"the type of this value must be known in this context" is fatal

[pcwalton]

It shouldn't be.

[lkuper]

The "the type of this value must be known in this context" error comes from the typeck::check::structurally_resolved_type() function. structurally_resolved_type() is supposed to take a type and return a one-level-resolved version of it. Right now, it will do that or die trying. If we just make this a non-fatal error, then structurally_resolved_type() will have no choice but to return the original type with unresolved type variables, and let other code deal with the consequences.

In particular, this means that in code like compile-fail/issue-2151.rs, where we currently just get

/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3:20: 3:21 error: the type of this value must be known in this context
/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3         log (debug, i * 2);
                                                                            ^

we'd instead get

/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3:20: 3:21 error: the type of this value must be known in this context
/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3         log (debug, i * 2);
                                                                            ^
/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3:20: 3:25 error: binary operation * cannot be applied to type `<V1>`
/home/lkuper/rust/src/test/compile-fail/issue-2151.rs:3         log (debug, i * 2);
                                                                            ^~~~~

A similar thing happens in compile-fail/issue-2150.rs and in compile-fail/issue-2149.rs. In all three cases, after the "type of this value must be known" error, we reach another error that's essentially a more specific version of it. Is this an improvement? Specificity = probably good, duplicate errors = meh, having to print type variables = probably bad...

[pcwalton]

At the very least it should bail out of the current function and go on to typecheck other functions.

[catamorphism]

This should be fixed by my work on derived errors. Report any further duplicate errors as a new bug.