"internal compiler error: trans_lvalue reached fall-through case" with tuple assignment

[jorendorff]

This program triggers an ICE:

fn main() {
    let mut a = 1;
    let mut b = 1;
    (a, b) = (b, a + b);
}

The output is:

$ rustc --version
rustc 0.8-pre (8f65529 2013-09-18 15:05:56 -0700)
host: x86_64-apple-darwin
$ rustc -o err err.rs
err.rs:2:12: 2:13 warning: variable does not need to be mutable [-W unused-mut (default)]
err.rs:2     let mut a = 1;
                     ^
err.rs:3:12: 3:13 warning: variable does not need to be mutable [-W unused-mut (default)]
err.rs:3     let mut b = 1;
                     ^
err.rs:4:4: 4:10 error: internal compiler error: trans_lvalue reached fall-through case: ExprTup(~[@syntax::ast::Expr{id: 17, node: ExprPath(syntax::ast::Path{span: syntax::codemap::Span{lo: syntax::codemap::BytePos(55u), hi: syntax::codemap::BytePos(56u), expn_info: None}, global: false, segments: ~[syntax::ast::PathSegment{identifier: syntax::ast::Ident{name: 68u, ctxt: 59u}, lifetime: None, types: Empty}]}), span: syntax::codemap::Span{lo: syntax::codemap::BytePos(55u), hi: syntax::codemap::BytePos(56u), expn_info: None}}, @syntax::ast::Expr{id: 18, node: ExprPath(syntax::ast::Path{span: syntax::codemap::Span{lo: syntax::codemap::BytePos(58u), hi: syntax::codemap::BytePos(59u), expn_info: None}, global: false, segments: ~[syntax::ast::PathSegment{identifier: syntax::ast::Ident{name: 69u, ctxt: 59u}, lifetime: None, types: Empty}]}), span: syntax::codemap::Span{lo: syntax::codemap::BytePos(58u), hi: syntax::codemap::BytePos(59u), expn_info: None}}])
err.rs:4     (a, b) = (b, a + b);
             ^~~~~~

[alexcrichton]

Closing as a dupe of #7507. (this should be a legit compiler error I believe, not an internal compiler error)