Auto merge of #11373 - Red-Rapious:master, r=blyxyas,y21

Added new lint: `reserve_after_initialization`

Closes https://github.com/rust-lang/rust-clippy/issues/11330.

A new lint that informs the user about a more concise way to create a vector with a known capacity.
Example:
```rust
let mut v: Vec<usize> = vec![];
v.reserve(10);
```

Produces the following help:
```rust
  |
2 | /     let mut v: Vec<usize> = vec![];
3 | |     v.reserve(10);
  | |__________________^ help: consider using `Vec::with_capacity(space_hint)`: `let v: Vec<usize> = Vec::with_capacity(10);`
  |
```

And can be rewritten as:
```rust
let v: Vec<usize> = Vec::with_capacity(10);
```

changelog: new lint [`reserve_after_initialization`]

7 files changed, 262 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe64283462d..4e33cb7b457 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5299,6 +5299,7 @@ Released 2018-09-13
 [`regex_macro`]: https://rust-lang.github.io/rust-clippy/master/index.html#regex_macro
 [`repeat_once`]: https://rust-lang.github.io/rust-clippy/master/index.html#repeat_once
 [`replace_consts`]: https://rust-lang.github.io/rust-clippy/master/index.html#replace_consts
+[`reserve_after_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#reserve_after_initialization
 [`rest_pat_in_fully_bound_structs`]: https://rust-lang.github.io/rust-clippy/master/index.html#rest_pat_in_fully_bound_structs
 [`result_expect_used`]: https://rust-lang.github.io/rust-clippy/master/index.html#result_expect_used
 [`result_large_err`]: https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index bab77f91269..1be9720fbbf 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -579,6 +579,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::reference::DEREF_ADDROF_INFO,
     crate::regex::INVALID_REGEX_INFO,
     crate::regex::TRIVIAL_REGEX_INFO,
+    crate::reserve_after_initialization::RESERVE_AFTER_INITIALIZATION_INFO,
     crate::return_self_not_must_use::RETURN_SELF_NOT_MUST_USE_INFO,
     crate::returns::LET_AND_RETURN_INFO,
     crate::returns::NEEDLESS_RETURN_INFO,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 358004cf460..f50019f3cf7 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -285,6 +285,7 @@ mod ref_option_ref;
 mod ref_patterns;
 mod reference;
 mod regex;
+mod reserve_after_initialization;
 mod return_self_not_must_use;
 mod returns;
 mod same_name_method;
@@ -1095,6 +1096,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     });
     store.register_late_pass(|_| Box::new(redundant_locals::RedundantLocals));
     store.register_late_pass(|_| Box::new(ignored_unit_patterns::IgnoredUnitPatterns));
+    store.register_late_pass(|_| Box::<reserve_after_initialization::ReserveAfterInitialization>::default());
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_lints/src/reserve_after_initialization.rs b/clippy_lints/src/reserve_after_initialization.rs
new file mode 100644
index 00000000000..0c8c904e374
--- /dev/null
+++ b/clippy_lints/src/reserve_after_initialization.rs
@@ -0,0 +1,134 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::higher::{get_vec_init_kind, VecInitKind};
+use clippy_utils::source::snippet;
+use clippy_utils::{is_from_proc_macro, path_to_local_id};
+use rustc_errors::Applicability;
+use rustc_hir::def::Res;
+use rustc_hir::{BindingAnnotation, Block, Expr, ExprKind, HirId, Local, PatKind, QPath, Stmt, StmtKind};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+use rustc_span::Span;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Informs the user about a more concise way to create a vector with a known capacity.
+    ///
+    /// ### Why is this bad?
+    /// The `Vec::with_capacity` constructor is less complex.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let mut v: Vec<usize> = vec![];
+    /// v.reserve(10);
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let mut v: Vec<usize> = Vec::with_capacity(10);
