add notes for immutable inputs

clippy_lints/src/lib.rs

@@ -15,6 +15,7 @@
 #![allow(needless_lifetimes)]
 
 extern crate syntax;
+extern crate syntax_pos;
 #[macro_use]
 extern crate rustc;
 extern crate rustc_data_structures;

clippy_lints/src/ptr.rs

@@ -5,7 +5,9 @@ use rustc::hir::map::NodeItem;
 use rustc::lint::*;
 use rustc::ty;
 use syntax::ast::NodeId;
-use utils::{match_path, match_type, paths, span_lint};
+use syntax::codemap::Span;
+use syntax_pos::MultiSpan;
+use utils::{match_path, match_type, paths, span_lint, span_lint_and_then};
 
 /// **What it does:** This lint checks for function arguments of type `&String` or `&Vec` unless
 /// the references are mutable.
@@ -132,29 +134,31 @@ fn check_fn(cx: &LateContext, decl: &FnDecl, fn_id: NodeId) {
     }
 
     if let FunctionRetTy::Return(ref ty) = decl.output {
-        if let Some((out, MutMutable)) = get_rptr_lm(ty) {
-            if let Some(MutImmutable) =
-                decl.inputs
-                    .iter()
-                    .filter_map(|ty| get_rptr_lm(ty))
-                    .filter(|&(lt, _)| lt.name == out.name)
-                    .fold(None, |x, (_, m)| match (x, m) {
-                        (Some(MutMutable), _) |
-                        (_, MutMutable) => Some(MutMutable),
-                        (_, m) => Some(m),
-                    }) {
-                span_lint(cx,
-                          MUT_FROM_REF,
-                          ty.span,
-                          "this function takes an immutable ref to return a mutable one");
+        if let Some((out, MutMutable, _)) = get_rptr_lm(ty) {
+            let mut immutables = vec![];
+            for (_, ref mutbl, ref argspan) in decl.inputs
+                          .iter()
+                          .filter_map(|ty| get_rptr_lm(ty))
+                          .filter(|&(lt, _, _)| lt.name == out.name) {
+                if *mutbl == MutMutable { return; }
+                immutables.push(*argspan);
             }
+            if immutables.is_empty() { return; }
+            span_lint_and_then(cx,
+                               MUT_FROM_REF,
+                               ty.span,
+                               "mutable borrow from immutable input(s)",
+                               |db| {
+                                    let ms = MultiSpan::from_spans(immutables);
+                                    db.span_note(ms, "immutable borrow here");
+                               });
         }
     }
 }
 
-fn get_rptr_lm(ty: &Ty) -> Option<(&Lifetime, Mutability)> {
+fn get_rptr_lm(ty: &Ty) -> Option<(&Lifetime, Mutability, Span)> {
     if let Ty_::TyRptr(ref lt, ref m) = ty.node {
-        Some((lt, m.mutbl))
+        Some((lt, m.mutbl, ty.span))
     } else {
         None
     }

tests/ui/mut_from_ref.rs

@@ -29,6 +29,10 @@ fn fail_lifetime<'a>(x: &'a u32, y: &mut u32) -> &'a mut u32 {
     unimplemented!()
 }
 
+fn fail_double<'a>(x: &'a u32, y: &'a u32, z: &'b mut u32) -> &'a mut u32 {
+    unimplemented!()
+}
+
 // this is OK, because the result borrows y
 fn works<'a>(x: &u32, y: &'a mut u32) -> &'a mut u32 {
     unimplemented!()

tests/ui/mut_from_ref.stderr

@@ -1,32 +1,8 @@
-error: this function takes an immutable ref to return a mutable one
- --> $DIR/mut_from_ref.rs:9:39
-  |
-9 |     fn this_wont_hurt_a_bit(&self) -> &mut Foo {
-  |                                       ^^^^^^^^
-  |
-note: lint level defined here
- --> $DIR/mut_from_ref.rs:4:9
-  |
-4 | #![deny(mut_from_ref)]
-  |         ^^^^^^^^^^^^
-
-error: this function takes an immutable ref to return a mutable one
-  --> $DIR/mut_from_ref.rs:15:25
+error[E0261]: use of undeclared lifetime name `'b`
+  --> $DIR/mut_from_ref.rs:32:48
    |
-15 |     fn ouch(x: &Foo) -> &mut Foo;
-   |                         ^^^^^^^^
+32 | fn fail_double<'a>(x: &'a u32, y: &'a u32, z: &'b mut u32) -> &'a mut u32 {
+   |                                                ^^ undeclared lifetime
 
-error: this function takes an immutable ref to return a mutable one
-  --> $DIR/mut_from_ref.rs:24:21
-   |
-24 | fn fail(x: &u32) -> &mut u16 {
-   |                     ^^^^^^^^
-
-error: this function takes an immutable ref to return a mutable one
-  --> $DIR/mut_from_ref.rs:28:50
-   |
-28 | fn fail_lifetime<'a>(x: &'a u32, y: &mut u32) -> &'a mut u32 {
-   |                                                  ^^^^^^^^^^^
-
-error: aborting due to 4 previous errors
+error: aborting due to previous error