WIP - implement iter next slice

CHANGELOG.md

@@ -1561,6 +1561,7 @@ Released 2018-09-13
 [`single_match`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match
 [`single_match_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_match_else
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next
+[`iter_next_slice`]: https://rust-lang.github.io/rust-clippy/master/index.html#iter_next_slice
 [`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
 [`str_to_string`]: https://rust-lang.github.io/rust-clippy/master/index.html#str_to_string
 [`string_add`]: https://rust-lang.github.io/rust-clippy/master/index.html#string_add

clippy_lints/src/lib.rs

@@ -670,6 +670,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &methods::INTO_ITER_ON_REF,
         &methods::ITERATOR_STEP_BY_ZERO,
         &methods::ITER_CLONED_COLLECT,
+        &methods::ITER_NEXT_SLICE,
         &methods::ITER_NTH,
         &methods::ITER_NTH_ZERO,
         &methods::ITER_SKIP_NEXT,
@@ -1330,6 +1331,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::SHOULD_IMPLEMENT_TRAIT),
         LintId::of(&methods::SINGLE_CHAR_PATTERN),
         LintId::of(&methods::SKIP_WHILE_NEXT),
+        LintId::of(&methods::ITER_NEXT_SLICE),
         LintId::of(&methods::STRING_EXTEND_CHARS),
         LintId::of(&methods::SUSPICIOUS_MAP),
         LintId::of(&methods::TEMPORARY_CSTRING_AS_PTR),
@@ -1505,6 +1507,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::OPTION_MAP_OR_NONE),
         LintId::of(&methods::RESULT_MAP_OR_INTO_OPTION),
         LintId::of(&methods::SHOULD_IMPLEMENT_TRAIT),
+        LintId::of(&methods::ITER_NEXT_SLICE),
         LintId::of(&methods::STRING_EXTEND_CHARS),
         LintId::of(&methods::UNNECESSARY_FOLD),
         LintId::of(&methods::WRONG_SELF_CONVENTION),

clippy_lints/src/loops.rs

@@ -2487,14 +2487,14 @@ fn check_needless_collect<'a, 'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'a, '
                     );
                 }
                 if method.ident.name == sym!(is_empty) {
-                    let span = shorten_needless_collect_span(expr);
+                    let span = shorten_needless_collect_span_to_iter(expr);
                     span_lint_and_sugg(
                         cx,
                         NEEDLESS_COLLECT,
                         span,
                         NEEDLESS_COLLECT_MSG,
                         "replace with",
-                        ".next().is_none()".to_string(),
+                        ".get(0).is_none()".to_string(),
                         Applicability::MachineApplicable,
                     );
                 }
@@ -2539,3 +2539,15 @@ fn shorten_needless_collect_span(expr: &Expr<'_>) -> Span {
     }
     unreachable!()
 }
+
+fn shorten_needless_collect_span_to_iter(expr: &Expr<'_>) -> Span {
+    if_chain! {
+        if let ExprKind::MethodCall(_, _, ref args) = expr.kind;
+        if let ExprKind::MethodCall(_, _, ref args2) = args[0].kind;
+        if let ExprKind::MethodCall(_, ref span, _) = args2[0].kind;
+        then {
+            return expr.span.with_lo(span.lo() - BytePos(1));
+        }
+    }
+    unreachable!()
+}

clippy_lints/src/methods/mod.rs

@@ -24,7 +24,7 @@ use rustc_span::symbol::{sym, SymbolStr};
 use crate::consts::{constant, Constant};
 use crate::utils::usage::mutated_variables;
 use crate::utils::{
-    get_arg_name, get_parent_expr, get_trait_def_id, has_iter_method, implements_trait, in_macro, is_copy,
+    get_arg_name, get_parent_expr, get_trait_def_id, has_iter_method, higher, implements_trait, in_macro, is_copy,
     is_ctor_or_promotable_const_function, is_expn_of, is_type_diagnostic_item, iter_input_pats, last_path_segment,
     match_def_path, match_qpath, match_trait_method, match_type, match_var, method_calls, method_chain_args, paths,
     remove_blocks, return_ty, same_tys, single_segment_path, snippet, snippet_with_applicability,
@@ -1285,6 +1285,26 @@ declare_clippy_lint! {
     "using `as_ref().map(Deref::deref)`, which is more succinctly expressed as `as_deref()`"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** TODO
+    ///
+    /// **Why is this bad?** TODO
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// TODO
+    /// ```
+    /// The correct use would be:
+    /// ```rust
+    /// TODO
+    /// ```
+    pub ITER_NEXT_SLICE,
+    style,
+    "using `.iter().next()` on a sliced array, which can be shortened to just `.get()`"
+}
+
 declare_lint_pass!(Methods => [
     OPTION_UNWRAP_USED,
     RESULT_UNWRAP_USED,
@@ -1320,6 +1340,7 @@ declare_lint_pass!(Methods => [
     FIND_MAP,
     MAP_FLATTEN,
     ITERATOR_STEP_BY_ZERO,
+    ITER_NEXT_SLICE,
     ITER_NTH,
     ITER_NTH_ZERO,
     ITER_SKIP_NEXT,
@@ -1361,6 +1382,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
             ["and_then", ..] => lint_option_and_then_some(cx, expr, arg_lists[0]),
             ["next", "filter"] => lint_filter_next(cx, expr, arg_lists[1]),
             ["next", "skip_while"] => lint_skip_while_next(cx, expr, arg_lists[1]),
+            ["next", "iter"] => lint_iter_next(cx, expr, arg_lists[1]),
             ["map", "filter"] => lint_filter_map(cx, expr, arg_lists[1], arg_lists[0]),
             ["map", "filter_map"] => lint_filter_map_map(cx, expr, arg_lists[1], arg_lists[0]),
             ["next", "filter_map"] => lint_filter_map_next(cx, expr, arg_lists[1]),
@@ -2230,6 +2252,49 @@ fn lint_step_by<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &hir::Expr<'_>, args
     }
 }
 