+    /// ```
+    #[clippy::version = "1.73.0"]
+    pub RESERVE_AFTER_INITIALIZATION,
+    complexity,
+    "`reserve` called immediatly after `Vec` creation"
+}
+impl_lint_pass!(ReserveAfterInitialization => [RESERVE_AFTER_INITIALIZATION]);
+
+#[derive(Default)]
+pub struct ReserveAfterInitialization {
+    searcher: Option<VecReserveSearcher>,
+}
+
+struct VecReserveSearcher {
+    local_id: HirId,
+    err_span: Span,
+    init_part: String,
+    space_hint: String,
+}
+impl VecReserveSearcher {
+    fn display_err(&self, cx: &LateContext<'_>) {
+        if self.space_hint.is_empty() {
+            return;
+        }
+
+        let s = format!("{}Vec::with_capacity({});", self.init_part, self.space_hint);
+
+        span_lint_and_sugg(
+            cx,
+            RESERVE_AFTER_INITIALIZATION,
+            self.err_span,
+            "call to `reserve` immediately after creation",
+            "consider using `Vec::with_capacity(/* Space hint */)`",
+            s,
+            Applicability::HasPlaceholders,
+        );
+    }
+}
+
+impl<'tcx> LateLintPass<'tcx> for ReserveAfterInitialization {
+    fn check_block(&mut self, _: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
+        self.searcher = None;
+    }
+
+    fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx Local<'tcx>) {
+        if let Some(init_expr) = local.init
+            && let PatKind::Binding(BindingAnnotation::MUT, id, _, None) = local.pat.kind
+            && !in_external_macro(cx.sess(), local.span)
+            && let Some(init) = get_vec_init_kind(cx, init_expr)
+            && !matches!(init, VecInitKind::WithExprCapacity(_) | VecInitKind::WithConstCapacity(_))
+        {
+            self.searcher = Some(VecReserveSearcher {
+                local_id: id,
+                err_span: local.span,
+                init_part: snippet(cx, local.span.shrink_to_lo()
+                    .to(init_expr.span.source_callsite().shrink_to_lo()), "..")
+                    .into_owned(),
+                space_hint: String::new()
+            });
+        }
+    }
+
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if self.searcher.is_none()
+            && let ExprKind::Assign(left, right, _) = expr.kind
+            && let ExprKind::Path(QPath::Resolved(None, path)) = left.kind
+            && let Res::Local(id) = path.res
+            && !in_external_macro(cx.sess(), expr.span)
+            && let Some(init) = get_vec_init_kind(cx, right)
+            && !matches!(init, VecInitKind::WithExprCapacity(_) | VecInitKind::WithConstCapacity(_))
+        {
+            self.searcher = Some(VecReserveSearcher {
+                local_id: id,
+                err_span: expr.span,
+                init_part: snippet(cx, left.span.shrink_to_lo()
+                    .to(right.span.source_callsite().shrink_to_lo()), "..")
+                    .into_owned(), // see `assign_expression` test
+                space_hint: String::new()
+            });
+        }
+    }
+
+    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'_>) {
+        if let Some(searcher) = self.searcher.take() {
+            if let StmtKind::Expr(expr) | StmtKind::Semi(expr) = stmt.kind
+                && let ExprKind::MethodCall(name, self_arg, [space_hint], _) = expr.kind
+                && path_to_local_id(self_arg, searcher.local_id)
+                && name.ident.as_str() == "reserve"
+                && !is_from_proc_macro(cx, expr)
+            {
+                self.searcher = Some(VecReserveSearcher {
+                    err_span: searcher.err_span.to(stmt.span),
+                    space_hint: snippet(cx, space_hint.span, "..").into_owned(),
+                    .. searcher
+                });
+            } else {
+                searcher.display_err(cx);
+            }
+        }
+    }
