<p dir="auto">Regarding <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="4703438" data-permission-text="Title is private" data-url="https://github.com/rust-lang/rust/issues/2429" data-hovercard-type="issue" data-hovercard-url="/rust-lang/rust/issues/2429/hovercard" href="https://github.com/rust-lang/rust/issues/2429">#2429</a>, I have observed the same error occurring in the recent incoming (<code class="notranslate">5f13e9c 2013-04-02 13:36:51 -0700</code>):</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="use core::hashmap::linear::LinearMap;
fn main() {
    let mut map = LinearMap::new();
    let set_default = |k: int, v: int| {
        // N.B. we can't use match here due to the other bug #4666
        let insert =
            match map.find_mut(&amp;k) {
                Some(vs) =&gt; { /*let vs: &amp;mut ~[int] = vs;*/ vs.push(v); false }
                None =&gt; true
            };
        if insert { map.insert(k, ~[v]); }
    };
    set_default(1, 2);
    set_default(2, 3);
    set_default(1, 4);
    for map.each |&amp;(&amp;k, &amp;v)| { io::println(fmt!(&quot;%? -&gt; %?&quot;, k, v)); }
}"><pre lang=".rust" class="notranslate"><code class="notranslate">use core::hashmap::linear::LinearMap;
fn main() {
    let mut map = LinearMap::new();
    let set_default = |k: int, v: int| {
        // N.B. we can't use match here due to the other bug #4666
        let insert =
            match map.find_mut(&amp;k) {
                Some(vs) =&gt; { /*let vs: &amp;mut ~[int] = vs;*/ vs.push(v); false }
                None =&gt; true
            };
        if insert { map.insert(k, ~[v]); }
    };
    set_default(1, 2);
    set_default(2, 3);
    set_default(1, 4);
    for map.each |&amp;(&amp;k, &amp;v)| { io::println(fmt!("%? -&gt; %?", k, v)); }
}
</code></pre></div>
<p dir="auto">...resulting in:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="t.rs:6:24: 6:26 error: the type of this value must be known in this context
t.rs:6             Some(vs) =&gt; vs.push(v),
                               ^~"><pre class="notranslate"><code class="notranslate">t.rs:6:24: 6:26 error: the type of this value must be known in this context
t.rs:6             Some(vs) =&gt; vs.push(v),
                               ^~
</code></pre></div>
<p dir="auto">It compiles (and works) correctly if we uncomment <code class="notranslate">let vs: ... = vs;</code> part.</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="~ &gt; cat test.rs
trait Foo {}

trait Bar {}

impl&lt;T: Bar&gt; Foo for T {}

impl Foo for int {}
~ &gt; rustc --lib test.rs
test.rs:7:0: 7:19 error: conflicting implementations for a trait
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 note: note conflicting implementation here
test.rs:5 impl&lt;T: Bar&gt; Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 error: conflicting implementations for a trait
test.rs:5 impl&lt;T: Bar&gt; Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:7:0: 7:19 note: note conflicting implementation here
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors"><pre class="notranslate"><code class="notranslate">~ &gt; cat test.rs
trait Foo {}

trait Bar {}

impl&lt;T: Bar&gt; Foo for T {}

impl Foo for int {}
~ &gt; rustc --lib test.rs
test.rs:7:0: 7:19 error: conflicting implementations for a trait
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 note: note conflicting implementation here
test.rs:5 impl&lt;T: Bar&gt; Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:5:0: 5:25 error: conflicting implementations for a trait
test.rs:5 impl&lt;T: Bar&gt; Foo for T {}
          ^~~~~~~~~~~~~~~~~~~~~~~~~
test.rs:7:0: 7:19 note: note conflicting implementation here
test.rs:7 impl Foo for int {}
          ^~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors
</code></pre></div>
<p dir="auto">I think this is related to <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="6731351" data-permission-text="Title is private" data-url="https://github.com/rust-lang/rust/issues/3429" data-hovercard-type="issue" data-hovercard-url="/rust-lang/rust/issues/3429/hovercard" href="https://github.com/rust-lang/rust/issues/3429">#3429</a> but is far more general.</p>