Use ident instead of def_span in dead-code pass

According to @estebank, def_span scans forward on the line until it finds a {,
and if it can't find one, fallse back to the span for the whole item.  This
was apparently written before the identifier span was explicitly tracked on
each node.

This means that if an unused function signature spans multiple lines, the
entire function (potentially hundreds of lines) gets flagged as dead code.
This could, for example, cause IDEs to add error squiggly's to the whole
function.

By using the span from the ident instead, we narrow the scope of this in
most cases.  In a wider sense, it's probably safe to use ident.span
instead of def_span in most locations throughout the whole code base,
but since this is my first contribution, I kept it small.

Some interesting points that came up while I was working on this:
 - I reorganized the tests a bit to bring some of the dead code ones all
   into the same location
 - A few tests were for things unrelated to dead code (like the
   path-lookahead for parens), so I added #![allow(dead_code)] and
   cleaned up the stderr file to reduce noise in the future
 - The same fix doesn't apply to const and static declarations.  I tried
   adding these cases to the match expression, but that created a much
   wider change to tests and error messages, so I left it off until I
   could get some code review to validate the approach.

21 files changed, 114 insertions, 98 deletions

diff --git a/src/librustc_passes/dead.rs b/src/librustc_passes/dead.rs
index f2aef2c12c7..9ad63f28a8e 100644
--- a/src/librustc_passes/dead.rs
+++ b/src/librustc_passes/dead.rs
@@ -578,7 +578,7 @@ impl Visitor<'tcx> for DeadVisitor<'tcx> {
                 hir::ItemKind::Struct(..) |
                 hir::ItemKind::Union(..) |
                 hir::ItemKind::Trait(..) |
-                hir::ItemKind::Impl(..) => self.tcx.sess.source_map().def_span(item.span),
+                hir::ItemKind::Impl(..) => item.ident.span,
                 _ => item.span,
             };
             let participle = match item.kind {
diff --git a/src/test/ui-fulldeps/lint-plugin-cmdline-allow.stderr b/src/test/ui-fulldeps/lint-plugin-cmdline-allow.stderr
index c6d198dc458..5ab3dfb2449 100644
--- a/src/test/ui-fulldeps/lint-plugin-cmdline-allow.stderr
+++ b/src/test/ui-fulldeps/lint-plugin-cmdline-allow.stderr
@@ -7,10 +7,10 @@ LL | #![plugin(lint_plugin_test)]
    = note: `#[warn(deprecated)]` on by default
 
 warning: function is never used: `lintme`
-  --> $DIR/lint-plugin-cmdline-allow.rs:10:1
+  --> $DIR/lint-plugin-cmdline-allow.rs:10:4
    |
 LL | fn lintme() { }
-   | ^^^^^^^^^^^
+   |    ^^^^^^
    |
 note: lint level defined here
   --> $DIR/lint-plugin-cmdline-allow.rs:7:9
diff --git a/src/test/ui-fulldeps/lint-tool-cmdline-allow.stderr b/src/test/ui-fulldeps/lint-tool-cmdline-allow.stderr
index 239732521d5..3a9fd7c2867 100644
--- a/src/test/ui-fulldeps/lint-tool-cmdline-allow.stderr
+++ b/src/test/ui-fulldeps/lint-tool-cmdline-allow.stderr
@@ -19,10 +19,10 @@ LL | fn lintme() {}
    = note: `#[warn(clippy::test_lint)]` on by default
 