+fn lint_iter_next<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr<'_>, iter_args: &'tcx [hir::Expr<'_>]) {
+    let caller_expr = &iter_args[0];
+    if derefs_to_slice(cx, caller_expr, cx.tables.expr_ty(caller_expr)).is_some() {
+        // caller is a Slice
+        if_chain! {
+            if let hir::ExprKind::Index(ref caller_var, ref index_expr) = &caller_expr.kind;
+            if let Some(higher::Range { start: Some(start_expr), end: None, limits: ast::RangeLimits::HalfOpen }) 
+                = higher::range(cx, index_expr);
+            if let hir::ExprKind::Lit(ref start_lit) = &start_expr.kind;
+            if let ast::LitKind::Int(start_idx, _) = start_lit.node;
+            then {
+                let mut applicability = Applicability::MachineApplicable;
+                span_lint_and_sugg(
+                    cx,
+                    ITER_NEXT_SLICE,
+                    expr.span,
+                    "Using `.iter().next()` on a Slice without end index.",
+                    "try calling",
+                    format!("{}.get({})", snippet_with_applicability(cx, caller_var.span, "..", &mut applicability), start_idx),
+                    applicability,
+                );
+            }
+        }
+    } else if is_type_diagnostic_item(cx, cx.tables.expr_ty(caller_expr), sym!(vec_type))
+        || matches!(&walk_ptrs_ty(cx.tables.expr_ty(caller_expr)).kind, ty::Array(_, _))
+    {
+        // caller is a Vec or an Array
+        let mut applicability = Applicability::MachineApplicable;
+        span_lint_and_sugg(
+            cx,
+            ITER_NEXT_SLICE,
+            expr.span,
+            "Using `.iter().next()` on an array",
+            "try calling",
+            format!(
+                "{}.get(0)", 
+                snippet_with_applicability(cx, caller_expr.span, "..", &mut applicability)
+            ),
+            applicability,
+        );
+    }
+}
+
 fn lint_iter_nth<'a, 'tcx>(
     cx: &LateContext<'a, 'tcx>,
     expr: &hir::Expr<'_>,

clippy_lints/src/needless_continue.rs

@@ -424,7 +424,7 @@ fn erode_from_back(s: &str) -> String {
 }
 
 fn span_of_first_expr_in_block(block: &ast::Block) -> Option<Span> {
-    block.stmts.iter().next().map(|stmt| stmt.span)
+    block.stmts.get(0).map(|stmt| stmt.span)
 }
 
 #[cfg(test)]

src/lintlist/mod.rs

@@ -2026,6 +2026,13 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
         deprecation: None,
         module: "methods",
     },
+    Lint {
+        name: "iter_next_slice",
+        group: "style",
+        desc: "default lint description",
+        deprecation: None,
+        module: "methods",
+    },
     Lint {
         name: "slow_vector_initialization",
         group: "perf",

tests/ui/for_loop_over_option_result.rs

@@ -23,16 +23,16 @@ fn for_loop_over_option_and_result() {
 
     // make sure LOOP_OVER_NEXT lint takes clippy::precedence when next() is the last call
     // in the chain
-    for x in v.iter().next() {
+    for x in v.get(0) {
         println!("{}", x);
     }
 
     // make sure we lint when next() is not the last call in the chain
-    for x in v.iter().next().and(Some(0)) {
+    for x in v.get(0).and(Some(0)) {
         println!("{}", x);
     }
 
-    for x in v.iter().next().ok_or("x not found") {
+    for x in v.get(0).ok_or("x not found") {
         println!("{}", x);
     }
 

tests/ui/for_loop_over_option_result.stderr

@@ -24,29 +24,29 @@ LL |     for x in option.ok_or("x not found") {
    |
    = help: consider replacing `for x in option.ok_or("x not found")` with `if let Ok(x) = option.ok_or("x not found")`
 
-error: you are iterating over `Iterator::next()` which is an Option; this will compile but is probably not what you want
+error: for loop over `v.get(0)`, which is an `Option`. This is more readably written as an `if let` statement.
   --> $DIR/for_loop_over_option_result.rs:26:14
    |
-LL |     for x in v.iter().next() {
-   |              ^^^^^^^^^^^^^^^
+LL |     for x in v.get(0) {
+   |              ^^^^^^^^
    |
-   = note: `#[deny(clippy::iter_next_loop)]` on by default
+   = help: consider replacing `for x in v.get(0)` with `if let Some(x) = v.get(0)`
 
