Make the unused_mut lint smarter with respect to locals.

Fixes #26332

Author: arielb1

src/libcore/iter.rs

@@ -2655,8 +2655,8 @@ macro_rules! step_impl_signed {
             #[allow(trivial_numeric_casts)]
             fn steps_between(start: &$t, end: &$t, by: &$t) -> Option<usize> {
                 if *by == 0 { return None; }
-                let mut diff: usize;
-                let mut by_u: usize;
+                let diff: usize;
+                let by_u: usize;
                 if *by > 0 {
                     if *start >= *end {
                         return Some(0);

src/libfmt_macros/lib.rs

@@ -399,7 +399,7 @@ impl<'a> Parser<'a> {
             }
             Some(..) | None => { return &self.input[..0]; }
         };
-        let mut end;
+        let end;
         loop {
             match self.cur.clone().next() {
                 Some((_, c)) if c.is_xid_continue() => {

src/librustc/middle/infer/error_reporting.rs

@@ -1854,7 +1854,7 @@ impl LifeGiver {
     }
 
     fn give_lifetime(&self) -> ast::Lifetime {
-        let mut lifetime;
+        let lifetime;
         loop {
             let mut s = String::from("'");
             s.push_str(&num_to_string(self.counter.get()));

src/librustc_borrowck/borrowck/check_loans.rs

@@ -182,7 +182,7 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for CheckLoanCtxt<'a, 'tcx> {
             None => { }
         }
 
-        self.check_assignment(assignment_id, assignment_span, assignee_cmt, mode);
+        self.check_assignment(assignment_id, assignment_span, assignee_cmt);
     }
 
     fn decl_without_init(&mut self, _id: ast::NodeId, _span: Span) { }
@@ -782,16 +782,9 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
     fn check_assignment(&self,
                         assignment_id: ast::NodeId,
                         assignment_span: Span,
-                        assignee_cmt: mc::cmt<'tcx>,
-                        mode: euv::MutateMode) {
+                        assignee_cmt: mc::cmt<'tcx>) {
         debug!("check_assignment(assignee_cmt={:?})", assignee_cmt);
 
-        // Initializations never cause borrow errors as they only
-        // affect a fresh local.
-        if mode == euv::Init {
-            return
-        }
-
         // Check that we don't invalidate any outstanding loans
         if let Some(loan_path) = opt_loan_path(&assignee_cmt) {
             let scope = region::CodeExtent::from_node_id(assignment_id);
@@ -801,17 +794,15 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
             });
         }
 
-        // Local variables can always be assigned to, expect for reassignments
-        // of immutable variables (or assignments that invalidate loans,
-        // of course).
+        // Check for reassignments to (immutable) local variables. This
+        // needs to be done here instead of in check_loans because we
+        // depend on move data.
         if let mc::cat_local(local_id) = assignee_cmt.cat {
-            if assignee_cmt.mutbl.is_mutable() {
-                self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
-            }
-
             let lp = opt_loan_path(&assignee_cmt).unwrap();
             self.move_data.each_assignment_of(assignment_id, &lp, |assign| {
-                if !assignee_cmt.mutbl.is_mutable() {
+                if assignee_cmt.mutbl.is_mutable() {
+                    self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
+                } else {
                     self.bccx.report_reassigned_immutable_variable(
                         assignment_span,
                         &*lp,

src/librustc_trans/save/dump_csv.rs

@@ -308,7 +308,7 @@ impl <'l, 'tcx> DumpCsvVisitor<'l, 'tcx> {
 
         debug!("process_method: {}:{}", id, token::get_name(name));
 
-        let mut scope_id;
+        let scope_id;
         // The qualname for a method is the trait name or name of the struct in an impl in
         // which the method is declared in, followed by the method's name.
         let qualname = match self.tcx.impl_of_method(ast_util::local_def(id)) {

src/libsyntax/parse/lexer/mod.rs

@@ -598,7 +598,7 @@ impl<'a> StringReader<'a> {
 
     /// Lex a LIT_INTEGER or a LIT_FLOAT
     fn scan_number(&mut self, c: char) -> token::Lit {
-        let mut num_digits;
+        let num_digits;
         let mut base = 10;
         let start_bpos = self.last_pos;
 

src/test/compile-fail/lint-unused-mut-variables.rs

@@ -23,6 +23,15 @@ fn main() {
     let mut b = 3; //~ ERROR: variable does not need to be mutable
     let mut a = vec!(3); //~ ERROR: variable does not need to be mutable
     let (mut a, b) = (1, 2); //~ ERROR: variable does not need to be mutable
+    let mut a; //~ ERROR: variable does not need to be mutable
+    a = 3;
+
+    let mut b; //~ ERROR: variable does not need to be mutable
+    if true {
+        b = 3;
+    } else {
+        b = 4;
+    }
 
     match 30 {
         mut x => {} //~ ERROR: variable does not need to be mutable