,
None => return,
}
} else {
&body.value
};
lint_implicit_returns(cx, expr, expr.span.ctxt(), None);
}
}
40 changes: 37 additions & 3 deletions clippy_utils/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -61,12 +61,12 @@ use rustc_data_structures::fx::FxHashMap;
use rustc_hir as hir;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::{DefId, LOCAL_CRATE};
use rustc_hir::intravisit::{self, walk_expr, ErasedMap, NestedVisitorMap, Visitor};
use rustc_hir::intravisit::{self, walk_expr, ErasedMap, FnKind, NestedVisitorMap, Visitor};
use rustc_hir::LangItem::{ResultErr, ResultOk};
use rustc_hir::{
def, Arm, BindingAnnotation, Block, Body, Constness, Destination, Expr, ExprKind, FnDecl, GenericArgs, HirId, Impl,
ImplItem, ImplItemKind, Item, ItemKind, LangItem, Local, MatchSource, Node, Param, Pat, PatKind, Path, PathSegment,
QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitRef, TyKind,
ImplItem, ImplItemKind, IsAsync, Item, ItemKind, LangItem, Local, MatchSource, Node, Param, Pat, PatKind, Path,
PathSegment, QPath, Stmt, StmtKind, TraitItem, TraitItemKind, TraitRef, TyKind,
};
use rustc_lint::{LateContext, Level, Lint, LintContext};
use rustc_middle::hir::exports::Export;
Expand Down Expand Up @@ -1301,6 +1301,40 @@ pub fn if_sequence<'tcx>(mut expr: &'tcx Expr<'tcx>) -> (Vec<&'tcx Expr<'tcx>>,
(conds, blocks)
}

/// Checks if the given function kind is an async function.
pub fn is_async_fn(kind: FnKind) -> bool {
matches!(kind, FnKind::ItemFn(_, _, header, _) if header.asyncness == IsAsync::Async)
}

/// Peels away all the compiler generated code surrounding the body of an async function,
pub fn get_async_fn_body(tcx: TyCtxt<'tcx>, body: &Body<'_>) -> Option<&'tcx Expr<'tcx>> {
if let ExprKind::Call(
_,
&[Expr {
kind: ExprKind::Closure(_, _, body, _, _),
..
}],
) = body.value.kind
{
if let ExprKind::Block(
Block {
stmts: [],
expr:
Some(Expr {
kind: ExprKind::DropTemps(expr),
..
}),
..
},
_,
) = tcx.hir().body(body).value.kind
{
return Some(expr);
}
};
None
}

// Finds the `#[must_use]` attribute, if any
pub fn must_use_attr(attrs: &[Attribute]) -> Option<&Attribute> {
attrs.iter().find(|a| a.has_name(sym::must_use))
Expand Down
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