 warning: function is never used: `lintme`
-  --> $DIR/lint-tool-cmdline-allow.rs:10:1
+  --> $DIR/lint-tool-cmdline-allow.rs:10:4
    |
 LL | fn lintme() {}
-   | ^^^^^^^^^^^
+   |    ^^^^^^
    |
 note: lint level defined here
   --> $DIR/lint-tool-cmdline-allow.rs:7:9
diff --git a/src/test/ui/lint/dead-code/basic.stderr b/src/test/ui/lint/dead-code/basic.stderr
index e5d1af12cf2..194398e5a07 100644
--- a/src/test/ui/lint/dead-code/basic.stderr
+++ b/src/test/ui/lint/dead-code/basic.stderr
@@ -1,8 +1,8 @@
 error: function is never used: `foo`
-  --> $DIR/basic.rs:4:1
+  --> $DIR/basic.rs:4:4
    |
 LL | fn foo() {
-   | ^^^^^^^^
+   |    ^^^
    |
 note: lint level defined here
   --> $DIR/basic.rs:1:9
diff --git a/src/test/ui/lint/dead-code/empty-unused-enum.stderr b/src/test/ui/lint/dead-code/empty-unused-enum.stderr
index 17bbdfc71a2..44b0a8f613e 100644
--- a/src/test/ui/lint/dead-code/empty-unused-enum.stderr
+++ b/src/test/ui/lint/dead-code/empty-unused-enum.stderr
@@ -1,8 +1,8 @@
 error: enum is never used: `E`
-  --> $DIR/empty-unused-enum.rs:3:1
+  --> $DIR/empty-unused-enum.rs:3:6
    |
 LL | enum E {}
-   | ^^^^^^
+   |      ^
    |
 note: lint level defined here
   --> $DIR/empty-unused-enum.rs:1:9
diff --git a/src/test/ui/lint/dead-code/lint-dead-code-1.stderr b/src/test/ui/lint/dead-code/lint-dead-code-1.stderr
index be96c697d99..bac46a2e843 100644
--- a/src/test/ui/lint/dead-code/lint-dead-code-1.stderr
+++ b/src/test/ui/lint/dead-code/lint-dead-code-1.stderr
@@ -1,8 +1,8 @@
 error: struct is never constructed: `Bar`
-  --> $DIR/lint-dead-code-1.rs:12:5
+  --> $DIR/lint-dead-code-1.rs:12:16
    |
 LL |     pub struct Bar;
-   |     ^^^^^^^^^^^^^^^
+   |                ^^^
    |
 note: lint level defined here
   --> $DIR/lint-dead-code-1.rs:5:9
@@ -23,16 +23,16 @@ LL | const priv_const: isize = 0;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: struct is never constructed: `PrivStruct`
-  --> $DIR/lint-dead-code-1.rs:35:1
+  --> $DIR/lint-dead-code-1.rs:35:8
    |
 LL | struct PrivStruct;
-   | ^^^^^^^^^^^^^^^^^^
+   |        ^^^^^^^^^^
 
 error: enum is never used: `priv_enum`
-  --> $DIR/lint-dead-code-1.rs:64:1
+  --> $DIR/lint-dead-code-1.rs:64:6
    |
 LL | enum priv_enum { foo2, bar2 }
-   | ^^^^^^^^^^^^^^
+   |      ^^^^^^^^^
 
 error: variant is never constructed: `bar3`
   --> $DIR/lint-dead-code-1.rs:67:5
@@ -41,28 +41,28 @@ LL |     bar3
    |     ^^^^
 
 error: function is never used: `priv_fn`
-  --> $DIR/lint-dead-code-1.rs:88:1
+  --> $DIR/lint-dead-code-1.rs:88:4
    |
 LL | fn priv_fn() {
-   | ^^^^^^^^^^^^
+   |    ^^^^^^^
 
 error: function is never used: `foo`
-  --> $DIR/lint-dead-code-1.rs:93:1
+  --> $DIR/lint-dead-code-1.rs:93:4
    |
 LL | fn foo() {
-   | ^^^^^^^^
+   |    ^^^
 
 error: function is never used: `bar`
-  --> $DIR/lint-dead-code-1.rs:98:1
+  --> $DIR/lint-dead-code-1.rs:98:4
    |
 LL | fn bar() {
-   | ^^^^^^^^
+   |    ^^^
 