+
+    fn check_block_post(&mut self, cx: &LateContext<'tcx>, _: &'tcx Block<'tcx>) {
+        if let Some(searcher) = self.searcher.take() {
+            searcher.display_err(cx);
+        }
+    }
+}
diff --git a/tests/ui/reserve_after_initialization.fixed b/tests/ui/reserve_after_initialization.fixed
new file mode 100644
index 00000000000..0675277849a
--- /dev/null
+++ b/tests/ui/reserve_after_initialization.fixed
@@ -0,0 +1,48 @@
+//@aux-build:proc_macros.rs
+#![warn(clippy::reserve_after_initialization)]
+#![no_main]
+
+extern crate proc_macros;
+use proc_macros::{external, with_span};
+
+// Should lint
+fn standard() {
+    let mut v1: Vec<usize> = Vec::with_capacity(10);
+}
+
+// Should lint
+fn capacity_as_expr() {
+    let capacity = 10;
+    let mut v2: Vec<usize> = Vec::with_capacity(capacity);
+}
+
+// Shouldn't lint
+fn vec_init_with_argument() {
+    let mut v3 = vec![1];
+    v3.reserve(10);
+}
+
+// Shouldn't lint
+fn called_with_capacity() {
+    let _v4: Vec<usize> = Vec::with_capacity(10);
+}
+
+// Should lint
+fn assign_expression() {
+    let mut v5: Vec<usize> = Vec::new();
+    v5 = Vec::with_capacity(10);
+}
+
+fn in_macros() {
+    external! {
+        let mut v: Vec<usize> = vec![];
+        v.reserve(10);
+    }
+
+    with_span! {
+        span
+
+        let mut v: Vec<usize> = vec![];
+        v.reserve(10);
+    }
+}
diff --git a/tests/ui/reserve_after_initialization.rs b/tests/ui/reserve_after_initialization.rs
new file mode 100644
index 00000000000..b57a8e162c5
--- /dev/null
+++ b/tests/ui/reserve_after_initialization.rs
@@ -0,0 +1,51 @@
+//@aux-build:proc_macros.rs
+#![warn(clippy::reserve_after_initialization)]
+#![no_main]
+
+extern crate proc_macros;
+use proc_macros::{external, with_span};
+
+// Should lint
+fn standard() {
+    let mut v1: Vec<usize> = vec![];
+    v1.reserve(10);
+}
+
+// Should lint
+fn capacity_as_expr() {
+    let capacity = 10;
+    let mut v2: Vec<usize> = vec![];
+    v2.reserve(capacity);
+}
+
+// Shouldn't lint
+fn vec_init_with_argument() {
+    let mut v3 = vec![1];
+    v3.reserve(10);
+}
+
+// Shouldn't lint
+fn called_with_capacity() {
+    let _v4: Vec<usize> = Vec::with_capacity(10);
+}
+
+// Should lint
+fn assign_expression() {
+    let mut v5: Vec<usize> = Vec::new();
+    v5 = Vec::new();
+    v5.reserve(10);
+}
+
+fn in_macros() {
+    external! {
+        let mut v: Vec<usize> = vec![];
+        v.reserve(10);
+    }
+
+    with_span! {
+        span
+
+        let mut v: Vec<usize> = vec![];
+        v.reserve(10);
+    }
+}
diff --git a/tests/ui/reserve_after_initialization.stderr b/tests/ui/reserve_after_initialization.stderr
new file mode 100644
index 00000000000..4a6164d8ebc
--- /dev/null
+++ b/tests/ui/reserve_after_initialization.stderr
@@ -0,0 +1,25 @@
+error: call to `reserve` immediately after creation
+  --> $DIR/reserve_after_initialization.rs:10:5
+   |
+LL | /     let mut v1: Vec<usize> = vec![];
+LL | |     v1.reserve(10);
+   | |___________________^ help: consider using `Vec::with_capacity(/* Space hint */)`: `let mut v1: Vec<usize> = Vec::with_capacity(10);`
+   |
+   = note: `-D clippy::reserve-after-initialization` implied by `-D warnings`
+
+error: call to `reserve` immediately after creation
+  --> $DIR/reserve_after_initialization.rs:17:5
+   |
+LL | /     let mut v2: Vec<usize> = vec![];
+LL | |     v2.reserve(capacity);
+   | |_________________________^ help: consider using `Vec::with_capacity(/* Space hint */)`: `let mut v2: Vec<usize> = Vec::with_capacity(capacity);`
+
+error: call to `reserve` immediately after creation
+  --> $DIR/reserve_after_initialization.rs:35:5
+   |
+LL | /     v5 = Vec::new();
+LL | |     v5.reserve(10);
+   | |___________________^ help: consider using `Vec::with_capacity(/* Space hint */)`: `v5 = Vec::with_capacity(10);`
+
+error: aborting due to 3 previous errors
+