Auto merge of #9295 - Guilherme-Vasconcelos:manual-empty-string-creation, r=dswij

Add `manual_empty_string_creations` lint

Closes #2972

- [x] Followed [lint naming conventions][lint_naming]
- [x] Added passing UI tests (including committed `.stderr` file)
- [x] `cargo test` passes locally
- [x] Executed `cargo dev update_lints`
- [x] Added lint documentation
- [x] Run `cargo dev fmt`

changelog: [`manual_empty_string_creations`]: Add lint for empty String not being created with `String::new()`

clippy_lints/src/lib.register_all.rs

@@ -124,6 +124,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(main_recursion::MAIN_RECURSION),
     LintId::of(manual_async_fn::MANUAL_ASYNC_FN),
     LintId::of(manual_bits::MANUAL_BITS),
+    LintId::of(manual_empty_string_creations::MANUAL_EMPTY_STRING_CREATIONS),
     LintId::of(manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE),
     LintId::of(manual_rem_euclid::MANUAL_REM_EUCLID),
     LintId::of(manual_retain::MANUAL_RETAIN),

clippy_lints/src/lib.register_lints.rs

@@ -244,6 +244,7 @@ store.register_lints(&[
     manual_assert::MANUAL_ASSERT,
     manual_async_fn::MANUAL_ASYNC_FN,
     manual_bits::MANUAL_BITS,
+    manual_empty_string_creations::MANUAL_EMPTY_STRING_CREATIONS,
     manual_instant_elapsed::MANUAL_INSTANT_ELAPSED,
     manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE,
     manual_ok_or::MANUAL_OK_OR,

clippy_lints/src/lib.register_style.rs

@@ -44,6 +44,7 @@ store.register_group(true, "clippy::style", Some("clippy_style"), vec![
     LintId::of(main_recursion::MAIN_RECURSION),
     LintId::of(manual_async_fn::MANUAL_ASYNC_FN),
     LintId::of(manual_bits::MANUAL_BITS),
+    LintId::of(manual_empty_string_creations::MANUAL_EMPTY_STRING_CREATIONS),
     LintId::of(manual_non_exhaustive::MANUAL_NON_EXHAUSTIVE),
     LintId::of(map_clone::MAP_CLONE),
     LintId::of(match_result_ok::MATCH_RESULT_OK),

clippy_lints/src/lib.rs

@@ -273,6 +273,7 @@ mod main_recursion;
 mod manual_assert;
 mod manual_async_fn;
 mod manual_bits;
+mod manual_empty_string_creations;
 mod manual_instant_elapsed;
 mod manual_non_exhaustive;
 mod manual_ok_or;
@@ -933,6 +934,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(std_instead_of_core::StdReexports::default()));
     store.register_late_pass(|| Box::new(manual_instant_elapsed::ManualInstantElapsed));
     store.register_late_pass(|| Box::new(partialeq_to_none::PartialeqToNone));
+    store.register_late_pass(|| Box::new(manual_empty_string_creations::ManualEmptyStringCreations));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/manual_async_fn.rs

@@ -192,7 +192,7 @@ fn suggested_ret(cx: &LateContext<'_>, output: &Ty<'_>) -> Option<(&'static str,
     match output.kind {
         TyKind::Tup(tys) if tys.is_empty() => {
             let sugg = "remove the return type";
-            Some((sugg, "".into()))
+            Some((sugg, String::new()))
         },
         _ => {
             let sugg = "return the output of the future directly";

