MIR always initializes variables with them in (debuginfo) scope.

[eddyb]

That is, given let x = expr;, MIR will produce the same code as let x; x = expr; (modulo borrows).
When breaking inside expr in a debugger, x will be in scope, uninitialized.
It can be really annoying when dealing with shadowing or arguments.
For example, if you break on the call in let x = ...; let x = f(x); and try to print x, you will get the second x, which is uninitialized at that point.

cc @michaelwoerister

[michaelwoerister]

This might be interesting here:

rust/src/librustc_trans/debuginfo/create_scope_map.rs

Line 246 in c9f1501

// LLVM does not properly generate 'DW_AT_start_scope' fields

How would I best be able to take a look at what the scoping structure of a function looks like (i.e. what expression is assigned to what scope)?

[eddyb]

@michaelwoerister MIR scopes? rustc --unpretty=mir -Z unstable-options on master should print scope IDs for all variables, statements and terminators.

[michaelwoerister]

@eddyb Thanks! I'll take a look.

[michaelwoerister]

r? @michaelwoerister
I'll review this if nobody minds.

Oops, wrong tab...

[michaelwoerister]

@eddyb This looks rather suspicious to me:

rust/src/librustc_mir/build/block.rs

Line 55 in 8b09372

let remainder_scope_id = this.push_scope(remainder_scope, block);

At least from a visibility point of view, the init-scope should not be nested within the remainder-scope.

[eddyb]

@michaelwoerister that nesting is necessary for MIR borrowck AFAICT - if a variable is not in scope, you can't actually initialize it, since it doesn't exist.

[michaelwoerister]

Hm, then I'm inclined to suggest that we do not re-use these scopes for debuginfo and instead build an additional visibility scope tree, as not to conflate two things that are not really the same.

[eddyb]

How terrible would it be to have two scopes, one starting before the let and one after it?
The visibility problems would be fixed by having each VarDecl track two scopes, the one in which it's declared and the one in which it's initialized.