Warn about unnecessary parentheses upon assignment

Closes #12366.

Parentheses around assignment statements such as

    let mut a = (0);
    a = (1);
    a += (2);

are not necessary and therefore an unnecessary_parens warning is raised when
statements like this occur.

The warning mechanism was refactored along the way to allow for code reuse
between the routines for checking expressions and statements.

Code had to be adopted throughout the compiler and standard libraries to comply
with this modification of the lint.

Author: milibopp

src/librustc/metadata/loader.rs

@@ -407,11 +407,8 @@ fn get_metadata_section_imp(os: Os, filename: &Path) -> Option<MetadataBlob> {
                 debug!("checking {} bytes of metadata-version stamp",
                        vlen);
                 let minsz = cmp::min(vlen, csz);
-                let mut version_ok = false;
-                vec::raw::buf_as_slice(cvbuf, minsz, |buf0| {
-                    version_ok = (buf0 ==
-                                  encoder::metadata_encoding_version);
-                });
+                let version_ok = vec::raw::buf_as_slice(cvbuf, minsz,
+                    |buf0| buf0 == encoder::metadata_encoding_version);
                 if !version_ok { return None; }
 
                 let cvbuf1 = cvbuf.offset(vlen as int);

src/librustc/middle/borrowck/gather_loans/lifetime.rs

@@ -95,11 +95,10 @@ impl<'a> GuaranteeLifetimeContext<'a> {
                 let base_scope = self.scope(base);
 
                 // L-Deref-Managed-Imm-User-Root
-                let omit_root = (
+                let omit_root =
                     self.bccx.is_subregion_of(self.loan_region, base_scope) &&
                     self.is_rvalue_or_immutable(base) &&
-                    !self.is_moved(base)
-                );
+                    !self.is_moved(base);
 
                 if !omit_root {
                     // L-Deref-Managed-Imm-Compiler-Root

src/librustc/middle/dataflow.rs

@@ -891,7 +891,7 @@ fn bitwise(out_vec: &mut [uint], in_vec: &[uint], op: |uint, uint| -> uint)
         let old_val = *out_elt;
         let new_val = op(old_val, *in_elt);
         *out_elt = new_val;
-        changed |= (old_val != new_val);
+        changed |= old_val != new_val;
     }
     changed
 }

src/librustc/middle/lint.rs

@@ -1167,22 +1167,41 @@ fn check_pat_non_uppercase_statics(cx: &Context, p: &ast::Pat) {
     }
 }
 
-fn check_unnecessary_parens(cx: &Context, e: &ast::Expr) {
+fn check_unnecessary_parens_core(cx: &Context, value: &ast::Expr, msg: &str) {
+    match value.node {
+        ast::ExprParen(_) => {
+            cx.span_lint(UnnecessaryParens, value.span,
+                         format!("unnecessary parentheses around {}", msg))
+        }
+        _ => {}
+    }
+}
+
+fn check_unnecessary_parens_expr(cx: &Context, e: &ast::Expr) {
     let (value, msg) = match e.node {
         ast::ExprIf(cond, _, _) => (cond, "`if` condition"),
         ast::ExprWhile(cond, _) => (cond, "`while` condition"),
         ast::ExprMatch(head, _) => (head, "`match` head expression"),
         ast::ExprRet(Some(value)) => (value, "`return` value"),
+        ast::ExprAssign(_, value) => (value, "assigned value"),
+        ast::ExprAssignOp(_, _, _, value) => (value, "assigned value"),
         _ => return
     };
+    check_unnecessary_parens_core(cx, value, msg);
+}
 
-    match value.node {
-        ast::ExprParen(_) => {
-            cx.span_lint(UnnecessaryParens, value.span,
-                         format!("unnecessary parentheses around {}", msg))
-        }
-        _ => {}
-    }
+fn check_unnecessary_parens_stmt(cx: &Context, s: &ast::Stmt) {
+    let (value, msg) = match s.node {
+        ast::StmtDecl(decl, _) => match decl.node {
+            ast::DeclLocal(local) => match local.init {
+                Some(value) => (value, "assigned value"),
+                None => return
+            },
+            _ => return
+        },
+        _ => return
+    };
+    check_unnecessary_parens_core(cx, value, msg);
 }
 
 fn check_unused_unsafe(cx: &Context, e: &ast::Expr) {
@@ -1534,7 +1553,7 @@ impl<'a> Visitor<()> for Context<'a> {
 
         check_while_true_expr(self, e);
         check_stability(self, e);
-        check_unnecessary_parens(self, e);
+        check_unnecessary_parens_expr(self, e);
         check_unused_unsafe(self, e);
         check_unsafe_block(self, e);
         check_unnecessary_allocation(self, e);
@@ -1549,6 +1568,7 @@ impl<'a> Visitor<()> for Context<'a> {
     fn visit_stmt(&mut self, s: &ast::Stmt, _: ()) {
         check_path_statement(self, s);
         check_unused_result(self, s);
+        check_unnecessary_parens_stmt(self, s);
 
         visit::walk_stmt(self, s, ());
     }

src/librustc/middle/trans/monomorphize.rs

@@ -143,10 +143,8 @@ pub fn monomorphic_fn(ccx: @CrateContext,
             // This is a bit unfortunate.
 
