effects: refine :nothrow when exct is known to be Bottom (#52270)

Implements effects refinement that goes in the reverse direction of what
was implemented in 8dd0cf5.

In the discussion at #52268, there's a debate on how to
deal with exceptions
like `StackOverflowError` or `InterruptException` that are caused by
environment. The
original purpose of #52268 was to fix issues where methods that could
cause a
`StackOverflowError` were mistakenly refined to `:nothrow`. But now it
feels like it's a bit
weird to model such environment-dependent exceptions as `:nothrow` in
the first place. If we
can exclude environment-dependent errors from the `:nothrow` modeling,
those methods might
be safely considered `:nothrow`. To prevent the concrete evaluation of
methods that may
cause `StackOverflowError`, it's enough to taint its `:teminates` after
all.
This commit excludes environment-depending errors from the `:nothrow`
modeling, reverting the changes from #52268 while making it explicit in 
the `Effects` and `Base.@assume_effects` docstrings.

Anyway, now the compile is able to prove `:nothrow`-ness of the
following frame:
```julia
function getindex_nothrow(xs::Vector{Int}, i::Int)
    try
        return xs[i]
    catch err
        err isa BoundsError && return nothing
        rethrow(err)
    end
end
```

Author: aviatesk

base/compiler/effects.jl

@@ -24,6 +24,11 @@ following meanings:
   * `EFFECT_FREE_IF_INACCESSIBLEMEMONLY`: the `:effect-free`-ness of this method can later be
     refined to `ALWAYS_TRUE` in a case when `:inaccessiblememonly` is proven.
 - `nothrow::Bool`: this method is guaranteed to not throw an exception.
+  If the execution of this method may raise `MethodError`s and similar exceptions, then
+  the method is not considered as `:nothrow`.
+  However, note that environment-dependent errors like `StackOverflowError` or `InterruptException`
+  are not modeled by this effect and thus a method that may result in `StackOverflowError`
+  does not necessarily need to taint `:nothrow` (although it should usually taint `:terminates` too).
 - `terminates::Bool`: this method is guaranteed to terminate.
 - `notaskstate::Bool`: this method does not access any state bound to the current
   task and may thus be moved to a different task without changing observable

base/compiler/typeinfer.jl

@@ -463,6 +463,11 @@ function adjust_effects(sv::InferenceState)
         # always throwing an error counts or never returning both count as consistent
         ipo_effects = Effects(ipo_effects; consistent=ALWAYS_TRUE)
     end
+    if sv.exc_bestguess === Bottom
+        # if the exception type of this frame is known to be `Bottom`,
+        # this frame is known to be safe
+        ipo_effects = Effects(ipo_effects; nothrow=true)
+    end
     if is_inaccessiblemem_or_argmemonly(ipo_effects) && all(1:narguments(sv, #=include_va=#true)) do i::Int
             return is_mutation_free_argtype(sv.slottypes[i])
         end
@@ -883,9 +888,6 @@ function typeinf_edge(interp::AbstractInterpreter, method::Method, @nospecialize
     update_valid_age!(caller, frame.valid_worlds)
     effects = adjust_effects(Effects(), method)
     exc_bestguess = refine_exception_type(frame.exc_bestguess, effects)
-    # this call can fail into an infinite cycle, so incorporate this fact into
-    # `exc_bestguess` by merging `StackOverflowError` into it
-    exc_bestguess = tmerge(typeinf_lattice(interp), Core.StackOverflowError, exc_bestguess)
     return EdgeCallResult(frame.bestguess, exc_bestguess, nothing, effects)
 end
 

base/expr.jl

@@ -552,7 +552,7 @@ were not executed.
 ---
 ## `:nothrow`
 
-The `:nothrow` settings asserts that this method does not terminate abnormally
+The `:nothrow` settings asserts that this method does not throw an exception
 (i.e. will either always return a value or never return).
 
 !!! note
@@ -561,7 +561,11 @@ The `:nothrow` settings asserts that this method does not terminate abnormally
     method itself.
 
 !!! note
-    `MethodErrors` and similar exceptions count as abnormal termination.
+    If the execution of a method may raise `MethodError`s and similar exceptions, then
+    the method is not considered as `:nothrow`.
+    However, note that environment-dependent errors like `StackOverflowError` or `InterruptException`
+    are not modeled by this effect and thus a method that may result in `StackOverflowError`
+    does not necessarily need to be `!:nothrow` (although it should usually be `!:terminates` too).
 
 ---
 ## `:terminates_globally`

test/compiler/effects.jl

@@ -1290,3 +1290,15 @@ end |> !Core.Compiler.is_noub
 @test Base.infer_effects((Vector{Any},Int)) do xs, i
     @inbounds getindex_dont_propagate(xs, i)
 end |> Core.Compiler.is_noub
+
+# refine `:nothrow` when `exct` is known to be `Bottom`
+@test Base.infer_exception_type(getindex, (Vector{Int},Int)) == BoundsError
+function getindex_nothrow(xs::Vector{Int}, i::Int)
+    try
+        return xs[i]
+    catch err
+        err isa BoundsError && return nothing
+        rethrow(err)
+    end
+end
+@test Core.Compiler.is_nothrow(Base.infer_effects(getindex_nothrow, (Vector{Int}, Int)))

test/compiler/inference.jl

@@ -423,13 +423,11 @@ A14009(a::T) where {T} = A14009{T}()
 f14009(a) = rand(Bool) ? f14009(A14009(a)) : a
 code_typed(f14009, (Int,))
 code_llvm(devnull, f14009, (Int,))
-@test Base.infer_exception_type(f14009, (Int,)) != Union{}
 
 mutable struct B14009{T}; end
 g14009(a) = g14009(B14009{a})
 code_typed(g14009, (Type{Int},))
 code_llvm(devnull, g14009, (Type{Int},))
-@test Base.infer_exception_type(g14009, (Type{Int},)) == StackOverflowError
 
 # issue #9232
 arithtype9232(::Type{T},::Type{T}) where {T<:Real} = arithtype9232(T)
@@ -5560,7 +5558,7 @@ function foo_typed_throw_metherr()
 end
 @test Base.return_types(foo_typed_throw_metherr) |> only === Float64
 
-# using `exct` information if `:nothrow` is proven
+# refine `exct` when `:nothrow` is proven
 Base.@assume_effects :nothrow function sin_nothrow(x::Float64)
     x == Inf && return zero(x)
     return sin(x)