Bug 12936 – Some more @nogc cases for immediately iterated array literal

This is a spinoff of Issue 12932 See this code: struct F { int x; } void main() @nogc { foreach ( a; [F(1)]) {} // Case#1 OK foreach (int[1] a; [[1]]) {} // Case#2 Error foreach (const ref int[1] a; [[1]]) {} // Case#3 Error foreach ( a; [[1]]) {} // Case#4 Error } With dmd2.066alpha gives: test.d(4,25): Error: array literal in @nogc function main may cause GC allocation test.d(5,5): Error: argument type mismatch, int[] to ref const(int[1]) test.d(6,25): Error: array literal in @nogc function main may cause GC allocation Currently the Case#1 works. I think Case#2 could work because it's supposed to be a dynamic array of int[1] values. I am not sure we can also support the Case#4, this is more complex, and perhaps needs escape analisys. So not supporting Case#4 is acceptable for now. Case#4 currently doesn't even compile even without @nogc, but perhaps it should without @nogc.

A different case where the compiler should avoid heap allocations and allow @nogc: void main() @nogc { int[3] a; a[] = [1, 2, 3]; a[0 .. 2] = [1, 2]; } temp.d(3,11): Error: array literal in @nogc function main may cause GC allocation temp.d(4,17): Error: array literal in @nogc function main may cause GC allocation

(In reply to bearophile_hugs from comment #0) > > struct F { int x; } > void main() @nogc { > foreach ( a; [F(1)]) {} // Case#1 OK > foreach (int[1] a; [[1]]) {} // Case#2 Error > foreach (const ref int[1] a; [[1]]) {} // Case#3 Error > foreach ( a; [[1]]) {} // Case#4 Error > } For case#2, it's possible enhancement because int[1] a is a copy of iterated array literal element. For case#3, it's not safe because an address of stack allocated array literal can escape out of the lifetime. const(int)* p; foreach (const ref int[1] a; [[1]]) { p = &a[0]; } // After the foreach, p wold point invalid stack address. Note that similar situation is properly rejected. void foo(ref int[1] sa) {} void main() { foo([1]); // array literal never become stack allocated because // its address can escape out via the ref parameter. } For case#4 supporting it would cause inconsistent type inference result. void foo() { foreach (a; [[1]]) { pragma(msg, typeof(a)); } // prints int[] } void bar() @nogc { foreach (a; [[1]]) { pragma(msg, typeof(a)); } // prints int[] or int[1]? } I opened a compiler improvement for case#2: https://github.com/dlang/dmd/pull/5795

Commits pushed to master at https://github.com/dlang/dmd https://github.com/dlang/dmd/commit/6a48d5bb77be35a3778c7ad673333de232b2e9c8 Issue 12936 - Some more @nogc cases for immediately iterated array literal Support case#2 in bugzilla. https://github.com/dlang/dmd/commit/46c35125b335354077c11973389c7e82686036ff Merge pull request #5795 from 9rnsr/fix12936 Issue 12936 - Some more @nogc cases for immediately iterated array literal

Commits pushed to stable at https://github.com/dlang/dmd https://github.com/dlang/dmd/commit/6a48d5bb77be35a3778c7ad673333de232b2e9c8 Issue 12936 - Some more @nogc cases for immediately iterated array literal https://github.com/dlang/dmd/commit/46c35125b335354077c11973389c7e82686036ff Merge pull request #5795 from 9rnsr/fix12936