Auto merge of #50675 - csmoe:var_span, r=oli-obk

Reduce span highlighted code in unused_variables lint

Fixes #50472
- [X] reduce var span
- [ ] mark applicable

Before:
```
mut unused_mut_var
^^^^^^^^^^^^^^^^^^
```
After:
```
mut unused_mut_var
    ^^^^^^^^^^^^^^
```

Author: bors

src/librustc/hir/pat_util.rs

@@ -141,6 +141,15 @@ impl hir::Pat {
         }
     }
 
+    pub fn simple_span(&self) -> Option<Span> {
+        match self.node {
+            PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ref path1, None) |
+            PatKind::Binding(hir::BindingAnnotation::Mutable, _, ref path1, None) =>
+                Some(path1.span),
+            _ => None,
+        }
+    }
+
     /// Return variants that are necessary to exist for the pattern to match.
     pub fn necessary_variants(&self) -> Vec<DefId> {
         let mut variants = vec![];

src/librustc/middle/liveness.rs

@@ -453,11 +453,11 @@ fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
             }
         }
 
-        pat.each_binding(|bm, p_id, sp, path1| {
+        pat.each_binding(|bm, p_id, _sp, path1| {
             debug!("adding local variable {} from match with bm {:?}",
                    p_id, bm);
             let name = path1.node;
-            ir.add_live_node_for_node(p_id, VarDefNode(sp));
+            ir.add_live_node_for_node(p_id, VarDefNode(path1.span));
             ir.add_variable(Local(LocalInfo {
                 id: p_id,
                 name: name,
@@ -628,10 +628,10 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
         F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
     {
-        pat.each_binding(|_bm, p_id, sp, _n| {
+        pat.each_binding(|_bm, p_id, sp, n| {
             let ln = self.live_node(p_id, sp);
-            let var = self.variable(p_id, sp);
-            f(self, ln, var, sp, p_id);
+            let var = self.variable(p_id, n.span);
+            f(self, ln, var, n.span, p_id);
         })
     }
 
@@ -1398,7 +1398,8 @@ fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local)
         },
         None => {
             this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
-                this.warn_about_unused(sp, id, ln, var);
+                let span = local.pat.simple_span().unwrap_or(sp);
+                this.warn_about_unused(span, id, ln, var);
             })
         }
     }
@@ -1497,7 +1498,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
     fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
         for arg in &body.arguments {
-            arg.pat.each_binding(|_bm, p_id, sp, path1| {
+            arg.pat.each_binding(|_bm, p_id, _, path1| {
+                let sp = path1.span;
                 let var = self.variable(p_id, sp);
                 // Ignore unused self.
                 let name = path1.node;
@@ -1541,6 +1543,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
                 let suggest_underscore_msg = format!("consider using `_{}` instead",
                                                      name);
+
                 if is_assigned {
                     self.ir.tcx
                         .lint_node_note(lint::builtin::UNUSED_VARIABLES, id, sp,

src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.rs

@@ -35,6 +35,10 @@ fn main() {
         endless_and_singing: true
     };
 
+    let mut mut_unused_var = 1;
+
+    let (mut var, unused_var) = (1, 2);
+
     if let SoulHistory { corridors_of_light,
                          mut hours_are_suns,
                          endless_and_singing: true } = who_from_the_womb_remembered {

src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr

@@ -11,22 +11,40 @@ LL | #![warn(unused)] // UI tests pass `-A unused` (#43896)
    |         ^^^^^^
    = note: #[warn(unused_variables)] implied by #[warn(unused)]
 
+warning: unused variable: `mut_unused_var`
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:38:13
+   |
+LL |     let mut mut_unused_var = 1;
+   |             ^^^^^^^^^^^^^^ help: consider using `_mut_unused_var` instead
+
+warning: unused variable: `var`
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:40:14
+   |
+LL |     let (mut var, unused_var) = (1, 2);
+   |              ^^^ help: consider using `_var` instead
+
+warning: unused variable: `unused_var`
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:40:19
+   |
+LL |     let (mut var, unused_var) = (1, 2);
+   |                   ^^^^^^^^^^ help: consider using `_unused_var` instead
+
 warning: unused variable: `corridors_of_light`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:38:26
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:42:26
    |
 LL |     if let SoulHistory { corridors_of_light,
    |                          ^^^^^^^^^^^^^^^^^^ help: try ignoring the field: `corridors_of_light: _`
 
 warning: variable `hours_are_suns` is assigned to, but never used
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:39:26
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:43:30
    |
 LL |                          mut hours_are_suns,
-   |                          ^^^^^^^^^^^^^^^^^^
+   |                              ^^^^^^^^^^^^^^
    |
    = note: consider using `_hours_are_suns` instead
 
 warning: value assigned to `hours_are_suns` is never read
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:41:9
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:45:9
    |
 LL |         hours_are_suns = false;
    |         ^^^^^^^^^^^^^^
@@ -39,38 +57,61 @@ LL | #![warn(unused)] // UI tests pass `-A unused` (#43896)
    = note: #[warn(unused_assignments)] implied by #[warn(unused)]
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:50:23
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:54:23
    |
 LL |         Large::Suit { case } => {}
    |                       ^^^^ help: try ignoring the field: `case: _`
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:55:24
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:59:24
    |
 LL |         &Large::Suit { case } => {}
    |                        ^^^^ help: try ignoring the field: `case: _`
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:60:27
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:64:27
    |
 LL |         box Large::Suit { case } => {}
    |                           ^^^^ help: try ignoring the field: `case: _`
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:65:24
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:69:24
    |
 LL |         (Large::Suit { case },) => {}
    |                        ^^^^ help: try ignoring the field: `case: _`
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:70:24
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:74:24
    |
 LL |         [Large::Suit { case }] => {}
    |                        ^^^^ help: try ignoring the field: `case: _`
 
 warning: unused variable: `case`
-  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:75:29
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:79:29
    |
 LL |         Tuple(Large::Suit { case }, ()) => {}
    |                             ^^^^ help: try ignoring the field: `case: _`
 
+warning: variable does not need to be mutable
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:38:9
+   |
+LL |     let mut mut_unused_var = 1;
+   |         ----^^^^^^^^^^^^^^
+   |         |
+   |         help: remove this `mut`
+   |
+note: lint level defined here
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:15:9
+   |
+LL | #![warn(unused)] // UI tests pass `-A unused` (#43896)
+   |         ^^^^^^
+   = note: #[warn(unused_mut)] implied by #[warn(unused)]
+
+warning: variable does not need to be mutable
+  --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:40:10
+   |
+LL |     let (mut var, unused_var) = (1, 2);
+   |          ----^^^
+   |          |
+   |          help: remove this `mut`
+