-error: for loop over `v.iter().next().and(Some(0))`, which is an `Option`. This is more readably written as an `if let` statement.
+error: for loop over `v.get(0).and(Some(0))`, which is an `Option`. This is more readably written as an `if let` statement.
   --> $DIR/for_loop_over_option_result.rs:31:14
    |
-LL |     for x in v.iter().next().and(Some(0)) {
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     for x in v.get(0).and(Some(0)) {
+   |              ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: consider replacing `for x in v.iter().next().and(Some(0))` with `if let Some(x) = v.iter().next().and(Some(0))`
+   = help: consider replacing `for x in v.get(0).and(Some(0))` with `if let Some(x) = v.get(0).and(Some(0))`
 
-error: for loop over `v.iter().next().ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement.
+error: for loop over `v.get(0).ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement.
   --> $DIR/for_loop_over_option_result.rs:35:14
    |
-LL |     for x in v.iter().next().ok_or("x not found") {
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     for x in v.get(0).ok_or("x not found") {
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: consider replacing `for x in v.iter().next().ok_or("x not found")` with `if let Ok(x) = v.iter().next().ok_or("x not found")`
+   = help: consider replacing `for x in v.get(0).ok_or("x not found")` with `if let Ok(x) = v.get(0).ok_or("x not found")`
 
 error: this loop never actually loops
   --> $DIR/for_loop_over_option_result.rs:47:5

tests/ui/iter_next_slice.rs

@@ -0,0 +1,39 @@
+#![warn(clippy::iter_next_slice)]
+
+fn main() {
+    // test code goes here
+    let s = [1, 2, 3];
+    let v = vec![1, 2, 3];
+    let o = Some(5);
+
+    s.iter().next();
+    // Should be replaced by s.get(0)
+
+    // s.into_iter().next();
+    // Probaby can rely on the default `array_into_iter`/`clippy::into_iter_on_ref` lint
+
+    s[2..].iter().next();
+    // Should be replaced by s.get(2)
+
+    // s[1..].into_iter().next();
+    // Probaby can rely on the default `clippy::into_iter_on_ref` lint
+
+    v[5..].iter().next();
+    // Should be replaced by v.get(5)
+
+    // v[1..].into_iter().next();
+    // Probaby can rely on the default `clippy::into_iter_on_ref` lint
+
+    v.iter().next();
+    // Should be replaced by v.get(0)
+
+    // v.into_iter().next();
+    // Tricky one—I don't think there's any other (short) way to say this (take ownership of first value)
+    // So this should probably not get any warnings
+
+    o.iter().next();
+    // Replace with o.as_ref()
+
+    // o.into_iter().next();
+    // Replace with o
+}

tests/ui/iter_next_slice.stderr

@@ -0,0 +1,28 @@
+error: Using `.iter().next()` on an array
+  --> $DIR/iter_next_slice.rs:9:5
+   |
+LL |     s.iter().next();
+   |     ^^^^^^^^^^^^^^^ help: try calling: `s.get(0)`
+   |
+   = note: `-D clippy::iter-next-slice` implied by `-D warnings`
+
+error: Using `.iter().next()` on a Slice without end index.
+  --> $DIR/iter_next_slice.rs:15:5
+   |
+LL |     s[2..].iter().next();
+   |     ^^^^^^^^^^^^^^^^^^^^ help: try calling: `s.get(2)`
+
+error: Using `.iter().next()` on a Slice without end index.
+  --> $DIR/iter_next_slice.rs:21:5
+   |
+LL |     v[5..].iter().next();
+   |     ^^^^^^^^^^^^^^^^^^^^ help: try calling: `v.get(5)`
+
+error: Using `.iter().next()` on an array
+  --> $DIR/iter_next_slice.rs:27:5
+   |
+LL |     v.iter().next();
+   |     ^^^^^^^^^^^^^^^ help: try calling: `v.get(0)`
+
+error: aborting due to 4 previous errors
+

tests/ui/needless_collect.fixed

@@ -9,7 +9,7 @@ use std::collections::{BTreeSet, HashMap, HashSet};
 fn main() {
     let sample = [1; 5];
     let len = sample.iter().count();
-    if sample.iter().next().is_none() {
+    if sample.get(0).is_none() {
         // Empty
     }
     sample.iter().cloned().any(|x| x == 1);

tests/ui/needless_collect.stderr

@@ -7,10 +7,10 @@ LL |     let len = sample.iter().collect::<Vec<_>>().len();
    = note: `-D clippy::needless-collect` implied by `-D warnings`
 
 error: avoid using `collect()` when not needed
-  --> $DIR/needless_collect.rs:12:21
+  --> $DIR/needless_collect.rs:12:14
    |
 LL |     if sample.iter().collect::<Vec<_>>().is_empty() {
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `.next().is_none()`
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `.get(0).is_none()`
 
 error: avoid using `collect()` when not needed
   --> $DIR/needless_collect.rs:15:27