clippy_lints/src/manual_empty_string_creations.rs

@@ -0,0 +1,141 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_ast::LitKind;
+use rustc_errors::Applicability::MachineApplicable;
+use rustc_hir::{Expr, ExprKind, PathSegment, QPath, TyKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{sym, symbol, Span};
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Checks for usage of `""` to create a `String`, such as `"".to_string()`, `"".to_owned()`,
+    /// `String::from("")` and others.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// Different ways of creating an empty string makes your code less standardized, which can
+    /// be confusing.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let a = "".to_string();
+    /// let b: String = "".into();
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let a = String::new();
+    /// let b = String::new();
+    /// ```
+    #[clippy::version = "1.65.0"]
+    pub MANUAL_EMPTY_STRING_CREATIONS,
+    style,
+    "empty String is being created manually"
+}
+declare_lint_pass!(ManualEmptyStringCreations => [MANUAL_EMPTY_STRING_CREATIONS]);
+
+impl LateLintPass<'_> for ManualEmptyStringCreations {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        let ty = cx.typeck_results().expr_ty(expr);
+        match ty.kind() {
+            ty::Adt(adt_def, _) if adt_def.is_struct() => {
+                if !cx.tcx.is_diagnostic_item(sym::String, adt_def.did()) {
+                    return;
+                }
+            },
+            _ => return,
+        }
+
+        match expr.kind {
+            ExprKind::Call(func, args) => {
+                parse_call(cx, expr.span, func, args);
+            },
+            ExprKind::MethodCall(path_segment, args, _) => {
+                parse_method_call(cx, expr.span, path_segment, args);
+            },
+            _ => (),
+        }
+    }
+}
+
+/// Checks if an expression's kind corresponds to an empty &str.
+fn is_expr_kind_empty_str(expr_kind: &ExprKind<'_>) -> bool {
+    if  let ExprKind::Lit(lit) = expr_kind &&
+        let LitKind::Str(value, _) = lit.node &&
+        value == symbol::kw::Empty
+    {
+        return true;
+    }
+
+    false
+}
+
+/// Emits the `MANUAL_EMPTY_STRING_CREATION` warning and suggests the appropriate fix.
+fn warn_then_suggest(cx: &LateContext<'_>, span: Span) {
+    span_lint_and_sugg(
+        cx,
+        MANUAL_EMPTY_STRING_CREATIONS,
+        span,
+        "empty String is being created manually",
+        "consider using",
+        "String::new()".into(),
+        MachineApplicable,
+    );
+}
+
+/// Tries to parse an expression as a method call, emiting the warning if necessary.
+fn parse_method_call(cx: &LateContext<'_>, span: Span, path_segment: &PathSegment<'_>, args: &[Expr<'_>]) {
+    if args.is_empty() {
+        // When parsing TryFrom::try_from(...).expect(...), we will have more than 1 arg.
+        return;
+    }
+
+    let ident = path_segment.ident.as_str();
+    let method_arg_kind = &args[0].kind;
+    if ["to_string", "to_owned", "into"].contains(&ident) && is_expr_kind_empty_str(method_arg_kind) {
+        warn_then_suggest(cx, span);
+    } else if let ExprKind::Call(func, args) = method_arg_kind {
+        // If our first argument is a function call itself, it could be an `unwrap`-like function.
+        // E.g. String::try_from("hello").unwrap(), TryFrom::try_from("").expect("hello"), etc.
+        parse_call(cx, span, func, args);
+    }
+}
+
+/// Tries to parse an expression as a function call, emiting the warning if necessary.
+fn parse_call(cx: &LateContext<'_>, span: Span, func: &Expr<'_>, args: &[Expr<'_>]) {
+    if args.len() != 1 {
+        return;
+    }
+
+    let arg_kind = &args[0].kind;
+    if let ExprKind::Path(qpath) = &func.kind {
+        if let QPath::TypeRelative(_, _) = qpath {
+            // String::from(...) or String::try_from(...)
+            if  let QPath::TypeRelative(ty, path_seg) = qpath &&
+                [sym::from, sym::try_from].contains(&path_seg.ident.name) &&
+                let TyKind::Path(qpath) = &ty.kind &&
+                let QPath::Resolved(_, path) = qpath &&
+                let [path_seg] = path.segments &&
+                path_seg.ident.name == sym::String &&
+                is_expr_kind_empty_str(arg_kind)
+            {
+                warn_then_suggest(cx, span);
+            }
+        } else if let QPath::Resolved(_, path) = qpath {
+            // From::from(...) or TryFrom::try_from(...)
+            if  let [path_seg1, path_seg2] = path.segments &&
+                is_expr_kind_empty_str(arg_kind) && (
+                    (path_seg1.ident.name == sym::From && path_seg2.ident.name == sym::from) ||
+                    (path_seg1.ident.name == sym::TryFrom && path_seg2.ident.name == sym::try_from)
+                )
+            {
+                warn_then_suggest(cx, span);
+            }
+        }
+    }
+}

clippy_lints/src/methods/option_map_unwrap_or.rs

@@ -78,7 +78,7 @@ pub(super) fn check<'tcx>(
                     map_span,
                     String::from(if unwrap_snippet_none { "and_then" } else { "map_or" }),
                 ),
-                (expr.span.with_lo(unwrap_recv.span.hi()), String::from("")),
+                (expr.span.with_lo(unwrap_recv.span.hi()), String::new()),
             ];
 
             if !unwrap_snippet_none {

clippy_lints/src/misc_early/unneeded_wildcard_pattern.rs

@@ -46,7 +46,7 @@ fn span_lint(cx: &EarlyContext<'_>, span: Span, only_one: bool) {
             "these patterns are unneeded as the `..` pattern can match those elements"
         },
         if only_one { "remove it" } else { "remove them" },
-        "".to_string(),
+        String::new(),
         Applicability::MachineApplicable,
     );
 }

clippy_lints/src/unnecessary_wraps.rs

@@ -130,7 +130,7 @@ impl<'tcx> LateLintPass<'tcx> for UnnecessaryWraps {
                         (
                             ret_expr.span,
                             if inner_type.is_unit() {
-                                "".to_string()
+                                String::new()
                             } else {
                                 snippet(cx, arg.span.source_callsite(), "..").to_string()
                             }