Reduce cognitive complexity lint span

Currently the cognitive complexity lint spans the entire function
body making it really difficult to read and refactor the code in
editors. To fix this we reduce the lint span to the function name.

clippy_lints/src/cognitive_complexity.rs

@@ -1,14 +1,15 @@
 //! calculate cognitive complexity and warn about overly complex functions
 
-use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc::hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
 use rustc::hir::*;
 use rustc::impl_lint_pass;
 use rustc::lint::{LateContext, LateLintPass, LintArray, LintContext, LintPass};
 use rustc_session::declare_tool_lint;
 use syntax::ast::Attribute;
 use syntax::source_map::Span;
+use syntax_pos::BytePos;
 
-use crate::utils::{match_type, paths, span_help_and_lint, LimitStack};
+use crate::utils::{match_type, paths, snippet_opt, span_help_and_lint, LimitStack};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for methods with high cognitive complexity.
@@ -41,8 +42,16 @@ impl CognitiveComplexity {
 impl_lint_pass!(CognitiveComplexity => [COGNITIVE_COMPLEXITY]);
 
 impl CognitiveComplexity {
-    fn check<'a, 'tcx>(&mut self, cx: &'a LateContext<'a, 'tcx>, body: &'tcx Body, span: Span) {
-        if span.from_expansion() {
+    #[allow(clippy::cast_possible_truncation)]
+    fn check<'a, 'tcx>(
+        &mut self,
+        cx: &'a LateContext<'a, 'tcx>,
+        kind: FnKind<'tcx>,
+        decl: &'tcx FnDecl,
+        body: &'tcx Body,
+        body_span: Span,
+    ) {
+        if body_span.from_expansion() {
             return;
         }
 
@@ -64,11 +73,33 @@ impl CognitiveComplexity {
         if rust_cc >= ret_adjust {
             rust_cc -= ret_adjust;
         }
+
         if rust_cc > self.limit.limit() {
+            let fn_span = match kind {
+                FnKind::ItemFn(ident, _, _, _, _) | FnKind::Method(ident, _, _, _) => ident.span,
+                FnKind::Closure(_) => {
+                    let header_span = body_span.with_hi(decl.output.span().lo());
+                    let pos = snippet_opt(cx, header_span).and_then(|snip| {
+                        let low_offset = snip.find('|')?;
+                        let high_offset = 1 + snip.get(low_offset + 1..)?.find('|')?;
+                        let low = header_span.lo() + BytePos(low_offset as u32);
+                        let high = low + BytePos(high_offset as u32 + 1);
+
+                        Some((low, high))
+                    });
+
+                    if let Some((low, high)) = pos {
+                        Span::new(low, high, header_span.ctxt())
+                    } else {
+                        return;
+                    }
+                },
+            };
+
             span_help_and_lint(
                 cx,
                 COGNITIVE_COMPLEXITY,
-                span,
+                fn_span,
                 &format!(
                     "the function has a cognitive complexity of ({}/{})",
                     rust_cc,
@@ -84,15 +115,15 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CognitiveComplexity {
     fn check_fn(
         &mut self,
         cx: &LateContext<'a, 'tcx>,
-        _: intravisit::FnKind<'tcx>,
-        _: &'tcx FnDecl,
+        kind: FnKind<'tcx>,
+        decl: &'tcx FnDecl,
         body: &'tcx Body,
         span: Span,
         hir_id: HirId,
     ) {
         let def_id = cx.tcx.hir().local_def_id(hir_id);
         if !cx.tcx.has_attr(def_id, sym!(test)) {
-            self.check(cx, body, span);
+            self.check(cx, kind, decl, body, span);
         }
     }