 error: function is never used: `baz`
-  --> $DIR/lint-dead-code-1.rs:102:1
+  --> $DIR/lint-dead-code-1.rs:102:4
    |
 LL | fn baz() -> impl Copy {
-   | ^^^^^^^^^^^^^^^^^^^^^
+   |    ^^^
 
 error: aborting due to 10 previous errors
 
diff --git a/src/test/ui/lint/dead-code/lint-dead-code-2.stderr b/src/test/ui/lint/dead-code/lint-dead-code-2.stderr
index 1226f9823ac..a578a76d9a0 100644
--- a/src/test/ui/lint/dead-code/lint-dead-code-2.stderr
+++ b/src/test/ui/lint/dead-code/lint-dead-code-2.stderr
@@ -1,8 +1,8 @@
 error: function is never used: `dead_fn`
-  --> $DIR/lint-dead-code-2.rs:22:1
+  --> $DIR/lint-dead-code-2.rs:22:4
    |
 LL | fn dead_fn() {}
-   | ^^^^^^^^^^^^
+   |    ^^^^^^^
    |
 note: lint level defined here
   --> $DIR/lint-dead-code-2.rs:2:9
@@ -11,16 +11,16 @@ LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
 error: function is never used: `dead_fn2`
-  --> $DIR/lint-dead-code-2.rs:25:1
+  --> $DIR/lint-dead-code-2.rs:25:4
    |
 LL | fn dead_fn2() {}
-   | ^^^^^^^^^^^^^
+   |    ^^^^^^^^
 
 error: function is never used: `main`
-  --> $DIR/lint-dead-code-2.rs:38:1
+  --> $DIR/lint-dead-code-2.rs:38:4
    |
 LL | fn main() {
-   | ^^^^^^^^^
+   |    ^^^^
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/lint/dead-code/lint-dead-code-3.stderr b/src/test/ui/lint/dead-code/lint-dead-code-3.stderr
index 2408da0af89..569196fffdd 100644
--- a/src/test/ui/lint/dead-code/lint-dead-code-3.stderr
+++ b/src/test/ui/lint/dead-code/lint-dead-code-3.stderr
@@ -1,8 +1,8 @@
 error: struct is never constructed: `Foo`
-  --> $DIR/lint-dead-code-3.rs:13:1
+  --> $DIR/lint-dead-code-3.rs:13:8
    |
 LL | struct Foo;
-   | ^^^^^^^^^^^
+   |        ^^^
    |
 note: lint level defined here
   --> $DIR/lint-dead-code-3.rs:3:9
@@ -17,16 +17,16 @@ LL |     fn foo(&self) {
    |     ^^^^^^^^^^^^^
 
 error: function is never used: `bar`
-  --> $DIR/lint-dead-code-3.rs:20:1
+  --> $DIR/lint-dead-code-3.rs:20:4
    |
 LL | fn bar() {
-   | ^^^^^^^^
+   |    ^^^
 
 error: enum is never used: `c_void`
-  --> $DIR/lint-dead-code-3.rs:59:1
+  --> $DIR/lint-dead-code-3.rs:59:6
    |
 LL | enum c_void {}
-   | ^^^^^^^^^^^
+   |      ^^^^^^
 
 error: foreign function is never used: `free`
   --> $DIR/lint-dead-code-3.rs:61:5
diff --git a/src/test/ui/lint/dead-code/lint-dead-code-4.stderr b/src/test/ui/lint/dead-code/lint-dead-code-4.stderr
index b7ceee99998..8eaf789f8f7 100644
--- a/src/test/ui/lint/dead-code/lint-dead-code-4.stderr
+++ b/src/test/ui/lint/dead-code/lint-dead-code-4.stderr
@@ -27,10 +27,10 @@ LL | |     },
    | |_____^
 
