Rollup merge of #145610 - GrigorenkoPV:char_max_len, r=Amanieu

Stabilize `char_max_len`

Tracking issue: rust-lang/rust#121714

r? t-libs-api

`@rustbot` label +needs-fcp -T-libs +T-libs-api

Closes rust-lang/rust#121714

Author: matthiaskrgr