Add initial handling for iterators

[yuxincs]

NilAway is panicking on code that ranges over iterators. This PR adds initial handling for such that we assume the results from the iterators are always nonnil, without panicking.

See #287

[github-actions]

Golden Test

Warning

❌ NilAway errors reported on stdlib are different 📈.

3211 errors on base branch (main, f74befd)
3215 errors on test branch (1e3e402)

Diffs

+ /opt/hostedtoolcache/go/1.23.2/x64/src/go/ast/commentmap_test.go:123:14: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- ast/commentmap.go:138:10: literal `nil` returned from `NewCommentMap()` in position 0
+ 	- ast/commentmap_test.go:123:14: result 0 of `NewCommentMap()` called `Comments()` via the assignment(s):
+ 		- `NewCommentMap(...)` to `cmap` at ast/commentmap_test.go:110:2
+ 
+ (Same nil source could also cause potential nil panic(s) at 2 other place(s): "ast/commentmap_test.go:160:8", and "ast/example_test.go:224:15".)
+ /opt/hostedtoolcache/go/1.23.2/x64/src/go/token/position.go:119:9: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- ast/issues_test.go:33:12: unassigned variable `tf` used as receiver to call `Base()`
+ 	- token/position.go:119:9: read by method receiver `f` accessed field `base`
+ /opt/hostedtoolcache/go/1.23.2/x64/src/go/token/position.go:119:9: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- ast/issues_test.go:39:86: unassigned variable `tf` used as receiver to call `Base()`
+ 	- token/position.go:119:9: read by method receiver `f` accessed field `base`
+ /opt/hostedtoolcache/go/1.23.2/x64/src/go/token/position.go:124:9: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- ast/issues_test.go:33:24: unassigned variable `tf` used as receiver to call `Size()`
+ 	- token/position.go:124:9: read by method receiver `f` accessed field `size`
+ /opt/hostedtoolcache/go/1.23.2/x64/src/go/token/position.go:124:9: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- ast/issues_test.go:39:97: unassigned variable `tf` used as receiver to call `Size()`
+ 	- token/position.go:124:9: read by method receiver `f` accessed field `size`
+ /opt/hostedtoolcache/go/1.23.2/x64/src/maps/example_test.go:18:2: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- maps/maps.go:53:10: literal `nil` returned from `Clone()` in position 0
+ 	- maps/example_test.go:18:2: result 0 of `Clone()` written to at an index via the assignment(s):
+ 		- `maps.Clone(m1)` to `m2` at maps/example_test.go:17:2
+ /opt/hostedtoolcache/go/1.23.2/x64/src/maps/maps_test.go:93:2: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- maps/maps.go:53:10: literal `nil` returned from `Clone()` in position 0
+ 	- maps/maps_test.go:93:2: result 0 of `Clone()` written to at an index via the assignment(s):
+ 		- `Clone(m1)` to `mc` at maps/maps_test.go:89:2
+ /opt/hostedtoolcache/go/1.23.2/x64/src/slices/slices.go:322:16: Potential nil panic detected. Observed nil flow from source to dereference point: 
+ 	- slices/slices.go:322:16: read directly from variadic parameter `v` sliced into
+ 
+ (Same nil source could also cause potential nil panic(s) at 3 other place(s): "slices/slices.go:323:20", "slices/slices.go:328:20", and "slices/slices.go:329:16".)
- /opt/hostedtoolcache/go/1.23.2/x64/src/internal/goarch/zgoarch_amd64.go:1:1: INTERNAL ERROR(s):
- nilaway_assertion_analyzer: nilaway_function_analyzer: analyzing function AppendSeq at /opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter.go:49.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter.go:50:6) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[E]
- /opt/hostedtoolcache/go/1.23.2/x64/src/internal/goarch/zgoarch_amd64.go:1:1: INTERNAL ERROR(s):
- nilaway_assertion_analyzer: nilaway_function_analyzer: analyzing function ExampleChunk at /opt/hostedtoolcache/go/1.23.2/x64/src/slices/example_test.go:388.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/slices/example_test.go:405:6) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[slices_test.People]
- analyzing function TestValues at /opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter_test.go:58.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter_test.go:66:7) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[int]
- analyzing function TestChunkRange at /opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter_test.go:275.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/slices/iter_test.go:278:6) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[[]int]
- /opt/hostedtoolcache/go/1.23.2/x64/src/internal/goarch/zgoarch_amd64.go:1:1: INTERNAL ERROR(s):
- nilaway_assertion_analyzer: nilaway_function_analyzer: analyzing function ExamplePreorder at /opt/hostedtoolcache/go/1.23.2/x64/src/go/ast/example_test.go:143.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/go/ast/example_test.go:159:6) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[go/ast.Node]
- analyzing function TestPreorderBreak at /opt/hostedtoolcache/go/1.23.2/x64/src/go/ast/walk_test.go:14.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/go/ast/walk_test.go:28:6) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[go/ast.Node]
- /opt/hostedtoolcache/go/1.23.2/x64/src/internal/goarch/zgoarch_amd64.go:1:1: INTERNAL ERROR(s):
- nilaway_assertion_analyzer: nilaway_function_analyzer: analyzing function TestKeys at /opt/hostedtoolcache/go/1.23.2/x64/src/maps/iter_test.go:32.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/maps/iter_test.go:42:7) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[int]
- analyzing function TestValues at /opt/hostedtoolcache/go/1.23.2/x64/src/maps/iter_test.go:52.1: backpropagation across node (/opt/hostedtoolcache/go/1.23.2/x64/src/maps/iter_test.go:62:7) of type *ast.AssignStmt failed for reason: unrecognized type of rhs in range statement: iter.Seq[int]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.70%. Comparing base (f74befd) to head (46f8fa9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
- Coverage   87.91%   87.70%   -0.21%     
==========================================
  Files          66       66              
  Lines        7935     7949      +14     
==========================================
- Hits         6976     6972       -4     
- Misses        785      796      +11     
- Partials      174      181       +7     

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[sonalmahajan15]

LGTM!