 error: enum is never used: `ABC`
-  --> $DIR/lint-dead-code-4.rs:24:1
+  --> $DIR/lint-dead-code-4.rs:24:6
    |
 LL | enum ABC {
-   | ^^^^^^^^
+   |      ^^^
 
 error: variant is never constructed: `I`
   --> $DIR/lint-dead-code-4.rs:36:5
diff --git a/src/test/ui/lint/dead-code/lint-dead-code-5.stderr b/src/test/ui/lint/dead-code/lint-dead-code-5.stderr
index 740cfde2c06..9670d8e7a32 100644
--- a/src/test/ui/lint/dead-code/lint-dead-code-5.stderr
+++ b/src/test/ui/lint/dead-code/lint-dead-code-5.stderr
@@ -23,10 +23,10 @@ LL |     Variant6(isize),
    |     ^^^^^^^^^^^^^^^
 
 error: enum is never used: `Enum3`
-  --> $DIR/lint-dead-code-5.rs:18:1
+  --> $DIR/lint-dead-code-5.rs:18:6
    |
 LL | enum Enum3 {
-   | ^^^^^^^^^^
+   |      ^^^^^
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/lint/dead-code/newline-span.rs b/src/test/ui/lint/dead-code/newline-span.rs
new file mode 100644
index 00000000000..a4342056419
--- /dev/null
+++ b/src/test/ui/lint/dead-code/newline-span.rs
@@ -0,0 +1,19 @@
+#![deny(dead_code)]
+
+fn unused() { //~ error: function is never used:
+    println!("blah");
+}
+
+fn unused2(var: i32) { //~ error: function is never used:
+    println!("foo {}", var);
+}
+
+fn unused3( //~ error: function is never used:
+    var: i32,
+) {
+    println!("bar {}", var);
+}
+
+fn main() {
+    println!("Hello world!");
+}
diff --git a/src/test/ui/lint/dead-code/newline-span.stderr b/src/test/ui/lint/dead-code/newline-span.stderr
new file mode 100644
index 00000000000..c5d0d605067
--- /dev/null
+++ b/src/test/ui/lint/dead-code/newline-span.stderr
@@ -0,0 +1,26 @@
+error: function is never used: `unused`
+  --> $DIR/newline-span.rs:3:4
+   |
+LL | fn unused() {
+   |    ^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/newline-span.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: function is never used: `unused2`
+  --> $DIR/newline-span.rs:7:4
+   |
+LL | fn unused2(var: i32) {
+   |    ^^^^^^^
+
+error: function is never used: `unused3`
+  --> $DIR/newline-span.rs:11:4
+   |
+LL | fn unused3(
+   |    ^^^^^^^
+
+error: aborting due to 3 previous errors
+
diff --git a/src/test/ui/lint/dead-code/unused-enum.stderr b/src/test/ui/lint/dead-code/unused-enum.stderr
index 682dfcf5881..142c2ccb99b 100644
--- a/src/test/ui/lint/dead-code/unused-enum.stderr
+++ b/src/test/ui/lint/dead-code/unused-enum.stderr
@@ -1,8 +1,8 @@
 error: struct is never constructed: `F`
-  --> $DIR/unused-enum.rs:3:1
+  --> $DIR/unused-enum.rs:3:8
    |
 LL | struct F;
-   | ^^^^^^^^^
+   |        ^
    |
 note: lint level defined here
   --> $DIR/unused-enum.rs:1:9
@@ -12,16 +12,16 @@ LL | #![deny(unused)]
    = note: `#[deny(dead_code)]` implied by `#[deny(unused)]`
 
 error: struct is never constructed: `B`
-  --> $DIR/unused-enum.rs:4:1
+  --> $DIR/unused-enum.rs:4:8
    |
 LL | struct B;
-   | ^^^^^^^^^
+   |        ^
 
