In `:break ident` allow out of scope and nested identifiers (Fix #3000)

This patch fixes the bug and implements the feature request of #3000.

1. If `Module` is a real module name and `identifier` a name of a
top-level function in `Module` then `:break Module.identifer` works
also for an `identifier` that is out of scope.

2. Extend the syntax for `:break identifier` to:

    :break [ModQual.]topLevelIdent[.nestedIdent]...[.nestedIdent]

`ModQual` is optional and is either the effective name of a module or
the local alias of a qualified import statement.

`topLevelIdent` is the name of a top level function in the module
referenced by `ModQual`.

`nestedIdent` is optional and the name of a function nested in a let or
where clause inside the previously mentioned function `nestedIdent` or
`topLevelIdent`.

If `ModQual` is a module name, then `topLevelIdent` can be any top level
identifier in this module. If `ModQual` is missing or a local alias of a
qualified import, then `topLevelIdent` must be in scope.

Breakpoints can be set on arbitrarily deeply nested functions, but the
whole chain of nested function names must be specified.

3. To support the new functionality rewrite the code to tab complete `:break`.

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/GHC/ByteCode/Types.hs b/compiler/GHC/ByteCode/Types.hs
index 55ad604447..296f886a29 100644
--- a/compiler/GHC/ByteCode/Types.hs
+++ b/compiler/GHC/ByteCode/Types.hs
@@ -154,6 +154,7 @@ data ModBreaks
         -- ^ An array giving the names of the free variables at each breakpoint.
    , modBreaks_decls :: !(Array BreakIndex [String])
         -- ^ An array giving the names of the declarations enclosing each breakpoint.
+        -- See Note [Field modBreaks_decls]
    , modBreaks_ccs :: !(Array BreakIndex (RemotePtr CostCentre))
         -- ^ Array pointing to cost centre for each breakpoint
    , modBreaks_breakInfo :: IntMap CgBreakInfo
@@ -180,3 +181,12 @@ emptyModBreaks = ModBreaks
    , modBreaks_ccs = array (0,-1) []
    , modBreaks_breakInfo = IntMap.empty
    }
+
+{-
+Note [Field modBreaks_decls]
+~~~~~~~~~~~~~~~~~~~~~~
+A value of eg ["foo", "bar", "baz"] in a `modBreaks_decls` field means:
+The breakpoint is in the function called "baz" that is declared in a `let`
+or `where` clause of a declaration called "bar", which itself is declared
+in a `let` or `where` clause of the top-level function called "foo".
+-}