             let idx = psubsts.tys.len() - num_method_ty_params;
-            let substs =
-                (psubsts.tys.slice(0, idx) +
-                 &[psubsts.self_ty.unwrap()] +
-                 psubsts.tys.tailn(idx));
+            let substs = psubsts.tys.slice(0, idx) +
+                &[psubsts.self_ty.unwrap()] + psubsts.tys.tailn(idx);
             debug!("static default: changed substitution to {}",
                    substs.repr(ccx.tcx));
 

src/librustc/middle/ty.rs

@@ -2293,8 +2293,8 @@ pub fn type_contents(cx: ctxt, ty: t) -> TypeContents {
                        bounds: BuiltinBounds)
                        -> TypeContents {
         // These are the type contents of the (opaque) interior
-        let contents = (TC::ReachesMutable.when(mutbl == ast::MutMutable) |
-                        kind_bounds_to_contents(cx, bounds, []));
+        let contents = TC::ReachesMutable.when(mutbl == ast::MutMutable) |
+            kind_bounds_to_contents(cx, bounds, []);
 
         match store {
             UniqTraitStore => {

src/libserialize/ebml.rs

@@ -674,7 +674,7 @@ pub mod writer {
             let last_size_pos = self.size_positions.pop().unwrap();
             let cur_pos = try!(self.writer.tell());
             try!(self.writer.seek(last_size_pos as i64, io::SeekSet));
-            let size = (cur_pos as uint - last_size_pos - 4);
+            let size = cur_pos as uint - last_size_pos - 4;
             write_sized_vuint(self.writer, size, 4u);
             try!(self.writer.seek(cur_pos as i64, io::SeekSet));
 

src/libstd/os.rs

@@ -119,7 +119,7 @@ pub mod win32 {
                 } else if k == n &&
                           libc::GetLastError() ==
                           libc::ERROR_INSUFFICIENT_BUFFER as DWORD {
-                    n *= (2 as DWORD);
+                    n *= 2 as DWORD;
                 } else if k >= n {
                     n = k;
                 } else {
@@ -225,7 +225,7 @@ pub fn env_as_bytes() -> ~[(~[u8],~[u8])] {
             for p in input.iter() {
                 let vs: ~[&[u8]] = p.splitn(1, |b| *b == '=' as u8).collect();
                 let key = vs[0].to_owned();
-                let val = (if vs.len() < 2 { ~[] } else { vs[1].to_owned() });
+                let val = if vs.len() < 2 { ~[] } else { vs[1].to_owned() };
                 pairs.push((key, val));
             }
             pairs

src/libsyntax/abi.rs

@@ -202,7 +202,7 @@ impl AbiSet {
     }
 
     pub fn add(&mut self, abi: Abi) {
-        self.bits |= (1 << abi.index());
+        self.bits |= 1 << abi.index();
     }
 
     pub fn each(&self, op: |abi: Abi| -> bool) -> bool {

src/libsyntax/ast.rs

@@ -1224,6 +1224,6 @@ mod test {
             },
         };
         // doesn't matter which encoder we use....
-        let _f = (&e as &serialize::Encodable<json::Encoder>);
+        let _f = &e as &serialize::Encodable<json::Encoder>;
     }
 }

src/libsyntax/diagnostic.rs

@@ -329,7 +329,7 @@ fn highlight_lines(cm: &codemap::CodeMap,
         for _ in range(0, skip) { s.push_char(' '); }
         let orig = fm.get_line(lines.lines[0] as int);
         for pos in range(0u, left-skip) {
-            let curChar = (orig[pos] as char);
+            let curChar = orig[pos] as char;
             // Whenever a tab occurs on the previous line, we insert one on
             // the error-point-squiggly-line as well (instead of a space).
             // That way the squiggly line will usually appear in the correct

src/libterm/terminfo/parm.rs

@@ -259,7 +259,7 @@ pub fn expand(cap: &[u8], params: &[Param], vars: &mut Variables)
                             ' ' => flags.space = true,
                             '.' => fstate = FormatStatePrecision,
                             '0'..'9' => {
-                                flags.width = (cur as uint - '0' as uint);
+                                flags.width = cur as uint - '0' as uint;
                                 fstate = FormatStateWidth;
                             }
                             _ => unreachable!()
@@ -359,7 +359,7 @@ pub fn expand(cap: &[u8], params: &[Param], vars: &mut Variables)
                         flags.space = true;
                     }
                     (FormatStateFlags,'0'..'9') => {
-                        flags.width = (cur as uint - '0' as uint);
+                        flags.width = cur as uint - '0' as uint;
                         *fstate = FormatStateWidth;
                     }
                     (FormatStateFlags,'.') => {

src/libtest/lib.rs

@@ -1047,7 +1047,7 @@ impl MetricMap {
             let r = match selfmap.find(k) {
                 None => MetricRemoved,
                 Some(v) => {
-                    let delta = (v.value - vold.value);
+                    let delta = v.value - vold.value;
                     let noise = match noise_pct {
                         None => f64::max(vold.noise.abs(), v.noise.abs()),
                         Some(pct) => vold.value * pct / 100.0

src/libuuid/lib.rs

@@ -296,7 +296,7 @@ impl Uuid {
     ///
     /// This represents the algorithm used to generate the contents
     pub fn get_version(&self) -> Option<UuidVersion> {
-        let v = (self.bytes[6] >> 4);
+        let v = self.bytes[6] >> 4;
         match v {
             1 => Some(Version1Mac),
             2 => Some(Version2Dce),

src/test/compile-fail/lint-unnecessary-parens.rs

@@ -22,4 +22,7 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
+    let mut _a = (0); //~ ERROR unnecessary parentheses around assigned value
+    _a = (0); //~ ERROR unnecessary parentheses around assigned value
+    _a += (1); //~ ERROR unnecessary parentheses around assigned value
 }