 error: enum is never used: `E`
-  --> $DIR/unused-enum.rs:6:1
+  --> $DIR/unused-enum.rs:6:6
    |
 LL | enum E {
-   | ^^^^^^
+   |      ^
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/lint/dead-code/with-core-crate.stderr b/src/test/ui/lint/dead-code/with-core-crate.stderr
index 30caac3cd01..0b6ab67d9bf 100644
--- a/src/test/ui/lint/dead-code/with-core-crate.stderr
+++ b/src/test/ui/lint/dead-code/with-core-crate.stderr
@@ -1,8 +1,8 @@
 error: function is never used: `foo`
-  --> $DIR/with-core-crate.rs:7:1
+  --> $DIR/with-core-crate.rs:7:4
    |
 LL | fn foo() {
-   | ^^^^^^^^
+   |    ^^^
    |
 note: lint level defined here
   --> $DIR/with-core-crate.rs:1:9
diff --git a/src/test/ui/path-lookahead.rs b/src/test/ui/path-lookahead.rs
index fd7509a623e..86bcb08de40 100644
--- a/src/test/ui/path-lookahead.rs
+++ b/src/test/ui/path-lookahead.rs
@@ -1,14 +1,14 @@
 // run-pass
-
-#![warn(unused)]
+#![allow(dead_code)]
+#![warn(unused_parens)]
 
 // Parser test for #37765
 
-fn with_parens<T: ToString>(arg: T) -> String { //~WARN function is never used: `with_parens`
+fn with_parens<T: ToString>(arg: T) -> String {
   return (<T as ToString>::to_string(&arg)); //~WARN unnecessary parentheses around `return` value
 }
 
-fn no_parens<T: ToString>(arg: T) -> String { //~WARN function is never used: `no_parens`
+fn no_parens<T: ToString>(arg: T) -> String {
   return <T as ToString>::to_string(&arg);
 }
 
diff --git a/src/test/ui/path-lookahead.stderr b/src/test/ui/path-lookahead.stderr
index 197848e428a..caf9e8303ea 100644
--- a/src/test/ui/path-lookahead.stderr
+++ b/src/test/ui/path-lookahead.stderr
@@ -7,26 +7,6 @@ LL |   return (<T as ToString>::to_string(&arg));
 note: lint level defined here
   --> $DIR/path-lookahead.rs:3:9
    |
-LL | #![warn(unused)]
-   |         ^^^^^^
-   = note: `#[warn(unused_parens)]` implied by `#[warn(unused)]`
-
-warning: function is never used: `with_parens`
-  --> $DIR/path-lookahead.rs:7:1
-   |
-LL | fn with_parens<T: ToString>(arg: T) -> String {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: lint level defined here
-  --> $DIR/path-lookahead.rs:3:9
-   |
-LL | #![warn(unused)]
-   |         ^^^^^^
-   = note: `#[warn(dead_code)]` implied by `#[warn(unused)]`
-
-warning: function is never used: `no_parens`
-  --> $DIR/path-lookahead.rs:11:1
-   |
-LL | fn no_parens<T: ToString>(arg: T) -> String {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | #![warn(unused_parens)]
+   |         ^^^^^^^^^^^^^
 
diff --git a/src/test/ui/span/macro-span-replacement.stderr b/src/test/ui/span/macro-span-replacement.stderr
index 8b65e798b6e..439aedb4da4 100644
--- a/src/test/ui/span/macro-span-replacement.stderr
+++ b/src/test/ui/span/macro-span-replacement.stderr
@@ -1,11 +1,8 @@
 warning: struct is never constructed: `S`
-  --> $DIR/macro-span-replacement.rs:7:14
+  --> $DIR/macro-span-replacement.rs:7:12
    |
 LL |         $b $a;
-   |              ^
-...
-LL |     m!(S struct);
-   |     ------------- in this macro invocation
+   |            ^^
    |
 note: lint level defined here
   --> $DIR/macro-span-replacement.rs:3:9
diff --git a/src/test/ui/span/unused-warning-point-at-signature.rs b/src/test/ui/span/unused-warning-point-at-identifier.rs
index 3af272a012e..d4d5bc1cbc8 100644
--- a/src/test/ui/span/unused-warning-point-at-signature.rs
+++ b/src/test/ui/span/unused-warning-point-at-identifier.rs
@@ -20,8 +20,8 @@ fn func() -> usize { //~ WARN function is never used
     3
 }
 
-fn //~ WARN function is never used
-func_complete_span()
+fn
+func_complete_span() //~ WARN function is never used
 -> usize
 {
     3
diff --git a/src/test/ui/span/unused-warning-point-at-signature.stderr b/src/test/ui/span/unused-warning-point-at-identifier.stderr
index 83e2ec1987b..a4715d4adeb 100644
--- a/src/test/ui/span/unused-warning-point-at-signature.stderr
+++ b/src/test/ui/span/unused-warning-point-at-identifier.stderr
@@ -1,36 +1,31 @@
 warning: enum is never used: `Enum`
-  --> $DIR/unused-warning-point-at-signature.rs:5:1
+  --> $DIR/unused-warning-point-at-identifier.rs:5:6
    |
 LL | enum Enum {
-   | ^^^^^^^^^
+   |      ^^^^
    |
 note: lint level defined here
-  --> $DIR/unused-warning-point-at-signature.rs:3:9
+  --> $DIR/unused-warning-point-at-identifier.rs:3:9
    |
 LL | #![warn(unused)]
    |         ^^^^^^
    = note: `#[warn(dead_code)]` implied by `#[warn(unused)]`
 
 warning: struct is never constructed: `Struct`
-  --> $DIR/unused-warning-point-at-signature.rs:12:1
+  --> $DIR/unused-warning-point-at-identifier.rs:12:8
    |
 LL | struct Struct {
-   | ^^^^^^^^^^^^^
+   |        ^^^^^^
 
 warning: function is never used: `func`
-  --> $DIR/unused-warning-point-at-signature.rs:19:1
+  --> $DIR/unused-warning-point-at-identifier.rs:19:4
    |
 LL | fn func() -> usize {
-   | ^^^^^^^^^^^^^^^^^^
+   |    ^^^^
 
 warning: function is never used: `func_complete_span`
-  --> $DIR/unused-warning-point-at-signature.rs:23:1
+  --> $DIR/unused-warning-point-at-identifier.rs:24:1
    |
-LL | / fn
-LL | | func_complete_span()
-LL | | -> usize
-LL | | {
-LL | |     3
-LL | | }
-   | |_^
+LL | func_complete_span()
+   | ^^^^^^^^^^^^^^^^^^
 
diff --git a/src/test/ui/test-attrs/test-warns-dead-code.stderr b/src/test/ui/test-attrs/test-warns-dead-code.stderr
index 62e99225dd9..cb118cdb410 100644
--- a/src/test/ui/test-attrs/test-warns-dead-code.stderr
+++ b/src/test/ui/test-attrs/test-warns-dead-code.stderr
@@ -1,8 +1,8 @@
 error: function is never used: `dead`
-  --> $DIR/test-warns-dead-code.rs:5:1
+  --> $DIR/test-warns-dead-code.rs:5:4
    |
 LL | fn dead() {}
-   | ^^^^^^^^^
+   |    ^^^^
    |
 note: lint level defined here
   --> $DIR/test-warns-dead-code.rs:3:9
diff --git a/src/test/ui/thread-local-not-in-prelude.rs b/src/test/ui/thread-local-not-in-prelude.rs
index 03974982625..e5ed09c600b 100644
--- a/src/test/ui/thread-local-not-in-prelude.rs
+++ b/src/test/ui/thread-local-not-in-prelude.rs
@@ -1,5 +1,4 @@
 // run-pass
-
 #![no_std]
 
 extern crate std;